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Sample records for dynamic shear stress

  1. Effects of dynamic shear and transmural pressure on wall shear stress sensitivity in collecting lymphatic vessels.

    PubMed

    Kornuta, Jeffrey A; Nepiyushchikh, Zhanna; Gasheva, Olga Y; Mukherjee, Anish; Zawieja, David C; Dixon, J Brandon

    2015-11-01

    Given the known mechanosensitivity of the lymphatic vasculature, we sought to investigate the effects of dynamic wall shear stress (WSS) on collecting lymphatic vessels while controlling for transmural pressure. Using a previously developed ex vivo lymphatic perfusion system (ELPS) capable of independently controlling both transaxial pressure gradient and average transmural pressure on an isolated lymphatic vessel, we imposed a multitude of flow conditions on rat thoracic ducts, while controlling for transmural pressure and measuring diameter changes. By gradually increasing the imposed flow through a vessel, we determined the WSS at which the vessel first shows sign of contraction inhibition, defining this point as the shear stress sensitivity of the vessel. The shear stress threshold that triggered a contractile response was significantly greater at a transmural pressure of 5 cmH2O (0.97 dyne/cm(2)) than at 3 cmH2O (0.64 dyne/cm(2)). While contraction frequency was reduced when a steady WSS was applied, this inhibition was reversed when the applied WSS oscillated, even though the mean wall shear stresses between the conditions were not significantly different. When the applied oscillatory WSS was large enough, flow itself synchronized the lymphatic contractions to the exact frequency of the applied waveform. Both transmural pressure and the rate of change of WSS have significant impacts on the contractile response of lymphatic vessels to flow. Specifically, time-varying shear stress can alter the inhibition of phasic contraction frequency and even coordinate contractions, providing evidence that dynamic shear could play an important role in the contractile function of collecting lymphatic vessels. PMID:26333787

  2. Experimental investigation of the wall shear stress and the vortex dynamics in a circular impinging jet

    NASA Astrophysics Data System (ADS)

    El Hassan, Mouhammad; Assoum, Hassan Hassan; Sobolik, Vaclav; Vétel, Jérôme; Abed-Meraim, Kamel; Garon, André; Sakout, Anas

    2012-06-01

    The wall shear stress and the vortex dynamics in a circular impinging jet are investigated experimentally for Re = 1,260 and 2,450. The wall shear stress is obtained at different radial locations from the stagnation point using the polarographic method. The velocity field is given from the time resolved particle image velocimetry (TR-PIV) technique in both the free jet region and near the wall in the impinging region. The distribution of the momentum thickness is also inspected from the jet exit toward the impinged wall. It is found that the wall shear stress is correlated with the large-scale vortex passing. Both the primary vortices and the secondary structures strongly affect the variation of the wall shear stress. The maximum mean wall shear stress is obtained just upstream from the secondary vortex generation where the primary structures impinge the wall. Spectral analysis and cross-correlations between the wall shear stress fluctuations show that the vortex passing influences the wall shear stress at different locations simultaneously. Analysis of cross-correlations between temporal fluctuations of the wall shear stress and the transverse vorticity brings out the role of different vortical structures on the wall shear stress distribution for the two Reynolds numbers.

  3. Evaluation of Shear-Induced Platelet Activation Models Under Constant and Dynamic Shear Stress Loading Conditions Relevant to Devices

    PubMed Central

    Sheriff, Jawaad; Soares, João Silva; Xenos, Michalis; Jesty, Jolyon; Bluestein, Danny

    2013-01-01

    The advent of implantable blood-recirculating devices such as left ventricular assist devices and prosthetic heart valves provides a viable therapy for patients with end-stage heart failure and valvular disease. However, device-generated pathological flow patterns result in thromboembolic complications that require complex and lifelong anticoagulant therapy, which entails hemorrhagic risks and is not appropriate for certain patients. Optimizing the thrombogenic performance of such devices utilizing numerical simulations requires the development of predictive platelet activation models that account for variations in shear-loading rates characterizing blood flow through such devices. Platelets were exposed in vitro to both dynamic and constant shear stress conditions emulating those found in blood-recirculating devices in order to determine their shear-induced activation and sensitization response. Both these behaviors were found to be dependent on the shear loading rates, in addition to shear stress magnitude and exposure time. We then critically examined several current models and evaluated their predictive capabilities using these results. Shear loading rate terms were then included to account for dynamic aspects that are either ignored or partially considered by these models, and model parameters were optimized. Independent optimization for each of the two types of shear stress exposure conditions tested resulted in different sets of best-fit constants, indicating that universal optimization may not be possible. Inherent limitations of the current models require a paradigm shift from these integral-based discretized power law models to better address the dynamic conditions encountered in blood-recirculating devices. PMID:23400312

  4. Stress dynamics of a 2D dense granular system near shear jamming

    NASA Astrophysics Data System (ADS)

    Ren, Jie; Dijksman, Joshua; Behringer, Robert

    2013-03-01

    We study the dynamics of pressure and shear stress in a frictional 2D dense granular system using a novel apparatus that can provide fixed-volume shear without generating inhomogeneities. Under increasing shear strain, the system's pressure shows a strong increase with strain, characterized by a ``Reynolds coefficient,'' R =d2 P / dγ2 . R depends only on packing fraction ϕ, and shows a strong increase as ϕ approaches ϕJ from below. In the meantime, the system's shear stress shows a non-monotonic behavior with increasing strain. It first increases with strain as the system is in ``fragile'' states and builds up long force chains along the compression direction. After a certain amount of strain, force chains along the dilation direction starts to build up, and the system transfers into a ``shear-jammed'' state and the shear stress starts to decrease with strain. Under oscillatory shear, both pressure and shear stress show limit-cycle behavior and reach steady states after many cycles. However, the limit cycles of pressure and shear stress are very different: the pressure exhibits a hysteresis-free parabolic curve, while the shear stress exhibits a strongly hysteretic loop. This work is funded by NSF grants: DMR0906908, DMS0835571, NASA grant NNX10AU01G and ARO grant W911NF-11-1-0110.

  5. Working Principle Simulations of a Dynamic Resonant Wall Shear Stress Sensor Concept

    PubMed Central

    Zhang, Xu; Naughton, Jonathan W.; Lindberg, William R.

    2008-01-01

    This paper discusses a novel dynamic resonant wall shear stress sensor concept based on an oscillating sensor operating near resonance. The interaction between the oscillating sensor surface and the fluid above it is modelled using the unsteady laminar boundary layer equations. The numerical experiment shows that the effect of the oscillating shear stress is well correlated by the Hummer number, the ratio of the steady shear force caused by the outside flow to the oscillating viscous force created by the sensor motion. The oscillating shear stress predicted by the fluid model is used in a mechanical model of the sensor to predict the sensor's dynamic motion. Static calibration curves for amplitude and frequency influences are predicted. These results agree with experimental results on some extent, and shows some expectation for further development of the dynamic resonant sensor concept.

  6. Dynamic response of micro-pillar sensors measuring fluctuating wall-shear-stress

    NASA Astrophysics Data System (ADS)

    Brücker, Ch.; Bauer, D.; Chaves, H.

    2007-05-01

    We present in this paper test results of flexible micro-pillars and pillar arrays for wall shear stress measurements in flows with fluctuating wall shear stress such as unsteady separated flows or turbulent flows. Previous papers reported on the sensing principle and fabrication process. Static calibrations have shown this sensor to have a maximum nonlinearity of 1% over two orders of wall-shear-stress. For measurements in flows with fluctuating wall shear stress the dynamic response has been experimentally verified in an oscillating pipe flow and compared to a calculated response based on Stokes’ and Oseen’s solution for unsteady flow around a cylinder. The results demonstrate good agreement under the given boundary conditions of cylindrical micro-pillars and the limit of viscous Stokes-flow around the pillar. Depending on the fluid and pillar geometry, different response curves result ranging from a flat low-pass filtered response to a strong resonant behavior. Two different methods are developed to detect the frequency content and the directional wall shear stress information from image processing of large sensor films with arrays of micro-pillars of different geometry. Design rules are given to achieve the optimal conditions with respect to signal-to-noise ratio, sensitivity and bandwidth for measurements in turbulent flows.

  7. Normal and shear impact of layered composite with a crack - Dynamic stress intensification

    NASA Technical Reports Server (NTRS)

    Sih, G. C.; Chen, E. P.

    1980-01-01

    The dynamic response of a layered composite under normal and shear impact is analyzed by assuming that the composite contains an initial flaw in the matrix material. One of the objectives was to develop an analytical method for determining dynamic stress solutions which should lead to a numerical method which utilizes Fourier transform for the space variable and Laplace transform for the time variable. The time-dependent angle loading is separated into two parts: a symmetric and a skew-symmetric with reference to the crack plane. By superposition, the transient boundary conditions consist of applying normal and shear tractions to a crack embedded in a layered composite; one phase of the composite could represent the fiber while the other could be the matrix. Mathematically, these conditions reduce the problem to a system of dual integral equations solved in the transform plane for the transform of the dynamic stress-intensity factor.

  8. Dynamic Deformation and Recovery Response of Red Blood Cells to a Cyclically Reversing Shear Flow: Effects of Frequency of Cyclically Reversing Shear Flow and Shear Stress Level

    PubMed Central

    Watanabe, Nobuo; Kataoka, Hiroyuki; Yasuda, Toshitaka; Takatani, Setsuo

    2006-01-01

    Dynamic deformation and recovery responses of red blood cells (RBCs) to a cyclically reversing shear flow generated in a 30-μm clearance, with the peak shear stress of 53, 108, 161, and 274 Pa at the frequency of 1, 2, 3, and 5 Hz, respectively, were studied. The RBCs' time-varying velocity varied after the glass plate velocity without any time lag, whereas the L/W change, where L and W were the major and minor axes of RBCs' ellipsoidal shape, exhibited a rapid increase and gradual decay during the deformation and recovery phase. The time of minimum L/W occurrence lagged behind the zero-velocity time of the glass plate (zero stress), and the delay time normalized to the one-cycle duration remained constant at 8.0%. The elongation of RBCs at zero stress time became larger with the reversing frequency. A simple mechanical model consisting of an elastic linear element during a rapid elongation period and a parallel combination of elements such as a spring and dashpot during the nonlinear recovery phase was suggested. The dynamic response behavior of RBCs under a cyclically reversing shear flow was different from the conventional shape change where a steplike force was applied to and completely released from the RBCs. PMID:16766612

  9. Yield shear stress and disaggregating shear stress of human blood

    NASA Astrophysics Data System (ADS)

    Jung, Jinmu; Lee, Byoung-Kwon; Shin, Sehyun

    2014-05-01

    This review presents two distinct rheological parameters of blood that have the potential to indicate blood circulation adequacy: yield shear stress (YSS) and disaggregating shear stress (DSS). YSS and DSS reflect the strength of red blood cell (RBC) aggregation in suspension under static and dynamic conditions, respectively. YSS, defined as the critical stress to disperse RBC aggregates under static conditions, was found to be dependent upon hematocrit, fibrinogen, and red cell deformability, but not temperature. DSS, defined as the minimum shear stress to disperse RBC aggregates under dynamic conditions, is dependent upon fibrinogen, red cell deformability, and temperature but not hematocrit. Owing to recent advances in measurement technology, these two parameters can be easily measured, and thus, their clinical significance in blood circulation can be verified.

  10. Measurement of shear stress-mediated intracellular calcium dynamics in human dermal lymphatic endothelial cells

    PubMed Central

    Jafarnejad, M.; Cromer, W. E.; Kaunas, R. R.; Zhang, S. L.; Zawieja, D. C.

    2015-01-01

    The shear stress applied to lymphatic endothelial cells (LEC) by lymph flow changes dramatically under normal conditions as well as in response to disease conditions and immune reactions. In general, LEC are known to regulate the contraction frequency and strength of lymphatic pumping in response to shear stress. Intracellular calcium concentration ([Ca2+]i) is an important factor that regulates lymphatic contraction characteristics. In this study, we measured changes in the [Ca2+]i under different shear stress levels and determined the source of this calcium signal. Briefly, human dermal LEC were cultured in custom-made microchannels for 3 days before loading with 2 µM fura-2 AM, a ratiometric calcium dye to measure [Ca2+]i. Step changes in shear stress resulted in a rapid increase in [Ca2+]i followed by a gradual return to the basal level and sometimes below the initial baseline (45.2 ± 2.2 nM). The [Ca2+]i reached a peak at 126.2 ± 5.6 nM for 10 dyn/cm2 stimulus, whereas the peak was only 71.8 ± 5.4 nM for 1 dyn/cm2 stimulus, indicating that the calcium signal depends on the magnitude of shear stress. Removal of the extracellular calcium from the buffer or pharmocological blockade of calcium release-activated calcium (CRAC) channels significantly reduced the peak [Ca2+]i, demonstrating a role of extracellular calcium entry. Inhibition of endoplasmic reticulum (ER) calcium pumps showed the importance of intracellular calcium stores in the initiation of this signal. In conclusion, we demonstrated that the shear-mediated calcium signal is dependent on the magnitude of the shear and involves ER store calcium release and extracellular calcium entry. PMID:25617358

  11. Dynamic deformation capability of a red blood cell under a cyclically reciprocating shear stress.

    PubMed

    Watanabe, N; Yasuda, T; Kataoka, H; Takatani, S

    2004-01-01

    Red blood cells (RBCs) in the cardiovascular devices are exposed to varying degree of the shear stress from all the directions. However the RBCs' deformability or the deformation capability under such a shear stress is not well understood. In this study, we designed and built a system that can induce a cyclically reciprocating shear stress to a RBC suspension. The arm of the cyclically reciprocating shear stress device was attached to the upper piece of the parallel glass plates between which a suspension of human RBCs (1% hematocrit whole blood diluted in a 32 weight% dextran phosphate buffer solution) was contained. The cyclic reciprocating motion of the upper glass plate of 3.0 mm stroke length was produced using a slider-crank shaft mechanism that was linked to an eccentric cam-motor system. Each rotation of the motor produced a 3.0 mm stroke each in the forward and backward direction of the slider block. The clearance between the two glass plates was adjusted to 30 micrometer. The cyclic reciprocating glass plate apparatus was attached to a light microscope stage (IX71 Olympus with x40 objective lens) for illumination with a 350 watt metal halide light source. A high speed camera (MEMREMCAM fx-K3 Nac, 5000 frames per second with shutter kept open) was attached to the microscope to capture the deformation process of the RBCs under cyclic shear stress. The preliminary result indicated that the correlation between the amplitude of the maximum shear stress and the RBCs' deformability. This indicates a potential application of the cyclic reciprocating device to evaluate the temporal response of the RBCs deformability prior to its destruction. The future study will focus on the study of the relative velocity of the erythrocytes with respect to the velocity of the reciprocating plate. PMID:17271457

  12. Dynamic shear-stress-enhanced rates of nutrient consumption in gas-liquid semi-continuous-flow suspensions

    NASA Astrophysics Data System (ADS)

    Belfiore, Laurence A.; Volpato, Fabio Z.; Paulino, Alexandre T.; Belfiore, Carol J.

    2011-12-01

    The primary objective of this investigation is to establish guidelines for generating significant mammalian cell density in suspension bioreactors when stress-sensitive kinetics enhance the rate of nutrient consumption. Ultra-low-frequency dynamic modulations of the impeller (i.e., 35104 Hz) introduce time-dependent oscillatory shear into this transient analysis of cell proliferation under semi-continuous creeping flow conditions. Greater nutrient consumption is predicted when the amplitude A of modulated impeller rotation increases, and stress-kinetic contributions to nutrient consumption rates increase linearly at higher modulation frequency via an application of fluctuation-dissipation response. Interphase mass transfer is required to replace dissolved oxygen as it is consumed by aerobic nutrient consumption in the liquid phase. The theory and predictions described herein could be important at small length scales in the creeping flow regime where viscous shear is significant at the interface between the nutrient medium and isolated cells in suspension. Two-dimensional flow around spherically shaped mammalian cells, suspended in a Newtonian culture medium, is analyzed to calculate the surface-averaged magnitude of the velocity gradient tensor and modify homogeneous rates of nutrient consumption that are stimulated by viscous shear, via the formalism of stress-kinetic reciprocal relations that obey Curie's theorem in non-equilibrium thermodynamics. Time constants for stress-free free and stress-sensitive stress nutrient consumption are defined and quantified to identify the threshold (i.e., stress,threshold) below which the effect of stress cannot be neglected in accurate predictions of bioreactor performance. Parametric studies reveal that the threshold time constant for stress-sensitive nutrient consumption stress,threshold decreases when the time constant for stress

  13. A Piezoelectric Shear Stress Sensor

    NASA Technical Reports Server (NTRS)

    Kim, Taeyang; Saini, Aditya; Kim, Jinwook; Gopalarathnam, Ashok; Zhu, Yong; Palmieri, Frank L.; Wohl, Christopher J.; Jiang, Xiaoning

    2016-01-01

    In this paper, a piezoelectric sensor with a floating element was developed for shear stress measurement. The piezoelectric sensor was designed to detect the pure shear stress suppressing effects of normal stress generated from the vortex lift-up by applying opposite poling vectors to the: piezoelectric elements. The sensor was first calibrated in the lab by applying shear forces and it showed high sensitivity to shear stress (=91.3 +/- 2.1 pC/Pa) due to the high piezoelectric coefficients of PMN-33%PT (d31=-1330 pC/N). The sensor also showed almost no sensitivity to normal stress (less than 1.2 pC/Pa) because of the electromechanical symmetry of the device. The usable frequency range of the sensor is 0-800 Hz. Keywords: Piezoelectric sensor, shear stress, floating element, electromechanical symmetry

  14. A piezoelectric shear stress sensor

    NASA Astrophysics Data System (ADS)

    Kim, Taeyang; Saini, Aditya; Kim, Jinwook; Gopalarathnam, Ashok; Zhu, Yong; Palmieri, Frank L.; Wohl, Christopher J.; Jiang, Xiaoning

    2016-04-01

    In this paper, a piezoelectric sensor with a floating element was developed for shear stress measurement. The piezoelectric sensor was designed to detect the pure shear stress, suppressing effects of normal stress components, by applying opposite poling vectors to the piezoelectric elements. The sensor was first calibrated in the lab by applying shear forces where it demonstrated high sensitivity to shear stress (91.3 +/- 2.1 pC/Pa) due to the high piezoelectric coefficients of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-33%PT, d31=-1330 pC/N). The sensor also exhibited negligible sensitivity to normal stress (less than 1.2 pC/Pa) because of the electromechanical symmetry of the device. The usable frequency range of the sensor is up to 800 Hz.

  15. Dynamic blood flow and wall shear stress in pulmonary hypertensive disease.

    PubMed

    Postles, Arthur; Clark, Alys R; Tawhai, Merryn H

    2014-01-01

    This study provides new model of pulsatile flow in the pulmonary circulation in health and pulmonary hypertensive disease. Structural vascular remodeling typical of pulmonary hypertensive disease was implemented in the model by progressively altering the mechanical properties of the arterial geometry and progressively increasing the inlet pulse pressure (PP). The transmission of PP throughout the tree was shown to increase in advanced stages of disease, creating the potential for a `vicious-cycle' of damage to vasculature. Wall shear stress (WSS) was shown to be highest in the terminal arteries of the model and increased significantly with disease. A further trend observed in WSS results was that high WSS values began to `climb' the arterial tree towards the proximal vessels as disease progressed. This suggests a link between WSS and distal remodeling in pulmonary hypertensive disease, which initiates in the small muscular arteries and arterioles and spreads into larger arteries as the disease progresses. PMID:25571282

  16. Analysis of structure and orientation of adsorbed polymers in solution subject to a dynamic shear stress

    SciTech Connect

    Smith, G.; Baker, S.; Toprakcioglu, C.

    1996-09-01

    This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Polymer-based separation techniques rely on the ability of a binding portion of the polymer to interact with a specific molecule in a solution flowing past the polymer. The location of the binding site within or out of the entangled polymer chains is thus crucial to the effectiveness of these methods. For this reason, the details of flow induced deformation of the polymer chains is important in such applications as exclusion chromatography, waste water treatment, ultrafiltration, enhanced oil recovery and microbial adhesion. Few techniques exist to examine the structure and orientation of polymeric materials, and even fewer to examine systems in a dynamic fluid flow. The goal of this project was to understand the molecular structure and orientation of adsorbed polymers with and without active binding ligands as a function of solvent shear rate, solvent power, polymer molecular weight, surface polymer coverage and heterogeneity of the surface polymer chains by neutron reflectometry in a newly designed shear cell. Geometrical effects on binding of molecules in the flow was also studied subject to the same parameters.

  17. Dynamics of a one-dimensional model for the emergence of the plasma edge shear flow layer with momentum-conserving Reynolds stress

    SciTech Connect

    Calvo, I.; Carreras, B. A.

    2007-10-15

    A one-dimensional version of the second-order transition model based on the sheared flow amplification by Reynolds stress and turbulence suppression by shearing is presented. The model discussed in this paper includes a form of the Reynolds stress which explicitly conserves momentum. A linear stability analysis of the critical point is performed. Then, it is shown that the dynamics of weakly unstable states is determined by a reduced equation for the shear flow. In the case in which the flow damping term is diffusive, the stationary solutions are those of the real Ginzburg-Landau equation.

  18. Observations of dynamic stall phenomena on an oscillating airfoil with shear-stress-sensitive liquid crystal coatings

    SciTech Connect

    Reda, D.C.

    1989-01-01

    Oscillating airfoil experiments were conducted using shear-stress- sensitive/temperature-insensitive liquid crystal coatings in order to investigate unsteady fluid physics associated with the dynamic-stall process. Laser-light-sheet/smoke-particle flow visualization and surface-mounted micro-tufts were also employed to complement the liquid crystal technique. Experiments were conducted under incompressible flow conditions at a freestream Reynolds number (based on chord) of {approximately}10{sup 6}. Angle-of-attack oscillations of {plus minus}19{degree} about 0{degree}, at several discrete frequencies, were used to induce the unsteady flows. Boundary layer transition and turbulent separation locations were seen to undergo extensive and rapid movements with changing angle of attack, particularly on the airfoil lee surface. Progression of turbulent separation to the immediate vicinity of the leading edge was observed (via the liquid crystal technique) to result in large-scale, high-frequency fluctuations in the surface shear stress distribution. Comparisons of transition and turbulent separation measurements with Eppler code predictions indicated that the empirically-based viscous flow modeling used in this design tool requires updates. 19 refs., 8 figs., 1 tab.

  19. A three-dimensional computational fluid dynamics model of shear stress distribution during neotissue growth in a perfusion bioreactor.

    PubMed

    Guyot, Y; Luyten, F P; Schrooten, J; Papantoniou, I; Geris, L

    2015-12-01

    Bone tissue engineering strategies use flow through perfusion bioreactors to apply mechanical stimuli to cells seeded on porous scaffolds. Cells grow on the scaffold surface but also by bridging the scaffold pores leading a fully filled scaffold following the scaffold's geometric characteristics. Current computational fluid dynamic approaches for tissue engineering bioreactor systems have been mostly carried out for empty scaffolds. The effect of 3D cell growth and extracellular matrix formation (termed in this study as neotissue growth), on its surrounding fluid flow field is a challenge yet to be tackled. In this work a combined approach was followed linking curvature driven cell growth to fluid dynamics modeling. The level-set method (LSM) was employed to capture neotissue growth driven by curvature, while the Stokes and Darcy equations, combined in the Brinkman equation, provided information regarding the distribution of the shear stress profile at the neotissue/medium interface and within the neotissue itself during growth. The neotissue was assumed to be micro-porous allowing flow through its structure while at the same time allowing the simulation of complete scaffold filling without numerical convergence issues. The results show a significant difference in the amplitude of shear stress for cells located within the micro-porous neo-tissue or at the neotissue/medium interface, demonstrating the importance of taking along the neotissue in the calculation of the mechanical stimulation of cells during culture.The presented computational framework is used on different scaffold pore geometries demonstrating its potential to be used a design as tool for scaffold architecture taking into account the growing neotissue. Biotechnol. Bioeng. 2015;112: 2591-2600. © 2015 Wiley Periodicals, Inc. PMID:26059101

  20. Computational Fluid Dynamics Analysis to Determine Shear Stresses and Rates in a Centrifugal Left Ventricular Assist Device

    PubMed Central

    Selgrade, Brian Paul; Truskey, George A.

    2014-01-01

    Axial flow left ventricular assist devices (LVADs) are a significant improvement in mechanical circulatory support. However, patients with these devices experience degradation of large von Willebrand factor (vWF) multimers, which is associated with bleeding and may be caused by high shear stresses within the LVAD. In this study, we used computational fluid mechanics to determine the wall shear stresses, shear rates, and residence times in a centrifugal LVAD and assess the impact on these variables caused by changing impeller speed and changing from a shrouded to a semi-open impeller. In both LVAD types, shear rates were well over 10 000/s in several regions. This is high enough to degrade vWF, but it is unclear if residence times, which were below 5 ms in high-shear regions, are long enough to allow vWF cleavage. Additionally, wall shear stresses were below the threshold stress of 10 Pa only in the outlet tube so it is feasible to endothelialize this region to enhance its biocompatibility. PMID:22360826

  1. Dynamics of Sheared Granular Materials

    NASA Astrophysics Data System (ADS)

    Kondic, Lou; Utter, Brian; Behringer, Robert P.

    2002-11-01

    This work focuses on the properties of sheared granular materials near the jamming transition. The project currently involves two aspects. The first of these is an experiment that is a prototype for a planned ISS (International Space Station) flight. The second is discrete element simulations (DES) that can give insight into the behavior one might expect in a reduced-g environment. The experimental arrangement consists of an annular channel that contains the granular material. One surface, say the upper surface, rotates so as to shear the material contained in the annulus. The lower surface controls the mean density/mean stress on the sample through an actuator or other control system. A novel feature under development is the ability to 'thermalize' the layer, i.e. create a larger amount of random motion in the material, by using the actuating system to provide vibrations as well control the mean volume of the annulus. The stress states of the system are determined by transducers on the non-rotating wall. These measure both shear and normal components of the stress on different size scales. Here, the idea is to characterize the system as the density varies through values spanning dense almost solid to relatively mobile granular states. This transition regime encompasses the regime usually thought of as the glass transition, and/or the jamming transition. Motivation for this experiment springs from ideas of a granular glass transition, a related jamming transition, and from recent experiments. In particular, we note recent experiments carried out by our group to characterize this type of transition and also to demonstrate/ characterize fluctuations in slowly sheared systems. These experiments give key insights into what one might expect in near-zero g. In particular, they show that the compressibility of granular systems diverges at a transition or critical point. It is this divergence, coupled to gravity, that makes it extremely difficult if not impossible to

  2. Dynamics of Sheared Granular Materials

    NASA Technical Reports Server (NTRS)

    Kondic, Lou; Utter, Brian; Behringer, Robert P.

    2002-01-01

    This work focuses on the properties of sheared granular materials near the jamming transition. The project currently involves two aspects. The first of these is an experiment that is a prototype for a planned ISS (International Space Station) flight. The second is discrete element simulations (DES) that can give insight into the behavior one might expect in a reduced-g environment. The experimental arrangement consists of an annular channel that contains the granular material. One surface, say the upper surface, rotates so as to shear the material contained in the annulus. The lower surface controls the mean density/mean stress on the sample through an actuator or other control system. A novel feature under development is the ability to 'thermalize' the layer, i.e. create a larger amount of random motion in the material, by using the actuating system to provide vibrations as well control the mean volume of the annulus. The stress states of the system are determined by transducers on the non-rotating wall. These measure both shear and normal components of the stress on different size scales. Here, the idea is to characterize the system as the density varies through values spanning dense almost solid to relatively mobile granular states. This transition regime encompasses the regime usually thought of as the glass transition, and/or the jamming transition. Motivation for this experiment springs from ideas of a granular glass transition, a related jamming transition, and from recent experiments. In particular, we note recent experiments carried out by our group to characterize this type of transition and also to demonstrate/ characterize fluctuations in slowly sheared systems. These experiments give key insights into what one might expect in near-zero g. In particular, they show that the compressibility of granular systems diverges at a transition or critical point. It is this divergence, coupled to gravity, that makes it extremely difficult if not impossible to

  3. Flow fields, bed shear stresses, and suspended bed sediment dynamics in bifurcations of a large river

    NASA Astrophysics Data System (ADS)

    Szupiany, R. N.; Amsler, M. L.; Hernandez, J.; Parsons, D. R.; Best, J. L.; Fornari, E.; Trento, A.

    2012-11-01

    Channel bifurcations associated with bars and islands are important nodes in braided rivers and may control flow partitioning and thus affect downstream confluences, as well as the formation and dynamics of bars. However, the morphodynamic processes associated with bar formation are poorly understood, and previous studies have largely concerned laboratory experiments, small natural streams, or numerical analyses with large Froude numbers, high slopes, and low Shields stresses. In these cases, the morphologic changes at bifurcations are relatively rapid, with predominant bed load transport and the suspended load playing a minor role. In this paper, the evolution of the flow structure and suspended bed sediment transport along four expansion-diffluence units in the Rio Paraná, Argentina, are described. The Rio Paraná is a large multichannel river with a bed composed of medium and fine sands and possesses low Froude numbers and high suspended bed material transport. Primary and secondary flow velocity components were measured with an acoustic Doppler current profiler (ADCP) along the expansion-diffluence units, and the backscatter signal of the ADCP was calibrated to allow simultaneous measurements of suspended bed sediment concentrations. The interactions between these variables show that the cores of primary flow velocity and suspended bed sediment concentration do not necessarily follow the thalweg at the bifurcation and that inertial effects on the suspended bed sediment may influence the morphodynamics of bar formation. It is suggested that changes in flow stage, as well as the presence of vegetation, may further increase the deposition of suspended bed sediment at the bar head. This study suggests that the ratio of suspended bed material to bed load is an important factor controlling the morphodynamics of bifurcations in large sand bed braided rivers.

  4. Dynamic pore-pressure fluctuations in rapidly shearing granular materials

    USGS Publications Warehouse

    Iverson, R.M.; LaHusen, R.G.

    1989-01-01

    Results from two types of experiments show that intergranular pore pressures fluctuated dynamically during rapid, steady shear deformation of water-saturated granular materials. During some fluctuations, the pore water locally supported all normal and shear stresses, while grain-contact stresses transiently fell to zero. Fluctuations also propagated outward from the shear zone; this process modifies grain-contact stresses in adjacent areas and potentially instigates shear-zone growth.

  5. Vortex dynamics and wall shear stress behaviour associated with an elliptic jet impinging upon a flat plate

    NASA Astrophysics Data System (ADS)

    Long, J.; New, T. H.

    2016-07-01

    Vortical structures and dynamics of a Re h = 2100 elliptic jet impinging upon a flat plate were studied at H/ d h = 1, 2 and 4 jet-to-plate separation distances. Flow investigations were conducted along both its major and minor planes using laser-induced fluorescence and digital particle image velocimetry techniques. Results show that the impingement process along the major plane largely consists of primary jet ring-vortex and wall-separated secondary vortex formations, where they subsequently separate from the flat plate at smaller H/ d h = 1 and 2 separation distances. Key vortex formation locations occur closer to the impingement point as the separation distance increases. Interestingly, braid vortices and rib structures begin to take part in the impingement process at H/ d h = 4 and wave instabilities dominate the flow field. In contrast, significantly more coherent primary and secondary vortices with physically larger vortex core sizes and higher vortex strengths are observed along the minor plane, with no signs of braid vortices and rib structures. Lastly, influences of these different flow dynamics on the major and minor plane instantaneous and mean skin friction coefficient levels are investigated to shed light on the effects of separation distance on the wall shear stress distributions.

  6. A dynamic jamming point for shear thickening suspensions

    NASA Astrophysics Data System (ADS)

    Brown, Eric; Jaeger, Heinrich

    2008-11-01

    Densely packed suspensions can shear thicken, in which the viscosity increases with shear rate. We performed rheometry measurements on two model systems: corn starch in water and glass spheres in oils. In both systems we observed shear thickening up to a critical packing fraction φc (=0.55 for spherical grains) above which the flow abruptly transitions to shear thinning. The viscosity and yield stress diverge as power laws at φc. Extrapolating the dynamic ranges of shear rate and stress in the shear thickening regime up to φc suggests a finite change in shear stress with zero change in shear rate. This is a dynamic analog to the jamming point with a yield stress at zero shear rate.

  7. Computational Fluid Dynamic Simulations of Maternal Circulation: Wall Shear Stress in the Human Placenta and Its Biological Implications

    PubMed Central

    Lecarpentier, E.; Bhatt, M.; Bertin, G. I.; Deloison, B.; Salomon, L. J.; Deloron, P.; Fournier, T.; Barakat, A. I.; Tsatsaris, V.

    2016-01-01

    Introduction In the human placenta the maternal blood circulates in the intervillous space (IVS). The syncytiotrophoblast (STB) is in direct contact with maternal blood. The wall shear stress (WSS) exerted by the maternal blood flow on the STB has not been evaluated. Our objective was to determine the physiological WSS exerted on the surface of the STB during the third trimester of pregnancy. Material and Methods To gain insight into the shear stress levels that the STB is expected to experience in vivo, we have formulated three different computational models of varying levels of complexity that reflect different physical representations of the IVS. Computations of the flow fields in all models were performed using the CFD module of the finite element code COMSOL Multiphysics 4.4. The mean velocity of maternal blood in the IVS during the third trimester was measured in vivo with dynamic MRI (0.94±0.14 mm.s-1). To investigate if the in silico results are consistent with physiological observations, we studied the cytoadhesion of human parasitized (Plasmodium falciparum) erythrocytes to primary human STB cultures, in flow conditions with different WSS values. Results The WSS applied to the STB is highly heterogeneous in the IVS. The estimated average values are relatively low (0.5±0.2 to 2.3±1.1 dyn.cm-2). The increase of WSS from 0.15 to 5 dyn.cm-2 was associated with a significant decrease of infected erythrocyte cytoadhesion. No cytoadhesion of infected erythrocytes was observed above 5 dyn.cm-2 applied for one hour. Conclusion Our study provides for the first time a WSS estimation in the maternal placental circulation. In spite of high maternal blood flow rates, the average WSS applied at the surface of the chorionic villi is low (<5 dyn.cm-2). These results provide the basis for future physiologically-relevant in vitro studies of the biological effects of WSS on the STB. PMID:26815115

  8. Study of flow behaviors on single-cell manipulation and shear stress reduction in microfluidic chips using computational fluid dynamics simulations

    PubMed Central

    Shen, Feng; Li, XiuJun; Li, Paul C. H.

    2014-01-01

    Various single-cell retention structures (SCRSs) were reported for analysis of single cells within microfluidic devices. Undesirable flow behaviors within micro-environments not only influence single-cell manipulation and retention significantly but also lead to cell damage, biochemical heterogeneity among different individual cells (e.g., different cell signaling pathways induced by shear stress). However, the fundamentals in flow behaviors for single-cell manipulation and shear stress reduction, especially comparison of these behaviors in different microstructures, were not fully investigated in previous reports. Herein, flow distribution and induced shear stress in two different single-cell retention structures (SCRS I and SCRS II) were investigated in detail to study their effects on single-cell trapping using computational fluid dynamics (CFD) methods. The results were successfully verified by experimental results. Comparison between these two SCRS shows that the wasp-waisted configuration of SCRS II has a better performance in trapping and manipulating long cylinder-shaped cardiac myocytes and provides a safer “harbor” for fragile cells to prevent cell damage due to the shear stress induced from strong flows. The simulation results have not only explained flow phenomena observed in experiments but also predict new flow phenomena, providing guidelines for new chip design and optimization, and a better understanding of the cell micro-environment and fundamentals of microfluidic flows in single-cell manipulation and analysis. PMID:24753729

  9. Micro-computed tomography based computational fluid dynamics for the determination of shear stresses in scaffolds within a perfusion bioreactor.

    PubMed

    Zermatten, Emilie; Vetsch, Jolanda Rita; Ruffoni, Davide; Hofmann, Sandra; Müller, Ralph; Steinfeld, Aldo

    2014-05-01

    Perfusion bioreactors are known to exert shear stresses on cultured cells, leading to cell differentiation and enhanced extracellular matrix deposition on scaffolds. The influence of the scaffold's porous microstructure is investigated for a polycaprolactone (PCL) scaffold with a regular microarchitecture and a silk fibroin (SF) scaffold with an irregular network of interconnected pores. Their complex 3D geometries are imaged by micro-computed tomography and used in direct pore-level simulations of the entire scaffold-bioreactor system to numerically solve the governing mass and momentum conservation equations for fluid flow through porous media. The velocity field and wall shear stress distribution are determined for both scaffolds. The PCL scaffold exhibited an asymmetric distribution with peak and plateau, while the SF scaffold exhibited a homogenous distribution and conditioned the flow more efficiently than the PCL scaffold. The methodology guides the design and optimization of the scaffold geometry. PMID:24492950

  10. Flexible Micropost Arrays for Shear Stress Measurement

    NASA Technical Reports Server (NTRS)

    Wohl, Christopher J.; Palmieri, Frank L.; Hopkins, John W.; Jackson, Allen M.; Connell, John W.; Lin, Yi; Cisotto, Alexxandra A.

    2015-01-01

    Increased fuel costs, heightened environmental protection requirements, and noise abatement continue to place drag reduction at the forefront of aerospace research priorities. Unfortunately, shortfalls still exist in the fundamental understanding of boundary-layer airflow over aerodynamic surfaces, especially regarding drag arising from skin friction. For example, there is insufficient availability of instrumentation to adequately characterize complex flows with strong pressure gradients, heat transfer, wall mass flux, three-dimensionality, separation, shock waves, and transient phenomena. One example is the acoustic liner efficacy on aircraft engine nacelle walls. Active measurement of shear stress in boundary layer airflow would enable a better understanding of how aircraft structure and flight dynamics affect skin friction. Current shear stress measurement techniques suffer from reliability, complexity, and airflow disruption, thereby compromising resultant shear stress data. The state-of-the-art for shear stress sensing uses indirect or direct measurement techniques. Indirect measurements (e.g., hot-wire, heat flux gages, oil interferometry, laser Doppler anemometry, small scale pressure drag surfaces, i.e., fences) require intricate knowledge of the studied flow, restrictive instrument arrangements, large surface areas, flow disruption, or seeding material; with smaller, higher bandwidth probes under development. Direct measurements involve strain displacement of a sensor element and require no prior knowledge of the flow. Unfortunately, conventional "floating" recessed components for direct measurements are mm to cm in size. Whispering gallery mode devices and Fiber Bragg Gratings are examples of recent additions to this type of sensor with much smaller (?m) sensor components. Direct detection techniques are often single point measurements and difficult to calibrate and implement in wind tunnel experiments. In addition, the wiring, packaging, and installation

  11. Determining Shear Stress Distribution in a Laminate

    NASA Technical Reports Server (NTRS)

    Bednarcyk, Brett A.; Aboudi, Jacob; Yarrington, Phillip W.

    2010-01-01

    A "simplified shear solution" method approximates the through-thickness shear stress distribution within a composite laminate based on an extension of laminated beam theory. The method does not consider the solution of a particular boundary value problem; rather, it requires only knowledge of the global shear loading, geometry, and material properties of the laminate or panel. It is thus analogous to lamination theory in that ply-level stresses can be efficiently determined from global load resultants at a given location in a structure and used to evaluate the margin of safety on a ply-by-ply basis. The simplified shear solution stress distribution is zero at free surfaces, continuous at ply boundaries, and integrates to the applied shear load. The method has been incorporated within the HyperSizer commercial structural sizing software to improve its predictive capability for designing composite structures. The HyperSizer structural sizing software is used extensively by NASA to design composite structures. In the case of through-thickness shear loading on panels, HyperSizer previously included a basic, industry-standard, method for approximating the resulting shear stress distribution in sandwich panels. However, no such method was employed for solid laminate panels. The purpose of the innovation is to provide an approximation of the through-thickness shear stresses in a solid laminate given the through-thickness shear loads (Qx and Qy) on the panel. The method was needed for implementation within the HyperSizer structural sizing software so that the approximated ply-level shear stresses could be utilized in a failure theory to assess the adequacy of a panel design. The simplified shear solution method was developed based on extending and generalizing bi-material beam theory to plate-like structures. It is assumed that the through-thickness shear stresses arise due to local bending of the laminate induced by the through-thickness shear load, and by imposing

  12. Turbulent shear stresses in compressible boundary layers

    NASA Technical Reports Server (NTRS)

    Laderman, A. J.; Demetriades, A.

    1979-01-01

    Hot-wire anemometer measurements of turbulent shear stresses in a Mach 3 compressible boundary layer were performed in order to investigate the effects of heat transfer on turbulence. Measurements were obtained by an x-probe in a flat plate, zero pressure gradient, two dimensional boundary layer in a wind tunnel with wall to freestream temperature ratios of 0.94 and 0.71. The measured shear stress distributions are found to be in good agreement with previous results, supporting the contention that the shear stress distribution is essentially independent of Mach number and heat transfer for Mach numbers from incompressible to hypersonic and wall to freestream temperature ratios of 0.4 to 1.0. It is also found that corrections for frequency response limitations of the electronic equipment are necessary to determine the correct shear stress distribution, particularly at the walls.

  13. Coupling curvature-dependent and shear stress-stimulated neotissue growth in dynamic bioreactor cultures: a 3D computational model of a complete scaffold.

    PubMed

    Guyot, Y; Papantoniou, I; Luyten, F P; Geris, L

    2016-02-01

    The main challenge in tissue engineering consists in understanding and controlling the growth process of in vitro cultured neotissues toward obtaining functional tissues. Computational models can provide crucial information on appropriate bioreactor and scaffold design but also on the bioprocess environment and culture conditions. In this study, the development of a 3D model using the level set method to capture the growth of a microporous neotissue domain in a dynamic culture environment (perfusion bioreactor) was pursued. In our model, neotissue growth velocity was influenced by scaffold geometry as well as by flow- induced shear stresses. The neotissue was modeled as a homogenous porous medium with a given permeability, and the Brinkman equation was used to calculate the flow profile in both neotissue and void space. Neotissue growth was modeled until the scaffold void volume was filled, thus capturing already established experimental observations, in particular the differences between scaffold filling under different flow regimes. This tool is envisaged as a scaffold shape and bioprocess optimization tool with predictive capacities. It will allow controlling fluid flow during long-term culture, whereby neotissue growth alters flow patterns, in order to provide shear stress profiles and magnitudes across the whole scaffold volume influencing, in turn, the neotissue growth. PMID:26758425

  14. Stress diffusion in models for shear banding

    NASA Astrophysics Data System (ADS)

    Masnada, Elian; Olmsted, Peter

    Understanding shear banding is of utmost importance from both theoretical and experimental point of view and consequently it has been studied for several decades. Despite this study numerous aspects of shear banding remains poorly understood. Because of the intrinsic inhomogeneity in the shear banded state, applicable constitutive models must be include spatial inhomogeneities, leading to a so-called 'diffusive' term in the equation of motion for the slow variables that carry stress. Such terms are also vital in describing the interaction of bulk shear banding flows with walls and incorporation of wall slip. In this work, we consider different sources of 'diffusion' in polymer models in which concentration degrees of freedom are negligible. The simplest models used are consistent with diffusive terms whose origin is intrinsically dissipative, such as due to hydrodynamic interactions. By contrast, models in which elastic effects such as finite chain stiffness contribute to stress diffusion are inconsistent with simple diffusive models, and we propose alternative consistent models

  15. Spatiotemporal dynamics of shear induced bands en route to rheochaos

    NASA Astrophysics Data System (ADS)

    Ganapathy, R.; Majumdar, S.; Sood, A. K.

    2008-08-01

    We show experimentally that the route to rheochaos in shear rate relaxation measurements is via Type-III intermittency and mixed mode oscillations in the shear-thinning wormlike micellar system of cetyltrimethylammonium tosylate in the presence of salt sodium chloride. Depolarised small angle light scattering measurements performed during flow show that scattered intensity temporally follows the shear rate/stress dynamics and portrays the crucial role played by nematic ordering. Direct visualization of the gap of the Couette cell, illuminated by an unpolarised laser sheet, in the (vorticity, velocity gradient) plane shows that the spatiotemporal dynamics of the shear induced structures is closely related to the temporal behaviour of shear rate/stress fluctuations.

  16. Stress enhanced shear yielding in aging polymer glasses

    NASA Astrophysics Data System (ADS)

    Rottler, Joerg; Liu, Amy Y.-H.

    2010-03-01

    The plastic response of polymer glasses is strongly dependent on the thermomechanical history of the material. We determine the molecular level origin of the enhancement of the shear yield stress reported in experiments of polymer glasses that undergo physical aging in the presence of a pre-stress. Molecular dynamics simulations are employed to show that the applied stress does not alter the physical aging rate, but instead induces a highly orientation-dependent mechanical response of the polymer glass. The change in yield stress with respect to polymers that have aged without pre-stress is directly proportional to the orientation of covalent bonds, which is proportional to strain and logarithmic aging time. We observe a pronounced Bauschinger effect, which amplifies or reduces the pressure dependence of shear yielding. Control simulations with a monovalent Lennard-Jones glass offer further evidence that these effects are distinct from other rejuvenation and overaging behavior reported for a broad class of amorphous solids.

  17. A multi-shear perfusion bioreactor for investigating shear stress effects in endothelial cell constructs.

    PubMed

    Rotenberg, Menahem Y; Ruvinov, Emil; Armoza, Anna; Cohen, Smadar

    2012-08-01

    Tissue engineering research is increasingly relying on the use of advanced cultivation technologies that provide rigorously-controlled cell microenvironments. Herein, we describe the features of a micro-fabricated Multi-Shear Perfusion Bioreactor (MSPB) designed to deliver up to six different levels of physiologically-relevant shear stresses (1-13 dyne cm(-2)) to six cell constructs simultaneously, during a single run. To attain a homogeneous fluid flow within each construct, flow-distributing nets photo-etched with a set of openings for fluid flow were placed up- and down-stream from each construct. Human umbilical vein endothelial cells (HUVECs) seeded in alginate scaffolds within the MSPB and subjected to three different levels of shear stress for 24 h, responded accordingly by expressing three different levels of the membranal marker Intercellular Adhesion Molecule 1 (ICAM-1) and the phosphorylated endothelial nitric oxide synthetase (eNOS). A longer period of cultivation, 17 d, under two different levels of shear stress resulted in different lengths of cell sprouts within the constructs. Collectively, the HUVEC behaviour within the different constructs confirms the feasibility of using the MSPB system for simultaneously imposing different shear stress levels, and for validating the flow regime in the bioreactor vessel as assessed by the computational fluid dynamic (CFD) model. PMID:22622237

  18. A Shearing-Stretching Device That Can Apply Physiological Fluid Shear Stress and Cyclic Stretch Concurrently to Endothelial Cells.

    PubMed

    Meza, Daphne; Abejar, Louie; Rubenstein, David A; Yin, Wei

    2016-03-01

    Endothelial cell (EC) morphology and functions can be highly impacted by the mechanical stresses that the cells experience in vivo. In most areas in the vasculature, ECs are continuously exposed to unsteady blood flow-induced shear stress and vasodilation-contraction-induced tensile stress/strain simultaneously. Investigations on how ECs respond to combined shear stress and tensile strain will help us to better understand how an altered mechanical environment affects EC mechanotransduction, dysfunction, and associated cardiovascular disease development. In the present study, a programmable shearing and stretching device that can apply dynamic fluid shear stress and cyclic tensile strain simultaneously to cultured ECs was developed. Flow and stress/strain conditions in the device were simulated using a fluid structure interaction (FSI) model. To characterize the performance of this device and the effect of combined shear stress-tensile strain on EC morphology, human coronary artery ECs (HCAECs) were exposed to concurrent shear stress and cyclic tensile strain in the device. Changes in EC morphology were evaluated through cell elongation, cell alignment, and cell junctional actin accumulation. Results obtained from the numerical simulation indicated that in the "in-plane" area of the device, both fluid shear stress and biaxial tensile strain were uniform. Results obtained from the in vitro experiments demonstrated that shear stress, alone or combined with cyclic tensile strain, induced significant cell elongation. While biaxial tensile strain alone did not induce any appreciable change in EC elongation. Fluid shear stress and cyclic tensile strain had different effects on EC actin filament alignment and accumulation. By combining various fluid shear stress and cyclic tensile strain conditions, this device can provide a physiologically relevant mechanical environment to study EC responses to physiological and pathological mechanical stimulation. PMID:26810848

  19. Shear Stress in Magnetorheological FInishing for Glasses

    SciTech Connect

    Miao, C.; Shafrir, S.N.; Lambropoulos, J.C.; Mici, J.; Jacobs, S.D.

    2009-04-28

    We report in situ, simultaneous measurements of both drag and normal forces in magnetorheological finishing (MRF) for what is believed to be the first time, using a spot taking machine (STM) as a test bed to take MRF spots on stationary parts. The measurements are carried out over the entire area where material is being removed, i.e., the projected area of the MRF removal function/spot on the part surface, using a dual force sensor. This approach experimentally addresses the mechanisms governing material removal in MRF for optical glasses in terms of the hydrodynamic pressure and shear stress, applied by the hydrodynamic flow of magnetorheological fluid at the gap between the part surface and the STM wheel. This work demonstrates that the volumetric removal rate shows a positive linear dependence on shear stress. Shear stress exhibits a positive linear dependence on a material figure of merit that depends upon Young’s modulus, fracture toughness, and hardness. A modified Preston’s equation is proposed that better estimates MRF material removal rate for optical glasses by incorporating mechanical properties, shear stress, and velocity.

  20. Shear stress in magnetorheological finishing for glasses.

    PubMed

    Miao, Chunlin; Shafrir, Shai N; Lambropoulos, John C; Mici, Joni; Jacobs, Stephen D

    2009-05-01

    We report in situ, simultaneous measurements of both drag and normal forces in magnetorheological finishing (MRF) for what is believed to be the first time, using a spot taking machine (STM) as a test bed to take MRF spots on stationary parts. The measurements are carried out over the entire area where material is being removed, i.e., the projected area of the MRF removal function/spot on the part surface, using a dual force sensor. This approach experimentally addresses the mechanisms governing material removal in MRF for optical glasses in terms of the hydrodynamic pressure and shear stress, applied by the hydrodynamic flow of magnetorheological fluid at the gap between the part surface and the STM wheel. This work demonstrates that the volumetric removal rate shows a positive linear dependence on shear stress. Shear stress exhibits a positive linear dependence on a material figure of merit that depends upon Young's modulus, fracture toughness, and hardness. A modified Preston's equation is proposed that better estimates MRF material removal rate for optical glasses by incorporating mechanical properties, shear stress, and velocity. PMID:19412219

  1. Integrated Shear Stress/Temperature Micromachined Sensors

    NASA Technical Reports Server (NTRS)

    Sheplak, Mark; Cattafesta, Louis N., III; Nishida, Toshikazu

    2002-01-01

    During this project we were able to design and initiate the fabrication of an integrated Micro ElectroMechanical Systems (MEMS)-based shear stress/temperature sensor for flow control applications. A brief summary of the completed activities during this project is presented.

  2. BOUNDARY SHEAR STRESS ALONG VEGETATED STREAMBANKS

    EPA Science Inventory

    This research is intended to improve our understanding of the role of riparian vegetation in stream morphology by evaluating the effects of vegetation on boundary shear stress, providing insight to the type and density of vegetation required for streambank stability. The resu...

  3. Dynamic Shear Modulus of Polymers from Molecular Dynamics Simulations

    NASA Astrophysics Data System (ADS)

    Byutner, Oleksiy; Smith, Grant

    2001-03-01

    In this work we describe the methodology for using equilibrium molecular dynamics simulations (MD) simulations to obtain the viscoelastic properties of polymers in the glassy regime. Specifically we show how the time dependent shear stress modulus and frequency dependent complex shear modulus in the high-frequency regime can be determined from the off-diagonal terms of the stress-tensor autocorrelation function obtained from MD trajectories using the Green-Kubo method and appropriate Fourier transforms. In order to test the methodology we have performed MD simulations of a low-molecular-weight polybutadiene system using quantum chemistry based potential functions. Values of the glassy modulus and the maximum loss frequency were found to be in good agreement with experimental data for polybutadiene at 298 K.

  4. ISIS: An Instrument for Measuring Erosion Shear Stress In Situ

    NASA Astrophysics Data System (ADS)

    Williamson, Helen; Ockenden, Mary

    1996-01-01

    An instrument for measuring shear stress for erosion in situ(ISIS) has been developed to measure the erosion shear stress of muddy sediments on intertidal mud flats. Erosion shear stress is defined in this paper as the minimum applied bed shear stress required to initiate erosion and remove sediment from the bed surface. An applied shear stress is generated by the flow through and around a specially shaped bell head, which draws water radially across the bed into the centre of the bell head. The applied shear stress is a function of the distance from the bell head to the bed surface and the discharge through the system. The design of ISIS was assisted by the use of a computational numerical flow modelling package. The operating conditions giving the most even shear stress across the whole test section were discharges of 0·01-0·6 ls -1, and bell-to-bed distance of 4-8 mm giving a shear stress of 0·02-5 Nm -2. The ISIS system was calibrated using hot film shear stress probes. The calibration data gave a 92% fit to the calibration function for shear stress. Laboratory measurements with ISIS of the erosion shear stress of mud beds consolidated for c. 1·5 days, showed surface shear stresses of 0·11-0·24 Nm -2. These were very similar to values of surface erosion shear stress measured for the same mud in an annular flume. The ISIS system was used to measure surface erosion shear stresses on the mud flats at Portishead and Blue Anchor Bay in the Severn Estuary, U.K. Surface erosion shear stresses at Portishead were generally in the range 0·2-0·5 Nm -2. The surface erosion shear stresses measured at Blue Anchor Bay, which included mud and sand, ranged between 0·1-1·9 Nm -2.

  5. Stent implantation influence wall shear stress evolution

    NASA Astrophysics Data System (ADS)

    Bernad, S. I.; Totorean, A. F.; Bosioc, A. I.; Petre, I.; Bernad, E. S.

    2016-06-01

    Local hemodynamic factors are known affect the natural history of the restenosis critically after coronary stenting of atherosclerosis. Stent-induced flows disturbance magnitude dependent directly on the strut design. The impact of flow alterations around struts vary as the strut geometrical parameters change. Our results provide data regarding the hemodynamic parameters for the blood flow in both stenosed and stented coronary artery under physiological conditions, namely wall shear stress and pressure drop.

  6. Shear Stress Sensing using Elastomer Micropillar Arrays

    NASA Technical Reports Server (NTRS)

    Wohl, Christopher J.; Palmieri, Frank L.; Lin, Yi; Jackson, Allen M.; Cissoto, Alexxandra; Sheplak, Mark; Connell, John W.

    2013-01-01

    The measurement of shear stress developed as a fluid moves around a solid body is difficult to measure. Stresses at the fluid-solid interface are very small and the nature of the fluid flow is easily disturbed by introducing sensor components to the interface. To address these challenges, an array of direct and indirect techniques have been investigated with various advantages and challenges. Hot wire sensors and other indirect sensors all protrude significantly into the fluid flow. Microelectromechanical systems (MEMS) devices, although facilitating very accurate measurements, are not durable, are prone to contamination, and are difficult to implement into existing model geometries. One promising approach is the use of engineered surfaces that interact with fluid flow in a detectable manner. To this end, standard lithographic techniques have been utilized to generate elastomeric micropillar arrays of various lengths and diameters. Micropillars of controlled length and width were generated in polydimethylsiloxane (PDMS) elastomer using a soft-lithography technique. The 3D mold for micropillar replication was fabricated using laser ablative micromachining and contact lithography. Micropillar dimensions and mechanical properties were characterized and compared to shear sensing requirements. The results of this characterization as well as shear stress detection techniques will be discussed.

  7. Open boundary molecular dynamics of sheared star-polymer melts.

    PubMed

    Sablić, Jurij; Praprotnik, Matej; Delgado-Buscalioni, Rafael

    2016-02-28

    Open boundary molecular dynamics (OBMD) simulations of a sheared star polymer melt under isothermal conditions are performed to study the rheology and molecular structure of the melt under a fixed normal load. Comparison is made with the standard molecular dynamics (MD) in periodic (closed) boxes at a fixed shear rate (using the SLLOD dynamics). The OBMD system exchanges mass and momentum with adjacent reservoirs (buffers) where the external pressure tensor is imposed. Insertion of molecules in the buffers is made feasible by implementing there a low resolution model (blob-molecules with soft effective interactions) and then using the adaptive resolution scheme (AdResS) to connect with the bulk MD. Straining with increasing shear stress induces melt expansion and a significantly different redistribution of pressure compared with the closed case. In the open sample, the shear viscosity is also a bit lowered but more stable against the viscous heating. At a given Weissenberg number, molecular deformations and material properties (recoverable shear strain and normal stress ratio) are found to be similar in both setups. We also study the modelling effect of normal and tangential friction between monomers implemented in a dissipative particle dynamics (DPD) thermostat. Interestingly, the tangential friction substantially enhances the elastic response of the melt due to a reduction of the kinetic stress viscous contribution. PMID:26820315

  8. Arrest stress of uniformly sheared wet granular matter

    NASA Astrophysics Data System (ADS)

    Ebrahimnazhad Rahbari, S. H.; Brinkmann, M.; Vollmer, J.

    2015-06-01

    We conduct extensive independent numerical experiments considering frictionless disks without internal degrees of freedom (rotation, etc.) in two dimensions. We report here that for a large range of the packing fractions below random-close packing, all components of the stress tensor of wet granular materials remain finite in the limit of zero shear rate. This is direct evidence for a fluid-to-solid arrest transition. The offset value of the shear stress characterizes plastic deformation of the arrested state which corresponds to dynamic yield stress of the system. Based on an analytical line of argument, we propose that the mean number of capillary bridges per particle, ν , follows a nontrivial dependence on the packing fraction, ϕ , and the capillary energy, ɛ . Most noticeably, we show that ν is a generic and universal quantity which does not depend on the driving protocol. Using this universal quantity, we calculate the arrest stress, σa, analytically based on a balance of the energy injection rate due to the external force driving the flow and the dissipation rate accounting for the rupture of capillary bridges. The resulting prediction of σa is a nonlinear function of the packing fraction, ϕ , and the capillary energy, ɛ . This formula provides an excellent, parameter-free prediction of the numerical data. Corrections to the theory for small and large packing fractions are connected to the emergence of shear bands and of contributions to the stress from repulsive particle interactions, respectively.

  9. Direct observation of dynamic shear jamming in dense suspensions.

    PubMed

    Peters, Ivo R; Majumdar, Sayantan; Jaeger, Heinrich M

    2016-04-14

    Liquid-like at rest, dense suspensions of hard particles can undergo striking transformations in behaviour when agitated or sheared. These phenomena include solidification during rapid impact, as well as strong shear thickening characterized by discontinuous, orders-of-magnitude increases in suspension viscosity. Much of this highly non-Newtonian behaviour has recently been interpreted within the framework of a jamming transition. However, although jamming indeed induces solid-like rigidity, even a strongly shear-thickened state still flows and thus cannot be fully jammed. Furthermore, although suspensions are incompressible, the onset of rigidity in the standard jamming scenario requires an increase in particle density. Finally, whereas shear thickening occurs in the steady state, impact-induced solidification is transient. As a result, it has remained unclear how these dense suspension phenomena are related and how they are connected to jamming. Here we resolve this by systematically exploring both the steady-state and transient regimes with the same experimental system. We demonstrate that a fully jammed, solid-like state can be reached without compression and instead purely with shear, as recently proposed for dry granular systems. This state is created by transient shear-jamming fronts, which we track directly. We also show that shear stress, rather than shear rate, is the key control parameter. From these findings we map out a state diagram with particle density and shear stress as variables. We identify discontinuous shear thickening with a marginally jammed regime just below the onset of full, solid-like jamming. This state diagram provides a unifying framework, compatible with prior experimental and simulation results on dense suspensions, that connects steady-state and transient behaviour in terms of a dynamic shear-jamming process. PMID:27042934

  10. Direct observation of dynamic shear jamming in dense suspensions

    NASA Astrophysics Data System (ADS)

    Peters, Ivo R.; Majumdar, Sayantan; Jaeger, Heinrich M.

    2016-04-01

    Liquid-like at rest, dense suspensions of hard particles can undergo striking transformations in behaviour when agitated or sheared. These phenomena include solidification during rapid impact, as well as strong shear thickening characterized by discontinuous, orders-of-magnitude increases in suspension viscosity. Much of this highly non-Newtonian behaviour has recently been interpreted within the framework of a jamming transition. However, although jamming indeed induces solid-like rigidity, even a strongly shear-thickened state still flows and thus cannot be fully jammed. Furthermore, although suspensions are incompressible, the onset of rigidity in the standard jamming scenario requires an increase in particle density. Finally, whereas shear thickening occurs in the steady state, impact-induced solidification is transient. As a result, it has remained unclear how these dense suspension phenomena are related and how they are connected to jamming. Here we resolve this by systematically exploring both the steady-state and transient regimes with the same experimental system. We demonstrate that a fully jammed, solid-like state can be reached without compression and instead purely with shear, as recently proposed for dry granular systems. This state is created by transient shear-jamming fronts, which we track directly. We also show that shear stress, rather than shear rate, is the key control parameter. From these findings we map out a state diagram with particle density and shear stress as variables. We identify discontinuous shear thickening with a marginally jammed regime just below the onset of full, solid-like jamming. This state diagram provides a unifying framework, compatible with prior experimental and simulation results on dense suspensions, that connects steady-state and transient behaviour in terms of a dynamic shear-jamming process.

  11. Dilation dynamics of granular suspensions during the shear thickening transition

    NASA Astrophysics Data System (ADS)

    Xu, Qin; Majumdar, Sayantan; Jaeger, Heinrich

    2014-03-01

    We experimentally investigate the dilation dynamics of dense granular (non-Brownian) suspensions under shear. We focus on the scenario where the packing fraction is close to the dynamic jamming point and combine oscillatory rheological measurements with in situ high-speed imaging to study the particle dynamics throughout the shear-thickening (ST) transition. By visualizing the shear profile at different strain amplitudes, we show that, although frustrated dilation is the dominant factor for ST in granular suspensions, viscous hydrodynamic stress τμ still plays an important role in determining the velocity profile and shear localization during the dilation process. Moreover, when the suspending liquid becomes highly viscous, τμ affects the magnitude of the stress increment. By imaging the air-suspension boundary during shear, we demonstrate that the upper stress limit of the observable ST regime in suspensions of hard particles corresponds to the point where the confining pressure due to capillary forces is exceeded, as signaled by movement of the contact line between suspension and substrate.

  12. Wrinkling of reinforced plates subjected to shear stresses

    NASA Technical Reports Server (NTRS)

    Seydel, Edgar

    1931-01-01

    An analysis is made here of the problem of long plates with transverse stiffeners subject to shear. A typical example would be a long Wagner beam. The shear stress is calculated at which the web wrinkles and shear stress becomes a maximum. The equation is solved for both a condition of free support and rigidity of support on the edges.

  13. Dynamic shear deformation in high purity Fe

    SciTech Connect

    Cerreta, Ellen K; Bingert, John F; Trujillo, Carl P; Lopez, Mike F; Gray, George T

    2009-01-01

    The forced shear test specimen, first developed by Meyer et al. [Meyer L. et al., Critical Adiabatic Shear Strength of Low Alloyed Steel Under Compressive Loading, Metallurgical Applications of Shock Wave and High Strain Rate Phenomena (Marcel Decker, 1986), 657; Hartmann K. et al., Metallurgical Effects on Impact Loaded Materials, Shock Waves and High Strain rate Phenomena in Metals (Plenum, 1981), 325-337.], has been utilized in a number of studies. While the geometry of this specimen does not allow for the microstructure to exactly define the location of shear band formation and the overall mechanical response of a specimen is highly sensitive to the geometry utilized, the forced shear specimen is useful for characterizing the influence of parameters such as strain rate, temperature, strain, and load on the microstructural evolution within a shear band. Additionally, many studies have utilized this geometry to advance the understanding of shear band development. In this study, by varying the geometry, specifically the ratio of the inner hole to the outer hat diameter, the dynamic shear localization response of high purity Fe was examined. Post mortem characterization was performed to quantify the width of the localizations and examine the microstructural and textural evolution of shear deformation in a bcc metal. Increased instability in mechanical response is strongly linked with development of enhanced intergranular misorientations, high angle boundaries, and classical shear textures characterized through orientation distribution functions.

  14. Liquid crystals for unsteady surface shear stress visualization

    NASA Astrophysics Data System (ADS)

    Reda, D. C.

    1988-04-01

    Oscillating airfoil experiments were conducted to test the frequency response of thermochromic liquid crystal coatings to unsteady surface shear stresses under isothermal-flow conditions. The model was an NACA-0015 airfoil, exposed to an incompressible flow at a freestream Reynolds number (based on chord) of 1.14 x 1000000. Angle-of-attack forcing functions were sine waves of amplitude + or - 10 deg about each of three mean angles of attack: 0 deg 10 deg, and 20 deg. Frequencies of oscillation were 0.2, 0.6 and 1.2 hertz, corresponding to reduced frequencies of 0.0055, 0.0164 and 0.0328. Data acquisition was accomplished by video recording. Observations showed the liquid crystal technique capable of visualizing high surface shear stress zones over the stated dynamic range in a continuous and reversible manner.

  15. Liquid crystals for unsteady surface shear stress visualization

    SciTech Connect

    Reda, D.C.

    1988-01-01

    Oscillating airfoil experiments were conducted to test the frequency response of thermochromic liquid crystal coatings to unsteady surface shear stresses under isothermal-flow conditions. The model was an NACA-0015 airfoil, exposed to an incompressible flow at a freestream Reynolds number (based on chord) of 1.14 x 10/sup 6/. Angle-of-attack forcing functions were sine waves of amplitude +- 10/degree/ about each of three mean angles of attack: 0/degree/, 10/degree/, and 20/degree/. Frequencies of oscillation were 0.2, 0.6 and 1.2 hertz, corresponding to reduced frequencies of 0.0055, 0.0164 and 0.0328. Data acquisition was accomplished by video recording. Observations showed the liquid crystal technique capable of visualizing high surface shear stress zones over the stated dynamic range in a continuous and reversible manner. 11 refs.

  16. Shear stress cleaning for surface departiculation

    NASA Technical Reports Server (NTRS)

    Musselman, R. P.; Yarbrough, T. W.

    1986-01-01

    A cleaning technique widely used by the nuclear utility industry for removal of radioactive surface contamination has proven effective at removing non-hazardous contaminant particles as small as 0.1 micrometer. The process employs a controlled high velocity liquid spray inside a vapor containment enclosure to remove particles from a surface. The viscous drag force generated by the cleaning fluid applies a shear stress greater than the adhesion force that holds small particles to a substrate. Fluid mechanics and field tests indicate general cleaning parameters.

  17. Shear-stress relaxation and ensemble transformation of shear-stress autocorrelation functions.

    PubMed

    Wittmer, J P; Xu, H; Baschnagel, J

    2015-02-01

    We revisit the relation between the shear-stress relaxation modulus G(t), computed at finite shear strain 0<γ≪1, and the shear-stress autocorrelation functions C(t)|(γ) and C(t)|(τ) computed, respectively, at imposed strain γ and mean stress τ. Focusing on permanent isotropic spring networks it is shown theoretically and computationally that in general G(t)=C(t)|(τ)=C(t)|(γ)+G(eq) for t>0 with G(eq) being the static equilibrium shear modulus. G(t) and C(t)|(γ) thus must become different for solids and it is impossible to obtain G(eq) alone from C(t)|(γ) as often assumed. We comment briefly on self-assembled transient networks where G(eq)(f) must vanish for a finite scission-recombination frequency f. We argue that G(t)=C(t)|(τ)=C(t)|(γ) should reveal an intermediate plateau set by the shear modulus G(eq)(f=0) of the quenched network. PMID:25768458

  18. Predicting bed shear stress and its role in sediment dynamics and restoration potential of the Everglades and other vegetated flow systems

    USGS Publications Warehouse

    Larsen, L.G.; Harvey, J.W.; Crimaldi, J.P.

    2009-01-01

    Entrainment of sediment by flowing water affects topography, habitat suitability, and nutrient cycling in vegetated floodplains and wetlands, impacting ecosystem evolution and the success of restoration projects. Nonetheless, restoration managers lack simple decision-support tools for predicting shear stresses and sediment redistribution potential in different vegetation communities. Using a field-validated numerical model, we developed state-space diagrams that provide these predictions over a range of water-surface slopes, depths, and associated velocities in Everglades ridge and slough vegetation communities. Diminished bed shear stresses and a consequent decrease in bed sediment redistribution are hypothesized causes of a recent reduction in the topographic and vegetation heterogeneity of this ecosystem. Results confirmed the inability of present-day flows to entrain bed sediment. Further, our diagrams showed bed shear stresses to be highly sensitive to emergent vegetation density and water-surface slope but less sensitive to water depth and periphyton or floating vegetation abundance. These findings suggested that instituting a pulsing flow regime could be the most effective means to restore sediment redistribution to the Everglades. However, pulsing flows will not be sufficient to erode sediment from sloughs with abundant spikerush, unless spikerush density first decreases by natural or managed processes. Our methods provide a novel tool for identifying restoration parameters and performance measures in many types of vegetated aquatic environments where sediment erosion and deposition are involved.

  19. Dynamic Rock Fragmentation in Grain Flow: Application to Geophysical Shearing

    NASA Astrophysics Data System (ADS)

    Davies, T. R.; McSaveney, M. J.

    2006-12-01

    Mechanical explanations for large-scale, hypermobile, geophysical phenomena (long-runout debris avalanches, low-angle blockslides, faulting) have had limited success. These phenomena all involve shearing of comminuted grain strata, but grain-flow mechanics provides no mechanism for reducing frictional resistance to grain shearing. We outline a mechanical explanation for increased debris mobility, based on the shearing grain strata in these situations being intensely comminuted by dynamic rock fragmentation, and fragmentation taking place throughout the motion. The work done in fragmentation is not lost to "fracture-surface energy": much of it is recycled to the motion of the grain mass as local isotropic dispersive pressure, which can be in the GPa range. We identify two classes of geological mass movement, in which grain fragmentation has different roles. In confined shear the fragmenting grain layer is thin and bounded by non-fragmenting material (fault motion, blocksliding, basal shear of volcanic debris avalanches, some laboratory experiments). Under high ambient stresses and strain rates, shear concentrates in thin "shear bands"; fragmentation reduces the confining stress in a band, and so reduces its effective intergranular direct stress and ability to resist boundary shear with a conventional friction coefficient. Thus we quantitatively explain the motion of the Waikaremoana blockslide and the Socompa volcanic debris avalanche; rupture stresses reported from the San Andreas fault; and data from laboratory rock friction experiments. In less confined shear (dry debris avalanches) the fragmenting layer is initially thin, but thickens in runout until it extends through the whole mass except for a ~ 10-m thick unfragmented surface carapace. The shear rate is lower and shear banding is less dominant than in confined shearing; motion is affected by reduced internal friction due to local, transient shear bands and by the isotropic dispersive pressure from

  20. Shear stress induced stimulation of mammalian cell metabolism

    NASA Technical Reports Server (NTRS)

    Mcintire, L. V.; Frangos, J. A.; Eskin, S. G.

    1988-01-01

    A flow apparatus was developed for the study of the metabolic response of anchorage dependent cells to a wide range of steady and pulsatile shear stresses under well controlled conditions. Human umbilical vein endothelial cell monolayers were subjected to steady shear stresses of up to 24 dynes/sq cm, and the production of prostacyclin was determined. The onset of flow led to a burst in prostacyclin production which decayed to a long term steady state rate (SSR). The SSR of cells exposed to flow was greater than the basal release level, and increased linearly with increasing shear stress. It is demonstrated that shear stresses in certain ranges may not be detrimental to mammalian cell metabolism. In fact, throughout the range of shear stresses studied, metabolite production is maximized by maximizing shear stress.

  1. Wall shear stress estimates in coronary artery constrictions

    NASA Technical Reports Server (NTRS)

    Back, L. H.; Crawford, D. W.

    1992-01-01

    Wall shear stress estimates from laminar boundary layer theory were found to agree fairly well with the magnitude of shear stress levels along coronary artery constrictions obtained from solutions of the Navier Stokes equations for both steady and pulsatile flow. The relatively simple method can be used for in vivo estimates of wall shear stress in constrictions by using a vessel shape function determined from a coronary angiogram, along with a knowledge of the flow rate.

  2. Colors Of Liquid Crystals Used To Measure Surface Shear Stresses

    NASA Technical Reports Server (NTRS)

    Reda, D. C.; Muratore, J. J., Jr.

    1996-01-01

    Developmental method of mapping shear stresses on aerodynamic surfaces involves observation, at multiple viewing angles, of colors of liquid-crystal surface coats illuminated by white light. Report describing method referenced in "Liquid Crystals Indicate Directions Of Surface Shear Stresses" (ARC-13379). Resulting maps of surface shear stresses contain valuable data on magnitudes and directions of skin friction forces associated with surface flows; data used to refine mathematical models of aerodynamics for research and design purposes.

  3. Temporal oscillations of the shear stress and scattered light in a shear-banding--shear-thickening micellar solution.

    PubMed

    Azzouzi, H; Decruppe, J P; Lerouge, S; Greffier, O

    2005-08-01

    The results of optical and rheological experiments performed on a viscoelastic solution (cetyltrimethylammonium bromide + sodium salicylate in water) are reported. The flow curve has a horizontal plateau extending between two critical shear rates characteristic of heterogeneous flows formed by two layers of fluid with different viscosities. These two bands which also have different optical anisotropy are clearly seen by direct observation in polarized light. At the end of the plateau, apparent shear thickening is observed in a narrow range of shear rates; in phase oscillations of the shear stress and of the first normal stress difference are recorded in a shearing device operating under controlled strain. The direct observation of the annular gap of a Couette cell in a direction perpendicular to a plane containing the vorticity shows that the turbidity of the whole sample also undergoes time dependent variations with the same period as the shear stress. However no banding is observed during the oscillations and the flow remains homogeneous. PMID:16132153

  4. Adjustable shear stress erosion and transport flume

    DOEpatents

    Roberts, Jesse D.; Jepsen, Richard A.

    2002-01-01

    A method and apparatus for measuring the total erosion rate and downstream transport of suspended and bedload sediments using an adjustable shear stress erosion and transport (ASSET) flume with a variable-depth sediment core sample. Water is forced past a variable-depth sediment core sample in a closed channel, eroding sediments, and introducing suspended and bedload sediments into the flow stream. The core sample is continuously pushed into the flow stream, while keeping the surface level with the bottom of the channel. Eroded bedload sediments are transported downstream and then gravitationally separated from the flow stream into one or more quiescent traps. The captured bedload sediments (particles and aggregates) are weighed and compared to the total mass of sediment eroded, and also to the concentration of sediments suspended in the flow stream.

  5. High-Shear Stress Sensitizes Platelets to Subsequent Low-Shear Conditions

    PubMed Central

    Sheriff, Jawaad; Bluestein, Danny; Girdhar, Gaurav; Jesty, Jolyon

    2010-01-01

    Individuals with mechanical heart valve implants are plagued by flow-induced thromboembolic complications, which are undoubtedly caused by platelet activation. Flow fields in or around the affected regions involve brief exposure to pathologically high-shear stresses on the order of 100 to 1000 dyne/cm2. Although high shear is known to activate platelets directly, their subsequent behavior is not known. We hypothesize that the post-high-shear activation behavior of platelets is particularly relevant in understanding the increased thrombotic risk associated with blood-recirculating prosthetic cardiovascular devices. Purified platelets were exposed to brief (5–40 s) periods of high-shear stress, and then exposed to longer periods (15–60 min) of low shear. Their activation state was measured using a prothrombinase-based assay. Platelets briefly exposed to an initial high-shear stress (e.g., 60 dyne/cm2 for 40 s) activate a little, but this study shows that they are now sensitized, and when exposed to subsequent low shear stress, they activate at least 20-fold faster than platelets not initially exposed to high shear. The results show that platelets in vitro exposed beyond a threshold of high-shear stress are primed for subsequent activation under normal cardiovascular circulation conditions, and they do not recover from the initial high-shear insult. PMID:20135353

  6. Research on measurement of bed shear stress under wave-current interaction

    NASA Astrophysics Data System (ADS)

    Xu, Hua; Xia, Yun-feng; Ma, Bing-he; Hao, Si-yu; Zhang, Shi-zhao; Du, De-jun

    2015-06-01

    The movement of sediment in estuary and on coast is directly restricted by the bed shear stress. Therefore, the research on the basic problem of sediment movement by the bed shear stress is an important way to research the theory of sediment movement. However, there is not a measuring and computing method to measure the bed shear stress under a complicated dynamic effect like wave and current. This paper describes the measurement and test research on the bed shear stress in a long launder of direct current by the new instrument named thermal shearometer based on micro-nanotechnology. As shown by the research results, the thermal shearometer has a high response frequency and strong stability. The measured results can reflect the basic change of the bed shear stress under wave and wave-current effect, and confirm that the method of measuring bed shear stress under wave-current effect with thermal shearometer is feasible. Meanwhile, a preliminary method to compute the shear stress compounded by wave-current is put forward according to the tested and measured results, and then a reference for further study on the basic theory of sediment movement under a complicated dynamic effect is provided.

  7. Granular dynamics under shear with deformable boundaries

    NASA Astrophysics Data System (ADS)

    Geller, Drew; Backhaus, Scott; Ecke, Robert

    2015-03-01

    Granular materials under shear develop complex patterns of stress as the result of granular positional rearrangements under an applied load. We consider the simple planar shear of a quasi two-dimensional granular material consisting of bi-dispersed nylon cylinders confined between deformable boundaries. The aspect ratio of the gap width to total system length is 50, and the ratio of particle diameter to gap width is about 10. This system, designed to model a long earthquake fault with long range elastic coupling through the plates, is an interesting model system for understanding effective granular friction because it essentially self tunes to the jamming condition owing to the hardness of the grains relative to that of the boundary material, a ratio of more than 1000 in elastic moduli. We measure the differential strain displacements of the plates, the inhomogeneous stress distribution in the plates, the positions and angular orientations of the individual grains, and the shear force, all as functions of the applied normal stress. There is significant stick-slip motion in this system that we quantify through our quantitative measurements of both the boundary and the grain motion, resulting in a good characterization of this sheared 2D hard sphere system.

  8. 4D shear stress maps of the developing heart using Doppler optical coherence tomography

    PubMed Central

    Peterson, Lindsy M.; Jenkins, Michael W.; Gu, Shi; Barwick, Lee; Watanabe, Michiko; Rollins, Andrew M.

    2012-01-01

    Accurate imaging and measurement of hemodynamic forces is vital for investigating how physical forces acting on the embryonic heart are transduced and influence developmental pathways. Of particular importance is blood flow-induced shear stress, which influences gene expression by endothelial cells and potentially leads to congenital heart defects through abnormal heart looping, septation, and valvulogenesis. However no imaging tool has been available to measure shear stress on the endocardium volumetrically and dynamically. Using 4D structural and Doppler OCT imaging, we are able to accurately measure the blood flow in the heart tube in vivo and to map endocardial shear stress throughout the heart cycle under physiological conditions for the first time. These measurements of the shear stress patterns will enable precise titration of experimental perturbations and accurate correlation of shear with the expression of molecules critical to heart development. PMID:23162737

  9. Stress Heterogeneities in Sheared Type-I Collagen Networks Revealed by Boundary Stress Microscopy

    PubMed Central

    Arevalo, Richard C.; Kumar, Pramukta; Urbach, Jeffrey S.; Blair, Daniel L.

    2015-01-01

    Disordered fiber networks provide structural support to a wide range of important materials, and the combination of spatial and dynamic complexity may produce large inhomogeneities in mechanical properties, an effect that is largely unexplored experimentally. In this work, we introduce Boundary Stress Microscopy to quantify the non-uniform surface stresses in sheared collagen gels. We find local stresses exceeding average stresses by an order of magnitude, with variations over length scales much larger than the network mesh size. The strain stiffening behavior observed over a wide range of network mesh sizes can be parameterized by a single characteristic strain and associated stress, which describes both the strain stiffening regime and network yielding. The characteristic stress is approximately proportional to network density, but the peak boundary stress at both the characteristic strain and at yielding are remarkably insensitive to concentration. PMID:25734484

  10. Two-axis direct fluid shear stress sensor

    NASA Technical Reports Server (NTRS)

    Bajikar, Sateesh (Inventor); Scott, Michael A. (Inventor); Adcock, Edward E. (Inventor)

    2011-01-01

    A micro sized multi-axis semiconductor skin friction/wall shear stress induced by fluid flow. The sensor design includes a shear/strain transduction gimble connected to a force collecting plate located at the flow boundary surface. The shear force collecting plate is interconnected by an arm to offset the tortional hinges from the fluid flow. The arm is connected to the shear force collecting plate through dual axis torsional hinges with piezoresistive torsional strain gauges. These gauges are disposed on the tortional hinges and provide a voltage output indicative of applied shear stress acting on the force collection plate proximate the flow boundary surface. Offsetting the torsional hinges creates a force concentration and resolution structure that enables the generation of a large stress on the strain gauge from small shear stress, or small displacement of the collecting plate. The design also isolates the torsional sensors from exposure to the fluid flow.

  11. Shear thinning behavior of linear polymer melts under shear flow via nonequilibrium molecular dynamics

    NASA Astrophysics Data System (ADS)

    Xu, Xiaolei; Chen, Jizhong; An, Lijia

    2014-05-01

    The properties of both untangled and entangled linear polymer melts under shear flow are studied by nonequilibrium molecular dynamics simulations. The results reveal that the dependence of shear viscosity η on shear rate dot{γ }, expressed by η ˜ dot{γ }^{-n}, exhibits three distinct regimes. The first is the well-known Newtonian regime, namely, η independent of shear rate at small shear rates dot{γ }<τ 0^{-1} (where τ0 is the longest polymer relaxation time at equilibrium). In the non-Newtonian regime (dot{γ }>τ 0^{-1}), the shear dependence of viscosity exhibits a crossover at a critical shear rate dot{γ }c dividing this regime into two different regimes, shear thinning regime I (ST-I) and II (ST-II), respectively. In the ST-I regime (τ ^{-1}_0dot{γ }c) a universal power law η ˜ dot{γ }^{-0.37} is found for considered chain lengths. Furthermore, the longer the polymer chain is, the smaller the shear viscosity for a given shear rate in the ST-II regime. The simulation also shows that a characteristic chain length, below which dot{γ }c will be equal to τ 0^{-1}, lies in the interval 30 < N < 50. For all considered chain lengths in the ST-II regime, we also find that the first and second normal stress differences N1 and N2 follow power laws of N1 ˜ dot{γ }^{2/3} and N2 ˜ dot{γ }^{0.82}, respectively; the orientation resistance parameter mG follows the relation mG ˜ dot{γ }^{0.75} and the tumbling frequency ftb follows f_{tb} ˜ dot{γ }^{0.75}. These results imply that the effects of entanglement on the shear dependences of these properties may be negligible in the ST-II regime. These findings may shed some light on the nature of shear thinning in flexible linear polymer melts.

  12. Shear-Panel Test Fixture Eliminates Corner Stresses

    NASA Technical Reports Server (NTRS)

    Kiss, J. J.; Farley, G. L.; Baker, D. J.

    1984-01-01

    New design eliminates corner stresses while maintaining uniform stress across panel. Shear panel test fixture includes eight frames and eight corner pins. Fixture assembled in two halves with shear panel sandwiched in between. Results generated from this fixture will result in good data base for design of efficient aircraft structures and other applications.

  13. A model of Barchan dunes including lateral shear stress.

    PubMed

    Schwämmle, V; Herrmann, H J

    2005-01-01

    Barchan dunes are found where sand availability is low and wind direction quite constant. The two dimensional shear stress of the wind field and the sand movement by saltation and avalanches over a barchan dune are simulated. The model with one dimensional shear stress is extended including surface diffusion and lateral shear stress. The resulting final shape is compared to the results of the model with a one dimensional shear stress and confirmed by comparison to measurements. We found agreement and improvements with respect to the model with one dimensional shear stress. Additionally, a characteristic edge at the center of the windward side is discovered which is also observed for big barchans. Diffusion effects reduce this effect for small dunes. PMID:15688141

  14. Regulation of Vascular Endothelium Inflammatory Signalling by Shear Stress.

    PubMed

    Zakkar, Mustafa; Angelini, Gianni D; Emanueli, Costanza

    2016-01-01

    The vascular endothelium plays a pivotal role in regulating vascular homeostasis. Blood flow exerts several mechanical forces on the luminal surface of the Endothelial Cell (EC) including pressure, circumferential stretch, and shear stress. It is widely believed that shear stress plays a central role in regulating EC inflammatory responses and the pathogenesis of atherosclerosis. High shear stress can induce an antiinflammatory status in EC, which is partially mediated by the production of proteins and transcription factors able to suppress different proinflammatory signalling pathways. In this review, we summarise the available evidence regarding the effect of shear stress on vascular EC and smooth muscle cells, the regulation of MAPK and NF-κB including the production of different negative regulators of inflammation such as MKP-1 and NRF2, and the production of microRNAs. We also discuss the possible links between shear stress and the development of atherosclerosis. PMID:26638798

  15. The shear-stress intensity factor for a centrally cracked stiff-flanged shear web

    NASA Technical Reports Server (NTRS)

    Fichter, W. B.

    1976-01-01

    By use of the principle of superposition the stiff-flanged shear web is modeled mathematically by an infinite elastic strip with fixed longitudinal edges. The shear-stress intensity factor for a central longitudinal crack is calculated for various values of the ratio of strip width to crack length, h/a, in the range 0.1-10. The interaction of the crack with the boundaries is illustrated by boundary shear-stress distributions for three values of h/a. Some implications of the results for the design of damage-tolerant shear webs are discussed briefly.

  16. A technique for combined dynamic compression-shear test

    NASA Astrophysics Data System (ADS)

    Zhao, P. D.; Lu, F. Y.; Chen, R.; Lin, Y. L.; Li, J. L.; Lu, L.; Sun, G. L.

    2011-03-01

    It is critically important to study the dynamic response of materials under a combined compression-shear loading for developing constitutive laws more accurately and fully. We present a novel technique to achieve the combined compression and shear loadings at high strain rates. The main apparatus consists of a strike bar, an incident bar, and two transmission bars. The close-to-specimen end of the incident bar is wedge-shaped with 90°. In each experiment, there are two identical specimens, respectively, agglutinated between one side of the wedge and one of transmission bars. When a loading impulse travels to specimens along the incident bar, because of the special geometrical shape, the specimen-incident bar interface gets an axial and a transverse velocity. Specimens endure a combined compression-shear loading at high strain rates. The compression stress and strain of the specimens are deduced from signals recorded by strain gages mounted on the bars. The shear stress is measured by two piezoelectric transducers of quartz (Y-cut with rotation angle 17.7°) embedded at the close-to-specimen end of transmission bars; the shear strain is measured with a novel optical technique, which is based on the luminous flux method. An analytic model was proposed and validated by the numerical simulations. The simulation results yield good agreement with the analytic results. The proposed technique was then validated through experiments carried out on lead specimens, by comparing experimental results with that of the split Hopkinson pressure bar experiments.

  17. Measurement of turbulent wall shear stress in air using micro-pillars

    NASA Astrophysics Data System (ADS)

    Gnanamanickam, Ebenezer; Kevin, Kevin; Monty, Jason; Hutchins, Nicholas

    2013-11-01

    The measurement of unsteady wall shear stress in a turbulent boundary layer, especially when the working medium is air, has been a historically challenging problem in experimental fluid mechanics. Recently the micro-pillar shear stress sensor (MPS3) has shown promise in this regard. The MPS3 is an array of micro-pillar mounted on the wall of a model. These micro-pillars deflect an amount proportional to the drag force it experiences. This drag force is proportional to the wall shear stress. The micro-pillar tip deflection is thus tracked using high-speed imaging to yield the unsteady wall shear stress. Here, the MPS3 is used to carry out unsteady wall shear stress measurements in a fully developed channel flow. Both static and dynamic calibrations of the sensor are presented. The wall shear stress statistics obtained in the fully developed channel flow are compared with those obtained from Direct Numerical Simulations (DNS) to provide an assessment of the sensor capabilities. Exemplary measurements such as two-dimensional temporal distribution of the wall shear stress are presented to highlight the capabilities of the sensor.

  18. Dynamic response of block copolymer wormlike micelles to shear flow

    NASA Astrophysics Data System (ADS)

    Lonetti, B.; Kohlbrecher, J.; Willner, L.; Dhont, J. K. G.; Lettinga, M. P.

    2008-10-01

    The linear and nonlinear dynamic response to an oscillatory shear flow of giant wormlike micelles consisting of Pb-Peo block copolymers is studied by means of Fourier transform rheology. Experiments are performed in the vicinity of the isotropic-nematic phase transition concentration, where the location of isotropic-nematic phase transition lines is determined independently. Strong shear-thinning behaviour is observed due to critical slowing down of orientational diffusion as a result of the vicinity of the isotropic-nematic spinodal. This severe shear-thinning behaviour is shown to result in gradient shear banding. Time-resolved small-angle neutron scattering experiments are used to obtain an insight into the microscopic phenomena that underlie the observed rheological response. An equation of motion for the order parameter tensor and an expression of the stress tensor in terms of the order parameter tensor are used to interpret the experimental data, both in the linear and nonlinear regimes. Scaling of the dynamic behaviour of the orientational order parameter and the stress is found when critical slowing down due to the vicinity of the isotropic-nematic spinodal is accounted for.

  19. Dynamic shear jamming in granular suspensions

    NASA Astrophysics Data System (ADS)

    Peters, Ivo; Majumdar, Sayantan; Jaeger, Heinrich

    2014-11-01

    Jamming by shear allows a frictional granular packing to transition from an unjammed state into a jammed state while keeping the system volume and average packing fraction constant. Shear jamming of dry granular media can occur quasi-statically, but boundaries are crucial to confine the material. We perform experiments in aqueous starch suspension where we apply shear using a rheometer with a large volume (400 ml) cylindrical Couette cell. In our suspensions the packing fraction is sufficiently low that quasi-static deformation does not induce a shear jammed state. Applying a shock-like deformation however, will turn the suspension into a jammed solid. A fully jammed state is reached within tens of microseconds, and can be sustained for at least several seconds. High speed imaging of the initial process reveals a jamming front propagating radially outward through the suspension, while the suspension near the outer boundary remains quiescent. This indicates that granular suspensions can be shear jammed without the need of confining solid boundaries. Instead, confinement is most likely provided by the dynamics in the front region.

  20. Edge Sheared Flows and Blob Dynamics

    NASA Astrophysics Data System (ADS)

    Myra, J. R.

    2012-10-01

    The dynamics of blob-filaments [S. I. Krasheninnikov, et al. J. Plasma Phys. 74, 679 (2008); D. A. D'Ippolito, et al., Phys. Plasmas 18, 060501 (2011)] in the strongly radially inhomogeneous edge and scrape-off-layer (SOL) region of a tokamak plasma is considered, with emphasis on sheared flow generation and interaction. The work is motivated by the potential importance of edge sheared flows for turbulence regulation, (e.g. the L-H transition), and the influence of flows on the character of emitted blob-filament structures which ultimately contact plasma-facing components. To study the dynamics of blobs and sheared flows, we employ both numerical simulations and experimental data analysis. The simulations use the fluid-based 2D curvature-interchange model embedded in the SOLT (SOL turbulence) code [D. A. Russell, et al, Phys. Plasmas 16, 122304 (2009)]. A blob-tracking algorithm has also been developed and applied to NSTX and Alcator C-Mod data. The algorithm is based on 2D time-resolved images from the gas puff imaging (GPI) diagnostic [S. J. Zweben, et al. Phys. Plasmas 9, 1981 (2002)]. The algorithm is able to track the blob motion and changes in blob structure, such as elliptical deformations, that can be affected by sheared flows. Results of seeded blob simulations are compared with the experimental data to determine the role of plasma parameters on the blob tracks and to evaluate the exchange of momentum between the blobs and flows. Seeded blob simulations are shown to reproduce many qualitative and quantitative features of the data including size, scale and direction of perpendicular (approximately poloidal) flows and the inferred Reynolds forces, poloidal reversal of blob tracks, and blob trapping and/or ejection. Simulation and experimental data comparisons permit the inference of dynamical mechanisms associated with blob motion and sheared flow generation in these shots, and their relation to previous theoretical work.

  1. The origin of persistent shear stress in supercooled liquids

    NASA Astrophysics Data System (ADS)

    Abraham, Sneha; Harrowell, Peter

    2012-07-01

    The persistence of shear stress fluctuations in viscous liquids is a direct consequence of the non-zero shear stress of the local potential minima which couples stress relaxation to transitions between inherent structures. In simulations of 2D and 3D glass forming mixtures, we calculate the distribution of this inherent shear stress and demonstrate that the variance is independent of temperature and obeys a power law in density. The inherent stress is shown to involve only long wavelength fluctuations, evidence of the central role of the static boundary conditions in determining the residual stress left after the minimization of the potential energy. A temperature Tη is defined to characterise the crossover from stress relaxation governed by binary collisions at high temperatures to low temperature relaxation dominated by the relaxation of the inherent stress. Tη is found to coincide with the breakdown of the Stokes-Einstein scaling of diffusion and viscosity.

  2. Application and improvement of Raupach's shear stress partitioning model

    NASA Astrophysics Data System (ADS)

    Walter, B. A.; Lehning, M.; Gromke, C.

    2012-12-01

    .27. It was also found that the model parameter m, which relates τS'' with τS' and which is needed to determine the peak surface shear stress τS'', is rather impractically defined in the original model formulation, because m is identified to be a function of the wind velocity, the roughness element shape and the roughness density. We propose an alternative, more universal definition of an a-parameter as a substitute for the m-parameter simply linearly relating τS'' with τS'. This strong linear relation is supported by the measurements and can be made plausible with simple fluid dynamical arguments. The advantage of a over m is that a is solely a function of the roughness element shape. Finally, a method is presented to determine the new a-parameter for different kinds of roughness elements with relatively simple experimental methods.

  3. The stress profile in a sheared granular column

    NASA Astrophysics Data System (ADS)

    Nott, Prabhu; Mehandia, Vishwajeet; Jyotsna Gutam, Kamala

    2011-11-01

    It has been known for several centuries that the normal stress at the base of a column of granular material deviates from the value dictated by the hydrostatic balance. This was explained by Janssen (1895) as being due to the shear stress imposed by the confining walls on the granular column, as a result of grain-wall friction. The question we address in this presentation is, what is the stress field when the column is sheared? Depending on the assumptions on the kinematics, plasticity theories predict that the stress profile is similar either to that in a static column, or to that in a sheared fluid column. Here, we report the results of our experimental study of slow shear of a granular material in a cylindrical Couette cell, in which all components of the stress were measured at the stationary outer cylinder. The stress was measured as a function of distance from the free surface. The results of our experiments are intriguing: the radial normal stress deviates strongly from the predictions of all available theories and previous experimental measurements. The axial shear stress changes sign when a static column is sheared. We describe these results, and speculate as to which type of theory might explain the observations. Support from the DST, India is gratefully acknowledged.

  4. Dynamics of flexible fibers in shear flow

    NASA Astrophysics Data System (ADS)

    Słowicka, Agnieszka M.; Wajnryb, Eligiusz; Ekiel-JeŻewska, Maria L.

    2015-09-01

    Dynamics of flexible non-Brownian fibers in shear flow at low-Reynolds-number are analyzed numerically for a wide range of the ratios A of the fiber bending force to the viscous drag force. Initially, the fibers are aligned with the flow, and later they move in the plane perpendicular to the flow vorticity. A surprisingly rich spectrum of different modes is observed when the value of A is systematically changed, with sharp transitions between coiled and straightening out modes, period-doubling bifurcations from periodic to migrating solutions, irregular dynamics, and chaos.

  5. Dynamics of flexible fibers in shear flow.

    PubMed

    Słowicka, Agnieszka M; Wajnryb, Eligiusz; Ekiel-Jeżewska, Maria L

    2015-09-28

    Dynamics of flexible non-Brownian fibers in shear flow at low-Reynolds-number are analyzed numerically for a wide range of the ratios A of the fiber bending force to the viscous drag force. Initially, the fibers are aligned with the flow, and later they move in the plane perpendicular to the flow vorticity. A surprisingly rich spectrum of different modes is observed when the value of A is systematically changed, with sharp transitions between coiled and straightening out modes, period-doubling bifurcations from periodic to migrating solutions, irregular dynamics, and chaos. PMID:26429038

  6. Dynamics of flexible fibers in shear flow

    SciTech Connect

    Słowicka, Agnieszka M.; Wajnryb, Eligiusz; Ekiel-Jeżewska, Maria L.

    2015-09-28

    Dynamics of flexible non-Brownian fibers in shear flow at low-Reynolds-number are analyzed numerically for a wide range of the ratios A of the fiber bending force to the viscous drag force. Initially, the fibers are aligned with the flow, and later they move in the plane perpendicular to the flow vorticity. A surprisingly rich spectrum of different modes is observed when the value of A is systematically changed, with sharp transitions between coiled and straightening out modes, period-doubling bifurcations from periodic to migrating solutions, irregular dynamics, and chaos.

  7. Endovascular Treatment of Thoracic Aortic Dissection: Hemodynamic Shear Stress Study

    NASA Astrophysics Data System (ADS)

    Tang, Yik Sau; Lai, Siu Kai; Cheng, Stephen Wing Keung; Chow, Kwok Wing

    2012-11-01

    Thoracic Aortic Dissection (TAD), a life threatening cardiovascular disease, occurs when blood intrudes into the layers of the aortic wall, creating a new artificial channel (the false lumen) beside the original true lumen. The weakened false lumen wall may expand, enhancing the risk of rupture and resulting in high mortality. Endovascular treatment involves the deployment of a stent graft into the aorta, thus blocking blood from entering the false lumen. Due to the irregular geometry of the aorta, the stent graft, however, may fail to conform to the vessel curvature, and would create a ``bird-beak'' configuration, a wedge-shaped domain between the graft and the vessel wall. Computational fluid dynamics analysis is employed to study the hemodynamics of this pathological condition. With the `beaking' configuration, the local hemodynamic shear stress will drop below the threshold of safety reported earlier in the literature. The oscillating behavior of the shear stress might lead to local inflammation, atherosclerosis and other undesirable consequences. Supported by the Innovation and Technology Fund of the Hong Kong Government.

  8. Cake properties in ultrafiltration of TiO2 fine particles combined with HA: in situ measurement of cake thickness by fluid dynamic gauging and CFD calculation of imposed shear stress for cake controlling.

    PubMed

    Du, Xing; Qu, Fangshu; Liang, Heng; Li, Kai; Chang, Haiqing; Li, Guibai

    2016-05-01

    In this study, the cake buildup of TiO2 fine particles in the presence of humid acid (HA) and cake layer controlling during ultrafiltration (UF) were investigated. Specifically, we measured the cake thickness using fluid dynamic gauging (FDG) method under various solution conditions, including TiO2 concentration (0.1-0.5 g/L), HA concentration (0-5 mg/L, total organic carbon (TOC)), and pH values (e.g., 4, 6 and 10), and calculated the shear stress distribution induced by stirring using computational fluid dynamics (CFD) to analyze the cake layer controlling conditions, including the operation flux (50-200 L m(-2) h(-1)) and TiO2 concentration (0.1-0.5 g/L). It was found that lower TiO2/HA concentration ratio could lead to exceedingly severe membrane fouling because of the formation of a relatively denser cake layer by filling the voids of cake layer with HA, and pH was essential for cake layer formation owing to the net repulsion between particles. Additionally, it was observed that shear stress was rewarding for mitigating cake growth under lower operation flux as a result of sufficient back-transport forces, and exhibited an excellent performance on cake layer controlling in lower TiO2 concentrations due to slight interaction forces on the vicinity of membrane. PMID:26810663

  9. Production of functional proteins: balance of shear stress and gravity

    NASA Technical Reports Server (NTRS)

    Goodwin, Thomas John (Inventor); Hammond, Timothy Grant (Inventor); Kaysen, James Howard (Inventor)

    2011-01-01

    A method for the production of functional proteins including hormones by renal cells in a three dimensional culturing process responsive to shear stress uses a rotating wall vessel. Natural mixture of renal cells expresses the enzyme 1-.alpha.-hydroxylase which can be used to generate the active form of vitamin D: 1,25-diOH vitamin D.sub.3. The fibroblast cultures and co-culture of renal cortical cells express the gene for erythropoietin and secrete erythropoietin into the culture supernatant. Other shear stress response genes are also modulated by shear stress, such as toxin receptors megalin and cubulin (gp280). Also provided is a method of treating an in-need individual with the functional proteins produced in a three dimensional co-culture process responsive to shear stress using a rotating wall vessel.

  10. Production of functional proteins: balance of shear stress and gravity

    NASA Technical Reports Server (NTRS)

    Goodwin, Thomas John (Inventor); Hammond, Timothy Grant (Inventor); Kaysen, James Howard (Inventor)

    2007-01-01

    The present invention provides a method for production of functional proteins including hormones by renal cells in a three dimensional co-culture process responsive to shear stress using a rotating wall vessel. Natural mixture of renal cells expresses the enzyme 1-a-hydroxylase which can be used to generate the active form of vitamin D: 1,25-diOH vitamin D3. The fibroblast cultures and co-culture of renal cortical cells express the gene for erythropoietin and secrete erythropoietin into the culture supernatant. Other shear stress response genes are also modulated by shear stress, such as toxin receptors megalin and cubulin (gp280). Also provided is a method of treating in-need individual with the functional proteins produced in a three dimensional co-culture process responsive to shear stress using a rotating wall vessel.

  11. Production of functional proteins: balance of shear stress and gravity

    NASA Technical Reports Server (NTRS)

    Goodwin, Thomas John (Inventor); Hammond, Timothy Grant (Inventor); Kaysen, James Howard (Inventor)

    2004-01-01

    The present invention provides a method for production of functional proteins including hormones by renal cells in a three dimensional co-culture process responsive to shear stress using a rotating wall vessel. Natural mixture of renal cells expresses the enzyme 1-a-hydroxylase which can be used to generate the active form of vitamin D: 1,25-diOH vitamin D3. The fibroblast cultures and co-culture of renal cortical cells express the gene for erythropoietin and secrete erythropoietin into the culture supernatant. Other shear stress response genes are also modulated by shear stress, such as toxin receptors megalin and cubulin (gp280). Also provided is a method of treating in-need individual with the functional proteins produced in a three dimensional co-culture process responsive to shear stress using a rotating wall vessel.

  12. Shear-stress sensitive lenticular vesicles for targeted drug delivery

    NASA Astrophysics Data System (ADS)

    Holme, Margaret N.; Fedotenko, Illya A.; Abegg, Daniel; Althaus, Jasmin; Babel, Lucille; Favarger, France; Reiter, Renate; Tanasescu, Radu; Zaffalon, Pierre-Léonard; Ziegler, André; Müller, Bert; Saxer, Till; Zumbuehl, Andreas

    2012-08-01

    Atherosclerosis results in the narrowing of arterial blood vessels and this causes significant changes in the endogenous shear stress between healthy and constricted arteries. Nanocontainers that can release drugs locally with such rheological changes can be very useful. Here, we show that vesicles made from an artificial 1,3-diaminophospholipid are stable under static conditions but release their contents at elevated shear stress. These vesicles have a lenticular morphology, which potentially leads to instabilities along their equator. Using a model cardiovascular system based on polymer tubes and an external pump to represent shear stress in healthy and constricted vessels of the heart, we show that drugs preferentially release from the vesicles in constricted vessels that have high shear stress.

  13. Shear-stress sensitive lenticular vesicles for targeted drug delivery.

    PubMed

    Holme, Margaret N; Fedotenko, Illya A; Abegg, Daniel; Althaus, Jasmin; Babel, Lucille; Favarger, France; Reiter, Renate; Tanasescu, Radu; Zaffalon, Pierre-Léonard; Ziegler, André; Müller, Bert; Saxer, Till; Zumbuehl, Andreas

    2012-08-01

    Atherosclerosis results in the narrowing of arterial blood vessels and this causes significant changes in the endogenous shear stress between healthy and constricted arteries. Nanocontainers that can release drugs locally with such rheological changes can be very useful. Here, we show that vesicles made from an artificial 1,3-diaminophospholipid are stable under static conditions but release their contents at elevated shear stress. These vesicles have a lenticular morphology, which potentially leads to instabilities along their equator. Using a model cardiovascular system based on polymer tubes and an external pump to represent shear stress in healthy and constricted vessels of the heart, we show that drugs preferentially release from the vesicles in constricted vessels that have high shear stress. PMID:22683843

  14. Determination of the Shear Stress Distribution in a Laminate from the Applied Shear Resultant--A Simplified Shear Solution

    NASA Technical Reports Server (NTRS)

    Bednarcyk, Brett A.; Aboudi, Jacob; Yarrington, Phillip W.

    2007-01-01

    The simplified shear solution method is presented for approximating the through-thickness shear stress distribution within a composite laminate based on laminated beam theory. The method does not consider the solution of a particular boundary value problem, rather it requires only knowledge of the global shear loading, geometry, and material properties of the laminate or panel. It is thus analogous to lamination theory in that ply level stresses can be efficiently determined from global load resultants (as determined, for instance, by finite element analysis) at a given location in a structure and used to evaluate the margin of safety on a ply by ply basis. The simplified shear solution stress distribution is zero at free surfaces, continuous at ply boundaries, and integrates to the applied shear load. Comparisons to existing theories are made for a variety of laminates, and design examples are provided illustrating the use of the method for determining through-thickness shear stress margins in several types of composite panels and in the context of a finite element structural analysis.

  15. Simplified Shear Solution for Determination of the Shear Stress Distribution in a Composite Panel from the Applied Shear Resultant

    NASA Technical Reports Server (NTRS)

    Bednarcyk, Brett A.; Aboudi, Jacob; Yarrington, Phillip W.; Collier, Craig S.

    2008-01-01

    The simplified shear solution method is presented for approximating the through-thickness shear stress distribution within a composite laminate or panel based on laminated beam theory. The method does not consider the solution of a particular boundary value problem; rather it requires only knowledge of the global shear loading, geometry, and material properties of the laminate or panel. It is thus analogous to lamination theory in that ply level stresses can be efficiently determined from global load resultants (as determined, for instance, by finite element analysis) at a given location in a structure and used to evaluate the margin of safety on a ply by ply basis. The simplified shear solution stress distribution is zero at free surfaces, continuous at ply boundaries, and integrates to the applied shear load. Comparisons to existing theories are made for a variety of laminates, and design examples are provided illustrating the use of the method for determining through-thickness shear stress margins in several types of composite panels and in the context of a finite element structural analysis.

  16. Impaired endothelial shear stress induces podosome assembly via VEGF up-regulation.

    PubMed

    Fey, Theres; Schubert, Kai Michael; Schneider, Holger; Fein, Evelyn; Kleinert, Eike; Pohl, Ulrich; Dendorfer, Andreas

    2016-08-01

    Podosomes are dynamic cytoskeletal membrane structures with local adhesive and proteolytic activity. They are critically involved in angiogenesis and vascular adaptive growth. Here, we studied in HUVECs and murine small vessels whether shear stress controls podosome assembly and local proteolytic activity. Podosomes were characterized by immunohistochemistry, and their proteolytic activity was assessed as degradation imprints in fluorescent gelatin that was used as growth substrate. Compared with controls (10 dyn/cm(2)), the number of podosomes formed per time was doubled when cells were exposed to low shear stress (0.3 dyn/cm(2)) or even increased 5-fold under static conditions. This was a result of an enhanced expression of VEGF after reduction of shear stress. Consequently, enhanced podosome formation could be prevented by a VEGF receptor antagonist as well by interruption of VEGF signaling via inhibition of PI3K, Src, or p38. Increase of podosome assembly went along with significantly augmented cell motility. In vivo experiments in mouse arteries confirmed increased endothelial podosome numbers when shear stress was abolished by vessel occlusion. We conclude that shear stress, by reducing VEGF release, inhibits podosome assembly. Hence, endothelial cell-mediated matrix proteolysis and migratory activity are inhibited, thereby stabilizing the structure of the vessel wall.-Fey, T., Schubert, K. M., Schneider, H., Fein, E., Kleinert, E., Pohl, U., Dendorfer, A. Impaired endothelial shear stress induces podosome assembly via VEGF up-regulation. PMID:27103579

  17. ESTIMATION OF SHEAR STRESS WORKING ON SUBMERGED HOLLOW FIBRE MEMBRANE BY CFD METHOD IN MBRs

    NASA Astrophysics Data System (ADS)

    Zaw, Hlwan Moe; Li, Tairi; Nagaoka, Hiroshi

    This study was conducted to evaluate shear stress working on submerged hollow fibre membrane by CFD (Computation Fluid Dynamics) method in MBRs. Shear stress on hollow fibre membrane caused by aeration was measured directly using a two-direction load sensor. The measurement of water-phase flow velocity was done also by using laser doppler velocimeter. It was confirmed that the shear stress was possible to be evaluated from the water-phase flow velocityby the result of comparison of time average shear stress actually measured with one hollow fibre membrane and the one calculated by the water-phase flow velocity. In the estimation of the water-phase flow velocity using the CFD method, time average water-phase flow velocity estimated by consideration of the fluid resistance of the membrane module nearly coincided with the measured values, and it was shown that it was possible to be estimated also within the membrane module. Moreover, the measured shear stress and drag force well coincided with the values calculated from the estimated water-phase flow velocity outside of membrane module and in the center of membrane module, and it was suggested that the shear stress on the hollow fibre membrane could be estimated by the CFD method in MBRs.

  18. Wall shear stress distributions in a model of normal and constricted small airways.

    PubMed

    Evans, David J; Green, Anthony S; Thomas, Nicholas K

    2014-04-01

    Previous studies have highlighted flow shear stress as a possible damage mechanism for small airways, in particular those liable to constriction through disease or injury due to mechanical ventilation. Flow experiments in vitro have implicated shear stress as a relevant factor for mechanotransduction pathways with respect to airway epithelial cell function. Using computational fluid dynamics analysis, this study reports velocity profiles and calculations for wall shear stress distributions in a three-generation, asymmetric section of the small airways subjected to a steady, inspiratory flow. The results show distal variation of wall shear stress distributions due to velocity gradients on the carina side of each daughter airway branch. The maximum wall shear stresses in both normal and constricted small airways are shown to exceed those calculated using data from previous simpler one-dimensional experimental analyses. These findings have implications for lung cell flow experiments involving shear stress in the consideration of both normal airway function and pathology due to mechanotransduction mechanisms. PMID:24618983

  19. Effects of Fluid Shear Stress on Cancer Stem Cell Viability

    NASA Astrophysics Data System (ADS)

    Sunday, Brittney; Triantafillu, Ursula; Domier, Ria; Kim, Yonghyun

    2014-11-01

    Cancer stem cells (CSCs), which are believed to be the source of tumor formation, are exposed to fluid shear stress as a result of blood flow within the blood vessels. It was theorized that CSCs would be less susceptible to cell death than non-CSCs after both types of cell were exposed to a fluid shear stress, and that higher levels of fluid shear stress would result in lower levels of cell viability for both cell types. To test this hypothesis, U87 glioblastoma cells were cultured adherently (containing smaller populations of CSCs) and spherically (containing larger populations of CSCs). They were exposed to fluid shear stress in a simulated blood flow through a 125-micrometer diameter polyetheretherketone (PEEK) tubing using a syringe pump. After exposure, cell viability data was collected using a BioRad TC20 Automated Cell Counter. Each cell type was tested at three physiological shear stress values: 5, 20, and 60 dynes per centimeter squared. In general, it was found that the CSC-enriched U87 sphere cells had higher cell viability than the CSC-depleted U87 adherent cancer cells. Interestingly, it was also observed that the cell viability was not negatively affected by the higher fluid shear stress values in the tested range. In future follow-up studies, higher shear stresses will be tested. Furthermore, CSCs from different tumor origins (e.g. breast tumor, prostate tumor) will be tested to determine cell-specific shear sensitivity. National Science Foundation Grant #1358991 supported the first author as an REU student.

  20. Dimensionless critical shear stress in gravel-bed rivers

    NASA Astrophysics Data System (ADS)

    Petit, François; Houbrechts, Geoffrey; Peeters, Alexandre; Hallot, Eric; Van Campenhout, Jean; Denis, Anne-Cécile

    2015-12-01

    This paper first compiles critical shear stress values from 26 studies of gravel-bed rivers (GBRs) worldwide. The most frequently proposed value of the Shields criterion (θc) is 0.045, but three major groups with θc values ranging from < 0.030 to > 0.100 were identified. Second, dimensionless critical shear stresses (the Shields criterion) were evaluated for 14 GBRs (18 sites) with watershed areas ranging from 12 to 3000 km2. Different approaches were used to identify the initial movement of the bed material: painted and PIT-tag pebbles, sediment traps, and bedload samplers. The Shields criterion (θc) was estimated using the total shear stress (τ) and the grain shear stress (τ‧). Several shear stresses were also estimated using shear velocities. For bedload transport, we obtained an average Shields criterion (θc) of 0.040. The values were higher in small rivers (> 0.050) than larger rivers (< 0.030) because of more significant bedform shear stresses. The Shields criterion (θ‧c) was lower when the grain shear stress (τ‧) was used and only reached 0.019. Different values are also proposed in relation to the type of mobilization: the θc value for partial transport was ~ 0.025 and exceeded 0.040 for full transport (usually reached in association with discharges with a 10-year return period). The values based on the results of sediment traps and a bedload sampler were greater than those obtained using tracers, but these differences are smaller than those usually reported in the literature.

  1. Simple average expression for shear-stress relaxation modulus

    NASA Astrophysics Data System (ADS)

    Wittmer, J. P.; Xu, H.; Baschnagel, J.

    2016-01-01

    Focusing on isotropic elastic networks we propose a simple-average expression G (t ) =μA-h (t ) for the computational determination of the shear-stress relaxation modulus G (t ) of a classical elastic solid or fluid. Here, μA=G (0 ) characterizes the shear transformation of the system at t =0 and h (t ) the (rescaled) mean-square displacement of the instantaneous shear stress τ ̂(t ) as a function of time t . We discuss sampling time and ensemble effects and emphasize possible pitfalls of alternative expressions using the shear-stress autocorrelation function. We argue finally that our key relation may be readily adapted for more general linear response functions.

  2. Simple average expression for shear-stress relaxation modulus.

    PubMed

    Wittmer, J P; Xu, H; Baschnagel, J

    2016-01-01

    Focusing on isotropic elastic networks we propose a simple-average expression G(t)=μ_{A}-h(t) for the computational determination of the shear-stress relaxation modulus G(t) of a classical elastic solid or fluid. Here, μ_{A}=G(0) characterizes the shear transformation of the system at t=0 and h(t) the (rescaled) mean-square displacement of the instantaneous shear stress τ[over ̂](t) as a function of time t. We discuss sampling time and ensemble effects and emphasize possible pitfalls of alternative expressions using the shear-stress autocorrelation function. We argue finally that our key relation may be readily adapted for more general linear response functions. PMID:26871020

  3. Characterization of fractures subjected to normal and shear stress

    NASA Astrophysics Data System (ADS)

    Choi, Min-Kwang

    Results from a series of laboratory experiments to determine fracture specific stiffness, for a fracture subjected to shear and normal stress, are presented and analyzed. The experimental work focuses on the determination of relations between normal and shear fracture specific stiffness and between spatial distribution of fracture specific stiffness and fluid flow through the fracture The ratio of shear to normal fracture specific stiffness is experimentally investigated on a fracture subjected to shear as well as normal stress. Synthetic fractures made of gypsum and lucite were prepared with different fracture surface conditions: either well-mated or non-mated. For well-mated fracture surfaces, asperities were created by casting gypsum against sandpaper. A block of gypsum was cast against the sandpaper and then a second block was cast against the first block such that the two contact surfaces were well-mated. The surface roughness was controlled by using the sandpapers with different average grit size. Non-mated fracture surfaces were fabricated with two lucite blocks that were polished (lucite PL) or sand-blasted (lucite SB) along their contact surface. In the experiments, each specimen was subjected to normal and shear loading while the fracture was probed with transmitted and reflected compressional and shear waves. Shear and normal fracture specific stiffnesses were calculated using the displacement discontinuity theory. For non-mated fractures, the stiffness ratio was not sensitive to the application of shear stress and, as normal stress increased, approached a theoretical ratio which was determined assuming that the transmission of compressional and shear waves was equal. The stiffness ratio obtained from well-mated fractures ranged from 0.5 to 1.4, which deviated from the conventional assumption that shear and normal fracture specific stiffness are equal. The stiffness ratio increased with increasing surface roughness and with increasing shear stress. For

  4. Dynamics of Concentrated Silica Suspension under Oscillatory Shear Studied by SAXS and XPCS

    NASA Astrophysics Data System (ADS)

    Lee, Jonghun; Lin, Xiao-Min; Sandy, Alec; Narayanan, Suresh; X-ray Science Division Team; CenterNanomaterials Team

    2015-03-01

    The viscoelastic properties of complex fluids are often obtained by applying small amplitude oscillatory shear (SAOS). In this regime, their microstructure does not change by shear, and the shear stress linearly responds to the applied strain. However, in the real application, high shear strain or rate is applied, where the viscoelastic properties are affected by the microstructural deformation by this high shear. The rheological behavior of complex fluids under large amplitude oscillatory shear (LAOS) has been widely studied, but there is a lack of studies in microscopic dynamics of complex fluids under LAOS. X-ray scattering is a suitable method to understand microscopic perspective of rheology because of its proper length scales of tens to hundreds nm and time scales of millisecond to thousands second. Here, we studied the dynamics of the concentrated silica nanoparticle suspensions in PEG under different shear strain regimes using small angle x-ray scattering (SAXS) and x-ray photon correlation spectroscopy (XPCS). With strain increasing, these suspensions showed shear thinning and shear thickening behavior, and their microstructural change was observed by SAXS. In oscillatory shear, as the original scattering volume periodically comes back to the original position, we could better study the changes in autocorrelation function by shear and diffusion than steady shear study where correlation decays by transit.

  5. Significance of extensional stresses to red blood cell lysis in a shearing flow.

    PubMed

    Down, Linden A; Papavassiliou, Dimitrios V; O'Rear, Edgar A

    2011-06-01

    Traditionally, an empirical power-law model relating hemolysis to shear stress and exposure time has been used to estimate hemolysis related to flow--however, this basis alone has been insufficient in attempts to predict hemolysis through computational fluid dynamics. Because of this deficiency, we sought to re-examine flow features related to hemolysis in a shearing flow by computationally modeling a set of classic experiments performed in a capillary tube. Simulating 21 different flows of varying entrance contraction ratio, flowrate and viscosity, we identified hemolysis threshold streamlines and analyzed the stresses present. Constant damage thresholds for radial and axial extensional stresses of approximately 3000 Pa for exposure times on the order of microseconds were observed, while no such threshold was found for the maximum shear stress or gradient of the shear stress. The extensional flow seen at the entrance of the capillary appears to be most consistently related to hemolysis. An account of how extensional stresses can lead to lysis of a red cell undergoing tank-tread motion in a shearing flow is provided. This work shows that extensional components of the stress tensor are integral in causing hemolysis for some flows, and should be considered when attempting to predict hemolysis computationally. PMID:21298343

  6. Elevated Shear Stress in Arteriovenous Fistulae: Is There Mechanical Homeostasis?

    NASA Astrophysics Data System (ADS)

    McGah, Patrick; Leotta, Daniel; Beach, Kirk; Aliseda, Alberto

    2011-11-01

    Arteriovenous fistulae are created surgically to provide access for dialysis in patients with renal failure. The current hypothesis is that the rapid remodeling occurring after the fistula creation is in part a process to restore the mechanical stresses to some preferred level (i.e. mechanical homeostasis). Given that nearly 50% of fistulae require an intervention after one year, understanding the altered hemodynamic stress is important in improving clinical outcomes. We perform numerical simulations of four patient-specific models of functioning fistulae reconstructed from 3D Doppler ultrasound scans. Our results show that the vessels are subjected to `normal' shear stresses away from the anastomosis; about 1 Pa in the veins and about 2.5 Pa in the arteries. However, simulations show that part of the anastomoses are consistently subjected to very high shear stress (>10Pa) over the cardiac cycle. These elevated values shear stresses are caused by the transitional flows at the anastomoses including flow separation and quasiperiodic vortex shedding. This suggests that the remodeling process lowers shear stress in the fistula but that it is limited as evidenced by the elevated shear at the anastomoses. This constant insult on the arterialized venous wall may explain the process of late fistula failure in which the dialysis access become occluded after years of use. Supported by an R21 Grant from NIDDK (DK081823).

  7. Experimental investigation of the wall shear stress in a circular impinging jet

    NASA Astrophysics Data System (ADS)

    El Hassan, M.; Assoum, H. H.; Martinuzzi, R.; Sobolik, V.; Abed-Meraim, K.; Sakout, A.

    2013-07-01

    The influence of the large-scale vortical structures on the wall shear stress in a circular impinging jet is investigated experimentally for a Reynolds number of 1260. Time-resolved particle image velocimetry and polarographic measurements are performed simultaneously. It is found that the instantaneous wall shear stress is strongly dependent on the vortex dynamics, particularly for different parts of the transverse vortex. The influence of the vortex ring, the secondary and tertiary vortices on the ejection/sweep process near the wall is the main mechanism involved in the wall shear stress variation. In the region of the boundary layer separation, the wall shear stress amplitude increases just upstream of the separation and dramatically decreases in the recirculation zone downstream from the separation. The interaction between primary and secondary structures and their pairing process with the tertiary structure affects the sweep/ejection process near the wall and subsequently the wall shear stress variation. A comparison between the Finite Time Lyapunov Exponent (FTLE) method and the phase average technique is performed. It is shown that both methods describe the flow dynamics in the impinging region of the vortex ring. However, the FTLE method is more suitable for describing the unsteady separation of the boundary layer.

  8. Shear Stress Sensing with Elastic Microfence Structures

    NASA Technical Reports Server (NTRS)

    Cisotto, Alexxandra; Palmieri, Frank L.; Saini, Aditya; Lin, Yi; Thurman, Christopher S; Kim, Jinwook; Kim, Taeyang; Connell, John W.; Zhu, Yong; Gopalarathnam, Ashok; Jiang, Xiaoning; Wohl, Christopher J.

    2015-01-01

    In this work, elastic microfences were generated for the purpose of measuring shear forces acting on a wind tunnel model. The microfences were fabricated in a two part process involving laser ablation patterning to generate a template in a polymer film followed by soft lithography with a two-part silicone. Incorporation of a fluorescent dye was demonstrated as a method to enhance contrast between the sensing elements and the substrate. Sensing elements consisted of multiple microfences prepared at different orientations to enable determination of both shear force and directionality. Microfence arrays were integrated into an optical microscope with sub-micrometer resolution. Initial experiments were conducted on a flat plate wind tunnel model. Both image stabilization algorithms and digital image correlation were utilized to determine the amount of fence deflection as a result of airflow. Initial free jet experiments indicated that the microfences could be readily displaced and this displacement was recorded through the microscope.

  9. Gyrokinetic Simulation of Residual Stress from Diamagnetic Velocity Shears

    NASA Astrophysics Data System (ADS)

    Waltz, R. E.; Staebler, G. M.; Solomon, W. M.

    2010-11-01

    Residual stress refers to the remaining toroidal angular momentum (TAM) flux (divided by major radius) when the shear in the parallel velocity (and parallel velocity itself) vanishes. Previously [1] we demonstrated with gyrokinetic (GYRO) simulations that TAM pinching from the diamagnetic level shear in the ExB velocity could provide the residual stress needed for spontaneous toroidal rotation. Here we show that the shear in the diamagnetic velocities themselves provide comparable residual stress (and level of stabilization). The sign of the residual stress, quantified by the ratio of TAM flow to ion power flow (M/P), depends on the signs of the various velocity shears as well as ion (ITG) versus electron (TEM) mode directed turbulence. The residual stress from these temperature and density gradient diamagnetic velocity shears is demonstrated in global gyrokinetic simulation of ``null'' rotation DIIID discharges by matching M/P profiles within experimental error. 8pt [1] R.E. Waltz, G.M. Staebler, J. Candy, and F.L. Hinton, Phys. Plasmas 14, 122507 (2007); errata 16, 079902 (2009).

  10. Shear Load Transfer in High and Low Stress Tendons

    PubMed Central

    Kondratko-Mittnacht, Jaclyn; Duenwald-Kuehl, Sarah; Lakes, Roderic; Vanderby, Ray

    2016-01-01

    Background Tendon is an integral part of joint movement and stability, as it functions to transmit load from muscle to bone. It has an anisotropic, fibrous hierarchical structure that is generally loaded in the direction of its fibers/fascicles. Internal load distributions are altered when joint motion rotates an insertion site or when local damage disrupts fibers/fascicles, potentially causing inter-fiber (or inter-fascicular) shear. Tendons with different microstructure (helical versus linear) may redistribute loads differently. Method of Approach This study explored how shear redistributes axial loads in rat tail tendon (low stress tendons with linear microstructure) and porcine flexor tendon (high stress with helical microstructure) by creating lacerations on opposite sides of the tendon, ranging from about 20-60% of the tendon width, to create various magnitudes of shear. Differences in fascicular orientation were quantified using polarized light microscopy. Results and Conclusions Unexpectedly, both tendon types maintained about 20% of pre-laceration stress values after overlapping cuts of 60% of tendon width (no intact fibers end to end) suggesting that shear stress transfer can contribute more to overall tendon strength and stiffness than previously reported. All structural parameters for both tendon types decreased linearly with increasing laceration depth. The tail tendon had a more rapid decline in post-laceration elastic stress and modulus parameters as well as a more linear and less tightly packed fascicular structure, suggesting that positional tendons may be less well suited to redistribute loads via a shear mechanism. PMID:25700261

  11. Effects of shear stress on endothelial progenitor cells.

    PubMed

    Obi, Syotaro; Yamamoto, Kimiko; Ando, Joji

    2014-10-01

    Endothelial progenitor cells (EPCs) are adult stem cells that play a central role in neovascularization. EPCs are mobilized from bone marrow into peripheral blood, attach to existing endothelial cells, and then transmigrate across the endothelium into tissues, where they proliferate, differentiate, and form new blood vessels. In the process, EPCs are exposed to shear stress, a biomechanical force generated by flowing blood and tissue fluid flow. When cultured EPCs are exposed to controlled levels of shear stress in a flow-loading device, their bioactivities in terms of proliferation, anti-apoptosis, migration, production of bioactive substances, anti-thrombosis, and tube formation increase markedly. Expression of endothelial marker genes and proteins by EPCs also increases in response to shear stress, and they differentiate into mature endothelial cells. Great advances have been made in elucidating the mechanisms by which mature endothelial cells sense and respond to shear stress, but not in EPCs. Further study of EPC responses to shear stress will be necessary to better understand the physiological and pathophysiological roles of EPCs and to apply EPCs to new therapies in the field of regenerative medicine. PMID:25992410

  12. The Role of Shear Stress in Arteriovenous Fistula Maturation and Failure: A Systematic Review

    PubMed Central

    Browne, Leonard D.; Bashar, Khalid; Griffin, Philip; Kavanagh, Eamon G.; Walsh, Stewart R.; Walsh, Michael T.

    2015-01-01

    Introduction Non-maturation and post-maturation venous stenosis are the primary causes of failure within arteriovenous fistulae (AVFs). Although the exact mechanisms triggering failure remain unclear, abnormal hemodynamic profiles are thought to mediate vascular remodelling and can adversely impact on fistula patency. Aim The review aims to clarify the role of shear stress on outward remodelling during maturation and evaluate the evidence supporting theories related to the localisation and development of intimal hyperplasia within AVFs. Methods A systematic review of studies comparing remodelling data with hemodynamic data obtained from computational fluid dynamics of AVFs during and after maturation was conducted. Results Outward remodelling occurred to reduce or normalise the level of shear stress over time in fistulae with a large radius of curvature (curved) whereas shear stress was found to augment over time in fistulae with a small radius of curvature (straight) coinciding with minimal to no increases in lumen area. Although this review highlighted that there is a growing body of evidence suggesting low and oscillating shear stress may stimulate the initiation and development of intimal medial thickening within AVFs. Further lines of evidence are needed to support the disturbed flow theory and outward remodelling findings before surgical configurations and treatment strategies are optimised to conform to them. This review highlighted that variation between the time of analysis, classification of IH, resolution of simulations, data processing techniques and omission of various shear stress metrics prevented forming pooling of data amongst studies. Conclusion Standardised measurements and data processing techniques are needed to comprehensively evaluate the relationship between shear stress and intimal medial thickening. Advances in image acquisition and flow quantifications coupled with the increasing prevalence of longitudinal studies commencing from fistula

  13. High shear stress induces atherosclerotic vulnerable plaque formation through angiogenesis

    PubMed Central

    Wang, Yi; Qiu, Juhui; Luo, Shisui; Xie, Xiang; Zheng, Yiming; Zhang, Kang; Ye, Zhiyi; Liu, Wanqian; Gregersen, Hans; Wang, Guixue

    2016-01-01

    Rupture of atherosclerotic plaques causing thrombosis is the main cause of acute coronary syndrome and ischemic strokes. Inhibition of thrombosis is one of the important tasks developing biomedical materials such as intravascular stents and vascular grafts. Shear stress (SS) influences the formation and development of atherosclerosis. The current review focuses on the vulnerable plaques observed in the high shear stress (HSS) regions, which localizes at the proximal region of the plaque intruding into the lumen. The vascular outward remodelling occurs in the HSS region for vascular compensation and that angiogenesis is a critical factor for HSS which induces atherosclerotic vulnerable plaque formation. These results greatly challenge the established belief that low shear stress is important for expansive remodelling, which provides a new perspective for preventing the transition of stable plaques to high-risk atherosclerotic lesions. PMID:27482467

  14. High shear stress induces atherosclerotic vulnerable plaque formation through angiogenesis.

    PubMed

    Wang, Yi; Qiu, Juhui; Luo, Shisui; Xie, Xiang; Zheng, Yiming; Zhang, Kang; Ye, Zhiyi; Liu, Wanqian; Gregersen, Hans; Wang, Guixue

    2016-12-01

    Rupture of atherosclerotic plaques causing thrombosis is the main cause of acute coronary syndrome and ischemic strokes. Inhibition of thrombosis is one of the important tasks developing biomedical materials such as intravascular stents and vascular grafts. Shear stress (SS) influences the formation and development of atherosclerosis. The current review focuses on the vulnerable plaques observed in the high shear stress (HSS) regions, which localizes at the proximal region of the plaque intruding into the lumen. The vascular outward remodelling occurs in the HSS region for vascular compensation and that angiogenesis is a critical factor for HSS which induces atherosclerotic vulnerable plaque formation. These results greatly challenge the established belief that low shear stress is important for expansive remodelling, which provides a new perspective for preventing the transition of stable plaques to high-risk atherosclerotic lesions. PMID:27482467

  15. Bed Shear Stress in Channels with Emergent Vegetation

    NASA Astrophysics Data System (ADS)

    Yang, Q.; Kerger, F.; Nepf, H. M.

    2014-12-01

    The shear stress at the bed of a channel influences important benthic processes such as sediment transport. Several methods exist to estimate the bed shear stress in open channel flow, but most of these are not appropriate for vegetated channels due to the impact of vegetation on the velocity profile and turbulence production. This study proposes a new model to estimate the bed shear stress in both vegetated and bare channels. The model is based on the observation that, for both bare and vegetated channels, within a viscous sub-layer at the bed, the viscous stress decreases linearly with increasing distance from the bed, resulting in a parabolic velocity profile at the bed. For emergent canopies of sufficient density, the thickness of this linear-stress layer is set by the stem diameter, leading to a simple estimate for bed shear stress. For bare channels, the model describes the velocity profile in the overlap region of the Law of the Wall. The model is supported by high-resolution experiments. Furthermore, the changes in turbulence isotropy and integral length across a range of vegetation density, from bare bed to dense canopy, have been explored.

  16. A Rotary Flow Channel for Shear Stress Sensor Calibration

    NASA Technical Reports Server (NTRS)

    Zuckerwar, Allan J.; Scott, Michael A.

    2004-01-01

    A proposed shear sensor calibrator consists of a rotating wheel with the sensor mounted tangential to the rim and positioned in close proximity to the rim. The shear stress generated by the flow at the sensor position is simply tau(sub omega) = (mu)r(omega)/h, where mu is the viscosity of the ambient gas, r the wheel radius, omega the angular velocity of the wheel, and h the width of the gap between the wheel rim and the sensor. With numerical values of mu = 31 (mu)Pa s (neon at room temperature), r = 0.5 m, omega = 754 /s (7200 rpm), and h = 50.8 m, a shear stress of tau(sub omega) = 231 Pa can be generated. An analysis based on one-dimensional flow, with the flow velocity having only an angular component as a function of the axial and radial coordinates, yields corrections to the above simple formula for the curvature of the wheel, flatness of the sensor, and finite width of the wheel. It is assumed that the sensor mount contains a trough (sidewalls) to render a velocity release boundary condition at the edges of the rim. The Taylor number under maximum flow conditions is found to be 62.3, sufficiently low to obviate flow instability. The fact that the parameters entering into the evaluation of the shear stress can be measured to high accuracy with well-defined uncertainties makes the proposed calibrator suitable for a physical standard for shear stress calibration.

  17. Discontinuous shear thickening in Brownian suspensions by dynamic simulation

    PubMed Central

    Mari, Romain; Seto, Ryohei; Morris, Jeffrey F.; Denn, Morton M.

    2015-01-01

    Dynamic particle-scale numerical simulations are used to show that the shear thickening observed in dense colloidal, or Brownian, suspensions is of a similar nature to that observed in noncolloidal suspensions, i.e., a stress-induced transition from a flow of lubricated near-contacting particles to a flow of a frictionally contacting network of particles. Abrupt (or discontinuous) shear thickening is found to be a geometric rather than hydrodynamic phenomenon; it stems from the strong sensitivity of the jamming volume fraction to the nature of contact forces between suspended particles. The thickening obtained in a colloidal suspension of purely hard frictional spheres is qualitatively similar to experimental observations. However, the agreement cannot be made quantitative with only hydrodynamics, frictional contacts, and Brownian forces. Therefore, the role of a short-range repulsive potential mimicking the stabilization of actual suspensions on the thickening is studied. The effects of Brownian and repulsive forces on the onset stress can be combined in an additive manner. The simulations including Brownian and stabilizing forces show excellent agreement with experimental data for the viscosity η and the second normal stress difference N2. PMID:26621744

  18. Nonlinear Reynolds stress model for turbulent shear flows

    NASA Technical Reports Server (NTRS)

    Barton, J. Michael; Rubinstein, R.; Kirtley, K. R.

    1991-01-01

    A nonlinear algebraic Reynolds stress model, derived using the renormalization group, is applied to equilibrium homogeneous shear flow and fully developed flow in a square duct. The model, which is quadratically nonlinear in the velocity gradients, successfully captures the large-scale inhomogeneity and anisotropy of the flows studied. The ratios of normal stresses, as well as the actual magnitudes of the stresses are correctly predicted for equilibrium homogeneous shear flow. Reynolds normal stress anisotropy and attendant turbulence driven secondary flow are predicted for a square duct. Profiles of mean velocity and normal stresses are in good agreement with measurements. Very close to walls, agreement with measurements diminishes. The model has the benefit of containing no arbitrary constants; all values are determined directly from the theory. It seems that near wall behavior is influenced by more than the large scale anisotropy accommodated in the current model. More accurate near wall calculations may well require a model for anisotropic dissipation.

  19. Buried wire gage for wall shear stress measurements

    NASA Technical Reports Server (NTRS)

    Murthy, V. S.; Rose, W. C.

    1978-01-01

    A buried wire gage for measuring wall shear stress in fluid flow was studied and further developed. Several methods of making this relatively new type of gage were examined to arrive at a successful technique that is well-suited for wind-tunnel testing. A series of measurements was made to demonstrate the adequacy of a two-point calibration procedure for these gages. The buried wire gage is also demonstrated to be ideally suited for quantitative measurement of wall shear stress in wind-tunnel testing.

  20. Shear Stress Partitioning in Airflow over Rough Surfaces: Roughness Form Effects and Influence on the Distribution of Shear Stress

    NASA Astrophysics Data System (ADS)

    Gillies, J. A.; Nickling, W. G.; King, J.

    2004-12-01

    Roughness elements distributed across a surface can significantly decrease the entrainment and transport of underlying fine-grained sediments by wind. The parameterization of roughness effects on wind erosion thresholds and sediment transport is critical to the development of models that can provide realistic predictions of sediment thresholds and fluxes due to wind erosion. Raupach et al. (1993) present a model for predicting the protective role of roughness elements in terms of a threshold friction velocity ratio as a function of the roughness geometry and the aerodynamic properties of the surface and roughness elements. The predictive capacity of this model remains uncertain and the work presented here represents part of an on-going effort of our group to improve the parameterization of the Raupach et al. (1993) model. To gain additional understanding of how roughness elements influence the magnitude and nature of the shear stress acting on the surface among the elements and evaluate strength and weaknesses of the roughness density parameter to characterize these effects, a wind tunnel study using model roughness arrays of similar roughness density composed of cube-shaped elements of different length dimensions was undertaken. Roughness density is defined as the total frontal area of all the elements to the total surface area that they occupy. Shear stress in the above element air flow was determined from vertical wind speed profile measurements. Point measurements of near surface shear stresses within the roughness array were made with simple omni-directional skin friction meters in order to investigate the partitioning of shear stress to the intervening surface. The results suggest that the roughness density parameter has severe limitations in describing the shear stress partitioning for these regularly arrayed rough surfaces. For surfaces with identical roughness densities, the surface composed of more and smaller elements was observed to have average and

  1. Modelling of peak-flow wall shear stress in major airways of the lung.

    PubMed

    Green, A S

    2004-05-01

    Some respiratory diseases result in the inflammation of the lung airway epithelium. An associated chronic cough, as found in many cases of asthma and in long-term smokers, can exacerbate damage to the epithelial layer. It has been proposed that wall shear stresses, created by peak expiratory flow-rates during a coughing episode, are responsible. The work here uses a computational fluid dynamics technique to model peak expiratory flow in the trachea and major lung bronchi. Calculated wall shear stress values are compared to a limited set of published measurements taken from a physical model. The measurements are discussed in the context of a flow study of a complex bronchial network. A more complete picture is achieved by the calculation method, indicating, in some cases, higher maximum wall shear stresses than measured, confirming the original findings of the experimental work. Recommendations are made as to where further work would be beneficial to medical applications. PMID:15046995

  2. Stress impacts telomere dynamics.

    PubMed

    Kotrschal, Alexander; Ilmonen, Petteri; Penn, Dustin J

    2007-04-22

    Telomeres are DNA-protein complexes at the ends of chromosomes that control genomic integrity but appear to become shorter with age and stress. To test whether stress causes telomere attrition, we exposed the offspring of wild-caught house mice (Mus musculus) to stressful conditions and examined the changes in telomere length over six months. We found that females exposed to males and reproductive stress (either with or without crowding) had significantly shorter telomeres than controls, and males exposed to crowding stress had shorter telomeres than males that were not crowded. Our results indicate that stress alters telomere dynamics, causing attrition and hindering restoration, and these effects are sex dependent. Telomeres may thus provide a biomarker for assessing an individual's cumulative exposure or ability to cope with stressful conditions. PMID:17264051

  3. Rheological properties of polymer melts in confined shear flow from dynamic Monte Carlo simulations.

    NASA Astrophysics Data System (ADS)

    Dorgan, John

    2007-03-01

    The viscoelastic properties of dense polymer melts in shear flow are examined using dynamic Monte Carlo simulation for plate spacings less than 10 times the molecular radius of gyration. The coarse graining methodology employed consists of the cooperative motion algorithm of Pakula and a derived biasing technique based on previous studies of Binder and Baushnagel. For relatively large plate spacings and slow flows, a uniform linear velocity profile is obtainable. Use of the Kramers form for entropic springs allows the calculation of stress in the simulation providing a means for exploring rheological properties including viscosity and normal stress differences. Results are in excellent agreement with well-established experimental facts; a shear thinning viscosity is obtained, the first normal stress difference increases with shear rate, and the first normal stress coefficient decreases with shear rate. Evidence of entanglements are present for longer chain lengths. For fast flows, the linear velocity profile is lost and shear banding is observed. A non-monotonic stress with shear rate is found in conjunction with the shear banding and mechanistically this is attributable to a cohesive failure with an excess of chain ends being found at the slip plane. Results for variable plate spacings shed some insight into novel confinement effects that are being exploited in emerging areas of nanotechnology.

  4. Controlling Shear Stress in 3D Bioprinting is a Key Factor to Balance Printing Resolution and Stem Cell Integrity.

    PubMed

    Blaeser, Andreas; Duarte Campos, Daniela Filipa; Puster, Uta; Richtering, Walter; Stevens, Molly M; Fischer, Horst

    2016-02-01

    A microvalve-based bioprinting system for the manufacturing of high-resolution, multimaterial 3D-structures is reported. Applying a straightforward fluid-dynamics model, the shear stress at the nozzle site can precisely be controlled. Using this system, a broad study on how cell viability and proliferation potential are affected by different levels of shear stress is conducted. Complex, multimaterial 3D structures are printed with high resolution. This work pioneers the investigation of shear stress-induced cell damage in 3D bioprinting and might help to comprehend and improve the outcome of cell-printing studies in the future. PMID:26626828

  5. A Two-Axis Direct Fluid Shear Stress Sensor

    NASA Technical Reports Server (NTRS)

    Adcock, Edward E.; Scott, Michael A.; Bajikar, Sateesh S.

    2010-01-01

    This innovation is a miniature or micro sized semiconductor sensor design that provides two axis direct non-intrusive measurement of skin friction or wall shear stress in fluid flow. The sensor is fabricated by micro-electro-mechanical system (MEMS) technology, enabling small size and low cost reproductions. The sensors have been fabricated by utilizing MEMS fabrication processes to bond a sensing element wafer to a fluid coupling wafer. This layering technique provides for an out of plane dimension that is on the same order of length as the inplane dimensions. The sensor design has the following characteristics: a shear force collecting plate with dimensions that can be tailored to various application specific requirements such as spatial resolution, temporal resolution and shear force range and resolution. This plate is located coplanar to both the sensor body and flow boundary, and is connected to a dual axis gimbal structure by a connecting column or lever arm. The dual axis gimbal structure has torsional hinges with embedded piezoresistive torsional strain gauges which provide a voltage output that is correlated to the applied shear stress (and excitation current) on force collection plate that is located on the flow boundary surface (hence the transduction method). This combination of design elements create a force concentration and resolution structure that enables the generation of a large stress on the strain gauge from the small shear stress on the flow boundary wall. This design as well as the use of back side electrical contacts establishes a non-intrusive method to quantitatively measure the shear force vector on aerodynamic bodies.

  6. WAVE ACTION AND BOTTOM SHEAR STRESSES IN LAKE ERIE

    EPA Science Inventory

    For Lake Erie, the amplitudes and periods of wind-driven, surface gravity waves were calculated by means of the SMB hindcasting method. Bottom orbital velocities and bottom shear stresses were then calculated using linear wave theory and Kajiura's (1968) turbulent oscillating bou...

  7. Calculation of Near-Bank Velocity and Boundary Shear Stress

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Detailed knowledge of the flow and boundary shear stress fields near the banks of natural channels is essential for making accurate calculations of rates of near-bank sediment transport and geomorphic adjustment. This paper presents a test of a relatively simple, fully predictive, numerical method f...

  8. Production of Functional Proteins: Balance of Shear Stress and Gravity

    NASA Technical Reports Server (NTRS)

    Goodwin, Thomas John (Inventor); Hammond, Timothy Grant (Inventor); Haysen, James Howard (Inventor)

    2005-01-01

    The present invention provides for a method of culturing cells and inducing the expression of at least one gene in the cell culture. The method provides for contacting the cell with a transcription factor decoy oligonucleotide sequence directed against a nucleotide sequence encoding a shear stress response element.

  9. Fluid shear stress modulation of hepatocyte-like cell function.

    PubMed

    Rashidi, Hassan; Alhaque, Sharmin; Szkolnicka, Dagmara; Flint, Oliver; Hay, David C

    2016-07-01

    Freshly isolated human adult hepatocytes are considered to be the gold standard tool for in vitro studies. However, primary hepatocyte scarcity, cell cycle arrest and the rapid loss of cell phenotype limit their widespread deployment. Human embryonic stem cells and induced pluripotent stem cells provide renewable sources of hepatocyte-like cells (HLCs). Despite the use of various differentiation methodologies, HLCs like primary human hepatocytes exhibit unstable phenotype in culture. It has been shown that the functional capacity can be improved by adding back elements of human physiology, such as cell co-culture or through the use of natural and/or synthetic surfaces. In this study, the effect of fluid shear stress on HLC performance was investigated. We studied two important liver functions, cytochrome P450 drug metabolism and serum protein secretion, in static cultures and those exposed to fluid shear stress. Our study demonstrates that fluid shear stress improved Cyp1A2 activity by approximately fivefold. This was paralleled by an approximate ninefold increase in sensitivity to a drug, primarily metabolised by Cyp2D6. In addition to metabolic capacity, fluid shear stress also improved hepatocyte phenotype with an approximate fourfold reduction in the secretion of a foetal marker, alpha-fetoprotein. We believe these studies highlight the importance of introducing physiologic cues in cell-based models to improve somatic cell phenotype. PMID:26979076

  10. Liquid Crystals Indicate Directions Of Surface Shear Stresses

    NASA Technical Reports Server (NTRS)

    Reda, Daniel C.

    1996-01-01

    Report consisting of main text of U.S. Patent 5,394,752 presents detailed information on one aspect of method of using changes in colors of liquid-crystal coatings to indicate instantaneous directions of flow-induced shear stresses (skin friction) on aerodynamic surfaces.

  11. A fluidized bed technique for estimating soil critical shear stress

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Soil erosion models, depending on how they are formulated, always have erodibilitiy parameters in the erosion equations. For a process-based model like the Water Erosion Prediction Project (WEPP) model, the erodibility parameters include rill and interrill erodibility and critical shear stress. Thes...

  12. An interlaminar shear stress continuity theory for both thin and thick composite laminates

    NASA Astrophysics Data System (ADS)

    Lu, Xianqiang; Liu, Dahsin

    1992-09-01

    The interlaminar shear stress plays a very important role in the damage of composite laminates. With higher interlaminar shear stress, delamination can easily occur on the composite interface. In order to calculate the interlaminar shear stress, a laminate theory, which accounts for both the interlaminar shear stress continuity and the transverse shear deformation, was presented in this study. Verification of the theory was performed by comparing the present theory with Pagano's elasticity analysis. It was found that the present theory was able to give excellent results for both stresses and displacements. More importantly, the interlaminar shear stress can be presented directly from the constitutive equations instead of being recovered from the equilibrium equations.

  13. A Multi-Phase Based Fluid-Structure-Microfluidic interaction sensor for Aerodynamic Shear Stress

    NASA Astrophysics Data System (ADS)

    Hughes, Christopher; Dutta, Diganta; Bashirzadeh, Yashar; Ahmed, Kareem; Qian, Shizhi

    2014-11-01

    A novel innovative microfluidic shear stress sensor is developed for measuring shear stress through multi-phase fluid-structure-microfluidic interaction. The device is composed of a microfluidic cavity filled with an electrolyte liquid. Inside the cavity, two electrodes make electrochemical velocimetry measurements of the induced convection. The cavity is sealed with a flexible superhydrophobic membrane. The membrane will dynamically stretch and flex as a result of direct shear cross-flow interaction with the seal structure, forming instability wave modes and inducing fluid motion within the microfluidic cavity. The shear stress on the membrane is measured by sensing the induced convection generated by membrane deflections. The advantages of the sensor over current MEMS based shear stress sensor technology are: a simplified design with no moving parts, optimum relationship between size and sensitivity, no gaps such as those created by micromachining sensors in MEMS processes. We present the findings of a feasibility study of the proposed sensor including wind-tunnel tests, microPIV measurements, electrochemical velocimetry, and simulation data results. The study investigates the sensor in the supersonic and subsonic flow regimes. Supported by a NASA SBIR phase 1 contract.

  14. The Need for a Shear Stress Calibration Standard

    NASA Technical Reports Server (NTRS)

    Scott, Michael A.

    2004-01-01

    By surveying current research of various micro-electro mechanical systems (MEMS) shear stress sensor development efforts we illustrate the wide variety of methods used to test and characterize these sensors. The different methods of testing these sensors make comparison of results difficult in some cases, and also this comparison is further complicated by the different formats used in reporting the results of these tests. The fact that making these comparisons can be so difficult at times clearly illustrates a need for standardized testing and reporting methodologies. This need indicates that the development of a national or international standard for the calibration of MEMS shear stress sensors should be undertaken. As a first step towards the development of this standard, two types of devices are compared and contrasted. The first type device is a laminar flow channel with two different versions considered: the first built with standard manufacturing techniques and the second with advanced precision manufacturing techniques. The second type of device is a new concept for creating a known shear stress consisting of a rotating wheel with the sensor mounted tangentially to the rim and positioned in close proximity to the rim. The shear stress generated by the flow at the sensor position is simply tau = (mu)r(omega)/h, where mu is the viscosity of the ambient gas, r the wheel radius, omega the angular velocity of the wheel, and h the width of the gap between the wheel rim and the sensor. Additionally, issues related to the development of a standard for shear stress calibration are identified and discussed.

  15. Flow instability and wall shear stress variation in intracranial aneurysms

    PubMed Central

    Baek, H.; Jayaraman, M. V.; Richardson, P. D.; Karniadakis, G. E.

    2010-01-01

    We investigate the flow dynamics and oscillatory behaviour of wall shear stress (WSS) vectors in intracranial aneurysms using high resolution numerical simulations. We analyse three representative patient-specific internal carotid arteries laden with aneurysms of different characteristics: (i) a wide-necked saccular aneurysm, (ii) a narrower-necked saccular aneurysm, and (iii) a case with two adjacent saccular aneurysms. Our simulations show that the pulsatile flow in aneurysms can be subject to a hydrodynamic instability during the decelerating systolic phase resulting in a high-frequency oscillation in the range of 20–50 Hz, even when the blood flow rate in the parent vessel is as low as 150 and 250 ml min−1 for cases (iii) and (i), respectively. The flow returns to its original laminar pulsatile state near the end of diastole. When the aneurysmal flow becomes unstable, both the magnitude and the directions of WSS vectors fluctuate at the aforementioned high frequencies. In particular, the WSS vectors around the flow impingement region exhibit significant spatio-temporal changes in direction as well as in magnitude. PMID:20022896

  16. Characterizations and Correlations of Wall Shear Stress in Aneurysmal Flow.

    PubMed

    Arzani, Amirhossein; Shadden, Shawn C

    2016-01-01

    Wall shear stress (WSS) is one of the most studied hemodynamic parameters, used in correlating blood flow to various diseases. The pulsatile nature of blood flow, along with the complex geometries of diseased arteries, produces complicated temporal and spatial WSS patterns. Moreover, WSS is a vector, which further complicates its quantification and interpretation. The goal of this study is to investigate WSS magnitude, angle, and vector changes in space and time in complex blood flow. Abdominal aortic aneurysm (AAA) was chosen as a setting to explore WSS quantification. Patient-specific computational fluid dynamics (CFD) simulations were performed in six AAAs. New WSS parameters are introduced, and the pointwise correlation among these, and more traditional WSS parameters, was explored. WSS magnitude had positive correlation with spatial/temporal gradients of WSS magnitude. This motivated the definition of relative WSS gradients. WSS vectorial gradients were highly correlated with magnitude gradients. A mix WSS spatial gradient and a mix WSS temporal gradient are proposed to equally account for variations in the WSS angle and magnitude in single measures. The important role that WSS plays in regulating near wall transport, and the high correlation among some of the WSS parameters motivates further attention in revisiting the traditional approaches used in WSS characterizations. PMID:26592536

  17. Shear stress related blood damage in laminar couette flow.

    PubMed

    Paul, Reinhard; Apel, Jörn; Klaus, Sebastian; Schügner, Frank; Schwindke, Peter; Reul, Helmut

    2003-06-01

    Artificial organs within the blood stream are generally associated with flow-induced blood damage, particularly hemolysis of red blood cells. These damaging effects are known to be dependent on shear forces and exposure times. The determination of a correlation between these flow-dependent properties and actual hemolysis is the subject of this study. For this purpose, a Couette device has been developed. A fluid seal based on fluorocarbon is used to separate blood from secondary external damage effects. The shear rate within the gap is controlled by the rotational speed of the inner cylinder, and the exposure time by the amount of blood that is axially pumped through the device per given time. Blood damage is quantified by the index of hemolysis (IH), which is calculated from photometric plasma hemoglobin measurements. Experiments are conducted at exposure times from texp=25 - 1250 ms and shear rates ranging from tau=30 up to 450 Pa ensuring Taylor-vortex free flow characteristics. Blood damage is remarkably low over a broad range of shear rates and exposure times. However, a significant increase in blood damage can be observed for shear stresses of tau>or= 425 Pa and exposure times of texp>or= 620 ms. Maximum hemolysis within the investigated range is IH=3.5%. The results indicate generally lower blood damage than reported in earlier studies with comparable devices, and the measurements clearly indicate a rather abrupt (i.e., critical levels of shear stresses and exposure times) than gradual increase in hemolysis, at least for the investigated range of shear rates and exposure times. PMID:12780506

  18. Dynamic Triggering Stress Modeling

    NASA Astrophysics Data System (ADS)

    Gonzalez-Huizar, H.; Velasco, A. A.

    2008-12-01

    It has been well established that static (permanent) stress changes can trigger nearby earthquakes, within a few fault lengths from the causative event, whereas triggering by dynamic (transient) stresses carried by seismic waves both nearby and at remote distances has not been as well documented nor understood. An analysis of the change in the local stress caused by the passing of surfaces waves is important for the understanding of this phenomenon. In this study, we modeled the change in the stress that the passing of Rayleigh and Loves waves causes on a fault plane of arbitrary orientation, and applied a Coulomb failure criteria to calculate the potential of these stress changes to trigger reverse, normal or strike-slip failure. We preliminarily test these model results with data from dynamically triggering earthquakes in the Australian Bowen Basin. In the Bowen region, the modeling predicts a maximum triggering potential for Rayleigh waves arriving perpendicularly to the strike of the reverse faults present in the region. The modeled potentials agree with our observations, and give us an understanding of the dynamic stress orientation needed to trigger different type of earthquakes.

  19. Development of a shear stress sensor to analyse the influence of polymers on the turbulent wall shear stress.

    PubMed

    Nottebrock, Bernardo; Grosse, Sebastian; Schröder, Wolfgang

    2011-05-11

    The drag reducing effect of polymers in a channel flow is well known and it is assumed that the polymer filaments interfere with the turbulent structures in the very near-wall flow. To analyse their precise effect, a micro-pillar shear stress sensor (MPS³) measurement system is developed which allows the detection of wall shear stress at high spatial and temporal resolutions. Different manufacturing techniques for the required micro-pillars are discussed and their influence on the flow is investigated evidencing the non-intrusive character of the pillars. Subsequently, a complete calibration is presented to relate the recorded deflection to wall shear stress values and to assure the correct detection over the whole expected frequency spectrum. A feasibility study about the ability to visualize the two-dimensional wall shear stress distribution completes the discussion about the validity of MPS³. In the last step, the drag reduction of a polymer filament grafted on a micro-pillar compared to a plain pillar and the application of MPS³ in an ocean-type polymer solution are investigated. The results confirm the expected behaviour found in the literature. PMID:21508484

  20. The Role of Shear Failure on Stress Characterization

    NASA Astrophysics Data System (ADS)

    Chan, A. W.; Hauser, M.; Couzens-Schultz, B. A.; Gray, G.

    2014-09-01

    Leak-off pressure and lost circulation data are generally thought to be reflective of minimum stress. We propose an alternative interpretation should be considered where the data may reflect a shear failure along zones of pre-existing weakness rather than opening of tensile fractures against the minimum stress. This mechanism has been discussed in a small number of borehole stability and hydraulic fracture papers, but has not been widely applied to leak-off test or lost circulation interpretation. In this paper, we will revisit and expand the concept introduced recently by Couzens-Schultz and Chan (J Struct Geol, doi: 10.1016/j.jsg.2010.06.013, 2010) based on abnormally low leak-off tests in an active thrust belt to the analysis of lost circulation observations in modern-day deltaic environments. In the Gulf of Mexico, lost circulations historically are interpreted as a representation of the minimum horizontal stress due to initiating or reopening of a fracture in tensile mode. However, shear failure or fault reactivation can occur at pressures well below the minimum far-field stress that is typically considered a safe upper bound for mud pressure if pre-existing planes of weakness such as faults or fracture networks exist. We demonstrated a mud loss event is shown to be inconsistent with the tensile failure mode in a normal stress environment, but in good agreement with expectations for shear failure along pre-existing faults.

  1. Dynamic stress changes during earthquake rupture

    USGS Publications Warehouse

    Day, S.M.; Yu, G.; Wald, D.J.

    1998-01-01

    We assess two competing dynamic interpretations that have been proposed for the short slip durations characteristic of kinematic earthquake models derived by inversion of earthquake waveform and geodetic data. The first interpretation would require a fault constitutive relationship in which rapid dynamic restrengthening of the fault surface occurs after passage of the rupture front, a hypothesized mechanical behavior that has been referred to as "self-healing." The second interpretation would require sufficient spatial heterogeneity of stress drop to permit rapid equilibration of elastic stresses with the residual dynamic friction level, a condition we refer to as "geometrical constraint." These interpretations imply contrasting predictions for the time dependence of the fault-plane shear stresses. We compare these predictions with dynamic shear stress changes for the 1992 Landers (M 7.3), 1994 Northridge (M 6.7), and 1995 Kobe (M 6.9) earthquakes. Stress changes are computed from kinematic slip models of these earthquakes, using a finite-difference method. For each event, static stress drop is highly variable spatially, with high stress-drop patches embedded in a background of low, and largely negative, stress drop. The time histories of stress change show predominantly monotonic stress change after passage of the rupture front, settling to a residual level, without significant evidence for dynamic restrengthening. The stress change at the rupture front is usually gradual rather than abrupt, probably reflecting the limited resolution inherent in the underlying kinematic inversions. On the basis of this analysis, as well as recent similar results obtained independently for the Kobe and Morgan Hill earthquakes, we conclude that, at the present time, the self-healing hypothesis is unnecessary to explain earthquake kinematics.

  2. Sensor for Boundary Shear Stress in Fluid Flow

    NASA Technical Reports Server (NTRS)

    Bao, Xiaoqi; Badescu, Mircea; Sherrit, Stewart; Bar-Cohen, Yoseph; Lih, Shyh-Shiuh; Chang, Zensheu; Trease, Brian P.; Kerenyi, Kornel; Widholm, Scott E.; Ostlund, Patrick N.

    2012-01-01

    The formation of scour patterns at bridge piers is driven by the forces at the boundary of the water flow. In most experimental scour studies, indirect processes have been applied to estimate the shear stress using measured velocity profiles. The estimations are based on theoretical models and associated assumptions. However, the turbulence flow fields and boundary layer in the pier-scour region are very complex and lead to low-fidelity results. In addition, available turbulence models cannot account accurately for the bed roughness effect. Direct measurement of the boundary shear stress, normal stress, and their fluctuations are attractive alternatives. However, most direct-measurement shear sensors are bulky in size or not compatible to fluid flow. A sensor has been developed that consists of a floating plate with folded beam support and an optical grid on the back, combined with a high-resolution optical position probe. The folded beam support makes the floating plate more flexible in the sensing direction within a small footprint, while maintaining high stiffness in the other directions. The floating plate converts the shear force to displacement, and the optical probe detects the plate s position with nanometer resolution by sensing the pattern of the diffraction field of the grid through a glass window. This configuration makes the sensor compatible with liquid flow applications.

  3. A simple model to understand the role of membrane shear elasticity and stress-free shape on the motion of red blood cells in shear flow

    NASA Astrophysics Data System (ADS)

    Viallat, Annie; Abkarian, Manouk; Dupire, Jules

    2015-11-01

    The analytical model presented by Keller and Skalak on the dynamics of red blood cells in shear flow described the cell as a fluid ellipsoid of fixed shape. It was extended to introduce shear elasticity of the cell membrane. We further extend the model when the cell discoid physiological shape is not a stress-free shape. We show that spheroid stress-free shapes enables fitting experimental data with values of shear elasticity typical to that found with micropipettes and optical tweezers. For moderate shear rates (when RBCs keep their discoid shape) this model enables to quantitatively determine an effective cell viscosity, that combines membrane and hemoglobin viscosities and an effective shear modulus of the membrane that combines shear modulus and stress-free shape. This model allows determining RBC mechanical parameters both in the tanktreading regime for cells suspended in a high viscosity medium, and in the tumbling regime for cells suspended in a low viscosity medium. In this regime,a transition is predicted between a rigid-like tumbling motion and a fluid-like tumbling motion above a critical shear rate, which is directly related to the mechanical parameters of the cell. A*MIDEX (n ANR-11-IDEX-0001-02) funded by the ''Investissements d'Avenir'', Region Languedoc-Roussillon, Labex NUMEV (ANR-10-LABX-20), BPI France project DataDiag.

  4. Inference of dynamic shear modulus from Lotung downhole data

    SciTech Connect

    Chang, C.Y.; Mok, C.M.; Tang, H.T.

    1996-08-01

    Downhole ground motions recorded at the Lotung Large-Scale Seismic Test (LSST) site were used in this paper to infer in-situ dynamic soil properties. The purposes were (1) to provide field evidence of nonlinear soil behavior during earthquake excitation; and (2) to evaluate the accuracy of dynamic properties obtained from geophysical measurements and laboratory tests. For each horizontal component and event analyzed, representative shear-wave velocity and effective shear strain (defined as 65% of peak strain) between consecutive recording stations were estimated. The representative shear-wave velocities were estimated from fundamental resonant frequencies identifiable from the Fourier spectral ratios. The effective shear strains were estimated by linear ground response deconvolution analyses based on the inferred shear-wave velocity profiles. The inferred reduction in shear modulus with increasing effective shear strain was compared with laboratory test data. The degree of agreement between the inferred shear modulus reduction curves and the laboratory test data varied with different testing programs. The inferred low-strain shear-wave velocity profile agreed with geophysical measurements. These observations not only provide field evidence of nonlinear dynamic soil behavior during earthquakes, but also confirm the reasonableness of data provided by geophysical measurements and laboratory tests.

  5. Dynamic response of shear thickening fluid under laser induced shock

    SciTech Connect

    Wu, Xianqian Yin, Qiuyun; Huang, Chenguang; Zhong, Fachun

    2015-02-16

    The dynamic response of the 57 vol./vol. % dense spherical silica particle-polyethylene glycol suspension at high pressure was investigated through short pulsed laser induced shock experiments. The measured back free surface velocities by a photonic Doppler velocimetry showed that the shock and the particle velocities decreased while the shock wave transmitted in the shear thickening fluid (STF), from which an equation of state for the STF was obtained. In addition, the peak stress decreased and the absorbed energy increased rapidly with increasing the thickness for a thin layer of the STF, which should be attributed to the impact-jammed behavior through compression of particle matrix, the deformation or crack of the hard-sphere particles, and the volume compression of the particles and the polyethylene glycol.

  6. Fluid shear, intercellular stress, and endothelial cell alignment

    PubMed Central

    Steward, Robert; Tambe, Dhananjay; Hardin, C. Corey; Krishnan, Ramaswamy

    2015-01-01

    Endothelial cell alignment along the direction of laminar fluid flow is widely understood to be a defining morphological feature of vascular homeostasis. While the role of associated signaling and structural events have been well studied, associated intercellular stresses under laminar fluid shear have remained ill-defined and the role of these stresses in the alignment process has remained obscure. To fill this gap, we report here the tractions as well as the complete in-plane intercellular stress fields measured within the human umbilical vein endothelial cell (HUVEC) monolayer subjected to a steady laminar fluid shear of 1 Pa. Tractions, intercellular stresses, as well as their time course, heterogeneity, and anisotropy, were measured using monolayer traction microscopy and monolayer stress microscopy. Prior to application of laminar fluid flow, intercellular stresses were largely tensile but fluctuated dramatically in space and in time (317 ± 122 Pa). Within 12 h of the onset of laminar fluid flow, the intercellular stresses decreased substantially but continued to fluctuate dramatically (142 ± 84 Pa). Moreover, tractions and intercellular stresses aligned strongly and promptly (within 1 h) along the direction of fluid flow, whereas the endothelial cell body aligned less strongly and substantially more slowly (12 h). Taken together, these results reveal that steady laminar fluid flow induces prompt reduction in magnitude and alignment of tractions and intercellular stress tensor components followed by the retarded elongation and alignment of the endothelial cell body. Appreciably smaller intercellular stresses supported by cell-cell junctions logically favor smaller incidence of gap formation and thus improved barrier integrity. PMID:25652451

  7. [Exercise-induced shear stress: Physiological basis and clinical impact].

    PubMed

    Rodríguez-Núñez, Iván; Romero, Fernando; Saavedra, María Javiera

    2016-01-01

    The physiological regulation of vascular function is essential for cardiovascular health and depends on adequate control of molecular mechanisms triggered by endothelial cells in response to mechanical and chemical stimuli induced by blood flow. Endothelial dysfunction is one of the major risk factors for cardiovascular disease, where an imbalance between synthesis of vasodilator and vasoconstrictor molecules is one of its main mechanisms. In this context, the shear stress is one of the most important mechanical stimuli to improve vascular function, due to endothelial mechanotransduction, triggered by stimulation of various endothelial mechanosensors, induce signaling pathways culminating in increased bioavailability of vasodilators molecules such as nitric oxide, that finally trigger the angiogenic mechanisms. These mechanisms allow providing the physiological basis for the effects of exercise on vascular health. In this review it is discussed the molecular mechanisms involved in the vascular response induced by shear stress and its impact in reversing vascular injury associated with the most prevalent cardiovascular disease in our population. PMID:27118039

  8. Mathematical Modeling of Intravascular Blood Coagulation under Wall Shear Stress

    PubMed Central

    Rukhlenko, Oleksii S.; Dudchenko, Olga A.; Zlobina, Ksenia E.; Guria, Georgy Th.

    2015-01-01

    Increased shear stress such as observed at local stenosis may cause drastic changes in the permeability of the vessel wall to procoagulants and thus initiate intravascular blood coagulation. In this paper we suggest a mathematical model to investigate how shear stress-induced permeability influences the thrombogenic potential of atherosclerotic plaques. Numerical analysis of the model reveals the existence of two hydrodynamic thresholds for activation of blood coagulation in the system and unveils typical scenarios of thrombus formation. The dependence of blood coagulation development on the intensity of blood flow, as well as on geometrical parameters of atherosclerotic plaque is described. Relevant parametric diagrams are drawn. The results suggest a previously unrecognized role of relatively small plaques (resulting in less than 50% of the lumen area reduction) in atherothrombosis and have important implications for the existing stenting guidelines. PMID:26222505

  9. Calculation of turbulent shear stress in supersonic boundary layer flows

    NASA Technical Reports Server (NTRS)

    Sun, C. C.; Childs, M. E.

    1974-01-01

    An analysis of turbulent boundary layer flow characteristics and the computational procedure used are discussed. The integrated mass and momentum flux profiles and differentials of the integral quantities are used in the computations so that local evaluation of the streamwise velocity gradient is not necessary. The computed results are compared with measured shear stress data obtained by using hot wire anemometer and laser velocimeter techniques. The flow measurements were made upstream and downstream of an adiabatic unseparated interaction of an oblique shock wave with the turbulent boundary layer on the flat wall of a two dimensional wind tunnel. A comparison of the numerical analysis and actual measurements is made and the effects of small differences in mean flow profiles on the computed shear stress distributions are discussed.

  10. Wall shear stress measurements using a new transducer

    NASA Technical Reports Server (NTRS)

    Vakili, A. D.; Wu, J. M.; Lawing, P. L.

    1986-01-01

    A new instrument has been developed for direct measurement of wall shear stress. This instrument is simple and symmetric in design with small moving mass and no internal friction. Features employed in the design of this instrument eliminate most of the difficulties associated with the traditional floating element balances. Vibration problems associated with the floating element skin friction balances have been found to be minimized by the design features and optional damping provided. The unique design of this instrument eliminates or reduces the errors associated with conventional floating-element devices: such as errors due to gaps, pressure gradient, acceleration, heat transfer and temperature change. The instrument is equipped with various sensing systems and the output signal is a linear function of the wall shear stress. Measurement made in three different tunnels show good agreement with theory and data obtained by the floating element devices.

  11. Inverse method for estimating shear stress in machining

    NASA Astrophysics Data System (ADS)

    Burns, T. J.; Mates, S. P.; Rhorer, R. L.; Whitenton, E. P.; Basak, D.

    2016-01-01

    An inverse method is presented for estimating shear stress in the work material in the region of chip-tool contact along the rake face of the tool during orthogonal machining. The method is motivated by a model of heat generation in the chip, which is based on a two-zone contact model for friction along the rake face, and an estimate of the steady-state flow of heat into the cutting tool. Given an experimentally determined discrete set of steady-state temperature measurements along the rake face of the tool, it is shown how to estimate the corresponding shear stress distribution on the rake face, even when no friction model is specified.

  12. Structure-Enhanced Yield Shear Stress in Electrorheological Fluids

    NASA Astrophysics Data System (ADS)

    Tao, R.; Lan, Y. C.; Xu, X.

    A new technology, compression-assisted aggregation, is developed to enhance the strength of electrorheological (ER) fluids. The yield shear stress of ER fluids depends on the fluid microstructure. The unassisted electric-field induced ER structure mainly consists of single chains, whose weak points are at their ends. This new technology produces a structure consisting of robust thick columns with strong ends. As the weak points of the original ER structure are greatly enforced, the new structure makes ER fluids super-strong: At a moderate electric field and moderate pressure the yield shear stress of ER fluids at 35% volume fraction exceeds 100 kPa, well above any requirement for major industrial applications.

  13. Non-volcanic tremor driven by large transient shear stresses

    USGS Publications Warehouse

    Rubinstein, J.L.; Vidale, J.E.; Gomberg, J.; Bodin, P.; Creager, K.C.; Malone, S.D.

    2007-01-01

    Non-impulsive seismic radiation or 'tremor' has long been observed at volcanoes and more recently around subduction zones. Although the number of observations of non-volcanic tremor is steadily increasing, the causative mechanism remains unclear. Some have attributed non-volcanic tremor to the movement of fluids, while its coincidence with geodetically observed slow-slip events at regular intervals has led others to consider slip on the plate interface as its cause. Low-frequency earthquakes in Japan, which are believed to make up at least part of non-volcanic tremor, have focal mechanisms and locations that are consistent with tremor being generated by shear slip on the subduction interface. In Cascadia, however, tremor locations appear to be more distributed in depth than in Japan, making them harder to reconcile with a plate interface shear-slip model. Here we identify bursts of tremor that radiated from the Cascadia subduction zone near Vancouver Island, Canada, during the strongest shaking from the moment magnitude Mw = 7.8, 2002 Denali, Alaska, earthquake. Tremor occurs when the Love wave displacements are to the southwest (the direction of plate convergence of the overriding plate), implying that the Love waves trigger the tremor. We show that these displacements correspond to shear stresses of approximately 40 kPa on the plate interface, which suggests that the effective stress on the plate interface is very low. These observations indicate that tremor and possibly slow slip can be instantaneously induced by shear stress increases on the subduction interface - effectively a frictional failure response to the driving stress. ??2007 Nature Publishing Group.

  14. ENaC regulation by proteases and shear stress

    PubMed Central

    Shi, Shujie; Carattino, Marcelo D.; Hughey, Rebecca P.; Kleyman, Thomas R.

    2013-01-01

    Epithelial Na+ channels (ENaCs) are comprised of subunits that have large extracellular regions linked to membrane spanning domains where the channel pore and gate reside. A variety of external factors modify channel activity by interacting at sites within extracellular regions that lead to conformational changes that are transmitted to the channel gate and alter channel open probability. Our review addresses two external factors that have important roles in regulating channel activity, proteases and laminar shear stress. PMID:23547932

  15. Non-volcanic tremor driven by large transient shear stresses.

    PubMed

    Rubinstein, Justin L; Vidale, John E; Gomberg, Joan; Bodin, Paul; Creager, Kenneth C; Malone, Stephen D

    2007-08-01

    Non-impulsive seismic radiation or 'tremor' has long been observed at volcanoes and more recently around subduction zones. Although the number of observations of non-volcanic tremor is steadily increasing, the causative mechanism remains unclear. Some have attributed non-volcanic tremor to the movement of fluids, while its coincidence with geodetically observed slow-slip events at regular intervals has led others to consider slip on the plate interface as its cause. Low-frequency earthquakes in Japan, which are believed to make up at least part of non-volcanic tremor, have focal mechanisms and locations that are consistent with tremor being generated by shear slip on the subduction interface. In Cascadia, however, tremor locations appear to be more distributed in depth than in Japan, making them harder to reconcile with a plate interface shear-slip model. Here we identify bursts of tremor that radiated from the Cascadia subduction zone near Vancouver Island, Canada, during the strongest shaking from the moment magnitude M(w) = 7.8, 2002 Denali, Alaska, earthquake. Tremor occurs when the Love wave displacements are to the southwest (the direction of plate convergence of the overriding plate), implying that the Love waves trigger the tremor. We show that these displacements correspond to shear stresses of approximately 40 kPa on the plate interface, which suggests that the effective stress on the plate interface is very low. These observations indicate that tremor and possibly slow slip can be instantaneously induced by shear stress increases on the subduction interface-effectively a frictional failure response to the driving stress. PMID:17671500

  16. Shearing of a confined granular layer: tangential stress and dilatancy.

    PubMed

    Coste, C

    2004-11-01

    We study the behavior of a confined granular layer under shearing, in an annular cell, at low velocity. We give evidence that the response of the granular layer under shearing is described by characteristic length scales. The tangential stress reaches its steady state on the same length scale as the dilatancy. Stop-and-go experiments performed at several driving velocities show a logarithmic increase of the static friction coefficient with waiting time, followed by rejuvenation on a characteristic length of the order of the magnitude of a Hertz contact between adjacent grains. The dilatancy does not evolve during the stop, neither during the elastic reloading when the driving is resumed. There is a small variation when sliding sets anew, which corresponds to the rejuvenation of the layer, and this variation is independent of the waiting time. We argue that aging is due to the behavior of individual contacts between grains, not global evolution of the piling. Under an instantaneous increase of the velocity, the tangential stress reaches a new steady state, exhibiting velocity strengthening behavior. An increase of dilatancy is also observed. It is much larger than fluctuations in the steady state, variations in a stop and-go-experiment, but much less than for shearing of freshly poured grains. The dilatancy variation during a velocity jump is not due to structural rearrangements of the piling. The evolutions of tangential stress and dilatancy are logarithmic in the ratio of upper and lower velocities. PMID:15600598

  17. Pulse shear stress for anaerobic membrane bioreactor fouling control.

    PubMed

    Yang, Jixiang; Spanjers, Henri; van Lier, Jules B

    2011-01-01

    Increase of shear stress at membrane surfaces is a generally applied strategy to minimize membrane fouling. It has been reported that a two-phase flow, better known as slug flow, is an effective way to increase shear stress. Hence, slug flow was introduced into an anaerobic membrane bioreactor for membrane fouling control. Anaerobic suspended sludge was cultured in an anaerobic membrane bioreactor (AMBR) operated with a side stream inside-out tubular membrane unit applying sustainable flux flow regimes. The averaged particle diameter decreased from 20 to 5 microm during operation of the AMBR. However, the COD removal efficiency did not show any significant deterioration, whereas the specific methanogenic activity (SMA) increased from 0.16 to 0.41 gCOD/g VSS/day. Nevertheless, the imposed gas slug appeared to be insufficient for adequate fouling control, resulting in rapidly increasing trans membrane pressures (TMP) operating at a flux exceeding 16 L/m2/h. Addition of powdered activated carbon (PAC) enhanced the effect of slug flow on membrane fouling. However, the combined effect was still considered as not being significant. The tubular membrane was subsequently equipped with inert inserts for creating a locally increased shear stress for enhanced fouling control. Results show an increase in the membrane flux from 16 L/m2/h to 34 L/m2/h after the inserts were mounted in the membrane tube. PMID:22097007

  18. The conductivity dependence of the shear stress in electrorheological fluids

    NASA Astrophysics Data System (ADS)

    Lan, Yucheng; Xu, Xiaoyu; Men, Shouqiang; Lu, Kunquan

    1998-11-01

    A ferroelectric KNO3/silicone oil electrorheological (ER) fluid is introduced to investigate the conductivity dependence of the ER effect under dc electric fields where the ER effect is conductivity dominated. By measuring the temperature dependence of the shear stress across the Curie temperature of particles, the dependence of the ER effect on conductivity has been quantitatively obtained in experiments. There is a critical conductivity ratio Γc (or mismatch factor βc2): when Γ<Γc, the shear stress increases with Γ; when Γ>Γc, the shear stress decreases with Γ. An agreement is obtained between theory and experiment when Γ (or β2) is lower. In the higher Γ(or β2) range, the experimental data are not in agreement with the theoretical prediction and the interfacial effect should be taken into account. The experimental data are more reliable due to the same conditions, such as the chemical nature, the surfacial property of particles, and the interfacial property between particles and suspending liquid as well as the size and shape of the particles.

  19. FOXC2 and fluid shear stress stabilize postnatal lymphatic vasculature

    PubMed Central

    Sabine, Amélie; Bovay, Esther; Demir, Cansaran Saygili; Kimura, Wataru; Jaquet, Muriel; Agalarov, Yan; Zangger, Nadine; Scallan, Joshua P.; Graber, Werner; Gulpinar, Elgin; Kwak, Brenda R.; Mäkinen, Taija; Martinez-Corral, Inés; Ortega, Sagrario; Delorenzi, Mauro; Kiefer, Friedemann; Davis, Michael J.; Djonov, Valentin; Miura, Naoyuki; Petrova, Tatiana V.

    2015-01-01

    Biomechanical forces, such as fluid shear stress, govern multiple aspects of endothelial cell biology. In blood vessels, disturbed flow is associated with vascular diseases, such as atherosclerosis, and promotes endothelial cell proliferation and apoptosis. Here, we identified an important role for disturbed flow in lymphatic vessels, in which it cooperates with the transcription factor FOXC2 to ensure lifelong stability of the lymphatic vasculature. In cultured lymphatic endothelial cells, FOXC2 inactivation conferred abnormal shear stress sensing, promoting junction disassembly and entry into the cell cycle. Loss of FOXC2-dependent quiescence was mediated by the Hippo pathway transcriptional coactivator TAZ and, ultimately, led to cell death. In murine models, inducible deletion of Foxc2 within the lymphatic vasculature led to cell-cell junction defects, regression of valves, and focal vascular lumen collapse, which triggered generalized lymphatic vascular dysfunction and lethality. Together, our work describes a fundamental mechanism by which FOXC2 and oscillatory shear stress maintain lymphatic endothelial cell quiescence through intercellular junction and cytoskeleton stabilization and provides an essential link between biomechanical forces and endothelial cell identity that is necessary for postnatal vessel homeostasis. As FOXC2 is mutated in lymphedema-distichiasis syndrome, our data also underscore the role of impaired mechanotransduction in the pathology of this hereditary human disease. PMID:26389677

  20. Surface shear stress fluctuations in the atmospheric surface layer

    NASA Astrophysics Data System (ADS)

    Monty, Jason; Hutchins, Nick; Chong, Min

    2005-11-01

    A lightweight, high frequency response (25Hz), floating element sensor was used to measure wall shear stress fluctuations in the atmospheric surface layer formed over a salt flat at the SLTEST site, Utah, USA. The sensor uses a laser position measurement system to track the motion of the floating element which consisted of a 50mm diameter foam disc, as described by Heuer & Marusic (Meas. Sci. Tech., Vol. 16, 1644- -1649, 2005). The measurements were taken as part of an internationally coordinated experimental program designed to make extensive spatial and temporal measurements of velocity, temperature and wall shear stress of the surface layer. Velocity measurements were made with both a 30m high vertical array and a 100m wide horizontal array of sonic anemometers; 18 anemometers in total were employed. Cross-correlations of shear stress and streamwise velocity fluctuations were analysed in an attempt to identify structure angles in the flow. The results were also compared with experimental data from controlled, laboratory turbulent boundary layers having three orders of magnitude lower Reynolds number.

  1. Basal shear stress of debris flow in the runout phase

    NASA Astrophysics Data System (ADS)

    D'Agostino, V.; Bettella, F.; Cesca, M.

    2013-11-01

    A laboratory device is proposed to assess the basal shear stresses generated by debris-flow mixtures during their runout phase. The device consists of an inclinable box with a gate facing a deposition plane. The box is filled with a selected debris-flow mixture, and after sudden opening of the gate, the features of the dam-break deposit can be measured. Based on some simplified assumptions of the energy balance, a methodology is proposed to assess basal shear stresses. The device has been tested using sediment samples from debris-flow deposits generated by two catchments of the Dolomites (Cortina d'Ampezzo, Belluno, Italy) by carrying out runout tests for different sediment concentrations by volume. The results show how the static Coulomb friction law is valid in the runout phase, with friction angles on the order of the angle of repose of the same material in dry conditions. The data elaboration also yields an innovative constitutive equation for shear stresses. This relation merges the Coulomb mixture approach with the concept of a one-phase flow with a certain rheology. This integration offers a useful insight into the weaknesses of the rheological approach if it is not properly scaled up to the ambient pressure of interest.

  2. Rheological investigations of ferrofluids with a shear stress controlled rheometer.

    PubMed

    Shahnazian, Hamid; Odenbach, Stefan

    2008-05-21

    The appearance of field- and shear-dependent changes of viscosity-the magnetoviscous effect-is correlated to the formation of chains and structures of magnetic nanoparticles. Moreover, the formation of these structures leads to the appearance of viscoelastic effects or other non-Newtonian features in ferrofluids in the presence of a magnetic field. In order to describe these phenomena, different theoretical approaches have been developed which explain the mechanism of these effects with different assumptions. One point in which these models differ, and which has to be clarified, is the appearance of yield stress and its dependence on magnetic field strength. With this aim, a stress controlled rheometer has been designed to prove the existence of this very small field-dependent yield stress for ferrofluids. The results presented here show a dependence of the yield stress on the magnetic field strength as well as on the interparticle interaction and particle size distribution. Finally, yield stress experiments have been performed for different geometries of the shear cell in order to get more information about the microstructure formed by the magnetic particles. PMID:21694266

  3. Dynamic shear behavior of alumina-filled epoxy

    SciTech Connect

    Costin, L.S.

    1982-03-01

    Thin-walled tubular specimens of alumina-filled epoxy were loaded in torsion at a strain rate of approximately 10/sup 3/ s/sup -1/ using a stored-torque Kolsky bar. In addition to measuring the time resolved shear stress and shear strain in the specimen, the axial stress generated by the dilation of the material during shear deformation was also obtained as a function of time. Tests were conducted at room temperature and at -60/sup 0/C. At room temperature, a moderate amount of plastic deformation occurred before failure. Material dilation was associated with the plastic flow. At -60/sup 0/C, there was a marked increase in failure stress over the failure stress at room temperature. However, little or no plastic deformation or dilation occurred before failure.

  4. Turbulent Sediment Suspension and Induced Ripple Dynamics Absent Mean Shear

    NASA Astrophysics Data System (ADS)

    Johnson, B. A.; Cowen, E.

    2014-12-01

    The uprush and backwash phases in the swash zone, the region of the beach that is alternately covered and uncovered by wave run-up, are fundamentally different events. Backwash is dominated by a growing boundary layer where the turbulence is set by the bed shear stress. In this phase traditional boundary layer turbulence models and Shields-type critical stress pickup functions work well. However, the uprush phase, while often viewed in the context of traditional boundary layer turbulence models, has little in common with the backwash phase. During uprush, the entire water column is turbulent, as it rapidly advects well-stirred highly turbulent flow generated offshore from breaking waves or collapsing bores. Turbulence levels in the uprush are several times higher than turbulent boundary layer theory would predict and hence the use of a boundary layer model to predict turbulence levels during uprush grossly under predicts the turbulence and subsequent sediment suspension in the swash zone. To study the importance of this advected turbulence to sediment suspension we conduct experiments in a water tank designed to generate horizontally homogeneous isotropic turbulence absent mean shear using randomly actuated synthetic jet arrays suspended above both a solid glass plate and a narrowly graded sediment bed. Using jet arrays with different jet spacings allows the generation of high Reynolds number turbulence with variable integral length scales, which we hypothesize control the characteristic length scales in the induced ripple field. Particle image velocimetry and acoustic Doppler velocimetry measurements are used to characterize the near-bed flow and this unique turbulent boundary layer. Metrics include the mean flow and turbulence intensities and stresses, temporal and spatial spectra, dissipation of turbulent kinetic energy, and integral length scales of the turbulence. We leverage our unique dataset to compare the flows over impermeable fixed and permeable mobile

  5. Aeolian Shear Stress Ratio Measurements within Mesquite-Dominated Landscapes of the Chihuahuan Desert, New Mexico, USA

    NASA Technical Reports Server (NTRS)

    King, James; Nickling, W. G.; Gilliles, J. A.

    2006-01-01

    A field study was conducted to ascertain the amount of protection that mesquite-dominated communities provide to the surface from wind erosion. The dynamics of the locally accelerated evolution of a mesquite/coppice dune landscape and the undetermined spatial dependence of potential erosion by wind from a shear stress partition model were investigated. Sediment transport and dust emission processes are governed by the amount of protection that can be provided by roughness elements. Although shear stress partition models exist that can describe this, their accuracy has only been tested against a limited dataset because instrumentation has previously been unable to provide the necessary measurements. This study combines the use of meteorological towers and surface shear stress measurements with Irwin sensors to measure the partition of shear stress in situ. The surface shear stress within preferentially aligned vegetation (within coppice dune development) exhibited highly skewed distributions, while a more homogenous surface stress was recorded at a site with less developed coppice dunes. Above the vegetation, the logarithmic velocity profile deduced roughness length (based on 10-min averages) exhibited a distinct correlation with compass direction for the site with vegetation preferentially aligned, while the site with more homogenously distributed vegetation showed very little variation in the roughness length. This distribution in roughness length within an area, defines a distribution of a resolved shear stress partitioning model based on these measurements, ultimately providing potential closure to a previously uncorrelated model parameter.

  6. Shear banding of colloidal glasses: observation of a dynamic first-order transition.

    PubMed

    Chikkadi, V; Miedema, D M; Dang, M T; Nienhuis, B; Schall, P

    2014-11-14

    We demonstrate that application of an increasing shear field on a glass leads to an intriguing dynamic first-order transition in analogy with equilibrium transitions. By following the particle dynamics as a function of the driving field in a colloidal glass, we identify a critical shear rate upon which the diffusion time scale of the glass exhibits a sudden discontinuity. Using a new dynamic order parameter, we show that this discontinuity is analogous to a first-order transition, in which the applied stress acts as the conjugate field on the system's dynamic evolution. These results offer new perspectives to comprehend the generic shear-banding instability of a wide range of amorphous materials. PMID:25432056

  7. Imaging shear stress distribution and evaluating the stress concentration factor of the human eye

    PubMed Central

    Joseph Antony, S.

    2015-01-01

    Healthy eyes are vital for a better quality of human life. Historically, for man-made materials, scientists and engineers use stress concentration factors to characterise the effects of structural non-homogeneities on their mechanical strength. However, such information is scarce for the human eye. Here we present the shear stress distribution profiles of a healthy human cornea surface in vivo using photo-stress analysis tomography, which is a non-intrusive and non-X-ray based method. The corneal birefringent retardation measured here is comparable to that of previous studies. Using this, we derive eye stress concentration factors and the directional alignment of major principal stress on the surface of the cornea. Similar to thermometers being used for monitoring the general health in humans, this report provides a foundation to characterise the shear stress carrying capacity of the cornea, and a potential bench mark for validating theoretical modelling of stresses in the human eye in future. PMID:25754336

  8. Imaging shear stress distribution and evaluating the stress concentration factor of the human eye

    NASA Astrophysics Data System (ADS)

    Joseph Antony, S.

    2015-03-01

    Healthy eyes are vital for a better quality of human life. Historically, for man-made materials, scientists and engineers use stress concentration factors to characterise the effects of structural non-homogeneities on their mechanical strength. However, such information is scarce for the human eye. Here we present the shear stress distribution profiles of a healthy human cornea surface in vivo using photo-stress analysis tomography, which is a non-intrusive and non-X-ray based method. The corneal birefringent retardation measured here is comparable to that of previous studies. Using this, we derive eye stress concentration factors and the directional alignment of major principal stress on the surface of the cornea. Similar to thermometers being used for monitoring the general health in humans, this report provides a foundation to characterise the shear stress carrying capacity of the cornea, and a potential bench mark for validating theoretical modelling of stresses in the human eye in future.

  9. Imaging shear stress distribution and evaluating the stress concentration factor of the human eye.

    PubMed

    Joseph Antony, S

    2015-01-01

    Healthy eyes are vital for a better quality of human life. Historically, for man-made materials, scientists and engineers use stress concentration factors to characterise the effects of structural non-homogeneities on their mechanical strength. However, such information is scarce for the human eye. Here we present the shear stress distribution profiles of a healthy human cornea surface in vivo using photo-stress analysis tomography, which is a non-intrusive and non-X-ray based method. The corneal birefringent retardation measured here is comparable to that of previous studies. Using this, we derive eye stress concentration factors and the directional alignment of major principal stress on the surface of the cornea. Similar to thermometers being used for monitoring the general health in humans, this report provides a foundation to characterise the shear stress carrying capacity of the cornea, and a potential bench mark for validating theoretical modelling of stresses in the human eye in future. PMID:25754336

  10. Molecular Origins of Higher Harmonics in Large-Amplitude Oscillatory Shear Flow: Shear Stress Response

    NASA Astrophysics Data System (ADS)

    Gilbert, Peter; Giacomin, A. Jeffrey; Schmalzer, Andrew; Bird, R. B.

    Recent work has focused on understanding the molecular origins of higher harmonics that arise in the shear stress response of polymeric liquids in large-amplitude oscillatory shear flow. These higher harmonics have been explained using only the orientation distribution of a dilute suspension of rigid dumbbells in a Newtonian fluid, which neglects molecular interactions and is the simplest relevant molecular model of polymer viscoelasticity [R.B. Bird et al., J Chem Phys, 140, 074904 (2014)]. We explore these molecular interactions by examining the Curtiss-Bird model, a kinetic molecular theory that accounts for restricted polymer motions arising when chains are concentrated [Fan and Bird, JNNFM, 15, 341 (1984)]. For concentrated systems, the chain motion transverse to the chain axis is more restricted than along the axis. This anisotropy is described by the link tension coefficient, ɛ, for which several special cases arise: ɛ =0 corresponds to reptation, ɛ > 1 1 8 8 to rod-climbing, 1 1 2 2 >= ɛ >= 3 3 4 4 to reasonable shear-thinning predictions in steady simple shear flow, and ɛ =1 to a dilute solution of chains. We examine the shapes of the shear stress versus shear rate loops for the special cases, ɛ = 0 , 1 0 , 1 8 , 3 3 8 8 8 , 3 3 8 8 , 1 , of the Curtiss-Bird model, and we compare these with those

  11. ESTIMATION OF EFFECTIVE SHEAR STRESS WORKING ON FLAT SHEET MEMBRANE USING FLUIDIZED MEDIA IN MBRs

    NASA Astrophysics Data System (ADS)

    Zaw, Hlwan Moe; Li, Tairi; Nagaoka, Hiroshi; Mishima, Iori

    This study was aimed at estimating effective shear stress working on flat sheet membrane by the addition of fluidized media in MBRs. In both of laboratory-scale aeration tanks with and without fluidized media, shear stress variations on membrane surface and water phase velocity variations were measured and MBR operation was conducted. For the evaluation of the effective shear stress working on membrane surface to mitigate membrane surface, simulation of trans-membrane pressure increase was conducted. It was shown that the time-averaged absolute value of shear stress was smaller in the reactor with fluidized media than without fluidized media. However, due to strong turbulence in the reactor with fluidized media caused by interaction between water-phase and media and also due to the direct interaction between membrane surface and fluidized media, standard deviation of shear stress on membrane surface was larger in the reactor with fluidized media than without media. Histograms of shear stress variation data were fitted well to normal distribution curves and mean plus three times of standard deviation was defined to be a maximum shear stress value. By applying the defined maximum shear stress to a membrane fouling model, trans-membrane pressure curve in the MBR experiment was simulated well by the fouling model indicting that the maximum shear stress, not time-averaged shear stress, can be regarded as an effective shear stress to prevent membrane fouling in submerged flat-sheet MBRs.

  12. Wall shear stress indicators in abnormal aortic geometries

    NASA Astrophysics Data System (ADS)

    Prahl Wittberg, Lisa; van Wyk, Stevin; Fuchs, Laszlo; Gutmark, Ephraim; Gutmark-Little, Iris

    2015-11-01

    Cardiovascular disease, such as atherosclerosis, occurs at specific locations in the arterial tree. Characterizing flow and forces at these locations is crucial to understanding the genesis of disease. Measures such as time average wall shear stress, oscillatory shear index, relative residence time and temporal wall shear stress gradients have been shown to identify plaque prone regions. The present paper examines these indices in three aortic geometries obtained from patients whose aortas are deformed due to a genetic pathology and compared to one normal geometry. This patient group is known to be prone to aortic dissection and our study aims to identify early indicators that will enable timely intervention. Data obtained from cardiac magnetic resonance imaging is used to reconstruct the aortic arch. The local unsteady flow characteristics are calculated, fully resolving the flow field throughout the entire cardiac cycle. The Quemada model is applied to account for the non-Newtonian properties of blood, an empirical model valid for different red blood cell loading. The impact of the deformed aortic geometries is analyzed to identify flow patterns that could lead to arterial disease at certain locations.

  13. Effect of biomimetic shear stress on intracellular uptake and cell-killing efficiency of doxorubicin in a free and liposomal formulation.

    PubMed

    Kang, Taehee; Cho, Younhee; Park, Chulhun; Kim, Soo-Dong; Oh, Euichaul; Cui, Jing-Hao; Cao, Qing-Ri; Lee, Beom-Jin

    2016-08-20

    Shear stress could be considered in the context of cellular uptake and cell-killing efficiency. Thus, mimicking the dynamic characteristics of in vivo environment is important in targeted drug delivery. We investigated the intracellular uptake and cell-killing efficiency of doxorubicin (DOX) in a free and liposomal form (Doxil(®)) under biomimetic shear stress to mimic in vivo environment. In this dynamic environment, cells demonstrated significantly higher fluorescence intensity than that of the static environment, and fluorescence microscopy images indicated increased intracellular uptake of DOX in the presence of fluidic shear stress. In cells treated with free DOX and liposomal Doxil(®), cell-killing efficiency was affected by shear stress. Taken together, shear stress, affecting drug uptake and cell-killing efficiency, is important in intracellular drug targeting. PMID:27286636

  14. Interplay between shear stress and adhesion on neutrophil locomotion.

    PubMed

    Smith, Lee A; Aranda-Espinoza, Helim; Haun, Jered B; Hammer, Daniel A

    2007-01-15

    Leukocyte locomotion over the lumen of inflamed endothelial cells is a critical step, following firm adhesion, in the inflammatory response. Once firmly adherent, the cell will spread and will either undergo diapedesis through individual vascular endothelial cells or will migrate to tight junctions before extravasating to the site of injury or infection. Little is known about the mechanisms of neutrophil spreading or locomotion, or how motility is affected by the physical environment. We performed a systematic study to investigate the effect of the type of adhesive ligand and shear stress on neutrophil motility by employing a parallel-plate flow chamber with reconstituted protein surfaces of E-selectin, E-selectin/PECAM-1, and E-selectin/ICAM-1. We find that the level and type of adhesive ligand and the shear rate are intertwined in affecting several metrics of migration, such as the migration velocity, random motility, index of migration, and the percentage of cells moving in the direction of flow. On surfaces with high levels of PECAM-1, there is a near doubling in random motility at a shear rate of 180 s(-1) compared to the motility in the absence of flow. On surfaces with ICAM-1, neutrophil random motility exhibits a weaker response to shear rate, decreasing slightly when shear rate is increased from static conditions to 180 s(-1), and is only slightly higher at 1000 s(-1) than in the absence of flow. The random motility increases with increasing surface concentrations of E-selectin and PECAM-1 under static and flow conditions. Our findings illustrate that the endothelium may regulate neutrophil migration in postcapillary venules through the presentation of various adhesion ligands at sites of inflammation. PMID:17071667

  15. Effects of shear stress pattern and magnitude on mesenchymal transformation and invasion of aortic valve endothelial cells

    PubMed Central

    Mahler, Gretchen J.; Frendl, Christopher M.; Cao, Qingfeng; Butcher, Jonathan T.

    2015-01-01

    Understanding the role of mechanical forces on cell behavior is critical for tissue engineering, regenerative medicine, and disease initiation studies. Current hemodynamic bioreactors are largely limited to 2D substrates or the application of general flow conditions at a tissue level, which eliminates the investigation of some essential physiological and pathological responses. One example is the mesenchymal transformation of endothelial cells in response to shear stress. Endothelial to mesenchymal transformation (EndMT) is a valve morphogenic mechanism associated with aortic valve disease initiation. The aortic valve experiences oscillatory shear on the disease-susceptible fibrosa, and the role of hemodynamics on adult EndMT is unknown. The goal of this work was to develop and characterize a microfluidic bioreactor that applies physiologically relevant laminar or oscillatory shear stresses to endothelial cells and permits the quantitative analysis of 3D cell-extracellular matrix (ECM) interactions. In this study, porcine aortic valve endothelial cells were seeded onto 3D collagen I gels and exposed to different magnitudes of steady or oscillatory shear stress for 48 hours. Cells elongated and aligned perpendicular to laminar, but not oscillatory shear. Low steady shear stress (2 dyne/cm2) and oscillatory shear stress upregulated EndMT- (ACTA2, Snail, TGFB1) and inflammation- (ICAM1, NFKB1) related gene expression, EndMT-related (αSMA) protein expression, and matrix invasion when compared with static controls or cells exposed to high steady shear (10 and 20 dyne/cm2). Our system enables direct testing of the role of shear stress on endothelial cell mesenchymal transformation in a dynamic, 3D environment and shows that hemodynamics regulate EndMT in adult valve endothelial cells. PMID:24898772

  16. Effects of shear stress pattern and magnitude on mesenchymal transformation and invasion of aortic valve endothelial cells.

    PubMed

    Mahler, Gretchen J; Frendl, Christopher M; Cao, Qingfeng; Butcher, Jonathan T

    2014-11-01

    Understanding the role of mechanical forces on cell behavior is critical for tissue engineering, regenerative medicine, and disease initiation studies. Current hemodynamic bioreactors are largely limited to 2D substrates or the application of general flow conditions at a tissue level, which eliminates the investigation of some essential physiological and pathological responses. One example is the mesenchymal transformation of endothelial cells in response to shear stress. Endothelial to mesenchymal transformation (EndMT) is a valve morphogenic mechanism associated with aortic valve disease initiation. The aortic valve experiences oscillatory shear on the disease-susceptible fibrosa, and the role of hemodynamics on adult EndMT is unknown. The goal of this work was to develop and characterize a microfluidic bioreactor that applies physiologically relevant laminar or oscillatory shear stresses to endothelial cells and permits the quantitative analysis of 3D cell-extracellular matrix (ECM) interactions. In this study, porcine aortic valve endothelial cells were seeded onto 3D collagen I gels and exposed to different magnitudes of steady or oscillatory shear stress for 48 h. Cells elongated and aligned perpendicular to laminar, but not oscillatory shear. Low steady shear stress (2 dyne/cm(2) ) and oscillatory shear stress upregulated EndMT (ACTA2, Snail, TGFB1) and inflammation (ICAM1, NFKB1) related gene expression, EndMT-related (αSMA) protein expression, and matrix invasion when compared with static controls or cells exposed to high steady shear (10 and 20 dyne/cm(2) ). Our system enables direct testing of the role of shear stress on endothelial cell mesenchymal transformation in a dynamic, 3D environment and shows that hemodynamics regulate EndMT in adult valve endothelial cells. PMID:24898772

  17. Propose a Wall Shear Stress Divergence to Estimate the Risks of Intracranial Aneurysm Rupture

    PubMed Central

    Zhang, Y.; Takao, H.; Murayama, Y.; Qian, Y.

    2013-01-01

    Although wall shear stress (WSS) has long been considered a critical indicator of intracranial aneurysm rupture, there is still no definite conclusion as to whether a high or a low WSS results in aneurysm rupture. The reason may be that the effect of WSS direction has not been fully considered. The objectives of this study are to investigate the magnitude of WSS (|WSS|) and its divergence on the aneurysm surface and to test the significance of both in relation to the aneurysm rupture. Patient-specific computational fluid dynamics (CFD) was used to compute WSS and wall shear stress divergence (WSSD) on the aneurysm surface for nineteen patients. Our results revealed that if high |WSS| is stretching aneurysm luminal surface, and the stretching region is concentrated, the aneurysm is under a high risk of rupture. It seems that, by considering both direction and magnitude of WSS, WSSD may be a better indicator for the risk estimation of aneurysm rupture (154). PMID:24191140

  18. Differential Activation and Inhibition of RhoA by Fluid Flow Induced Shear Stress in Chondrocytes

    PubMed Central

    Wan, Qiaoqiao; Kim, Seung joon; Yokota, Hiroki; Na, Sungsoo

    2013-01-01

    Physical force environment is a major factor that influences cellular homeostasis and remodeling. It is not well understood, however, as a potential role of force intensities in the induction of cellular mechanotransduction. Using a fluorescence resonance energy transfer (FRET)-based approach, we asked whether activities of GTPase RhoA in chondrocytes are dependent on intensities of flow induced shear stress. We hypothesized that RhoA activities can be either elevated or reduced by selecting different levels of shear stress intensities. The result indicate that C28/I2 chondrocytes have increased RhoA activities in response to high shear stress (10 or 20 dyn/cm2), whereas a decrease in activity was seen with an intermediate shear stress of 5 dyn/cm2. No changes were seen under low shear stress (2 dyn/cm2). The observed 2-level switch of RhoA activities is closely linked to the shear stress-induced alterations in actin cytoskeleton and traction forces. In the presence of constitutively active RhoA (RhoA-V14), intermediate shear stress suppressed RhoA activities, while high shear stress failed to activate them. In chondrocytes, expression of various metalloproteinases is, in part, regulated by shear and normal stresses through a network of GTPases. Collectively, the data suggest that intensities of shear stress are critical in differential activation and inhibition of RhoA activities in chondrocytes. PMID:23408748

  19. Modeling flow and shear stress fields over unsteady three dimensional dunes

    NASA Astrophysics Data System (ADS)

    Hardy, Richard; Parsons, Dan; Ashworth, Phil; Reesink, Arjan; Best, Jim

    2014-05-01

    The flow field over dunes has been extensively measured in laboratory conditions and there is general understanding on the nature of the flow over dunes formed under equilibrium flow conditions. This has allowed an understanding of bed shear stress to be derived and the development of morpho-dynamic models. However, fluvial systems typically experience unsteady flow and therefore the sediment-water interface is constantly responding and reorganizing to these unsteady flows and stresses, over a range of both spatial and temporal scales. This is primarily through the adjustment of bed forms (including ripples, dunes and bar forms) which then subsequently alter the flow field. This paper investigates, through the application of a numerical model, the influence of these roughness elements on the overall flow and bed shear stress. A series of physical experiments were undertaken in a flume, 16m long and 2m wide, where a fine sand (D50 of 239µm) was water worked under a range of unsteady hydraulic conditions to generate a series of quasi-equilibrium three dimensional bed forms. During the experiments flow was measured with acoustic Doppler velocimeters, (aDv's). On four occasions the flume was drained and the bed topography measured with terrestrial LiDAR to create digital elevation models. This data provide the necessary boundary conditions and validation data for a numerical three dimensional flow model. The prediction of flow over the four static beds demonstrates the spatial distribution of shear stress and the potential sediment transport paths between the dune crests. These appear to be associated with coherent flow structures formed by localized shear flow. These flow predictions are currently being used to develop a fully three dimensional morphodynamic model to further understand dune dynamics under unsteady flow conditions.

  20. Permeability of fault gouge under confining pressure and shear stress.

    USGS Publications Warehouse

    Morrow, C.A.; Shi, L.Q.; Byerlee, J.D.

    1984-01-01

    The permeability of both clay-rich and non-clay gouges, as well as several pure clays, was studied as a function of confining pressures from 5 to 200 MPa and shear strain to 10. Permeability ranged over 4 orders of magnitude, from around 10-22 to 10-18 m2 (1 darcy = 0.987 X 10-12 m2). Grain size was an important factor in determining permeability, particularly for the clay-rich samples. The permeabilities of the non-clay samples were not significantly different than those of the clays. Strength of the saturated samples under drained (low pore pressure) conditions did not correlate with high or low permeability. However, the low permeabilities of these gouges could be a factor in the measured low shear stresses along fault regions if excess pore pressures were created as a result of shearing or compaction, and this pressure was unable to dissipate through a thick section of the material.-from Authors

  1. Adhesive dynamics simulations of the shear threshold effect for leukocytes.

    PubMed

    Caputo, Kelly E; Lee, Dooyoung; King, Michael R; Hammer, Daniel A

    2007-02-01

    Many experiments have measured the effect of force on the dissociation of single selectin bonds, but it is not yet clear how the force dependence of molecular dissociation can influence the rolling of cells expressing selectin molecules. Recent experiments using constant-force atomic force microscopy or high-resolution microscopic observations of pause-time distributions of cells in a flow chamber show that for some bonds, the dissociation rate is high at low force and initially decreases with force, indicating a catch bond. As the force continues to increase, the dissociation rate increases again, like a slip bond. It has been proposed that this catch-slip bond leads to the shear threshold effect, in which a certain level of shear rate is required to achieve rolling. We have incorporated a catch-slip dissociation rate into adhesive dynamics simulations of cell rolling. Using a relatively simple model for the shear-controlled association rate for selectin bonds, we were able to recreate characteristics of the shear threshold effect seen most prominently for rolling through L-selectin. The rolling velocity as a function of shear rate showed a minimum near 100 s-1. Furthermore, cells were observed to roll at a shear rate near the threshold, but detach and move more quickly when the shear rate was dropped below the threshold. Finally, using adhesive dynamics, we were able to determine ranges of parameters necessary to see the shear threshold effect in the rolling velocity. In summary, we found through simulation that the catch-slip behavior of selectin bonds can be responsible for the shear threshold effect. PMID:17085490

  2. Adhesive Dynamics Simulations of the Shear Threshold Effect for Leukocytes

    PubMed Central

    Caputo, Kelly E.; Lee, Dooyoung; King, Michael R.; Hammer, Daniel A.

    2007-01-01

    Many experiments have measured the effect of force on the dissociation of single selectin bonds, but it is not yet clear how the force dependence of molecular dissociation can influence the rolling of cells expressing selectin molecules. Recent experiments using constant-force atomic force microscopy or high-resolution microscopic observations of pause-time distributions of cells in a flow chamber show that for some bonds, the dissociation rate is high at low force and initially decreases with force, indicating a catch bond. As the force continues to increase, the dissociation rate increases again, like a slip bond. It has been proposed that this catch-slip bond leads to the shear threshold effect, in which a certain level of shear rate is required to achieve rolling. We have incorporated a catch-slip dissociation rate into adhesive dynamics simulations of cell rolling. Using a relatively simple model for the shear-controlled association rate for selectin bonds, we were able to recreate characteristics of the shear threshold effect seen most prominently for rolling through L-selectin. The rolling velocity as a function of shear rate showed a minimum near 100 s−1. Furthermore, cells were observed to roll at a shear rate near the threshold, but detach and move more quickly when the shear rate was dropped below the threshold. Finally, using adhesive dynamics, we were able to determine ranges of parameters necessary to see the shear threshold effect in the rolling velocity. In summary, we found through simulation that the catch-slip behavior of selectin bonds can be responsible for the shear threshold effect. PMID:17085490

  3. Interfacial shear stress in stratified flow in a horizontal rectangular duct

    SciTech Connect

    Lorencez, C.; Kawaji, M.; Murao, Y.

    1995-09-01

    Interfacial shear stress has been experimentally examined for both cocurrent and countercurrent stratified wavy flows in a horizontal interfacial shear stress from the measurements were examined and the results have been compared with existing correlations. Some differences were found in the estimated interfacial shear stress from the measurements were examined and the results have been compared with existing correlations. Some differences were found in the estimated interfacial shear stress values at high gas flow rates which could be attributed to the assumptions and procedures involved in each method. The interfacial waves and secondary motions were also found to have significant effects on the accuracy of Reynolds stress and turbulence kinetic energy extrapolation methods.

  4. A review of Reynolds stress models for turbulent shear flows

    NASA Technical Reports Server (NTRS)

    Speziale, Charles G.

    1995-01-01

    A detailed review of recent developments in Reynolds stress modeling for incompressible turbulent shear flows is provided. The mathematical foundations of both two-equation models and full second-order closures are explored in depth. It is shown how these models can be systematically derived for two-dimensional mean turbulent flows that are close to equilibrium. A variety of examples are provided to demonstrate how well properly calibrated versions of these models perform for such flows. However, substantial problems remain for the description of more complex turbulent flows where there are large departures from equilibrium. Recent efforts to extend Reynolds stress models to nonequilibrium turbulent flows are discussed briefly along with the major modeling issues relevant to practical naval hydrodynamics applications.

  5. Does low and oscillatory wall shear stress correlate spatially with early atherosclerosis? A systematic review.

    PubMed

    Peiffer, Veronique; Sherwin, Spencer J; Weinberg, Peter D

    2013-07-15

    Low and oscillatory wall shear stress is widely assumed to play a key role in the initiation and development of atherosclerosis. Indeed, some studies have relied on the low shear theory when developing diagnostic and treatment strategies for cardiovascular disease. We wished to ascertain if this consensus is justified by published data. We performed a systematic review of papers that compare the localization of atherosclerotic lesions with the distribution of haemodynamic indicators calculated using computational fluid dynamics. The review showed that although many articles claim their results conform to the theory, it has been interpreted in different ways: a range of metrics has been used to characterize the distribution of disease, and they have been compared with a range of haemodynamic factors. Several studies, including all of those making systematic point-by-point comparisons of shear and disease, failed to find the expected relation. The various pre- and post-processing techniques used by different groups have reduced the range of shears over which correlations were sought, and in some cases are mutually incompatible. Finally, only a subset of the known patterns of disease has been investigated. The evidence for the low/oscillatory shear theory is less robust than commonly assumed. Longitudinal studies starting from the healthy state, or the collection of average flow metrics derived from large numbers of healthy vessels, both in conjunction with point-by-point comparisons using appropriate statistical techniques, will be necessary to improve our understanding of the relation between blood flow and atherogenesis. PMID:23459102

  6. Probing the adhesion of particles to responsive polymer coatings with hydrodynamic shear stresses

    NASA Astrophysics Data System (ADS)

    Toomey, Ryan; Efe, Gulnur

    2015-03-01

    Lower critical solution temperature (LCST) polymers in confined geometries have found success in applications that benefit from reversible modulation of surface properties, including drug delivery, separations, tissue cultures, and chromatography. In this talk, we present the adhesion of polystyrene microspheres to cross-linked poly(N-isopropylacrylamide), or poly(NIPAAm) coatings, as studied with a spinning disk method. This method applies a linear range of hydrodynamic shear forces to physically adsorbed microspheres along the radius of a coated disk. Quantification of detachment is accomplished by optical microscopy to evaluate the minimum shear stress to remove adherent particles. Experiments were performed to assess the relationship between the surface chemistry of the microsphere, the thickness and cross-link density of the poly(NIPAAm) coating, the adsorption (or incubation) time, and the temperature on the detachment profiles of the microspheres. Results show that both the shear modulus and slow dynamic processes in the poly(NIPAAm) films strongly influence the detachment shear stresses. Moreover, whether an adsorbed microsphere can be released (through a modulation in the swelling of the poly(NIPAAm) coating by temperature) depends on both the surface chemistry of the microsphere and the extent of the adsorption time. Finally, the results show that the structure of the poly(NIPAAm) coating can significantly affect performance, which may explain several of the conflicting findings that have been reported in the literature.

  7. Shear flow behavior of a dynamically symmetric polymeric bicontinuous microemulsion

    NASA Astrophysics Data System (ADS)

    Zhou, Ning

    2005-03-01

    Soft materials with complex internal structure often exhibit fascinating rheological behavior. For example, under flow the poly (ethylethylene) (PEE)/poly(dimethyl siloxane) (PDMS)/PEE-PDMS polymeric bicontinuous microemulsion (BμE) showed shear-induced macrophase separation.^ 1 This was tentatively attributed to the extreme dynamical asymmetry of the two homopolymers, i.e., their viscosities differed by three orders of magnitude. To understand the role of the dynamic symmetry of a BμE when subjected to shear flow, we have developed a new ternary polymer blend system poly(butylene oxide) (PBO)/ poly(ethylenepropylene) (PEP)/PEP-PBO, which is dynamically almost symmetric. We will report on the shear flow behavior of this new BμE. Reference: [1] Krishnan et al. Phys. Rev. Lett. 2001, 87, 098301

  8. Complete Release of Horizontal Shear Stresses During Geothermal Reservoir Stimulation

    NASA Astrophysics Data System (ADS)

    Schoenball, M.; Gaucher, E.; Wellmann, F.; Kohl, T.

    2013-12-01

    Seismicity can be induced in previously seismically inactive regions by man-made changes of the stress field. Notable stress perturbations are created by injection or withdrawal of fluids such as wastewater, fresh water or hydrocarbons. Over the last decades our knowledge of the physical processes of induced seismicity has improved largely. However, the driving force of seismicity, i.e. the actual perturbation of the stress field in the reservoir during fluid injection, remains largely unknown up to now. Measurements of fluid pressure at the well are not enough to extrapolate the pressure change in the reservoir. Here we study the evolution of the stress field during a massive hydraulic stimulation of a 5 km deep well at the enhanced geothermal system at Soultz-sous-Forêts, France. Fresh water was pumped with rates of 30 to 50 ls-1 for 6 days. Locations of 7215 events with maximum magnitude of MW=2.5 were obtained, for 715 events with MW > 1 focal mechanism solutions were derived. At first we present observations of several peculiar phenomena of the seismicity migration, of fluid flow and earthquake mechanisms following the shut-in of the well, which indicate to yet not understood hydro-mechanical coupling mechanisms in connection with shut-in. In order to analyze the changes of the stress field during and after the stimulation we identify the fracture planes from the two nodal planes by a probability-based method where we incorporate structural geological information gained from well logs and uncertainties of the determination of focal mechanism solutions and independent estimates of the stress field. In principle, this approach is able to incorporate further uncertainties, if available. We then conduct stress inversions resolved in time and depth to study spatio-temporal changes of the stress tensor. Our results show an increasingly perturbed stress state with time with a strong reduction of the horizontal shear stresses in areas of highest seismic activity

  9. Interfacial shear-stress effects on transient capillary wedge flow

    NASA Astrophysics Data System (ADS)

    Su, Song-Kai; Lai, Chun-Liang

    2004-06-01

    The effects on the transient capillary flow in a wedge due to the interfacial shear-stress distribution S along the flow direction z is studied theoretically. With the assumptions of a slender liquid column and negligible gravitational and inertia effects, the problem is reduced to finding the axial velocity distribution at any cross section. The propagation of the liquid column h(z,t) and the tip location l(t) are then solved with the aid of the continuity equation. When the half-wedge angle α, the contact angle θ, and the shear-stress distribution on the free surface S are constant, analytic solutions exist. Otherwise, numerical simulation has to be applied. The results indicate that when S(z) is acting in the flow direction, the flow is strengthened and the liquid column propagates faster. When S(z) is opposing the flow direction, reverse flow may exist near the free surface and the propagation speed of the liquid column is reduced. Moreover, for a capillary flow in a wedge with constant α, θ, and S, both the analytic solutions and the numerical simulation predict that l(t)∝t3/5 for the constant-flow-rate stage and l(t)∝t1/2 for the constant-height flow stage. When S is a function of the flow direction z, the above functional relationship between l and t becomes no longer valid; it varies as the liquid column propagates along the wedge.

  10. Effects of wall shear stress and its gradient on tumor cell adhesion in curved microvessels

    PubMed Central

    Yan, W. W.; Cai, B.

    2016-01-01

    Tumor cell adhesion to vessel walls in the microcirculation is one critical step in cancer metastasis. In this paper, the hypothesis that tumor cells prefer to adhere at the microvessels with localized shear stresses and their gradients, such as in the curved microvessels, was examined both experimentally and computationally. Our in vivo experiments were performed on the microvessels (post-capillary venules, 30–50 μm diameter) of rat mesentery. A straight or curved microvessel was cannulated and perfused with tumor cells by a glass micropipette at a velocity of ~1mm/s. At less than 10 min after perfusion, there was a significant difference in cell adhesion to the straight and curved vessel walls. In 60 min, the averaged adhesion rate in the curved vessels (n = 14) was ~1.5-fold of that in the straight vessels (n = 19). In 51 curved segments, 45% of cell adhesion was initiated at the inner side, 25% at outer side, and 30% at both sides of the curved vessels. To investigate the mechanical mechanism by which tumor cells prefer adhering at curved sites, we performed a computational study, in which the fluid dynamics was carried out by the lattice Boltzmann method, and the tumor cell dynamics was governed by the Newton’s law of translation and rotation. A modified adhesive dynamics model that included the influence of wall shear stress/gradient on the association/dissociation rates of tumor celladhesion was proposed, in which the positive wall shear stress/gradient jump would enhance tumor cell adhesion while the negative wall shear stress/gradient jump would weaken tumor cell adhesion. It was found that the wall shear stress/gradient, over a threshold, had significant contribution to tumor cell adhesion by activating or inactivating cell adhesion molecules. Our results elucidated why the tumor cell adhesion prefers to occur at the positive curvature of curved microvessels with very low Reynolds number (in the order of 10−2) laminar flow. PMID:21818636

  11. Dynamic shear modulus of glycerol: Corrections due to instrument compliance

    NASA Astrophysics Data System (ADS)

    Schröter, K.; Hutcheson, S. A.; Shi, X.; Mandanici, A.; McKenna, G. B.

    2006-12-01

    A recent article by Shi et al. [J. Chem. Phys.123, 174507 (2005)] reports results from mechanical measurements on three simple inorganic glass formers: glycerol, m-toluidine, and sucrose benzoate. The experiments carried out were stress relaxation, aging, and dynamic (all in shear) using a torsional rheometer, an advanced rheometric expansion system (TA Instruments). The original force rebalance transducer (2KFRT) supplied with the system was replaced with a custom-made load cell (Sensotec) that had a capacity of 20000gcm in torque and 5000g in normal force. The replacement of the load cell was done due to the belief that the main source of compliance in this instrument was from the 2KFRT. With this assumption, the authors published their results for the three materials of interest and compared their results with the techniques of Schröter and Donth [J. Chem. Phys.113, 9101 (2000)] for the measurements on glycerol and reported important differences. These differences were disputed by one of the present authors (Schröter), and the present report shows that the results from Schröter and Donth are correct. We show that the reasons have to do with the instrument compliance being greater than originally thought by Shi et al. Here we examine the effects of platen diameter/geometry on the glycerol dynamic moduli, describe a means to correct dynamic data, present a revised comparison of the corrected data with that of Schröter and Donth, and provide a discussion of future work and conclusions.

  12. Numerical Investigations of the Dynamic Shear Behavior of Rough Rock Joints

    NASA Astrophysics Data System (ADS)

    Huang, Junyu; Xu, Songlin; Hu, Shisheng

    2014-09-01

    The dynamic shear behavior of rock joints is significant to both rock engineering and earthquake dynamics. With the discrete element method (DEM), the dynamic direct-shear tests on the rough rock joints with 3D (sinusoidal or random) surface morphologies are simulated and discussed. Evolution of the friction coefficient with the slip displacement shows that the 3D DEM joint model can accurately reproduce the initial strengthening, slip-weakening, and steady-sliding responses of real rock joints. Energy analyses show that the strengthening and weakening behavior of the rock joint are mainly attributed to the rapid accumulation and release of the elastic energy in the joint. Then, effects of the surface roughness and the normal stress on the friction coefficient and the micro shear deformation mechanisms, mainly volume change and asperity damage, of the rock joint are investigated. The results show that the peak friction coefficient increases logarithmically with the increasing surface roughness, but decreases exponentially with the increasing normal stress. In addition, the rougher rock joint exhibits both higher joint dilation and asperity degradation. However, high normal stress constrains the joint dilation, but promotes the degree of asperity degradation significantly. Lastly, the effects of the 3D surface morphology on the shear behavior of the rock joint are investigated with a directional roughness parameter. It is observed that the anisotropy of the surface roughness consequently results in the variation of the peak friction coefficient of the joint corresponding to different shearing directions as well as the micro shear deformation mechanisms, e.g., the extent of joint dilation.

  13. Mechanical characterization of brain tissue in simple shear at dynamic strain rates.

    PubMed

    Rashid, Badar; Destrade, Michel; Gilchrist, Michael D

    2013-12-01

    During severe impact conditions, brain tissue experiences a rapid and complex deformation, which can be seen as a mixture of compression, tension and shear. Diffuse axonal injury (DAI) occurs in animals and humans when both the strains and strain rates exceed 10% and 10/s, respectively. Knowing the mechanical properties of brain tissue in shear at these strains and strain rates is thus of particular importance, as they can be used in finite element simulations to predict the occurrence of brain injuries under different impact conditions. However, very few studies in the literature provide this information. In this research, an experimental setup was developed to perform simple shear tests on porcine brain tissue at strain rates ≤120/s. The maximum measured shear stress at strain rates of 30, 60, 90 and 120/s was 1.15±0.25kPa, 1.34±0.19kPa, 2.19±0.225kPa and 2.52±0.27kPa, (mean±SD), respectively at the maximum amount of shear, K=1. Good agreement of experimental, theoretical (Ogden and Mooney-Rivlin models) and numerical shear stresses was achieved (p=0.7866-0.9935). Specimen thickness effects (2.0-10.0mm thick specimens) were also analyzed numerically and we found that there is no significant difference (p=0.9954) in the shear stress magnitudes, indicating a homogeneous deformation of the specimens during simple shear tests. Stress relaxation tests in simple shear were also conducted at different strain magnitudes (10-60% strain) with the average rise time of 14ms. This allowed us to estimate elastic and viscoelastic parameters (initial shear modulus, μ=4942.0Pa, and Prony parameters: g1=0.520, g2=0.3057, τ1=0.0264s, and τ2=0.011s) that can be used in FE software to analyze the non-linear viscoelastic behavior of brain tissue. This study provides new insight into the behavior in finite shear of brain tissue under dynamic impact conditions, which will assist in developing effective brain injury criteria and adopting efficient countermeasures against

  14. Polymer Brushes under Shear: Molecular Dynamics Simulations Compared to Experiments.

    PubMed

    Singh, Manjesh K; Ilg, Patrick; Espinosa-Marzal, Rosa M; Kröger, Martin; Spencer, Nicholas D

    2015-04-28

    Surfaces coated with polymer brushes in a good solvent are known to exhibit excellent tribological properties. We have performed coarse-grained equilibrium and nonequilibrium molecular dynamics (MD) simulations to investigate dextran polymer brushes in an aqueous environment in molecular detail. In a first step, we determined simulation parameters and units by matching experimental results for a single dextran chain. Analyzing this model when applied to a multichain system, density profiles of end-tethered polymer brushes obtained from equilibrium MD simulations compare very well with expectations based on self-consistent field theory. Simulation results were further validated against and correlated with available experimental results. The simulated compression curves (normal force as a function of surface separation) compare successfully with results obtained with a surface forces apparatus. Shear stress (friction) obtained via nonequilibrium MD is contrasted with nanoscale friction studies employing colloidal-probe lateral force microscopy. We find good agreement in the hydrodynamic regime and explain the observed leveling-off of the friction forces in the boundary regime by means of an effective polymer-wall attraction. PMID:25830715

  15. Dynamics of prolate spheroidal elastic particles in confined shear flow

    NASA Astrophysics Data System (ADS)

    Villone, M. M.; D'Avino, G.; Hulsen, M. A.; Maffettone, P. L.

    2015-12-01

    We investigate through numerical simulations the dynamics of a neo-Hookean elastic prolate spheroid suspended in a Newtonian fluid under shear flow. Both initial orientations of the particle within and outside the shear plane and both unbounded and confined flow geometries are considered. In unbounded flow, when the particle starts on the shear plane, two stable regimes of motion are found, i.e., trembling, where the particle shape periodically elongates and compresses in the shear plane and the angle between its major semiaxis and the flow direction oscillates around a positive mean value, and tumbling, where the particle shape periodically changes and its major axis performs complete revolutions around the vorticity axis. When the particle is initially oriented out of the shear plane, more complex dynamics arise. Geometric confinement of the particle between the moving walls also influences its deformation and regime of motion. In addition, when the particle is initially located in an asymmetric position with respect to the moving walls, particle lateral migration is detected. The effects on the particle dynamics of the geometric and physical parameters that rule the system are investigated.

  16. Analysis of bonded joints. [shear stress and stress-strain diagrams

    NASA Technical Reports Server (NTRS)

    Srinivas, S.

    1975-01-01

    A refined elastic analysis of bonded joints which accounts for transverse shear deformation and transverse normal stress was developed to obtain the stresses and displacements in the adherends and in the bond. The displacements were expanded in terms of polynomials in the thicknesswise coordinate; the coefficients of these polynomials were functions of the axial coordinate. The stress distribution was obtained in terms of these coefficients by using strain-displacement and stress-strain relations. The governing differential equations were obtained by integrating the equations of equilibrium, and were solved. The boundary conditions (interface or support) were satisfied to complete the analysis. Single-lap, flush, and double-lap joints were analyzed, along with the effects of adhesive properties, plate thicknesses, material properties, and plate taper on maximum peel and shear stresses in the bond. The results obtained by using the thin-beam analysis available in the literature were compared with the results obtained by using the refined analysis. In general, thin-beam analysis yielded reasonably accurate results, but in certain cases the errors were high. Numerical investigations showed that the maximum peel and shear stresses in the bond can be reduced by (1) using a combination of flexible and stiff bonds, (2) using stiffer lap plates, and (3) tapering the plates.

  17. Dynamics and rheology of concentrated, finite-Reynolds-number suspensions in a homogeneous shear flow

    NASA Astrophysics Data System (ADS)

    Yeo, Kyongmin; Maxey, Martin R.

    2013-05-01

    We present the lubrication-corrected force-coupling method for the simulation of concentrated suspensions under finite inertia. Suspension dynamics are investigated as a function of the particle-scale Reynolds number Re_{dot{γ }} and the bulk volume fraction ϕ in a homogeneous linear shear flow, in which Re_{dot{γ }} is defined from the density ρf and dynamic viscosity μ of the fluid, particle radius a, and the shear rate dot{γ } as Re_{dot{γ }}= ρ _f dot{γ } a^2 / μ. It is shown that the velocity fluctuations in the velocity-gradient and vorticity directions decrease at larger Re_{dot{γ }}. However, the particle self-diffusivity is found to be an increasing function of Re_{dot{γ }} as the motion of the suspended particles develops a longer auto-correlation under finite fluid inertia. It is shown that finite-inertia suspension flows are shear-thickening and the particle stresses become highly intermittent as Re_{dot{γ }} increases. To study the detailed changes in the suspension microstructure and rheology, we introduce a particle-stress-weighted pair-distribution function. The stress-weighted pair-distribution function clearly shows that the increase of the effective viscosity at high Re_{dot{γ }} is mostly related to the strong normal lubrication interaction in the compressive principal axis of the shear flow.

  18. Imaging the cellular response to transient shear stress using time-resolved digital holography

    NASA Astrophysics Data System (ADS)

    Arita, Yoshihiko; Antkowiak, Maciej; Gunn-Moore, Frank; Dholakia, Kishan

    2014-02-01

    Shear stress has been recognized as one of the biophysical methods by which to permeabilize plasma membranes of cells. In particular, high pressure transient hydrodynamic flows created by laser-induced cavitation have been shown to lead to the uptake of fluorophores and plasmid DNA. While the mechanism and dynamics of cavitation have been extensively studied using a variety of time-resolved imaging techniques, the cellular response to the cavitation bubble and cavitation induced transient hydrodynamic flows has never been shown in detail. We use time-resolved quantitative phase microscopy to study cellular response to laser-induced cavitation bubbles. Laser-induced breakdown of an optically trapped polystyrene nanoparticle (500nm in diameter) irradiated with a single nanosecond laser pulse at 532nm creates transient shear stress to surrounding cells without causing cell lysis. A bi-directional transient displacement of cytoplasm is observed during expansion and collapse of the cavitation bubble. In some cases, cell deformation is only observable at the microsecond time scale without any permanent change in cell shape or optical thickness. On a time scale of seconds, the cellular response to shear stress and cytoplasm deformation typically leads to retraction of the cellular edge most exposed to the flow, rounding of the cell body and, in some cases, loss of cellular dry mass. These results give a new insight into the cellular response to laser-induced shear stress and related plasma membrane permeabilization. This study also demonstrates that laser-induced breakdown of an optically trapped nanoparticle offers localized cavitation (70 μm in diameter), which interacts with a single cell.

  19. Ageing under Shear: Effect of Stress and Temperature Field

    NASA Astrophysics Data System (ADS)

    Shukla, Asheesh; Joshi, Yogesh M.

    2008-07-01

    In this work we studied the effect of oscillatory stress and temperature on the ageing dynamics of aqueous suspension of laponite. At the higher magnitude of stress, elastic and viscous moduli of the system underwent a sharp rise with the ageing time. The age at the onset of rise and the sharpness of the same increased with the magnitude of stress. We propose that at the beginning of ageing, the strain associated with the oscillatory stress field affects the lower modes in the relaxation time distribution. The higher modes, which are not significantly affected by the deformation field, continue to grow increasing the viscosity of the system thereby lowering the magnitude of the deformation field. Progressive decrease in the later reduces the range of relaxation modes affected by it. This dynamics eventually leads to an auto-catalytic increase in the elastic and viscous moduli. An increase in temperature accelerates the ageing process by shifting the ageing dynamics to a lower ageing time. This is due the microscopic relaxation dynamics, which causes ageing, becomes faster with increase in the temperature.

  20. Numerical Modeling of Shear Bands and Dynamic Fracture in Metals

    NASA Astrophysics Data System (ADS)

    McAuliffe, Colin James

    Understanding the failure of metals at high strain rate is of utmost importance in the design of a broad range of engineering systems. Numerical methods offer the ability to analyze such complex physics and aid the design of structural systems. The objective of this research will be to develop reliable finite element models for high strain rate failure modelling, incorporating shear bands and fracture. Shear band modelling is explored first, and the subsequent developments are extended to incorporate fracture. Mesh sensitivity, the spurious dependence of failure on the discretization, is a well known hurdle in achieving reliable numerical results for shear bands and fracture, or any other strain softening model. Mesh sensitivity is overcome by regularization, and while details of regularization techniques may differ, all are similar in that a length scale is introduced which serves as a localization limiter. This dissertation contains two main contributions, the first of which presents several developments in shear band modeling. The importance of using a monolithic nonlinear solver in combination with a PDE model accounting for thermal diffusion is demonstrated. In contrast, excluding one or both of these components leads to unreliable numerical results. The Pian-Sumihara stress interpolants are also employed in small and finite deformation and shown to significantly improve the computational cost of shear band modelling. This is partly due to the fact that fewer unknowns than an irreducible discretization result from the same mesh, and more significantly, the fact that convergence of numerical results upon mesh refinement is improved drastically. This means coarser meshes are adequate to resolve shear bands, alleviating some of the computational cost of numerical modelling, which are notoriously significant. Since extremely large deformations are present during shear banding, a mesh to mesh transfer algorithm is presented for the Pian Sumihara element and used as

  1. Experimental and CFD simulation studies of wall shear stress for different impeller configurations and MBR activated sludge.

    PubMed

    Ratkovich, N; Chan, C C V; Bentzen, T R; Rasmussen, M R

    2012-01-01

    Membrane bioreactors (MBRs) have been used successfully in biological wastewater treatment for effective solids-liquid separation. However, a common problem encountered with MBR systems is fouling of the membrane resulting in frequent membrane cleaning and replacement which makes the system less appealing for full-scale applications. It has been widely demonstrated that the filtration performances in MBRs can be improved by understanding the shear stress over the membrane surface. Modern tools such as computational fluid dynamics (CFD) can be used to diagnose and understand the shear stress in an MBR. Nevertheless, proper experimental validation is required to validate CFD simulation. In this work experimental measurements of shear stress induced by impellers at a membrane surface were made with an electrochemical approach and the results were used to validate CFD simulations. As good results were obtained with the CFD model (<9% error), it was extrapolated to include the non-Newtonian behaviour of activated sludge. PMID:22592479

  2. Estimation of critical shear stress for erosion in the Changjiang Estuary: A synergy research of observation, GOCI sensing and modeling

    NASA Astrophysics Data System (ADS)

    Ge, Jianzhong; Shen, Fang; Guo, Wenyun; Chen, Changsheng; Ding, Pingxing

    2015-12-01

    Simulating the sediment transport in a high-turbidity region with spatially varying bed properties is challenging. A comprehensive strategy that integrates multiple methods is applied here to retrieve the critical shear stress for erosion, which plays a major role in suspended sediment dynamics in the Changjiang Estuary (CE). Time-series of sea surface suspended sediment concentration (SSC) were retrieved from the Geostationary Ocean Color Imager (GOCI) satellite data at hourly intervals (for 8 h each day) and combined with hydrodynamic modeling of high-resolution CE Finite-Volume Community Ocean Model (CE-FVCOM) to estimate the near-bed critical shear stress in the clay-dominated bed region (plasticity index > 7%). An experimental algorithm to determine the in situ critical shear stress via the plasticity index method was also used to verify the GOCI-derived critical shear stress. Implemented with this new critical shear stress, the sediment transport model significantly improved the simulation of the distribution and spatial variability of the SSC during the spring and neap tidal cycles in the CE. The results suggest that a significant lateral water exchange between channels and shoals occurred during the spring flood tide, which led to a broader high-SSC area in the CE throughout the water column.

  3. Shear stress reduces protease activated receptor-1 expression in human endothelial cells

    NASA Technical Reports Server (NTRS)

    Nguyen, K. T.; Eskin, S. G.; Patterson, C.; Runge, M. S.; McIntire, L. V.

    2001-01-01

    Shear stress has been shown to regulate several genes involved in the thrombotic and proliferative functions of endothelial cells. Thrombin receptor (protease-activated receptor-1: PAR-1) increases at sites of vascular injury, which suggests an important role for PAR-1 in vascular diseases. However, the effect of shear stress on PAR-1 expression has not been previously studied. This work investigates effects of shear stress on PAR-1 gene expression in both human umbilical vein endothelial cells (HUVECs) and microvascular endothelial cells (HMECs). Cells were exposed to different shear stresses using a parallel plate flow system. Northern blot and flow cytometry analysis showed that shear stress down-regulated PAR-1 messenger RNA (mRNA) and protein levels in both HUVECs and HMECs but with different thresholds. Furthermore, shear-reduced PAR-1 mRNA was due to a decrease of transcription rate, not increased mRNA degradation. Postshear stress release of endothelin-1 in response to thrombin was reduced in HUVECs and HMECs. Moreover, inhibitors of potential signaling pathways applied during shear stress indicated mediation of the shear-decreased PAR-1 expression by protein kinases. In conclusion, shear stress exposure reduces PAR-1 gene expression in HMECs and HUVECs through a mechanism dependent in part on protein kinases, leading to altered endothelial cell functional responses to thrombin.

  4. XIAP is essential for shear stress-enhanced Tyr-576 phosphorylation of FAK

    SciTech Connect

    Ahn, Sunyoung; Park, Heonyong

    2010-08-20

    Research highlights: {yields} Laminar shear stress phosphorylates Tyr-576 in FAK. {yields} XIAP is essential for shear stress-induced phosphorylation of Tyr-576. {yields} XIAP knockdown induces shear stress-triggered translocation of FAK into nucleus. {yields} XIAP regulates ERK activation by maintaining the Src-accessible location of FAK. -- Abstract: In endothelial cells, X-chromosome linked inhibitor of apoptosis protein (XIAP) regulates cell survival, migration and adhesion. We have recently found that XIAP recruits focal adhesion kinase (FAK) into integrin-associated focal adhesions, controlling cell migration. However, little is understood about the molecular mechanisms by which FAK modulation is controlled by XIAP. In this study, we show that XIAP modulates FAK activity through the control of FAK phosphorylation. In bovine aortic endothelial cells (BAEC), phosphorylation of Tyr-576 in FAK is elevated by laminar shear stress. This elevated phosphorylation appears to be responsible for shear stress-stimulated ERK activation. We found that XIAP knockdown reduces shear stress-enhanced phosphorylation of Tyr-576 and induces shear stress-triggered translocation of FAK into nucleus. Nuclear translocation of FAK reduces contact between FAK and Src, a kinase which phosphorylates Tyr-576. This spatial segregation of FAK from Src decreases Tyr-576 phosphorylation and thus shear-stimulated ERK activation. Taken together, our results demonstrate that XIAP plays a key role in shear stress-stimulated ERK activation by maintaining the Src-accessible location of FAK.

  5. Interlaminar shear stress effects on the postbuckling response of graphite-epoxy panels

    NASA Technical Reports Server (NTRS)

    Engelstad, S. P.; Knight, N. F., Jr.; Reddy, J. N.

    1990-01-01

    The influence of shear flexibility on overall postbuckling response was assessed, and transverse shear stress distributions in relation to panel failure were examined. Nonlinear postbuckling results are obtained for finite element models based on classical laminated plate theory and first-order shear deformation theory. Good correlation between test and analysis is obtained. The results presented analytically substantiate the experimentally observed failure mode.

  6. Wrinkling Phenomena of Thin Flat Plates Subjected to Shear Stresses

    NASA Technical Reports Server (NTRS)

    Bollenrath, F

    1931-01-01

    This report covers a series of tests on thin flat elastic strips restrained at two parallel edges and subjected to shear by conversely directed stresses. Theoretical treatments, particularly those of Lilly, Southwell and Skan, and Timoshenko are briefly outlined. The problem to be solved by these tests was to find out whether, and to what extent the conditions and assumptions upon which the calculations are based are complied with in the tests. Three materials were used: celluloid, duralumin, brass. Owing to the high elastic deformability of celluloid, it was not only possible to observe the beginning but also to ascertain the type of deflection. The test data on celluloid was affirmed by the experiments with duralumin and brass.

  7. Observations of wave shear stress on a steep beach

    NASA Astrophysics Data System (ADS)

    Wilson, G. W.; Hay, A. E.; Bowen, A. J.

    2014-11-01

    Observations are presented of the wave shear stress on a steeply sloping beach. Above the wave boundary layer (WBL), positive values of were observed and are attributed to a combination of both wave shoaling due to the large-scale bed slope, and dissipation due to wave breaking, in agreement with the wave theory of Zou et al. (2003). Within the WBL, observed vertical profiles of were also in good agreement with theory, in cases where the wave height was small. As wave heights increased, however, the WBL profile of generally did not agree with theory. Near-simultaneous rotary sonar observations of the bed suggest the disagreement with theory was due to the presence of orbital-scale ripples, which the present theory does not accommodate.

  8. Reynolds shear stress measurements in a separated boundary layer flow

    NASA Technical Reports Server (NTRS)

    Driver, David M.

    1991-01-01

    Turbulence measurements were obtained for two cases of boundary layer flow with an adverse pressure gradient, one attached and the other separated. A three-component laser Doppler velocimeter system was used to measure three mean velocity components, all six Reynolds stress components, and all ten velocity triple product correlations. Independent measurements of skin-friction obtained with a laser oil-flow interferometer were used to examine the law of the wall in adverse pressure gradient flows where p(+) is less than 0.05. Strong similiarities were seen between the two adverse pressure gradient flows and free shear layer type flows. Eddy viscosities, dissipation rates, and pressure-strain rates were deduced from the data and compared to various turbulence modeling assumptions.

  9. On the yielding behaviour in magnetorheology using ultrasounds, shear and normal stresses, and optical microscopy

    NASA Astrophysics Data System (ADS)

    Rodríguez-López, Jaime; Castro Blázquez, Pedro; Elvira, Luis; Montero de Espinosa, Francisco; Ramírez, Javier; de Vicente, Juan

    2015-11-01

    The yielding behaviour of magnetorheological fluids has been investigated by videomicroscopy, ultrasonic and rheometry techniques simultaneously. Particularly, the effect of different factors such as, the magnetic field strength, particle size, surface chemistry of the particles, particle concentration and carrier fluid viscosity has been studied. Special attention has been paid to correlate the yielding information obtained by acoustical, optical and mechanical techniques. As a general trend, independently of the particular field strength and suspension formulation, the steady shear flow curve exhibits three well differentiated regions. In the first region, at small stresses, field-induced structures remain quasistatic and all magnitudes remain constant. For larger stresses the number of aggregates decreases but their size increases. This is identified with the onset of flow, and corresponds to the classical static yield stress and a decrease in time-of-flight and normal stresses. For even larger stress values, the suspensions fully flow. This stress value corresponds to the classical dynamic yield stress and is associated to a minimum in the time-of-flight and normal stresses.

  10. Mechanical interaction between cells and fluid for bone tissue engineering scaffold: modulation of the interfacial shear stress.

    PubMed

    Blecha, L D; Rakotomanana, L; Razafimahery, F; Terrier, A; Pioletti, D P

    2010-03-22

    An analytical model of the fluid/cell mechanical interaction was developed. The interfacial shear stress, due to the coupling between the fluid and the cell deformation, was characterized by a new dimensionless number N(fs). For N(fs) above a critical value, the fluid/cell interaction had a damping effect on the interfacial shear stress. Conversely, for N(fs) below this critical value, interfacial shear stress was amplified. As illustration, the role of the dynamic fluid/cell mechanical coupling was studied in a specific biological situation involving cells seeded in a bone scaffold. For the particular bone scaffold chosen, the dimensionless number N(fs) was higher than the critical value. In this case, the dynamic shear stress at the fluid/cell interface is damped for increasing excitation frequency. Interestingly, this damping effect is correlated to the pore diameter of the scaffold, furnishing thus target values in the design of the scaffold. Correspondingly, an efficient cell stimulation might be achieved with a scaffold of pore size larger than 300 microm as no dynamic damping effect is likely to take place. The analytical model proposed in this study, while being a simplification of a fluid/cell mechanical interaction, brings complementary insights to numerical studies by analyzing the effect of different physical parameters. PMID:20004397

  11. Predicting boundary shear stress and sediment transport over bed forms

    USGS Publications Warehouse

    McLean, S.R.; Wolfe, S.R.; Nelson, J.M.

    1999-01-01

    To estimate bed-load sediment transport rates in flows over bed forms such as ripples and dunes, spatially averaged velocity profiles are frequently used to predict mean boundary shear stress. However, such averaging obscures the complex, nonlinear interaction of wake decay, boundary-layer development, and topographically induced acceleration downstream of flow separation and often leads to inaccurate estimates of boundary stress, particularly skin friction, which is critically important in predicting bed-load transport rates. This paper presents an alternative methodology for predicting skin friction over 2D bed forms. The approach is based on combining the equations describing the mechanics of the internal boundary layer with semiempirical structure functions to predict the velocity at the crest of a bedform, where the flow is most similar to a uniform boundary layer. Significantly, the methodology is directed toward making specific predictions only at the bed-form crest, and as a result it avoids the difficulty and questionable validity of spatial averaging. The model provides an accurate estimate of the skin friction at the crest where transport rates are highest. Simple geometric constraints can be used to derive the mean transport rates as long as bed load is dominant.To estimate bed-load sediment transport rates in flows over bed forms such as ripples and dunes, spatially averaged velocity profiles are frequently used to predict mean boundary shear stress. However, such averaging obscures the complex, nonlinear interaction of wake decay, boundary-layer development, and topographically induced acceleration downstream of flow separation and often leads to inaccurate estimates of boundary stress, particularly skin friction, which is critically important in predicting bed-load transport rates. This paper presents an alternative methodology for predicting skin friction over 2D bed forms. The approach is based on combining the equations describing the mechanics of

  12. Magnitude of shear stress on the san andreas fault: implications of a stress measurement profile at shallow depth.

    PubMed

    Zoback, M D; Roller, J C

    1979-10-26

    A profile of measurements of shear stress perpendicular to the San Andreas fault near Palmdale, California, shows a marked increase in stress with distance from the fault. The pattern suggests that shear stress on the fault increases slowly with depth and reaches a value on the order of the average stress released during earthquakes. This result has important implications for both long- and shortterm prediction of large earthquakes. PMID:17809367

  13. Dynamics of poloidal flows in enhanced reverse shear bifurcation

    SciTech Connect

    Srinivasan, R.; Avinash, K.

    2005-07-15

    A simple reduced enhanced reverse shear (RERS) model is constructed to study the dynamics of poloidal flows during the ERS transition. This model predicts that a reversal of poloidal flow shear occurs just prior to the transition, as seen in experiment [R. E. Bell et al., Phys. Rev. Lett. 81, 1429 (1998)]. This transition front propagates until the radial location where the safety factor (q) is minimum and becomes locked there due to insufficient input power to overcome the threshold requirement for the bifurcation. This study also reveals that there can be many routes to ERS transition depending upon various tunable parameters.

  14. Inverting for Shear Stress Rate on the Northern Cascadia Megathrust Using Geodetic Data

    NASA Astrophysics Data System (ADS)

    Bruhat, L.; Segall, P.; Bradley, A. M.

    2014-12-01

    Past physics-based models of slow slip events (SSE) have shown that, when averaged over many SSE cycles, the shear stress within the SSE zone remains roughly constant. Stress accumulates between SSE, and then is released during slow slip events. However, the predicted long-term deformation rates from such models, assuming the plate boundary is locked to the top of the ETS zone, do not fit well GPS velocities and uplift rates determined from leveling and tide-gauge data. These physics-based models particularly misfit the vertical rates. At the same time, previous kinematic inversions display a gap between the down-dip limit of the locked region and the top of the ETS zone. Our inversions of geodetic data for fault slip rates exhibit a steeper slip-rate profile at the top of the ETS zone, relative to the constant shear stress model, as well as creep up dip of the ETS zone. We explore physics-based models with velocity-strengthening regions of different length up dip the ETS zone, i.e. within the "gap" identified in kinematic inversions. However, this still does not match the observations well. We therefore try a new approach: we invert for shear stress rates on the megathrust that best fit the data. We show that a small decrease in shear stress within the top of the ETS zone, reaching 5 kPa/year at a depth of ~ 30 km, is required to fit the data. Possible explanations for this include a slow decrease in normal stress with time, possibly due to an increase in pore pressure, or a reduction in fault friction. We explore these hypotheses, using 2D quasi-dynamic simulations with rate-and-state friction and isothermal v-cutoff models for generating slow slip events. The potential for creep above the top of the ETS zone has important implications for the mechanical relationship between deep slow slip and dynamic events in the locked region.

  15. Pressure and shear stress in trabecular bone marrow during whole bone loading.

    PubMed

    Metzger, Thomas A; Schwaner, Stephen A; LaNeve, Anthony J; Kreipke, Tyler C; Niebur, Glen L

    2015-09-18

    Skeletal adaptation to mechanical loading is controlled by mechanobiological signaling. Osteocytes are highly responsive to applied strains, and are the key mechanosensory cells in bone. However, many cells residing in the marrow also respond to mechanical cues such as hydrostatic pressure and shear stress, and hence could play a role in skeletal adaptation. Trabecular bone encapsulates marrow, forming a poroelastic solid. According to the mechanical theory, deformation of the pores induces motion in the fluid-like marrow, resulting in pressure and velocity gradients. The latter results in shear stress acting between the components of the marrow. To characterize the mechanical environment of trabecular bone marrow in situ, pore pressure within the trabecular compartment of whole porcine femurs was measured with miniature pressure transducers during stress-relaxation and cyclic loading. Pressure gradients ranging from 0.013 to 0.46 kPa/mm were measured during loading. This range was consistent with calculated pressure gradients from continuum scale poroelastic models with the same permeability. Micro-scale computational fluid dynamics models created from computed tomography images were used to calculate the micromechanical stress in the marrow using the measured pressure differentials as boundary conditions. The volume averaged shear stress in the marrow ranged from 1.67 to 24.55 Pa during cyclic loading, which exceeds the mechanostimulatory threshold for mesenchymal lineage cells. Thus, the loading of bone through activities of daily living may be an essential component of bone marrow health and mechanobiology. Additional studies of cell-level interactions during loading in healthy and disease conditions will provide further incite into marrow mechanobiology. PMID:26283413

  16. Adaptive response of vascular endothelial cells to an acute increase in shear stress magnitude.

    PubMed

    Zhang, Ji; Friedman, Morton H

    2012-02-15

    The adaptation of vascular endothelial cells to shear stress alteration induced by global hemodynamic changes, such as those accompanying exercise or digestion, is an essential component of normal endothelial physiology in vivo. An understanding of the transient regulation of endothelial phenotype during adaptation to changes in mural shear will advance our understanding of endothelial biology and may yield new insights into the mechanism of atherogenesis. In this study, we characterized the adaptive response of arterial endothelial cells to an acute increase in shear stress magnitude in well-defined in vitro settings. Porcine endothelial cells were preconditioned by a basal level shear stress of 15 ± 15 dyn/cm(2) at 1 Hz for 24 h, after which an acute increase in shear stress to 30 ± 15 dyn/cm(2) was applied. Endothelial permeability nearly doubled after 40-min exposure to the elevated shear stress and then decreased gradually. Transcriptomics studies using microarray techniques identified 86 genes that were sensitive to the elevated shear. The acute increase in shear stress promoted the expression of a group of anti-inflammatory and antioxidative genes. The adaptive response of the global gene expression profile is triphasic, consisting of an induction period, an early adaptive response (ca. 45 min) and a late remodeling response. Our results suggest that endothelial cells exhibit a specific phenotype during the adaptive response to changes in shear stress; this phenotype is different than that of fully adapted endothelial cells. PMID:22140046

  17. Surface temperatures and glassy state investigations in tribology, part 3. [limiting shear stress rheological model

    NASA Technical Reports Server (NTRS)

    Bair, S.; Winer, W. O.

    1980-01-01

    Research related to the development of the limiting shear stress rheological model is reported. Techniques were developed for subjecting lubricants to isothermal compression in order to obtain relevant determinations of the limiting shear stress and elastic shear modulus. The isothermal compression limiting shear stress was found to predict very well the maximum traction for a given lubricant. Small amounts of side slip and twist incorporated in the model were shown to have great influence on the rising portion of the traction curve at low slide-roll ratio. The shear rheological model was also applied to a Grubin-like elastohydrodynamic inlet analysis for predicting film thicknesses when employing the limiting shear stress model material behavior.

  18. Spatial comparison between wall shear stress measures and porcine arterial endothelial permeability.

    PubMed

    Himburg, Heather A; Grzybowski, Deborah M; Hazel, Andrew L; LaMack, Jeffrey A; Li, Xue-Mei; Friedman, Morton H

    2004-05-01

    A better understanding of how hemodynamic factors affect the integrity and function of the vascular endothelium is necessary to appreciate more fully how atherosclerosis is initiated and promoted. A novel technique is presented to assess the relation between fluid dynamic variables and the permeability of the endothelium to macromolecules. Fully anesthetized, domestic swine were intravenously injected with the albumin marker Evans blue dye, which was allowed to circulate for 90 min. After the animals were euthanized, silicone casts were made of the abdominal aorta and its iliac branches. Pulsatile flow calculations were subsequently made in computational regions derived from the casts. The distribution of the calculated time-dependent wall shear stress in the external iliac branches was directly compared on a point-by-point basis with the spatially varying in vivo uptake of Evans blue dye in the same arteries. The results indicate that in vivo endothelial permeability to albumin decreases with increasing time-average shear stress over the normal range. Additionally, endothelial permeability increases slightly with oscillatory shear index. PMID:14715506

  19. Steady and dynamic shear characterization of cellulase-producing Trichoderma reesei suspensions

    SciTech Connect

    Marten, M.R.; Velkovska, S.; Khan, S.A.

    1995-12-31

    Suspension rheology of fungal fermentations is important in determination of mass transfer rates, as well as mixing quality. We have characterized Trichorderma reesei RUT-C30 suspension rheology during growth on xylose (soluble) and cellulose (particulate) substrates, using both steady and dynamic shear measurements. Biomass growth was monophasic on xylose and biphasic on cellulose; the latter behavior is consistent with relatively rapid, early growth on soluble sugars derived from rapidly hydrolyzed material, followed by a second, slower growth phase owing to hydrolysis of more recalcitrant cellulose by increasing cellulose concentrations. Steady shear measurements established the presence of a yield stress for fermentation broths when using a 10 (vol)% fungal inoculum. The Casson equation represented all data well. Casson parameters of viscosity and yield stress followed biomass evolution: two maxima in both parameters were observed with cellulose substrates, and a single maximum with xylose. Dynamic shear measurements on broths indicated a gel behavior at small strains and a shear thinning liquid behavior at larger displacements. These results indicate the need to include rheology and mixing considerations in the subsequent development of a full biological and physical kinetic description of T. reesei cellulose conversions.

  20. Wall Shear Stress, Wall Pressure and Near Wall Velocity Field Relationships in a Whirling Annular Seal

    NASA Technical Reports Server (NTRS)

    Morrison, Gerald L.; Winslow, Robert B.; Thames, H. Davis, III

    1996-01-01

    The mean and phase averaged pressure and wall shear stress distributions were measured on the stator wall of a 50% eccentric annular seal which was whirling in a circular orbit at the same speed as the shaft rotation. The shear stresses were measured using flush mounted hot-film probes. Four different operating conditions were considered consisting of Reynolds numbers of 12,000 and 24,000 and Taylor numbers of 3,300 and 6,600. At each of the operating conditions the axial distribution (from Z/L = -0.2 to 1.2) of the mean pressure, shear stress magnitude, and shear stress direction on the stator wall were measured. Also measured were the phase averaged pressure and shear stress. These data were combined to calculate the force distributions along the seal length. Integration of the force distributions result in the net forces and moments generated by the pressure and shear stresses. The flow field inside the seal operating at a Reynolds number of 24,000 and a Taylor number of 6,600 has been measured using a 3-D laser Doppler anemometer system. Phase averaged wall pressure and wall shear stress are presented along with phase averaged mean velocity and turbulence kinetic energy distributions located 0.16c from the stator wall where c is the seal clearance. The relationships between the velocity, turbulence, wall pressure and wall shear stress are very complex and do not follow simple bulk flow predictions.

  1. Possible effect from shear stress on maturation of somatic embryos of Norway spruce (Picea abies).

    PubMed

    Sun, Hong; Aidun, Cyrus K; Egertsdotter, Ulrika

    2011-05-01

    Somatic embryogenesis is the only method with the potential for industrial scale clonal propagation of conifers. Implementation of the method has so far been hampered by the extensive manual labor required for development of the somatic embryos into plants. The utilization of bioreactors is limited since the somatic embryos will not mature and germinate under liquid culture conditions. The negative effect on mature embryo yields from liquid culture conditions has been previously described. We have described the negative effects of shear stress on the development of early stage somatic embryos (proembryogenic masses; PEMs) at shear stresses of 0.086 and 0.14 N/m(2). In the present study, additional flow rates were studied to determine the effects of shear stress at lower rates resembling shear stress in a suspension culture flask. The results showed that shear stress at 0.009, 0.014, and 0.029 N/m(2) inhibited the PEM expansions comparing with the control group without shear stress. This study also provides validation for the cross-correlation method previously developed to show the effect of shear stress on early stage embryo suspensor cell formation and polarization. Furthermore, shear stress was shown to positively affect the uptake of water into the cells. The results indicate that the plasmolyzing effect from macromolecules added to liquid culture medium to stimulate maturation of the embryos are affected by liquid culture conditions and thus can affect the conversion of PEMs into mature somatic embryos. PMID:21449024

  2. Shear stress partitioning of overland flow on disturbed and undisturbed rangelands

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Physically-based hillslope erosion models commonly estimate soil detachment and transport capacity based on overland flow shear stress applied to soil aggregates. However, vegetation and rock cover counteract the shear stress of overland flow where they occur. Accordingly, partitioning of total sh...

  3. Shear stress partitioning of overland flow on disturbed and undisturbed rangelands

    Technology Transfer Automated Retrieval System (TEKTRAN)

    In physically-based hillslope erosion models, only overland flow shear stress exerted on soil aggregates (grains) is used to estimate concentrated flow soil detachment rates and sediment transport capacity. However, on vegetated hillslopes, only overland flow total shear stress can be obtained usin...

  4. Effect of cytoskeleton stress-free state on red blood cell responses in low shear rate flows

    NASA Astrophysics Data System (ADS)

    Zhu, Qiang; Peng, Zhangli; Mashayekh, Adel

    2013-11-01

    Inspired by the recent experiment on erythrocytes (red blood cells, or RBCs) in weak shear flows (Dupire et al. 2012), we conduct a numerical investigation to study the dynamics of RBCs in low shear rate flows by applying a multiscale fluid-structure interaction model. By employing a spheroidal stress-free state in the cytoskeleton we are able to numerically predict an important feature that the cell maintains its biconcave shape during tank treading motions. This has not been achieved by any existing models. Furthermore, we numerically confirm the hypothesis that as the stress-free state approaches a sphere, the threshold shear rates corresponding to the establishment of tank treading decrease. By comparing with the experimental measurements, our study suggests that the stress-free state of RBCs is a spheroid which is close to a sphere, rather than a biconcave shape applied in existing models (the implication is that the RBC skeleton is prestressed in its natural biconcave state). It also suggests that the response of RBCs in low shear rate flows may provide a measure to quantitatively determine the distribution of shear stress in RBC cytoskeleton at the natural state.

  5. Shear stress behavior in mesoscale simulations of granular materials

    NASA Astrophysics Data System (ADS)

    Fujino, Don; Lomov, Ilya; Antoun, Tarabay; Vitali, Efrem

    2012-03-01

    3D mesoscale simulations of shock propagation in porous solids and powders have been performed with the Eulerian hydrocode GEODYN. The results indicate that voids can have a profound effect on the stress state in the material behind the shock front. The simulations can explain experimentally observed wave profiles that are difficult to interpret in the context of the classical elastic-plastic theory. In particular, a quasielastic precursor is observed in reshock simulations. This effect persists even at extremely low porosity values, down to 0.1% by volume. Stress relaxation is pronounced in simulations involving wave propagation, but is not observed in uniform ramp loading. In this sense, the relaxation phenomenon is non-local in nature and classic continuum models are inadequate for its description. Simulations show that the response of highly porous powders is dominated by deviatoric stress relaxation in the shock regime. We propose an enhancement which can be easily integrated into most existing porous material continuum models for modeling the shockinduced relaxation phenomena observed in the mesoscale simulation. The model calculates the microkinetic energy generated by dynamic loading and stores it as an internal state variable. The rate of production and dissipation of microkinetic energy and other model parameters are calibrated based on the mesoscale results. The augmented continuum model represents the deviatoric stress behavior observed under different regimes of dynamic loading.

  6. Response of hot element wall shear stress gages in laminar oscillating flows

    NASA Technical Reports Server (NTRS)

    Cook, W. J.; Murphy, J. D.; Giddings, T. A.

    1986-01-01

    An experimental investigation of the time-dependent response of hot element wall shear stress gages in unsteady periodic air flows is reported. The study has focused on wall shear stress in laminar oscillating flows produced on a flat plate by a free stream velocity composed of a mean component and a superposed sinusoidal variation. Two types of hot element gages, platinum film and flush wire, were tested for values of reduced frequency ranging from 0.14 to 2.36. Values of the phase angle of the wall shear stress variation relative to the free stream velocity, as indicated by the hot element gages, are compared with numerical prediction. The comparisons show that the gages indicate a wall shear stress variation that lags the true variation, and that the gages will also not indicate the correct wall shear stress variation in periodic turbulent flows.

  7. Evaluation of Stress Anisotropy and Shearing Stress Using an Eddy Current Method with a Tangential-Rectangular Coil

    NASA Astrophysics Data System (ADS)

    Sekine, Yuichi; Soyama, Hitoshi

    In establishing a system to evaluate residual stress, it is important to design the system so that it can also evaluate the stress anisotropy, since this is introduced into metallic materials by surface processes such as grinding and polishing. The shearing stress is also an important parameter when the shear strength has to be considered, since tensile stress can cause stress corrosion cracking. Thus, a method to nondestructively evaluate the stress anisotropy and shearing stress in a short time is required. In this paper, a nondestructive eddy current method using a tangential-rectangular coil was used to accomplish this. The material under test was stainless steel, Japanese Industrial Standard (JIS) SUS316L, ground or polished by an angle grinder. The stress anisotropy caused by the grinding and polishing processes was evaluated by the eddy current method with the tangential-rectangular coil. To vary the stress state, some specimens were treated with cavitation peening after grinding with the angle grinder. The results demonstrate that the stress anisotropy, shearing stress and peening intensity can be evaluated by the eddy current method using the tangential-rectangular coil. From the results, it was concluded that the maximum shearing stress and the direction of the principal stress could be determined.

  8. Development of ionic polymer transducers as flow shear stress sensors: effects of electrode architecture

    NASA Astrophysics Data System (ADS)

    Griffiths, David; Dominic, Justin; Akle, Barbar J.; Vlachos, Pavlos P.; Leo, Donald J.

    2007-04-01

    Ionomeric polymer transducers (IPTs) have recently received a great deal of attention. As actuators, IPT have the ability to generate large bending strain and moderate stress at low applied voltages. Although the actuation capabilities of IPTs have been studied extensively, the sensing performance of these transducers has not received much attention. The work presented herein aims to develop a wall shear stress sensor for aero/hydrodynamic and biomedical applications. Ionic polymers are generally created by an impregnation-reduction process in an ion exchange membrane, typically Nafion, and then coated with a flexible electrode. The traditional impregnation-reduction fabrication technique of IPTs has little control on the electrode thickness. However, the new Direct Assembly Process (DAP) for fabrication of IPTs allows for experimentation with varying conducting materials and direct control of electrode architecture. The thickness of the electrode is controlled by altering the amount of the ionomer/metal mix sprayed on the membrane. Transducers with varied electrode and membrane thicknesses are fabricated. The sensitivity of the transducer is characterized using two basic experiments. First, the electric impedance of the transducer is measured and its capacitive properties are computed. Earlier studies have demonstrated that capacitance has been strongly correlated to actuation performance in IPTs. Subsequently, the sensing capability of the IPTs in bending is measured using a fixed-pined cantilever configuration. Finally the shear stress sensing performance in fluid flow is quantified through a detailed calibration procedure. This is accomplished using two dynamic shear stress calibration apparatuses. In this study we demonstrate a strong correlation between the electrode thickness and the sensing performance of an IPT.

  9. Effect of shear stress on the high-pressure behaviour of nitromethane: Raman spectroscopy in a shear diamond anvil cell

    NASA Astrophysics Data System (ADS)

    Hebert, Philippe; Isambert, Aude; Petitet, Jean-Pierre; Zerr, Andreas

    2009-06-01

    A detailed description of the reaction mechanisms occurring in shock-induced decomposition of condensed energetic materials is very important for a comprehensive understanding of detonation. Besides pressure and temperature effects, shear stress has also been proposed to play an important role in the initiation and decomposition mechanisms. In order to study this effect, a Shear Diamond Anvil Cell (SDAC) has been developed. It is actually a classical DAC with the upper diamond anvil rotating about the compression axis relative to the opposite anvil. In this paper, we present a Raman spectroscopy study of the effect of shear stress on the high-pressure behaviour of nitromethane. Two major effects of shear stress are observed in our experiments. The first one is a lowering of the pressures at which the different structural modifications that nitromethane undergoes are observed. The second effect is observed at 28 GPa where sudden decomposition of the sample occurs just after shear application. Observation of the sample after decomposition shows the presence of a black residue which is composed of carbon as indicated by the Raman spectrum. [1] Manaa, M. R., Fried, L. E., and Reed, E. J., Journal of Computer-Aided Materials Design, 10, pp 75-97, 2003.

  10. Critical combinations of shear and direct axial stress for curved rectangular panels

    NASA Technical Reports Server (NTRS)

    Schildcrout, Murry; Stein, Manuel

    1949-01-01

    A solution is presented for the problem of the buckling of curved rectangular panels subjected to combined shear and direct axial stress. Charts giving theoretical critical combinations of shear and direct axial stress are presented for panels having five different length-width ratios. Because the actual critical compressive stress of rectangular panels having substantial curvature is known to be much lower than the theoretical value, a semiempirical method of analysis of curved panels subjected to combined shear and direct axial stress is presented for use in design. (author

  11. Optimizing the rotor design for controlled-shear affinity filtration using computational fluid dynamics.

    PubMed

    Francis, Patrick; Martinez, D Mark; Taghipour, Fariborz; Bowen, Bruce D; Haynes, Charles A

    2006-12-20

    Controlled shear affinity filtration (CSAF) is a novel integrated processing technology that positions a rotor directly above an affinity membrane chromatography column to permit protein capture and purification directly from cell culture. The conical rotor is intended to provide a uniform and tunable shear stress at the membrane surface that inhibits membrane fouling and cell cake formation by providing a hydrodynamic force away from and a drag force parallel to the membrane surface. Computational fluid dynamics (CFD) simulations are used to show that the rotor in the original CSAF device (Vogel et al., 2002) does not provide uniform shear stress at the membrane surface. This results in the need to operate the system at unnecessarily high rotor speeds to reach a required shear stress of at least 0.17 Pa at every radial position of the membrane surface, compromising the scale-up of the technology. Results from CFD simulations are compared with particle image velocimetry (PIV) experiments and a numerical solution for low Reynolds number conditions to confirm that our CFD model accurately describes the hydrodynamics in the rotor chamber of the CSAF device over a range of rotor velocities, filtrate fluxes, and (both laminar and turbulent) retentate flows. CFD simulations were then carried out in combination with a root-finding method to optimize the shape of the CSAF rotor. The optimized rotor geometry produces a nearly constant shear stress of 0.17 Pa at a rotational velocity of 250 rpm, 60% lower than the original CSAF design. This permits the optimized CSAF device to be scaled up to a maximum rotor diameter 2.5 times larger than is permissible in the original device, thereby providing more than a sixfold increase in volumetric throughput. PMID:16937405

  12. Dynamically triggered slip and sustained fault gouge instability associated with unique slip behavior under laboratory shear conditions

    NASA Astrophysics Data System (ADS)

    Johnson, P. A.; Carmeliet, J.; Savage, H. M.; Scuderi, M.; Carpenter, B. M.; Guyer, R. A.; Daub, E. G.; Marone, C.

    2015-12-01

    We investigate dynamic-wave triggered slip under laboratory shear conditions. The experiment is comprised of a 3-block system containing two gouge layers composed of glass beads and held in place by a fixed load in a bi-axial configuration. When the system is sheared under steady state conditions at loads from 3-8 MPa, stick-slip exhibiting a characteristic recurrence time is observed. Under these load conditions, we find that shear failure may be instantaneously triggered by a brief dynamical wave if the system is in a critical shear-stress state, near failure. Dynamic triggering is only observed when the dynamic wave amplitude exceeds strains of 10^(-7). Following triggering, the gouge material remains in an unstable state for long periods of time as manifest by unique slip characteristics not observed during spontaneous events: the measured physical characteristics—the gouge material strength recovery, the gouge layer thickness, the gouge shear modulus and the stick-slip recurrence time recover over many stick-slip cycles following triggering. This work suggests that faults must be critically stressed to trigger under dynamic conditions and that the recovery process following a dynamically triggered event differs from the recovery following a spontaneous event.

  13. Additional shear resistance from fault roughness and stress levels on geometrically complex faults

    NASA Astrophysics Data System (ADS)

    Fang, Zijun; Dunham, Eric M.

    2013-07-01

    The majority of crustal faults host earthquakes when the ratio of average background shear stress τb to effective normal stress σeff is τb/σeff≈0.6. In contrast, mature plate-boundary faults like the San Andreas Fault (SAF) operate at τb/σeff≈0.2. Dynamic weakening, the dramatic reduction in frictional resistance at coseismic slip velocities that is commonly observed in laboratory experiments, provides a leading explanation for low stress levels on mature faults. Strongly velocity-weakening friction laws permit rupture propagation on flat faults above a critical stress level τpulse/σeff≈0.25. Provided that dynamic weakening is not restricted to mature faults, the higher stress levels on most faults are puzzling. In this work, we present a self-consistent explanation for the relatively high stress levels on immature faults that is compatible with low coseismic frictional resistance, from dynamic weakening, for all faults. We appeal to differences in structural complexity with the premise that geometric irregularities introduce resistance to slip in addition to frictional resistance. This general idea is quantified for the special case of self-similar fractal roughness of the fault surface. Natural faults have roughness characterized by amplitude-to-wavelength ratios α between 10-3 and 10-2. Through a second-order boundary perturbation analysis of quasi-static frictionless sliding across a band-limited self-similar interface in an ideally elastic solid, we demonstrate that roughness induces an additional shear resistance to slip, or roughness drag, given by τdrag=8π3α2G∗Δ/λmin, for G∗=G/(1-ν) with shear modulus Gand Poisson's ratio ν, slip Δ, and minimum roughness wavelength λmin. The influence of roughness drag on fault mechanics is verified through an extensive set of dynamic rupture simulations of earthquakes on strongly rate-weakening fractal faults with elastic-plastic off-fault response. The simulations suggest that fault rupture, in

  14. Shear Stress Behavior in Mesoscale Simulations of Granular Materials

    NASA Astrophysics Data System (ADS)

    Fujino, Don; Lomov, Ilya; Vitali, Efrem; Antoun, Tarabay

    2011-06-01

    3D mesoscale simulations of shock propagation in porous solids and powder have been performed with the hydrocode GEODYN. The results indicate that voids can have a profound effect on the stress state in the material behind the shock front. The simulations can explain experimentally observed quasielastic precursors in reshock profiles that are difficult to interpret in the context of the classical elastic-plastic theory. This effect persists even at extremely low porosity values, down to 0.01% by volume. Stress relaxation is pronounced in simulations involving wave propagation, but is not observed in uniform ramp loading. Thus this relaxation mechanism is non-local in nature and continuum models may not be inadequate for its description. Simulations show that response of highly porous powders are dominated by deviatoric stress relaxation in the shock regime. We propose an enhancement which can be easily integrated into most existing porous material continuum models for modeling the shock-induced relaxation phenomena observed in the mesoscale simulation. The model calculates microkinetic energy generated by dynamic loading and store it as an internal state variable. The rate of production and dissipation of microkinetic energy is calibrated based on the mesoscale results. The augmented continuum model represent deviatoric stress behavior observed under different loading regimes.

  15. The production of turbulent stress in a shear flow by irrotational fluctuations

    NASA Technical Reports Server (NTRS)

    Gartshore, I. S.; Durbin, P. A.; Hunt, J. C. R.

    1983-01-01

    Attention is given to the way in which external turbulence affects an initially turbulence-free region in which there is a mean velocity gradient. External turbulence induces irrotational fluctuations in the sheared region which interact with the shear to produce rotational velocity fluctuations and mean Reynolds stresses. Since the actual front between the initial external turbulence and the shear flow is a randomly contorted surface, the turbulence near the front is intermittent, and is presently included in the form of a simple statistical model. In wind tunnel tests, turbulent shear stress was found to grow from zero to significant values in the interaction region. Observed stress magnitude and extent agrees with predictions, and it is concluded that turbulent stresses can be produced by irrotational fluctuations in a region of mean shear.

  16. Sediment transport and shear stress partitioning in a vegetated flow

    NASA Astrophysics Data System (ADS)

    Le Bouteiller, Caroline; Venditti, J. G.

    2015-04-01

    Vegetation is a common feature in natural coastal and riverine water ways, interacting with both the water flow and sediment transport. However, the physical processes governing these interactions are still poorly understood, which makes it difficult to predict sediment transport and morphodynamics in a vegetated environment. We performed a simple experiment to study how sediment transport responds to the presence of flexible, single-blade vegetation, and how this response is influenced by the vegetation density. We found that the skin friction and sediment transport are reduced in a plant patch, and that this effect is larger for denser vegetation. We then evaluated several methods to calculate the skin friction in a vegetated flow, which is the key to sediment transport prediction. Among these, the inversion of bed load transport formulas and the Einstein and Banks (1950) methods appeared to produce the most reasonable values of the skin friction. Finally, we suggest using the parameter α, which is the ratio of the skin friction computed by these methods to the total bed shear stress, to make more realistic sediment transport predictions in morphodynamic models.

  17. Dynamic shear-influenced collagen self-assembly.

    PubMed

    Saeidi, Nima; Sander, Edward A; Ruberti, Jeffrey W

    2009-12-01

    The ability to influence the direction of polymerization of a self-assembling biomolecular system has the potential to generate materials with extremely high anisotropy. In biological systems where highly-oriented cellular populations give rise to aligned and often load-bearing tissue such organized molecular scaffolds could aid in the contact guidance of cells for engineered tissue constructs (e.g. cornea and tendon). In this investigation we examine the detailed dynamics of pepsin-extracted type I bovine collagen assembly on a glass surface under the influence of flow between two plates. Differential Interference Contrast (DIC) imaging (60x-1.4NA) with focal plane stabilization was used to resolve and track the growth of collagen aggregates on borosilicate glass for 4 different shear rates (500, 80, 20, and 9s(-1)). The detailed morphology of the collagen fibrils/aggregates was examined using Quick Freeze Deep Etch (QFDE) electron microscopy. Nucleation of fibrils on the glass was observed to occur rapidly (approximately 2 min) followed by continued growth of the fibrils. The growth rates were dependent on flow in a complex manner with the highest rate of axial growth (0.1 micro/s) occurring at a shear rate of 9s(-1). The lowest growth rate occurred at the highest shear. Fibrils were observed to both branch and join during the experiments. The best alignment of fibrils was observed at intermediate shear rates of 20 and 80s(-1). However, the investigation revealed that fibril directional growth was not stable. At high shear rates, fibrils would often turn downstream forming what we term "hooks" which are likely the combined result of monomer interaction with the initial collagen layer or "mat" and the high shear rate. Further, QFDE examination of fibril morphology demonstrated that the assembled fibrillar structure did not possess native D-periodicity. Instead, fibrils comprised a collection of generally aligned, monomers which were self-assembled to form a fibril

  18. Nonlinear dynamics and anisotropic structure of rotating sheared turbulence.

    PubMed

    Salhi, A; Jacobitz, F G; Schneider, K; Cambon, C

    2014-01-01

    Homogeneous turbulence in rotating shear flows is studied by means of pseudospectral direct numerical simulation and analytical spectral linear theory (SLT). The ratio of the Coriolis parameter to shear rate is varied over a wide range by changing the rotation strength, while a constant moderate shear rate is used to enable significant contributions to the nonlinear interscale energy transfer and to the nonlinear intercomponental redistribution terms. In the destabilized and neutral cases, in the sense of kinetic energy evolution, nonlinearity cannot saturate the growth of the largest scales. It permits the smallest scale to stabilize by a scale-by-scale quasibalance between the nonlinear energy transfer and the dissipation spectrum. In the stabilized cases, the role of rotation is mainly nonlinear, and interacting inertial waves can affect almost all scales as in purely rotating flows. In order to isolate the nonlinear effect of rotation, the two-dimensional manifold with vanishing spanwise wave number is revisited and both two-component spectra and single-point two-dimensional energy components exhibit an important effect of rotation, whereas the SLT as well as the purely two-dimensional nonlinear analysis are unaffected by rotation as stated by the Proudman theorem. The other two-dimensional manifold with vanishing streamwise wave number is analyzed with similar tools because it is essential for any shear flow. Finally, the spectral approach is used to disentangle, in an analytical way, the linear and nonlinear terms in the dynamical equations. PMID:24580333

  19. Rac1 and Cdc42 GTPases regulate shear stress-driven β-catenin signaling in osteoblasts

    SciTech Connect

    Wan, Qiaoqiao; Cho, Eunhye; Yokota, Hiroki; Na, Sungsoo

    2013-04-19

    Highlights: •Shear stress increased TCF/LEF activity and stimulated β-catenin nuclear localization. •Rac1, Cdc42, and RhoA displayed distinct dynamic activity patterns under flow. •Rac1 and Cdc42, but not RhoA, regulate shear stress-driven TCF/LEF activation. •Cytoskeleton did not significantly affect shear stress-induced TCF/LEF activation. -- Abstract: Beta-catenin-dependent TCF/LEF (T-cell factor/lymphocyte enhancing factor) is known to be mechanosensitive and an important regulator for promoting bone formation. However, the functional connection between TCF/LEF activity and Rho family GTPases is not well understood in osteoblasts. Herein we investigated the molecular mechanisms underlying oscillatory shear stress-induced TCF/LEF activity in MC3T3-E1 osteoblast cells using live cell imaging. We employed fluorescence resonance energy transfer (FRET)-based and green fluorescent protein (GFP)-based biosensors, which allowed us to monitor signal transduction in living cells in real time. Oscillatory (1 Hz) shear stress (10 dynes/cm{sup 2}) increased TCF/LEF activity and stimulated translocation of β-catenin to the nucleus with the distinct activity patterns of Rac1 and Cdc42. The shear stress-induced TCF/LEF activity was blocked by the inhibition of Rac1 and Cdc42 with their dominant negative mutants or selective drugs, but not by a dominant negative mutant of RhoA. In contrast, constitutively active Rac1 and Cdc42 mutants caused a significant enhancement of TCF/LEF activity. Moreover, activation of Rac1 and Cdc42 increased the basal level of TCF/LEF activity, while their inhibition decreased the basal level. Interestingly, disruption of cytoskeletal structures or inhibition of myosin activity did not significantly affect shear stress-induced TCF/LEF activity. Although Rac1 is reported to be involved in β-catenin in cancer cells, the involvement of Cdc42 in β-catenin signaling in osteoblasts has not been identified. Our findings in this study demonstrate

  20. Dynamics of a polyelectrolyte in simple shear flow.

    PubMed

    Jayasree, Kandiledath; Manna, Raj Kumar; Banerjee, Debapriya; Kumar, P B Sunil

    2013-12-14

    The configurational dynamics of a polyelectrolyte (PE), subjected to a simple shear flow, is studied using Brownian dynamics (BD) and Dissipative Particle Dynamics (DPD) simulations of a bead-spring model with explicit counterions. We explore the effect of counterion condensation on the tumbling and extension of PEs by varying the shear rates for a range of values of the electrostatic coupling parameter A (which is defined as the ratio of the Bjerrum length to the size of the monomer). In all cases, the power spectrum of Rs(t) (which characterizes the projected length of the PE in the flow direction as a function of time) exhibits a power law decay at high frequencies, similar to that for a dumbbell in shear flow. For lower values of A (A ~ 2), the tumbling of the PE is periodic and is always associated with folding and stretching, which is in contrast to the oscillatory transition between the extended and globular states seen at higher values of A (A ~ 15). We observe that for A ~ 2 the tumbling frequency decreases and the average tumbling time increases with hydrodynamic interaction (HI). For A > 15, we observe a critical shear rate γ[combining dot]c below which there is considerable counterion condensation and the PE remains in the globular state with a structure akin to that of a neutral polymer in poor solvent. The γ[combining dot]c and the behavior of the PE above the critical shear rate are dependent on the HI. For a given shear rate, when there is considerable condensed counterion fluctuation, the PE extends as a whole and then collapses by the formation of folds with no observable periodicity in tumbling. When the condensed counterion fluctuations are suppressed, the polymer exhibits periodic tumbling. Simulation artifacts resulting from the implicit nature of the solvent and that due to boundary conditions are discussed by comparing the BD results with that obtained from the DPD simulations incorporating Ewald summation for electrostatics. PMID:24329088

  1. Shear stress activation of SREBP1 in endothelial cells is mediated by integrins.

    PubMed

    Liu, Yi; Chen, Benjamin P-C; Lu, Min; Zhu, Yi; Stemerman, Michael B; Chien, Shu; Shyy, John Y-J

    2002-01-01

    We investigated the effect of shear stress on the sterol regulatory element-binding protein 1 (SREBP1) in vascular endothelial cells (ECs) and the mechanotransduction mechanism involved. Application of a shear stress (12 dyn/cm(2)) caused the proteolytic cleavage of SREBP1 and the ensuing translocation of its transcription factor domain into the nucleus. As a result, shear stress increased the mRNAs encoding the low density lipoprotein receptor (LDLR), as well as the binding of (125)I-LDL. Using a step flow channel, we showed that SREBP1 activation in ECs under laminar flow is transient, but disturbed flow causes sustained activation. In studying the shear stress-elicited molecular signaling that activates SREBP1, we found that blocking the beta(1)-integrin with the AIIB2 blocking-type monoclonal antibody inhibited SREBP1 activation induced by shear stress. EC attachment to fibronectin or the activation of beta(1)-integrin in the suspended ECs by the TS2/16 monoclonal antibody was sufficient for SREBP1 activation. Furthermore, transient transfection assays showed that dominant-negative mutants of focal adhesion kinase and c-Src attenuated the shear stress-increased LDLR promoter activity. These results demonstrate that integrin signaling plays a critical role in the modulation of SREBP in ECs in response to shear stress. PMID:11788464

  2. Shear stress-induced NO production is dependent on ATP autocrine signaling and capacitative calcium entry

    PubMed Central

    Andrews, Allison M.; Jaron, Dov; Buerk, Donald G.; Barbee, Kenneth A.

    2014-01-01

    Flow-induced production of nitric oxide (NO) by endothelial cells plays a fundamental role in vascular homeostasis. However, the mechanisms by which shear stress activates NO production remain unclear due in part to limitations in measuring NO, especially under flow conditions. Shear stress elicits the release of ATP, but the relative contribution of autocrine stimulation by ATP to flow-induced NO production has not been established. Furthermore, the importance of calcium in shear stress-induced NO production remains controversial, and in particular the role of capacitive calcium entry (CCE) has yet to be determined. We have utilized our unique NO measurement device to investigate the role of ATP autocrine signaling and CCE in shear stress-induced NO production. We found that endogenously released ATP and downstream activation of purinergic receptors and CCE plays a significant role in shear stress-induced NO production. ATP-induced eNOS phophorylation under static conditions is also dependent on CCE. Inhibition of protein kinase C significantly inhibited eNOS phosphorylation and the calcium response. To our knowledge, we are the first to report on the role of CCE in the mechanism of acute shear stress-induced NO response. In addition, our work highlights the importance of ATP autocrine signaling in shear stress-induced NO production. PMID:25386222

  3. Properties of the shear stress peak radiated ahead of rapidly accelerating rupture fronts that mediate frictional slip

    PubMed Central

    Svetlizky, Ilya; Pino Muñoz, Daniel; Radiguet, Mathilde; Kammer, David S.; Molinari, Jean-François; Fineberg, Jay

    2016-01-01

    We study rapidly accelerating rupture fronts at the onset of frictional motion by performing high-temporal-resolution measurements of both the real contact area and the strain fields surrounding the propagating rupture tip. We observe large-amplitude and localized shear stress peaks that precede rupture fronts and propagate at the shear-wave speed. These localized stress waves, which retain a well-defined form, are initiated during the rapid rupture acceleration phase. They transport considerable energy and are capable of nucleating a secondary supershear rupture. The amplitude of these localized waves roughly scales with the dynamic stress drop and does not decrease as long as the rupture front driving it continues to propagate. Only upon rupture arrest does decay initiate, although the stress wave both continues to propagate and retains its characteristic form. These experimental results are qualitatively described by a self-similar model: a simplified analytical solution of a suddenly expanding shear crack. Quantitative agreement with experiment is provided by realistic finite-element simulations that demonstrate that the radiated stress waves are strongly focused in the direction of the rupture front propagation and describe both their amplitude growth and spatial scaling. Our results demonstrate the extensive applicability of brittle fracture theory to fundamental understanding of friction. Implications for earthquake dynamics are discussed. PMID:26729877

  4. Properties of the shear stress peak radiated ahead of rapidly accelerating rupture fronts that mediate frictional slip.

    PubMed

    Svetlizky, Ilya; Pino Muñoz, Daniel; Radiguet, Mathilde; Kammer, David S; Molinari, Jean-François; Fineberg, Jay

    2016-01-19

    We study rapidly accelerating rupture fronts at the onset of frictional motion by performing high-temporal-resolution measurements of both the real contact area and the strain fields surrounding the propagating rupture tip. We observe large-amplitude and localized shear stress peaks that precede rupture fronts and propagate at the shear-wave speed. These localized stress waves, which retain a well-defined form, are initiated during the rapid rupture acceleration phase. They transport considerable energy and are capable of nucleating a secondary supershear rupture. The amplitude of these localized waves roughly scales with the dynamic stress drop and does not decrease as long as the rupture front driving it continues to propagate. Only upon rupture arrest does decay initiate, although the stress wave both continues to propagate and retains its characteristic form. These experimental results are qualitatively described by a self-similar model: a simplified analytical solution of a suddenly expanding shear crack. Quantitative agreement with experiment is provided by realistic finite-element simulations that demonstrate that the radiated stress waves are strongly focused in the direction of the rupture front propagation and describe both their amplitude growth and spatial scaling. Our results demonstrate the extensive applicability of brittle fracture theory to fundamental understanding of friction. Implications for earthquake dynamics are discussed. PMID:26729877

  5. Flow visualization and wall shear stress of a flapping model hummingbird wing

    NASA Astrophysics Data System (ADS)

    Swanton, Erik W. M.; Vanier, Blake A.; Mohseni, Kamran

    2010-09-01

    The unsteady low Reynolds number aerodynamics of flapping flight was investigated experimentally through flow visualization by suspended particle imagery and wall shear stress measurement from micro-array hot-film anemometry. In conjunction, a mechanism was developed to create a flapping motion with three degrees of freedom and adjustable flapping frequency. The flapping kinematics and wing shape were selected for dynamic similarity to a hummingbird during hovering flight. Flow visualization was used to validate the anemometry observations of leading edge vortex (LEV) characteristics and to investigate the necessity of spanwise flow in LEV stability. The shear sensors determined LEV characteristics throughout the translation section of the stroke period for various wing speeds. It was observed that a minimum frequency between 2 and 3.5 Hz is required for the formation and stabilization of a LEV. The vortex strength peaked around 30% of the flapping cycle (corresponding to just past the translation midpoint), which agrees with results from previous studies conducted by others. The shear sensors also indicated a mild growth in LEV size during translation sections of the wing’s motion. This growth magnitude was nearly constant through a range of operating frequencies.

  6. Shear Stress induced Stretching of Red Blood Cells by Oscillating Bubbles within a Narrow Gap

    NASA Astrophysics Data System (ADS)

    Li, Fenfang; Mohammadzadeh, Milad; Ohl, Claus-Dieter; Claus-Dieter Ohl Team

    2013-11-01

    The flow pattern, especially the boundary layer caused by the expanding/contracting bubble in a narrow gap (15 μm) and the resultant stretching of red blood cells is investigated in this work. High speed recordings show that a red blood cell (biconcave shape, thickness of 1-2 μm) can be elongated to five times its original length by a laser-induced cavitation bubble within the narrow gap. However, flexible cancer cells in suspension (RKO, spherical shape, diameter of 10-15 μm) are hardly elongated under the same experimental condition. We hypothesize that the shear stress at the boundary layer is crucial for this elongation to occur. Therefore, in order to resolve the related fluid dynamics, we conducted numerical simulations using the finite element method (Fluent). The rapidly expanding/contracting vapor bubble is successfully modeled by employing viscosity and surface tension. The transient pressure inside the bubble and the velocity profile of the flow is obtained. We observe strong shear near the upper and lower boundary during the bubble oscillation. The flow fields are compared with analytical solutions to transient and pulsating flows in 2D. In the experiment the red blood cells sit within the lower boundary layer, thus are probably elongated by this strong shear flow. In contrast, the spherical cancer cells are of comparable size to the gap height so that they are lesser affected by this boundary layer flow.

  7. Dynamic mechanoelectrochemistry of polypyrrole membranes via shear-force tracking.

    PubMed

    Northcutt, Robert G; Heinemann, Christian; Sundaresan, Vishnu Baba

    2016-07-14

    Mechanoelectrochemistry is the study of elastic and plastic deformation of materials during reversible reduction and oxidation processes. In this article, we introduce shear-force tracking as a method to dynamically measure mechanical (strain), chemical (ion transport), and electrical (applied redox potentials) responses of the conducting polymer polypyrrole (PPy) during redox reactions. This tracking technique uses a control algorithm to maintain a set distance between a ultramicroelectrode (UME) tip and a surface via shear-force regulation. Due to the sensitivity of shear-force signals in the near field of substrate surfaces, a significantly improved signal to noise ratio (20 : 1) is possible and allows for nanoscale measurement of redox events. Chemomechanical coupling (the ratio of ion transport to resultant extensional actuation) is calculated for PPy-based membranes of various thicknesses based on a mechanistic interpretation of charge storage in redox active conducting polymers. The measured dynamic response demonstrates that chemomechanical coupling is not a constant, as assumed in literature, but is dependent on the polymers state of charge and the direction (ingress/egress) of ion transport. PMID:27263628

  8. Acute Shear Stress Direction Dictates Adherent Cell Remodeling and Verifies Shear Profile of Spinning Disc Assays

    PubMed Central

    Fuhrmann, Alexander; Engler, Adam J.

    2015-01-01

    Several methods have been developed to quantify population level changes in cell attachment strength given its large heterogeneity. One such method is the rotating disc chamber or “spinning disc” in which a range of shear forces are applied to attached cells to quantify detachment force, i.e. attachment strength, which can be heterogeneous within cell populations. However, computing the exact force vectors that act upon cells is complicated by complex flow fields and variable cell morphologies. Recent observations suggest that cells may remodel their morphology and align during acute shear exposure, but contrary to intuition, shear is not orthogonal to the radial direction. Here we theoretically derive the magnitude and direction of applied shear and demonstrate that cells, under certain physiological conditions, align in this direction within minutes. Shear force magnitude is also experimentally verified which validates that for spread cells shear forces and not torque or drag dominate in this assay, and demonstrates that the applied force per cell area is largely independent of initial morphology. These findings suggest that direct quantified comparison of the effects of shear on a wide array of cell types and conditions can be made with confidence using this assay without the need for computational or numerical modeling. PMID:25619322

  9. Dynamic thermal field-induced gradient soft-shear for highly oriented block copolymer thin films.

    PubMed

    Singh, Gurpreet; Yager, Kevin G; Berry, Brian; Kim, Ho-Cheol; Karim, Alamgir

    2012-11-27

    As demand for smaller, more powerful, and energy-efficient devices continues, conventional patterning technologies are pushing up against fundamental limits. Block copolymers (BCPs) are considered prime candidates for a potential solution via directed self-assembly of nanostructures. We introduce here a facile directed self-assembly method to rapidly fabricate unidirectionally aligned BCP nanopatterns at large scale, on rigid or flexible template-free substrates via a thermally induced dynamic gradient soft-shear field. A localized differential thermal expansion at the interface between a BCP film and a confining polydimethylsiloxane (PDMS) layer due to a dynamic thermal field imposes the gradient soft-shear field. PDMS undergoes directional expansion (along the annealing direction) in the heating zone and contracts back in the cooling zone, thus setting up a single cycle of oscillatory shear (maximum lateral shear stress ∼12 × 10(4) Pa) in the system. We successfully apply this process to create unidirectional alignment of BCP thin films over a wide range of thicknesses (nm to μm) and processing speeds (μm/s to mm/s) using both a flat and patterned PDMS layer. Grazing incidence small-angle X-ray scattering measurements show absolutely no sign of isotropic population and reveal ≥99% aligned orientational order with an angular spread Δθ(fwhm) ≤ 5° (full width at half-maximum). This method may pave the way to practical industrial use of hierarchically patterned BCP nanostructures. PMID:23106286

  10. Energy consumption in terms of shear stress for two types of membrane bioreactors used for municipal wastewater treatment processes

    NASA Astrophysics Data System (ADS)

    Ratkovich, Nicolas; Bentzen, Thomas R.; Rasmussen, Michael R.

    2012-10-01

    Two types of submerged membrane bioreactors (MBR): hollow fiber (HF) and hollow sheet (HS), have been studied and compared in terms of energy consumption and average shear stress over the membrane wall. The analysis of energy consumption was made using the correlation to determine the blower power and the blower power demand per unit of permeate volume. Results showed that for the system geometries considered, in terms the of the blower power, the HF MBR requires less power compared to HS MBR. However, in terms of blower power per unit of permeate volume, the HS MBR requires less energy. The analysis of shear stress over the membrane surface was made using computational fluid dynamics (CFD) modelling. Experimental measurements for the HF MBR were compared with the CFD model and an error less that 8% was obtained. For the HS MBR, experimental measurements of velocity profiles were made and an error of 11% was found. This work uses an empirical relationship to determine the shear stress based on the ratio of aeration blower power to tank volume. This relationship is used in bubble column reactors and it is extrapolate to determine shear stress on MBR systems. This relationship proved to be overestimated by 28% compared to experimental measurements and CFD results. Therefore, a corrective factor is included in the relationship in order to account for the membrane placed inside the bioreactor.

  11. Shear stress induced reorganization of the keratin intermediate filament network requires phosphorylation by protein kinase C zeta.

    PubMed

    Sivaramakrishnan, Sivaraj; Schneider, Jaime L; Sitikov, Albert; Goldman, Robert D; Ridge, Karen M

    2009-06-01

    Keratin intermediate filaments (KIFs) form a fibrous polymer network that helps epithelial cells withstand external mechanical forces. Recently, we established a correlation between the structure of the KIF network and its local mechanical properties in alveolar epithelial cells. Shear stress applied across the cell surface resulted in the structural remodeling of KIF and a substantial increase in the elastic modulus of the network. This study examines the mechanosignaling that regulates the structural remodeling of the KIF network. We report that the shear stress-mediated remodeling of the KIF network is facilitated by a twofold increase in the dynamic exchange rate of KIF subunits, which is regulated in a PKC zeta and 14-3-3-dependent manner. PKC zeta phosphorylates K18pSer33, and this is required for the structural reorganization because the KIF network in A549 cells transfected with a dominant negative PKC zeta, or expressing the K18Ser33Ala mutation, is unchanged. Blocking the shear stress-mediated reorganization results in reduced cellular viability and increased apoptotic levels. These data suggest that shear stress mediates the phosphorylation of K18pSer33, which is required for the reorganization of the KIF network, resulting in changes in mechanical properties of the cell that help maintain the integrity of alveolar epithelial cells. PMID:19357195

  12. Shear stress induced by a gas bubble pulsating in an ultrasonic field near a wall.

    PubMed

    Krasovitski, Boris; Kimmel, Eitan

    2004-08-01

    Some of the effects that therapeutic ultrasound has in medicine and biology may be associated with steady oscillations of gas bubbles in liquid, very close to tissue surface. The bubble oscillations induce on the surface steady shear stress attributed to microstreaming. A mathematical simulation of the problem for both free and capsulated bubbles, known as contrast agents, is presented here. The simulation is based on a solution of Laplace's equation for potential flow and existing models for microstreaming. The solution for potential flow was obtained numerically using a boundary integral method. The solution provides the evolution of the bubble shape, the distribution of the velocity potential on the surface, and the shear stress along the surface. The simulation shows that significant shear stresses develop on the surface when the bubble bounces near the tissue surface. In this case, pressure amplitude of 20 kPa generates maximal steady shear stress of several kilo Pascal. Substantial shear stress on the tissue surface takes place inside a circular zone with a radius about half of the bubble radius. The predicted shear stress is greater than stress that causes hemolysis in blood and several orders of magnitude greater than the physiological stress induced on the vessel wall by the flowing blood. PMID:15344403

  13. Dynamics of model blood cells in shear flow

    NASA Astrophysics Data System (ADS)

    Podgorski, Thomas; Callens, Natacha; Minetti, Christophe; Coupier, Gwennou; Dubois, Frank; Misbah, Chaouqi

    The dynamics of a vesicle suspension in shear flow was investigated by digital holographic microscopy [1] in parabolic flights and in the MASER 11 sounding rocket. Vesicles are lipid membranes which mimic the mechanical behaviour of cells, such as red blood cells in flow. In a simple shear flow between parallel walls, a lift force of purely viscous origin pushes vesicles away from walls. Our parabolic flight experiments [2] reveal that the lift velocity in a dilute suspen-sion is well described by theoretical predictions by Olla. As vesicles gather near the center of the flow chamber due to lift forces from both walls, one expects hydrodynamic interactions of pairs of vesicles to result in shear induced diffusion in the suspension. The BIOMICS experi-ment in the MASER 11 sounding rocket revealed a complex spatial structure of a polydisperse vesicle suspension due to the interplay between lift forces from the walls and hydrodynamic interactions. These phenomena have a strong impact on the structure and rheology of blood in small vessels, and a precise knowledge of the dynamics of migration and diffusion of soft particles in flow can lead to alternative ways to separate and sort blood cells. 1. Dubois, F., Schockaert, C., Callens, N., Yourrassowsky, C., "Focus plane detection criteria in digital holography microscopy by amplitude analysis", Opt. Express, Vol. 14, pp 5895-5908, 2006 2. Callens, N., Minetti, C., Coupier, G., Mader, M.-A., Dubois, F., Misbah, C., Podgorski, T., "Hydrodynamics lift of vesicles under shear flow in microgravity", Europhys. Lett., Vol. 83, p. 24002, 2008

  14. Multiscale approach to link red blood cell dynamics, shear viscosity, and ATP release

    PubMed Central

    Forsyth, Alison M.; Wan, Jiandi; Owrutsky, Philip D.; Abkarian, Manouk; Stone, Howard A.

    2011-01-01

    RBCs are known to release ATP, which acts as a signaling molecule to cause dilation of blood vessels. A reduction in the release of ATP from RBCs has been linked to diseases such as type II diabetes and cystic fibrosis. Furthermore, reduced deformation of RBCs has been correlated with myocardial infarction and coronary heart disease. Because ATP release has been linked to cell deformation, we undertook a multiscale approach to understand the links between single RBC dynamics, ATP release, and macroscopic viscosity all at physiological shear rates. Our experimental approach included microfluidics, ATP measurements using a bioluminescent reaction, and rheology. Using microfluidics technology with high-speed imaging, we visualize the deformation and dynamics of single cells, which are known to undergo motions such as tumbling, swinging, tanktreading, and deformation. We report that shear thinning is not due to cellular deformation as previously believed, but rather it is due to the tumbling-to-tanktreading transition. In addition, our results indicate that ATP release is constant at shear stresses below a threshold (3 Pa), whereas above the threshold ATP release is increased and accompanied by large cellular deformations. Finally, performing experiments with well-known inhibitors, we show that the Pannexin 1 hemichannel is the main avenue for ATP release both above and below the threshold, whereas, the cystic fibrosis transmembrane conductance regulator only contributes to deformation-dependent ATP release above the stress threshold. PMID:21690355

  15. Real-Time Intravascular Shear Stress in the Rabbit Abdominal Aorta

    PubMed Central

    Ai, Lisong; Yu, Hongyu; Dai, Wangde; Hale, Sharon L.; Kloner, Robert A.

    2012-01-01

    Fluid shear stress is intimately linked with the biological activities of vascular cells. A flexible microelectromechanical system (MEMS) sensor was developed to assess spatial- and temporal-varying components of intravascular shear stress (ISS) in the abdominal aorta of adult New Zealand white (NZW) rabbits. Real-time ISS (ISSreal-time) was analyzed in comparison with computational fluid dynamics (CFD) simulations for wall shear stress (WSS). Three-dimensional abdominal arterial geometry and mesh were created using the GAMBIT software. Simulation of arterial flow profiles was established by FLUENT. The Navier–Stokes equations were solved for non-Newtonian blood flow. The coaxial-wire-based MEMS sensor was deployed into the abdominal arteries of rabbits via a femoral artery cutdown. Based on the CFD analysis, the entrance length of the sensor on the coaxial wire (0.4 mm in diameter) was less than 10 mm. Three-dimensional fluoroscope and contrast dye allowed for visualization of the positions of the sensor and ratios of vessel to coaxial wire diameters. Doppler ultrasound provided the velocity profiles for the CFD boundary conditions. If the coaxial wire were positioned at the center of vessel, the CFD analysis revealed a mean ISS value of 31.1 with a systolic peak at 102.8 dyn · cm−2. The mean WSS was computed to be 10.1 dyn · cm−2 with a systolic peak at 33.2 dyn · cm−2, and the introduction of coaxial wire increased the mean WSS by 5.4 dyn · cm−2 and systolic peak by 18.0 dyn · cm−2. Experimentally, the mean ISS was 11.9 dyn · cm−2 with a systolic peak at 47.0 dyn · cm−2. The waveform of experimental ISS was similar to that of CFD solution with a 30.2% difference in mean and 8.9% in peak systolic shear stress. Despite the difference between CD and experimental results, the flexible coaxial-wire-based MEMS sensors provided a possibility to assess real-time ISS in the abdominal aorta of NZW rabbits. PMID:19527952

  16. Interlaminar shear stress effects on the postbuckling response of graphite-epoxy panels

    NASA Technical Reports Server (NTRS)

    Engelstad, S. P.; Reddy, J. N.; Knight, N. F., Jr.

    1990-01-01

    The objectives of the study are to assess the influence of shear flexibility on overall postbuckling response, and to examine transverse shear stress distributions in relation to panel failure. Nonlinear postbuckling results are obtained for finite element models based on classical laminated plate theory and first-order shear deformation theory. Good correlation between test and analysis is obtained. The results presented in this paper analytically substantiate the experimentally observed failure mode.

  17. Method for measuring surface shear stress magnitude and direction using liquid crystal coatings

    NASA Technical Reports Server (NTRS)

    Reda, Daniel C. (Inventor)

    1995-01-01

    A method is provided for determining surface shear magnitude and direction at every point on a surface. The surface is covered with a shear stress sensitive liquid crystal coating and illuminated by white light from a normal direction. A video camera is positioned at an oblique angle above the surface to observe the color of the liquid crystal at that angle. The shear magnitude and direction are derived from the color information. A method of calibrating the device is also provided.

  18. Effect of Wall Shear Stress on Corrosion Inhibitor Film Performance

    NASA Astrophysics Data System (ADS)

    Canto Maya, Christian M.

    In oil and gas production, internal corrosion of pipelines causes the highest incidence of recurring failures. Ensuring the integrity of ageing pipeline infrastructure is an increasingly important requirement. One of the most widely applied methods to reduce internal corrosion rates is the continuous injection of chemicals in very small quantities, called corrosion inhibitors. These chemical substances form thin films at the pipeline internal surface that reduce the magnitude of the cathodic and/or anodic reactions. However, the efficacy of such corrosion inhibitor films can be reduced by different factors such as multiphase flow, due to enhanced shear stress and mass transfer effects, loss of inhibitor due to adsorption on other interfaces such as solid particles, bubbles and droplets entrained by the bulk phase, and due to chemical interaction with other incompatible substances present in the stream. The first part of the present project investigated the electrochemical behavior of two organic corrosion inhibitors (a TOFA/DETA imidazolinium, and an alkylbenzyl dimethyl ammonium chloride), with and without an inorganic salt (sodium thiosulfate), and the resulting enhancement. The second part of the work explored the performance of corrosion inhibitor under multiphase (gas/liquid, solid/liquid) flow. The effect of gas/liquid multiphase flow was investigated using small and large scale apparatus. The small scale tests were conducted using a glass cell and a submersed jet impingement attachment with three different hydrodynamic patterns (water jet, CO 2 bubbles impact, and water vapor cavitation). The large scale experiments were conducted applying different flow loops (hilly terrain and standing slug systems). Measurements of weight loss, linear polarization resistance (LPR), and adsorption mass (using an electrochemical quartz crystal microbalance, EQCM) were used to quantify the effect of wall shear stress on the performance and integrity of corrosion inhibitor

  19. On the Bauschinger effect in supercooled melts under shear: Results from mode coupling theory and molecular dynamics simulations

    NASA Astrophysics Data System (ADS)

    Frahsa, Fabian; Bhattacharjee, Amit Kumar; Horbach, Jürgen; Fuchs, Matthias; Voigtmann, Thomas

    2013-03-01

    We study the nonlinear rheology of a glass-forming binary mixture under the reversal of shear flow using molecular dynamics simulations and a schematic model of the mode-coupling theory of the glass transition (MCT). Memory effects lead to a history-dependent response, as exemplified by the vanishing of a stress-overshoot phenomenon in the stress-strain curves of the sheared liquid, and a change in the apparent elastic coefficients around states with zero stress. We investigate the various retarded contributions to the stress response at a given time schematically within MCT. The connection of this macroscopic response to single-particle motion is demonstrated using molecular-dynamics simulation.

  20. On the Bauschinger effect in supercooled melts under shear: results from mode coupling theory and molecular dynamics simulations.

    PubMed

    Frahsa, Fabian; Bhattacharjee, Amit Kumar; Horbach, Jürgen; Fuchs, Matthias; Voigtmann, Thomas

    2013-03-28

    We study the nonlinear rheology of a glass-forming binary mixture under the reversal of shear flow using molecular dynamics simulations and a schematic model of the mode-coupling theory of the glass transition (MCT). Memory effects lead to a history-dependent response, as exemplified by the vanishing of a stress-overshoot phenomenon in the stress-strain curves of the sheared liquid, and a change in the apparent elastic coefficients around states with zero stress. We investigate the various retarded contributions to the stress response at a given time schematically within MCT. The connection of this macroscopic response to single-particle motion is demonstrated using molecular-dynamics simulation. PMID:23556764

  1. Hemodynamic Shear Stress and Endothelial Dysfunction in Hemodialysis Access

    PubMed Central

    Fitts, Michelle K.; Pike, Daniel B.; Anderson, Kasey; Shiu, Yan-Ting

    2014-01-01

    Surgically-created blood conduits used for chronic hemodialysis, including native arteriovenous fistulas (AVFs) and synthetic AV grafts (AVGs), are the lifeline for kidney failure patients. Unfortunately, each has its own limitations: AVFs often fail to mature to become useful for dialysis and AVGs often fail due to stenosis as a result of neointimal hyperplasia, which preferentially forms at the graft-venous anastomosis. No clinical therapies are currently available to significantly promote AVF maturation or prevent neointimal hyperplasia in AVGs. Central to devising strategies to solve these problems is a complete mechanistic understanding of the pathophysiological processes. The pathology of arteriovenous access problems is likely multi-factorial. This review focuses on the roles of fluid-wall shear stress (WSS) and endothelial cells (ECs). In arteriovenous access, shunting of arterial blood flow directly into the vein drastically alters the hemodynamics in the vein. These hemodynamic changes are likely major contributors to non-maturation of an AVF vein and/or formation of neointimal hyperplasia at the venous anastomosis of an AVG. ECs separate blood from other vascular wall cells and also influence the phenotype of these other cells. In arteriovenous access, the responses of ECs to aberrant WSS may subsequently lead to AVF non-maturation and/or AVG stenosis. This review provides an overview of the methods for characterizing blood flow and calculating WSS in arteriovenous access and discusses EC responses to arteriovenous hemodynamics. This review also discusses the role of WSS in the pathology of arteriovenous access, as well as confounding factors that modulate the impact of WSS. PMID:25309636

  2. Time-Resolved Particle Image Velocimetry Measurements with Wall Shear Stress and Uncertainty Quantification for the FDA Nozzle Model.

    PubMed

    Raben, Jaime S; Hariharan, Prasanna; Robinson, Ronald; Malinauskas, Richard; Vlachos, Pavlos P

    2016-03-01

    We present advanced particle image velocimetry (PIV) processing, post-processing, and uncertainty estimation techniques to support the validation of computational fluid dynamics analyses of medical devices. This work is an extension of a previous FDA-sponsored multi-laboratory study, which used a medical device mimicking geometry referred to as the FDA benchmark nozzle model. Experimental measurements were performed using time-resolved PIV at five overlapping regions of the model for Reynolds numbers in the nozzle throat of 500, 2000, 5000, and 8000. Images included a twofold increase in spatial resolution in comparison to the previous study. Data was processed using ensemble correlation, dynamic range enhancement, and phase correlations to increase signal-to-noise ratios and measurement accuracy, and to resolve flow regions with large velocity ranges and gradients, which is typical of many blood-contacting medical devices. Parameters relevant to device safety, including shear stress at the wall and in bulk flow, were computed using radial basis functions. In addition, in-field spatially resolved pressure distributions, Reynolds stresses, and energy dissipation rates were computed from PIV measurements. Velocity measurement uncertainty was estimated directly from the PIV correlation plane, and uncertainty analysis for wall shear stress at each measurement location was performed using a Monte Carlo model. Local velocity uncertainty varied greatly and depended largely on local conditions such as particle seeding, velocity gradients, and particle displacements. Uncertainty in low velocity regions in the sudden expansion section of the nozzle was greatly reduced by over an order of magnitude when dynamic range enhancement was applied. Wall shear stress uncertainty was dominated by uncertainty contributions from velocity estimations, which were shown to account for 90-99% of the total uncertainty. This study provides advancements in the PIV processing methodologies over

  3. On the expected relationships among apparent stress, static stress drop, effective shear fracture energy, and efficiency

    USGS Publications Warehouse

    Beeler, N.M.; Wong, T.-F.; Hickman, S.H.

    2003-01-01

    We consider expected relationships between apparent stress ??a and static stress drop ????s using a standard energy balance and find ??a = ????s (0.5 - ??), where ?? is stress overshoot. A simple implementation of this balance is to assume overshoot is constant; then apparent stress should vary linearly with stress drop, consistent with spectral theories (Brune, 1970) and dynamic crack models (Madariaga, 1976). Normalizing this expression by the static stress drop defines an efficiency ??sw = ??sa/????s as follows from Savage and Wood (1971). We use this measure of efficiency to analyze data from one of a number of observational studies that find apparent stress to increase with seismic moment, namely earthquakes recorded in the Cajon Pass borehole by Abercrombie (1995). Increases in apparent stress with event size could reflect an increase in seismic efficiency; however, ??sw for the Cajon earthquakes shows no such increase and is approximately constant over the entire moment range. Thus, apparent stress and stress drop co-vary, as expected from the energy balance at constant overshoot. The median value of ??sw for the Cajon earthquakes is four times lower than ??sw for laboratory events. Thus, these Cajon-recorded earthquakes have relatively low and approximately constant efficiency. As the energy balance requires ??sw = 0.5 - ??, overshoot can be estimated directly from the Savage-Wood efficiency; overshoot is positive for Cajon Pass earthquakes. Variations in apparent stress with seismic moment for these earthquakes result primarily from systematic variations in static stress drop with seismic moment and do not require a relative decrease in sliding resistance with increasing event size (dynamic weakening). Based on the comparison of field and lab determinations of the Savage-Wood efficiency, we suggest the criterion ??sw > 0.3 as a test for dynamic weakening in excess of that seen in the lab.

  4. Evolution of the Reynolds shear stresses in highly accelerated turbulent boundary layers

    NASA Astrophysics Data System (ADS)

    Araya, Guillermo; Castillo, Luciano; Hussain, Fazle

    2014-11-01

    Turbulent boundary layers subjected to severe acceleration or strong Favorable Pressure Gradients (FPG) are of great fundamental and technological importance; examples of the latter include nozzle design, underwater bodies and drag reduction applications. Scientifically, they pose great interest from the point of view of scaling laws, the complex interaction between the outer and inner regions, and relaminarization phenomena. Direct Numerical Simulations (DNS) of highly accelerated turbulent boundary layers are performed by means of the Dynamic Multi-scale Approach (DMA) recently developed by [Araya et al. JFM 670, 581 (2011)]. It is shown that the Reynolds shear stress monotonically decreases and exhibits a logarithmic layer in the meso-layer region during the laminarization process. In addition, the local maxima of streamwise velocity fluctuations in wall units remain almost constant in the very strong FPG region, which prevents the flow to become completely laminar. Furthermore, the re-distribution of Reynolds shear stresses due to sweeps and ejections in the FPG region is performed and a physical mechanism is proposed.

  5. Liquid crystal coatings for surface shear stress visualization in hypersonic flows

    SciTech Connect

    Reda, D.C.; Aeschliman, D.P.

    1990-01-01

    Experiments were conducted to test the surface-shear-stress visualization capabilities of shear-stress-sensitive/temperature- insensitive liquid crystal compounds in hypersonic flow. Liquid crystal coatings were applied to the surface of a conical model, which was then exposed to a high-unit-Reynolds-number (2.3 {times} 10{sup 7}/m) Mach 5 flow. The coating was illuminated by white light, and its response to the various flow situations was monitored and recorded with standard video and high-speed movie cameras. Boundary layer transition to turbulence was clearly demarcated by the technique. The dynamic location of the transition front as a function of model angle of attack (for sharp and blunt cones, with and without boundary-layer trips) was recorded, and observations were found to be consistent with established (published) trends for hypersonic flows over conical bodies. Normal shock passage over the model during tunnel shutdown was recorded (at 400 frames/second), and the liquid crystal coating was observed to respond to this event in a time interval less than or equal to the time between sequential movie frame exposures ({le} 0.0025 seconds). The liquid crystal technique has thus been demonstrated as a viable diagnostic tool for use in transient/compressible flows. 18 refs., 3 figs.

  6. Visualization and Measurement of Surface Shear Stress Vector Distributions Using Liquid Crystal Coatings

    NASA Technical Reports Server (NTRS)

    Reda, Daniel C.; Wilder, Michael C.

    1998-01-01

    When a shear-sensitive liquid crystal coating is illuminated from the normal direction by white light and observed from an oblique above-plane view angle, its color-change response to shear depends on both shear stress vector magnitude and the direction of the applied shear vector relative to the observer's in-plane line of sight. At any point, the maximum color change is always seen or measured when the local shear vector is aligned with, and directed away from, the observer; the magnitude of the color change at this vector/observer aligned orientation scales directly with shear stress magnitude. Conversely, any point exposed to a shear vector with a component directed toward the observer exhibits a noncolor-change response, always characterized by a rusty red or brown color, independent of both shear magnitude and direction. Based on this knowledge, full-surface shear stress vector visualization and measurement methodologies were formulated and successfully demonstrated. The present paper reviews the observations and measurements that led to the development of these methodologies and applications of both are discussed.

  7. Steady shear rheometry of dissipative particle dynamics models of polymer fluids in reverse Poiseuille flow

    PubMed Central

    Fedosov, Dmitry A.; Karniadakis, George Em; Caswell, Bruce

    2010-01-01

    Polymer fluids are modeled with dissipative particle dynamics (DPD) as undiluted bead-spring chains and their solutions. The models are assessed by investigating their steady shear-rate properties. Non-Newtonian viscosity and normal stress coefficients, for shear rates from the lower to the upper Newtonian regimes, are calculated from both plane Couette and plane Poiseuille flows. The latter is realized as reverse Poiseuille flow (RPF) generated from two Poiseuille flows driven by uniform body forces in opposite directions along two-halves of a computational domain. Periodic boundary conditions ensure the RPF wall velocity to be zero without density fluctuations. In overlapping shear-rate regimes the RPF properties are confirmed to be in good agreement with those calculated from plane Couette flow with Lees–Edwards periodic boundary conditions (LECs), the standard virtual rheometer for steady shear-rate properties. The concentration and the temperature dependence of the properties of the model fluids are shown to satisfy the principles of concentration and temperature superposition commonly employed in the empirical correlation of real polymer-fluid properties. The thermodynamic validity of the equation of state is found to be a crucial factor for the achievement of time-temperature superposition. With these models, RPF is demonstrated to be an accurate and convenient virtual rheometer for the acquisition of steady shear-rate rheological properties. It complements, confirms, and extends the results obtained with the standard LEC configuration, and it can be used with the output from other particle-based methods, including molecular dynamics, Brownian dynamics, smooth particle hydrodynamics, and the lattice Boltzmann method. PMID:20405981

  8. Field measurements of the linear and nonlinear shear moduli of cemented alluvium using dynamically loaded surface footings

    NASA Astrophysics Data System (ADS)

    Park, Kwangsoo

    In this dissertation, a research effort aimed at development and implementation of a direct field test method to evaluate the linear and nonlinear shear modulus of soil is presented. The field method utilizes a surface footing that is dynamically loaded horizontally. The test procedure involves applying static and dynamic loads to the surface footing and measuring the soil response beneath the loaded area using embedded geophones. A wide range in dynamic loads under a constant static load permits measurements of linear and nonlinear shear wave propagation from which shear moduli and associated shearing strains are evaluated. Shear wave velocities in the linear and nonlinear strain ranges are calculated from time delays in waveforms monitored by geophone pairs. Shear moduli are then obtained using the shear wave velocities and the mass density of a soil. Shear strains are determined using particle displacements calculated from particle velocities measured at the geophones by assuming a linear variation between geophone pairs. The field test method was validated by conducting an initial field experiment at sandy site in Austin, Texas. Then, field experiments were performed on cemented alluvium, a complex, hard-to-sample material. Three separate locations at Yucca Mountain, Nevada were tested. The tests successfully measured: (1) the effect of confining pressure on shear and compression moduli in the linear strain range and (2) the effect of strain on shear moduli at various states of stress in the field. The field measurements were first compared with empirical relationships for uncemented gravel. This comparison showed that the alluvium was clearly cemented. The field measurements were then compared to other independent measurements including laboratory resonant column tests and field seismic tests using the spectral-analysis-of-surface-waves method. The results from the field tests were generally in good agreement with the other independent test results, indicating

  9. FLOCCULATION OF FINE-GRAINED LAKE SEDIMENTS DUE TO A UNIFORM SHEAR STRESS

    EPA Science Inventory

    Experiments were performed to investigate the effects of fluid shear on the flocculation of fine-grained lake sediments in fresh water. In these experiments, a Couette viscometer was used to apply a uniform shear stress to a sediment suspension. he sediments were from the Detroit...

  10. Frequency-dependent response of the vascular endothelium to pulsatile shear stress

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Most cells of the circulatory system are exposed to shear forces that occur at the frequency of the heartbeat. However, as a result of the complicated blood flow patterns that occur at arterial branches, small regions of the arterial wall experience fluctuations in shear stress that are dominated by...

  11. Role of fluid shear stress in regulating VWF structure, function and related blood disorders

    PubMed Central

    Gogia, Shobhit; Neelamegham, Sriram

    2015-01-01

    Von Willebrand factor (VWF) is the largest glycoprotein in blood. It plays a crucial role in primary hemostasis via its binding interaction with platelet and endothelial cell surface receptors, other blood proteins and extra-cellular matrix components. This protein is found as a series of repeat units that are disulfide bonded to form multimeric structures. Once in blood, the protein multimer distribution is dynamically regulated by fluid shear stress which has two opposing effects: it promotes the aggregation or self-association of multiple VWF units, and it simultaneously reduces multimer size by facilitating the force-dependent cleavage of the protein by various proteases, most notably ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type repeats, motif 1 type 13). In addition to these effects, fluid shear also controls the solution and substrate-immobilized structure of VWF, the nature of contact between blood platelets and substrates, and the biomechanics of the GpIbα–VWF bond. These features together regulate different physiological and pathological processes including normal hemostasis, arterial and venous thrombosis, von Willebrand disease, thrombotic thrombocytopenic purpura and acquired von Willebrand syndrome. This article discusses current knowledge of VWF structure–function relationships with emphasis on the effects of hydrodynamic shear, including rapid methods to estimate the nature and magnitude of these forces in selected conditions. It shows that observations made by many investigators using solution and substrate-based shearing devices can be reconciled upon considering the physical size of VWF and the applied mechanical force in these different geometries. PMID:26600266

  12. Role of fluid shear stress in regulating VWF structure, function and related blood disorders.

    PubMed

    Gogia, Shobhit; Neelamegham, Sriram

    2015-01-01

    Von Willebrand factor (VWF) is the largest glycoprotein in blood. It plays a crucial role in primary hemostasis via its binding interaction with platelet and endothelial cell surface receptors, other blood proteins and extra-cellular matrix components. This protein is found as a series of repeat units that are disulfide bonded to form multimeric structures. Once in blood, the protein multimer distribution is dynamically regulated by fluid shear stress which has two opposing effects: it promotes the aggregation or self-association of multiple VWF units, and it simultaneously reduces multimer size by facilitating the force-dependent cleavage of the protein by various proteases, most notably ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type repeats, motif 1 type 13). In addition to these effects, fluid shear also controls the solution and substrate-immobilized structure of VWF, the nature of contact between blood platelets and substrates, and the biomechanics of the GpIbα-VWF bond. These features together regulate different physiological and pathological processes including normal hemostasis, arterial and venous thrombosis, von Willebrand disease, thrombotic thrombocytopenic purpura and acquired von Willebrand syndrome. This article discusses current knowledge of VWF structure-function relationships with emphasis on the effects of hydrodynamic shear, including rapid methods to estimate the nature and magnitude of these forces in selected conditions. It shows that observations made by many investigators using solution and substrate-based shearing devices can be reconciled upon considering the physical size of VWF and the applied mechanical force in these different geometries. PMID:26600266

  13. A High shear stress segment along the San Andreas Fault: Inferences based on near-field stress direction and stress magnitude observations in the Carrizo Plain Area

    SciTech Connect

    Castillo, D. A.,; Younker, L.W.

    1997-01-30

    Nearly 200 new in-situ determinations of stress directions and stress magnitudes near the Carrizo plain segment of the San Andreas fault indicate a marked change in stress state occurring within 20 km of this principal transform plate boundary. A natural consequence of this stress transition is that if the observed near-field ``fault-oblique`` stress directions are representative of the fault stress state, the Mohr-Coulomb shear stresses resolved on San Andreas sub-parallel planes are substantially greater than previously inferred based on fault-normal compression. Although the directional stress data and near-hydrostatic pore pressures, which exist within 15 km of the fault, support a high shear stress environment near the fault, appealing to elevated pore pressures in the fault zone (Byerlee-Rice Model) merely enhances the likelihood of shear failure. These near-field stress observations raise important questions regarding what previous stress observations have actually been measuring. The ``fault-normal`` stress direction measured out to 70 km from the fault can be interpreted as representing a comparable depth average shear strength of the principal plate boundary. Stress measurements closer to the fault reflect a shallower depth-average representation of the fault zone shear strength. If this is true, only stress observations at fault distances comparable to the seismogenic depth will be representative of the fault zone shear strength. This is consistent with results from dislocation monitoring where there is pronounced shear stress accumulation out to 20 km of the fault as a result of aseismic slip within the lower crust loading the upper locked section. Beyond about 20 km, the shear stress resolved on San Andreas fault-parallel planes becomes negligible. 65 refs., 15 figs.

  14. Gyrokinetic simulation of momentum transport with residual stress from diamagnetic level velocity shears

    SciTech Connect

    Waltz, R. E.; Staebler, G. M.; Solomon, W. M.

    2011-04-15

    Residual stress refers to the remaining toroidal angular momentum (TAM) flux (divided by major radius) when the shear in the equilibrium fluid toroidal velocity (and the velocity itself) vanishes. Previously [Waltz et al., Phys. Plasmas 14, 122507 (2007); errata 16, 079902 (2009)], we demonstrated with GYRO [Candy and Waltz, J. Comp. Phys. 186, 545 (2003)] gyrokinetic simulations that TAM pinching from (ion pressure gradient supported or diamagnetic level) equilibrium ExB velocity shear could provide some of the residual stress needed to support spontaneous toroidal rotation against normal diffusive loss. Here we show that diamagnetic level shear in the intrinsic drift wave velocities (or ''profile shear'' in the ion and electron density and temperature gradients) provides a comparable residual stress. The individual signed contributions of these small (rho-star level) ExB and profile velocity shear rates to the turbulence level and (rho-star squared) ion energy transport stabilization are additive if the rates are of the same sign. However because of the additive stabilization effect, the contributions to the small (rho-star cubed) residual stress is not always simply additive. If the rates differ in sign, the residual stress from one can buck out that from the other (and in some cases reduce the stabilization.) The residual stress from these diamagnetic velocity shear rates is quantified by the ratio of TAM flow to ion energy (power) flow (M/P) in a global GYRO core simulation of a ''null'' toroidal rotation DIII-D [Mahdavi and Luxon, Fusion Sci. Technol. 48, 2 (2005)] discharge by matching M/P profiles within experimental uncertainty. Comparison of global GYRO (ion and electron energy as well as particle) transport flow balance simulations of TAM transport flow in a high-rotation DIII-D L-mode quantifies and isolates the ExB shear and parallel velocity (Coriolis force) pinching components from the larger ''diffusive'' parallel velocity shear driven component and

  15. Concurrent shear stress and chemical stimulation of mechano-sensitive cells by discontinuous dielectrophoresis.

    PubMed

    Soffe, Rebecca; Baratchi, Sara; Tang, Shi-Yang; Mitchell, Arnan; McIntyre, Peter; Khoshmanesh, Khashayar

    2016-03-01

    Microfluidic platforms enable a variety of physical or chemical stimulation of single or multiple cells to be examined and monitored in real-time. To date, intracellular calcium signalling research is, however, predominantly focused on observing the response of cells to a single mode of stimulation; consequently, the sensitising/desensitising of cell responses under concurrent stimuli is not well studied. In this paper, we provide an extended Discontinuous Dielectrophoresis procedure to investigate the sensitising of chemical stimulation, over an extensive range of shear stress, up to 63 dyn/cm(2), which encompasses shear stresses experienced in the arterial and venus systems (10 to 60 dyn/cm(2)). Furthermore, the TRPV4-selective agonist GSK1016790A, a form of chemical stimulation, did not influence the ability of the cells' to remain immobilised under high levels of shear stress; thus, enabling us to investigate shear stress stimulation on agonism. Our experiments revealed that shear stress sensitises GSK1016790A-evoked intracellular calcium signalling of cells in a shear-stimulus dependent manner, as observed through a reduction in the cellular response time and an increase in the pharmacological efficacy. Consequently, suggesting that the role of TRPV4 may be underestimated in endothelial cells-which experience high levels of shear stress. This study highlights the importance of conducting studies at high levels of shear stress. Additionally, our approach will be valuable for examining the effect of high levels of shear on different cell types under different conditions, as presented here for agonist activation. PMID:27099646

  16. Dynamics of vorticity defects in layered stratified shear flows

    NASA Astrophysics Data System (ADS)

    Caulfield, C. P.; Roy, A.; Balmforth, N. J.

    2011-11-01

    Layered stratified flows, where relatively deep regions of weak stratification are separated by thinner interfacial layers of substantially stronger density gradient are commonly observed in nature. If such flows are subjected to vertical shear, it is well-known that a wide range of qualitatively different instabilities may develop. For example, the three-layer, two interface case is susceptible to a ``Taylor'' instability which, although superficially similar to the classic Kelvin-Helmholtz instability, is actually qualitatively different in its growth mechanism. The investigation of the nonlinear dynamics of this instability, and to a lesser extent the single-interface ``Holmboe'' instability, has proved difficult, as the need to resolve the associated sharp density gradients places heavy demands on the required numerical resolutions for simulation. However, we show that it is possible to gain insight into the key nonlinear dynamics of such layered stratified shear flows by generalizing a reduced matched asymptotic ``vorticity defect'' model (N. J. Balmforth et al. J. Fluid Mech. 333, 197 [1997]) to include the dynamical effects of density variations. We particularly focus on investigating the finite amplitude structure of the saturated primary Taylor instability, and the properties of the secondary instabilities to which Taylor and Holmboe instabilities are susceptible.

  17. Dynamic characteristics of piezoelectric shear deformable composite plates

    NASA Astrophysics Data System (ADS)

    Kolar, Ramesh

    2004-07-01

    Layered composites have attracted attention for their high specific stiffness, high specific strength, and application specific tailoring of their properties. It is also recognized that layered composites are prone to delamination failure in addition to other failure modes. Consideration of transverse shear on the deformation behavior of the composites is an important aspect in the study of delamination mode failure of such plates. In this paper, we consider the effects of including the transverse shear deformation on the vibration characteristics of layered piezoelectric composites. The formulation is based on the Raleigh-Ritz method using the beam characteristic functions. MATLAB based symbollic math tool box is used in evaluating th eintegrals resulting from the Raleigh Ritz approach. Various commonly occuring boundary conditions are discussed. Results are provided showing the effects of the shear deformation on the dynamics of layered laminated composites. The effects of laminate thickness, fiber orientation, and the plate aspect ratios on the free vibration characteristics of the composite laminates are given to demonstrate the methodology described.

  18. Universality of scaling laws in correlation between velocity and shear stress in turbulent boundary layers

    NASA Astrophysics Data System (ADS)

    v., V.; Porte-Agel, F.; Heuer, W.; Marusic, I.

    2007-05-01

    In this study, we analyse simultaneous measurements (at 50 Hz) of velocity at several heights and shear stress at the surface made during the Utah field campaign for the presence of ranges of scales, where distinct scale-to-scale interactions between velocity and shear stress can be identified. We find that our results are similar to those obtained in a previous study [Venugopal et al., 2003] (contrary to the claim in V2003, that the scaling relations might be dependent on Reynolds number) where wind tunnel measurements of velocity and shear stress were analysed. We use a wavelet-based scale-to-scale cross-correlation to detect three ranges of scales of interaction between velocity and shear stress, namely, (a) inertial subrange, where the correlation is negligible; (b) energy production range, where the correlation follows a logarithmic law; and (c) for scales larger than the boundary layer height, the correlation reaches a plateau.

  19. An equilibrium method for prediction of transverse shear stresses in a thick laminated plate

    NASA Technical Reports Server (NTRS)

    Chaudhuri, R. Z.

    1986-01-01

    First two equations of equilibrium are utilized to compute the transverse shear stress variation through thickness of a thick laminated plate after in-plane stresses have been computed using an assumed quadratic displacement triangular element based on transverse inextensibility and layerwise constant shear angle theory (LCST). Centroid of the triangle is the point of exceptional accuracy for transverse shear stresses. Numerical results indicate close agreement with elasticity theory. An interesting comparison between the present theory and that based on assumed stress hybrid finite element approach suggests that the latter does not satisfy the condition of free normal traction at the edge. Comparison with numerical results obtained by using constant shear angle theory suggests that LCST is close to the elasticity solution while the CST is closer to classical (CLT) solution. It is also demonstrated that the reduced integration gives faster convergence when the present theory is applied to a thin plate.

  20. Wall shear stress measurement in blade end-wall corner region

    NASA Technical Reports Server (NTRS)

    Bhargava, R.; Raj, R.; Boldman, D. R.

    1987-01-01

    The magnitude and the direction of wall shear stress and surface pressure in the blade end-wall corner region were investigated. The measurements were obtained on a specially designed Preston tube, the tip of which could be concentrically rotated about its axis of rotation at the measurement location. The magnitude of wall shear stress in the vicinity of the corner was observed to increase significantly (170 percent) compared to its far-upstream value; the increase was consistently higher on the blade surface compared to the value on the plate surface of the blade end-wall corner. On both surfaces in the blade end-wall corner, the variation of the wall shear stress direction was found to be more predominant in the vicinity of the blade leading-edge location. The trend of the measured wall shear stress direction showed good agreement with the limiting streamline directions obtained from the flow visualization studies.

  1. Relationship between Microtubule Network Structure and Intracellular Transport in Cultured Endothelial Cells Affected by Shear Stress

    NASA Astrophysics Data System (ADS)

    Kudo, Susumu; Ikezawa, Kenji; Ikeda, Mariko; Tanishita, Kazuo

    Endothelial cells (ECs) that line the inner surface of blood vessels are barriers to the transport of various substances into or from vessel walls, and are continuously exposed to shear stress induced by blood flow in vivo. Shear stress affects the cytoskeleton (e.g., microtubules, microfilaments, intermediate filaments), and affects the transport of macromolecules. Here, the relationship between the microtubule network structure and this transport process for albumin uptake within cultured aortic endothelial cells affected by shear stress was studied. Based on fluorescent images of albumin uptake obtained by using confocal laser scanning microscopy (CLSM), both the microtubule network and albumin uptake in ECs were disrupted by colchicine and were affected by shear stress loading.

  2. Instrument for measuring the wall shearing stress of turbulent boundary layers

    NASA Technical Reports Server (NTRS)

    Ludwieg, H

    1950-01-01

    It is shown that at a smooth wall in a turbulent boundary layer the velocity profile next to the wall is dependent, aside from the material constants of the flowing medium, only on the shearing stress transmitted to the wall, even with pressure rise or with pressure drop. Consequently, the heat transfer of a small element that is built into the wall and has a higher temperature than that of the flowing medium is a measure of the wall shearing stress. Theoretical considerations indicate that the wall shearing stress of the boundary layer can be defined by means of a heat-transfer measurement with an instrument mounted in the wall. Such an instrument is described. The calibration curve and its directional sensitivity curve are indicated. It permits the determination of the wall shearing stress in magnitude and direction.

  3. Theory to Predict Shear Stress on Cells in Turbulent Blood Flow

    PubMed Central

    Morshed, Khandakar Niaz; Bark Jr., David; Forleo, Marcio; Dasi, Lakshmi Prasad

    2014-01-01

    Shear stress on blood cells and platelets transported in a turbulent flow dictates the fate and biological activity of these cells. We present a theoretical link between energy dissipation in turbulent flows to the shear stress that cells experience and show that for the case of physiological turbulent blood flow: (a) the Newtonian assumption is valid, (b) turbulent eddies are universal for the most complex of blood flow problems, and (c) shear stress distribution on turbulent blood flows is possibly universal. Further we resolve a long standing inconsistency in hemolysis between laminar and turbulent flow using the theoretical framework. This work demonstrates that energy dissipation as opposed to bulk shear stress in laminar or turbulent blood flow dictates local mechanical environment of blood cells and platelets universally. PMID:25171175

  4. Experimental investigations of the time and flow-direction responses of shear-stress-sensitive liquid crystal coatings

    NASA Technical Reports Server (NTRS)

    Reda, Daniel C.; Muratore, Joseph J., Jr.; Heineck, James T.

    1993-01-01

    Time and flow-direction responses of shearstress-sensitive liquid crystal coatings were explored experimentally. For the time-response experiments, coatings were exposed to transient, compressible flows created during the startup and off-design operation of an injector-driven supersonic wind tunnel. Flow transients were visualized with a focusing Schlieren system and recorded with a 1000 frame/sec color video camera. Liquid crystal responses to these changing-shear environments were then recorded with the same video system, documenting color-play response times equal to, or faster than, the time interval between sequential frames (i.e., 1 millisecond). For the flow-direction experiments, a planar test surface was exposed to equal-magnitude and known-direction surface shear stresses generated by both normal and tangential subsonic jet-impingement flows. Under shear, the sense of the angular displacement of the liquid crystal dispersed (reflected) spectrum was found to be a function of the instantaneous direction of the applied shear. This technique thus renders dynamic flow reversals or flow divergences visible over entire test surfaces at image recording rates up to 1 KHz. Extensions of the technique to visualize relatively small changes in surface shear stress direction appear feasible.

  5. Quantification of Interfibrillar Shear Stress in Aligned Soft Collagenous Tissues via Notch Tension Testing

    NASA Astrophysics Data System (ADS)

    Szczesny, Spencer E.; Caplan, Jeffrey L.; Pedersen, Pal; Elliott, Dawn M.

    2015-10-01

    The mechanical function of soft collagenous tissues is largely determined by their hierarchical organization of collagen molecules. While collagen fibrils are believed to be discontinuous and transfer load through shearing of the interfibrillar matrix, interfibrillar shear stresses have never been quantified. Scaling traditional shear testing procedures down to the fibrillar length scale is impractical and would introduce substantial artifacts. Here, through the use of a novel microscopic variation of notch tension testing, we explicitly demonstrate the existence of interfibrillar shear stresses within tendon fascicles and provide the first measurement of their magnitude. Axial stress gradients along the sample length generated by notch tension testing were measured and used to calculate a value of 32 kPa for the interfibrillar shear stress. This estimate is comparable to the interfibrillar shear stress predicted by previous multiscale modeling of tendon fascicles, which supports the hypothesis that fibrils are discontinuous and transmit load through interfibrillar shear. This information regarding the structure-function relationships of tendon and other soft collagenous tissues is necessary to identify potential causes for tissue impairment with degeneration and provide the foundation for developing regenerative repair strategies or engineering biomaterials for tissue replacement.

  6. Shear Stress Drives Local Variation in Invertebrate Drift in a Large River

    NASA Astrophysics Data System (ADS)

    Muehlbauer, J. D.; Kennedy, T.; Yackulic, C. B.

    2013-12-01

    Recent advances in physical stream flow measurements using acoustic Doppler current profilers (ADCPs) have yielded important insights in hydrology and geomorphology related to discharge and processes such as bed sediment incipient motion. These measurements also have underappreciated potential for use in ecological studies. For example, invertebrate drift, or the downstream transport of benthic-derived invertebrates, is a fundamental process in streams and rivers: it is both critical to the maintenance of benthic invertebrate populations and provides a key mechanism of resource delivery to drift-feeding fishes. However, there is substantial uncertainty regarding the factors that drive spatial variation in invertebrate drift, particularly in large rivers. While laboratory studies in flumes have demonstrated the importance of shear stress in initiating invertebrate drift (similar to studies of bed sediment critical shear stress in fluvial geomorphology), field-based evaluations of the relationship between shear stress and drift would be beneficial. Such field studies, however, are rare. Here, we evaluate the relationship between localized shear stress (N/m2) and invertebrate drift concentrations (#/m3) for the Colorado River downstream of Glen Canyon Dam (steady discharge of 228 m3/s during study). Invertebrate drift was quantified at 25 stations throughout the 25 km long Glen Canyon tailwater segment. We link these drift measurements to empirical measurements of water column shear stress derived from ADCP data, taken at the location of each drift sample and 250 m upstream of each drift sampling location (50 total profiles). Invertebrate drift concentrations varied strongly throughout the 25 km reach, and much of this variation can be explained by localized differences in shear stress. Species composition in the drift also varied with shear stress, suggesting that shear stress exerts a differential control on drift initiation for individual taxa. These results

  7. Evolution of crustal stress, pressure and temperature around shear zones during orogenic wedge formation: a 2D thermo-mechanical numerical study

    NASA Astrophysics Data System (ADS)

    Markus Schmalholz, Stefan; Jaquet, Yoann

    2016-04-01

    We study the formation of an orogenic wedge during lithospheric shortening with 2D numerical simulations. We consider a viscoelastoplastic rheology, thermo-mechanical coupling by shear heating and temperature-dependent viscosities, gravity and erosion. In the initial model configuration there is either a lateral temperature variation at the model base or a lateral variation in crustal thickness to generate slight stress variations during lithospheric shortening. These stress variations can trigger the formation of shear zones which are caused by thermal softening associated with shear heating. We do not apply any kind of strain softening, such as reduction of friction angle with progressive plastic strain. The first major shear zone that appears during shortening crosscuts the entire crust and initiates the asymmetric subduction/underthrusting of mainly the mechanically strong lower crust. After some deformation, the first shear zone in the upper crust is abandoned, the deformation propagates towards the foreland and a new shear zone forms only in the upper crust. The shear zone propagation occurs several times where new shear zones form in the upper crust and the mechanically strong top of the lower crust acts as detachment horizon. We calculate the magnitudes of the maximal and minimal principal stresses and of the mean stress (or dynamic pressure), and we record also the temperature for several marker points in the upper and lower crust. We analyse the evolution of stresses and temperature with burial depth and time. Deviatoric stresses (half the differential stress) in the upper crust are up to 200 MPa and associated shear heating in shear zones ranges between 40 - 80 °C. Lower crustal rocks remain either at the base of the orogenic wedge at depths of around 50 km or are subducted to depths of up to 120 km, depending on their position when the first shear zone formed. Largest deviatotric stresses in the strong part of the lower crust are about 1000 MPa and

  8. Dynamics of edge dislocations in a sheared lamellar mesophase

    NASA Astrophysics Data System (ADS)

    Kumaran, V.

    2013-10-01

    The dynamics and interactions of edge dislocations in a nearly aligned sheared lamellar mesophase is analysed to provide insights into the relationship between disorder and rheology. First, the mesoscale permeation and momentum equations for the displacement field in the presence of external forces are derived from the model H equations for the concentration and momentum field. The secondary flow generated due to the mean shear around an isolated defect is calculated, and the excess viscosity due to the presence of the defect is determined from the excess energy dissipation due to the secondary flow. The excess viscosity for an isolated defect is found to increase with system size in the cross-stream direction as L3/2 for an isolated defect, though this divergence is cut-off due to interactions in a defect suspension. As the defects are sheared past each other due to the mean flow, the Peach-Koehler force due to elastic interaction between pairs of defects is found to cause no net displacement relative to each other as they approach from large separation to the distance of closest approach. The equivalent force due to viscous interactions is found to increase the separation for defects of opposite sign, and decrease the separation for defects of same sign. During defect interactions, we find that there is no buckling instability due to dilation of layers for systems of realistic size. However, there is another mechanism, which is the velocity difference generated across a slightly deformed bilayer due to the mean shear, which could result in the creation of new defects.

  9. Micromechanical processes of frictional aging and the affect of shear stress on fault healing: insights from material characterization and ultrasonic velocity measurements

    NASA Astrophysics Data System (ADS)

    Ryan, K. L.; Marone, C.

    2015-12-01

    During the seismic cycle, faults repeatedly fail and regain strength. The gradual strength recovery is often referred to as frictional healing, and existing works suggest that healing can play an important role in determining the mode of fault slip ranging from dynamic rupture to slow earthquakes. Laboratory studies can play an important role in identifying the processes of frictional healing and their evolution with shear strain during the seismic cycle. These studies also provide data for laboratory-derived friction constitutive laws, which can improve dynamic earthquake models. Previous work shows that frictional healing varies with shear stress on a fault during the interseismic period. Unfortunately, the micromechanical processes that cause shear stress dependent frictional healing are not well understood and cannot be incorporated into current earthquake models. In fault gouge, frictional healing involves compaction and particle rearrangement within sheared granular layers. Therefore, to address these issues, we investigate the role grain size reduction plays in frictional re-strengthening processes at different levels of shear stress. Sample material was preserved from biaxial deformation experiments on granular Westerly granite. The normal stress was held constant at 25 MPa and we performed several 100 second slide-hold-slide tests in each experiment. We conducted a series of 5 experiments each with a different value of normalized shear stress (ranging from 0 to 1), defined as the ratio of the pre-hold shear stress to the shear stress during the hold. The particle size distribution for each sample was analyzed. In addition, acoustic measurements were recorded throughout our experiments to investigate variations in ultrasonic velocity and signal amplitude that reflect changes in the elastic moduli of the layer. Acoustic monitoring provides information about healing mechanisms and can provide a link between laboratory studies and tectonic fault zones.

  10. Computational simulations predict a key role for oscillatory fluid shear stress in de novo valvular tissue formation.

    PubMed

    Salinas, Manuel; Ramaswamy, Sharan

    2014-11-01

    Previous efforts in heart valve tissue engineering demonstrated that the combined effect of cyclic flexure and steady flow on bone marrow derived stem cell-seeded scaffolds resulted in significant increases in engineered collagen formation [Engelmayr et al. Cyclic flexure and laminar flow synergistically accelerate mesenchymal stem cell-mediated engineered tissue formation: Implications for engineered heart valve tissues. Biomaterials 2006; 27(36): 6083-95]. Here, we provide a new interpretation for the underlying reason for this observed effect. In addition, another related investigation demonstrated the impact of fluid flow on DNA content and quantified the fluid-induced shear stresses on the engineered heart valve tissue specimens [Engelmayr et al. A Novel Flex-Stretch-Flow Bioreactor for the Study of Engineered Heart Valve Tissue Mechanobiology]. Annals of Biomedical Engineering 2008, 36, 1-13]. In this study, we performed more advanced CFD analysis with an emphasis on oscillatory wall shear stresses imparted on specimens when mechanically conditioned by a combination of cyclic flexure and steady flow. Specifically, we hypothesized that the dominant stimulatory regulator of the bone marrow stem cells is fluid-induced and depends on both the magnitude and temporal directionality of surface stresses, i.e., oscillatory shear stresses (OSS) acting on the developing tissues. Therefore, we computationally quantified the (i) magnitude of fluid-induced shear stresses as well as (ii) the extent of temporal fluid oscillations in the flow field using the oscillatory shear index (OSI) parameter. Noting that sample cyclic flexure induces a high degree of OSS, we incorporated moving boundary computational fluid dynamic simulations of samples housed within a bioreactor to consider the effects of: (1) No Flow, No Flexure (control group), (2) Steady Flow-alone, (3) Cyclic Flexure-alone and (4) Combined Steady flow and Cyclic Flexure environments. Indeed we found that the

  11. Activation and shedding of platelet glycoprotein IIb/IIIa under non-physiological shear stress.

    PubMed

    Chen, Zengsheng; Mondal, Nandan K; Ding, Jun; Koenig, Steven C; Slaughter, Mark S; Griffith, Bartley P; Wu, Zhongjun J

    2015-11-01

    The purpose of this study was to investigate the influence of non-physiological high shear stress on activation and shedding of platelet GP IIb/IIIa receptors. The healthy donor blood was exposed to three levels of high shear stresses (25, 75, 125 Pa) from the physiological to non-physiological status with three short exposure time (0.05, 0.5, 1.5 s), created by a specific blood shearing system. The activation and shedding of the platelet GPIIb/IIIa were analyzed using flow cytometry and enzyme-linked immunosorbent assay. In addition, platelet P-selectin expression of sheared blood, which is a marker for activated platelets, was also analyzed. The results from the present study showed that the number of activated platelets, as indicated by the surface GPIIb/IIIa activation and P-selectin expression, increased with increasing the shear stress level and exposure time. However, the mean fluorescence of GPIIb/IIIa on the platelet surface, decreased with increasing the shear stress level and exposure time. The reduction of GPIIb/IIIa on the platelet surface was further proved by the reduction of further activated platelet GPIIb/IIIa surface expression induced by ADP and the increase in GPIIb/IIIa concentration in microparticle-free plasma with increasing the applied shear stress and exposure time. It is clear that non-physiological shear stress induce a paradoxical phenomenon, in which both activation and shedding of the GPIIb/IIIa on the platelet surface occur simultaneously. This study may offer a new perspective to explain the reason of both increased thrombosis and bleeding events in patients implanted with high shear blood-contacting medical devices. PMID:26160282

  12. Orbital fluid shear stress promotes osteoblast metabolism, proliferation and alkaline phosphates activity in vitro.

    PubMed

    Aisha, M D; Nor-Ashikin, M N K; Sharaniza, A B R; Nawawi, H; Froemming, G R A

    2015-09-10

    Prolonged disuse of the musculoskeletal system is associated with reduced mechanical loading and lack of anabolic stimulus. As a form of mechanical signal, the multidirectional orbital fluid shear stress transmits anabolic signal to bone forming cells in promoting cell differentiation, metabolism and proliferation. Signals are channeled through the cytoskeleton framework, directly modifying gene and protein expression. For that reason, we aimed to study the organization of Normal Human Osteoblast (NHOst) cytoskeleton with regards to orbital fluid shear (OFS) stress. Of special interest were the consequences of cytoskeletal reorganization on NHOst metabolism, proliferation, and osteogenic functional markers. Cells stimulated at 250 RPM in a shaking incubator resulted in the rearrangement of actin and tubulin fibers after 72 h. Orbital shear stress increased NHOst mitochondrial metabolism and proliferation, simultaneously preventing apoptosis. The ratio of RANKL/OPG was reduced, suggesting that orbital shear stress has the potential to inhibit osteoclastogenesis and osteoclast activity. Increase in ALP activity and OCN protein production suggests that stimulation retained osteoblast function. Shear stress possibly generated through actin seemed to hold an anabolic response as osteoblast metabolism and functional markers were enhanced. We hypothesize that by applying orbital shear stress with suitable magnitude and duration as a non-drug anabolic treatment can help improve bone regeneration in prolonged disuse cases. PMID:26163894

  13. Low Shear Stress Inhibited Endothelial Cell Autophagy Through TET2 Downregulation.

    PubMed

    Yang, Qin; Li, Xiaohong; Li, Rongqing; Peng, Juan; Wang, Zuo; Jiang, Zhisheng; Tang, Xiaoqing; Peng, Zhao; Wang, Yu; Wei, Dangheng

    2016-07-01

    Low shear stress plays a crucial role in the initiation and progression of atherosclerotic lesions. However, the detailed mechanisms of these processes remain unclear. In this study, the effect of low shear stress on endothelial cell autophagy and its potential mechanism were investigated. Results showed autophagy dysfunction and ten-eleven translocation 2 (TET2) protein downregulation during atherosclerotic lesion progression. Autophagic markers BECLIN 1 and LC3II/LC3I under low shear stress (5 dyne/cm(2)) obviously decreased compared with those under physiological shear stress (15 dyne/cm(2)), whereas autophagic substrate p62 increased. TET2 expression was also downregulated under low shear stress. Endothelial cell autophagy was improved with TET2 overexpression but was impaired by TET2 siRNA treatment. Moreover, TET2 overexpression upregulated the expression of endothelial cell nitric oxide synthase (eNOS) and downregulated the expression of endothelin-1 (ET-1). TET2 siRNA further attenuated eNOS expression and stimulated ET-1 expression. Overall, the results showed that low shear stress downregulated endothelial cell autophagy by impaired TET2 expression, which might contribute to the atherogenic process. PMID:26493943

  14. Arterial Shear Stress Reduces Eph-B4 Expression in Adult Human Veins

    PubMed Central

    Model, Lynn S.; Hall, Michael R.; Wong, Daniel J.; Muto, Akihito; Kondo, Yuka; Ziegler, Kenneth R.; Feigel, Amanda; Quint, Clay; Niklason, Laura; Dardik, Alan

    2014-01-01

    Vein graft adaptation to the arterial environment is characterized by loss of venous identity, with reduced Ephrin type-B receptor 4 (Eph-B4) expression but without increased Ephrin-B2 expression. We examined changes of vessel identity of human saphenous veins in a flow circuit in which shear stress could be precisely controlled. Medium circulated at arterial or venous magnitudes of laminar shear stress for 24 hours; histologic, protein, and RNA analyses of vein segments were performed. Vein endothelium remained viable and functional, with platelet endothelial cell adhesion molecule (PECAM)-expressing cells on the luminal surface. Venous Eph-B4 expression diminished (p = .002), Ephrin-B2 expression was not induced (p = .268), and expression of osteopontin (p = .002) was increased with exposure to arterial magnitudes of shear stress. Similar changes were not found in veins placed under venous flow or static conditions. These data show that human saphenous veins remain viable during ex vivo application of shear stress in a bioreactor, without loss of the venous endothelium. Arterial magnitudes of shear stress cause loss of venous identity without gain of arterial identity in human veins perfused ex vivo. Shear stress alone, without immunologic or hormonal influence, is capable of inducing changes in vessel identity and, specifically, loss of venous identity. PMID:25191151

  15. Application of Entropy Concept for Shear Stress Distribution in Laminar Pipe Flow

    NASA Astrophysics Data System (ADS)

    Choo, Yeon Moon; Choo, Tai Ho; Jung, Donghwi; Seon, Yun Gwan; Kim, Joong Hoon

    2016-04-01

    In the river fluid mechanics, shear stress is calculated from frictional force caused by viscosity and fluctuating velocity. Traditional shear stress distribution equations have been widely used because of their simplicity. However, they have a critical limitation of requiring energy gradient which is generally difficult to estimate in practice. Especially, measuring velocity/velocity gradient on the boundary layer is difficult in practice. It requires point velocity throughout the entire cross section to calculate velocity gradient. This study proposes shear stress distribution equations for laminar flow based on entropy theory using mean velocity and entropy coefficient. The proposed equations are demonstrated and compared with measured shear stress distribution using Nikuradse's data. Results showed that the coefficient of determination is around 0.99 indicating that the proposed method well describes the true shear stress distribution. Therefore, it was proved that shear stress distribution can be easily and accurately estimated by using the proposed equations. (This research was supported by a gran(13AWMP-B066744-01) from Advanced Water Management Research Program funded by Ministry of Land, Infrastructure and Transport of Korean Government)

  16. Effect of Varying Fluid Shear Stress on Cancer Stem Cell Viability & Protein Expression

    NASA Astrophysics Data System (ADS)

    Domier, Ria; Kim, Yonghyun; Dozier, David; Triantafillu, Ursula

    2013-11-01

    Cancer stem cells cultured in vitro in stirred bioreactors are exposed to shear stress. By observing the effect of shear stress on cancer stem cell viability, laboratory cell growth could be optimized. In addition, metastasized cancer stem cells in vivo are naturally exposed to shear stress, a factor influencing stem cell differentiation, while circulating in the bloodstream. Changes in protein expression after exposure to shear stress could allow for identification and targeting of circulating cancer cells. In this study, blood flow through capillaries was simulated by using a syringe pump to inject suspensions of Kasumi-1 leukemia stem cells into model blood vessels composed of PEEK tubing 125 microns in diameter. The Hagen-Poisseuille equation was used to solve for operating flow rates based on specified amounts of shear stress. After exposure, cell counts and viabilities were observed using an optical microscope and proteins were analyzed using Western blotting. It was observed that at a one minute exposure to stress, cell viability increased as the amount of shear was increased from 10 to 60 dynes per square centimeter. Results from this research are applicable to optimization of large-scale stem cell growth in bioreactors as well as to the design of targeted cancer therapies. Funding from NSF REU grant #1062611 is gratefully acknowledged.

  17. Shear stress sensing with Bragg grating-based sensors in microstructured optical fibers.

    PubMed

    Sulejmani, Sanne; Sonnenfeld, Camille; Geernaert, Thomas; Luyckx, Geert; Van Hemelrijck, Danny; Mergo, Pawel; Urbanczyk, Waclaw; Chah, Karima; Caucheteur, Christophe; Mégret, Patrice; Thienpont, Hugo; Berghmans, Francis

    2013-08-26

    We demonstrate shear stress sensing with a Bragg grating-based microstructured optical fiber sensor embedded in a single lap adhesive joint. We achieved an unprecedented shear stress sensitivity of 59.8 pm/MPa when the joint is loaded in tension. This corresponds to a shear strain sensitivity of 0.01 pm/µε. We verified these results with 2D and 3D finite element modeling. A comparative FEM study with conventional highly birefringent side-hole and bow-tie fibers shows that our dedicated fiber design yields a fourfold sensitivity improvement. PMID:24105585

  18. Impact of shear stress and pH changes on floc size and removal of dissolved organic matter (DOM).

    PubMed

    Slavik, Irene; Müller, Susanne; Mokosch, Regina; Azongbilla, Joseph Abanga; Uhl, Wolfgang

    2012-12-01

    The impact of shear stress and increases in pH on the release of natural dissolved organic matter (DOM) from Fe-DOM and Al-DOM flocs was investigated for a high organic matter, low turbidity raw water by application of a dynamic extinction probe (DEP) and liquid chromatography organic carbon detection (LC-OCD). It was shown that high shear forces resulted in a breakage of Fe-DOM flocs. Re-growth took place during subsequent low shear phases. However, re-growth was limited. The flocs regained a size of about 50% of the size after initial coagulation. Cyclic shearing resulted in slower re-growth rates. A new insight was that when enough time was given, similar sizes of the re-grown flocs were regained. As shown by bulk DOC, only an insignificant release of DOM took place when flocs were exposed to shear. Increase in shear stress resulted in smaller flocs with higher specific outer surface area. However, DOM removal did not change. Thus, there was no increase in adsorption capacity due to floc breakage. Consequently, DOM must be adsorbed inside the amorphous flocs rather than on the outer surface. Also, as shear results in more compact flocs, compaction does not have an effect on DOM removal. A pH increase of 0.5, as it can happen during water treatment after coagulation, resulted in a release of DOM. Humic substances accounted for the largest proportion of total DOM released. The increase in pH did not affect floc size. Consequently, DOM removal is mainly governed by the dependence of DOM properties on pH with the final pH determining the degree of DOM removal and not the path on which this pH is reached. The physical properties of the flocs have no impact on DOM removal. PMID:23047054

  19. Simulating Fiber Ordering and Aggregation In Shear Flow Using Dissipative Particle Dynamics

    NASA Astrophysics Data System (ADS)

    Stimatze, Justin T.

    We have developed a mesoscale simulation of fiber aggregation in shear flow using LAMMPS and its implementation of dissipative particle dynamics. Understanding fiber aggregation in shear flow and flow-induced microstructural fiber networks is critical to our interest in high-performance composite materials. Dissipative particle dynamics enables the consideration of hydrodynamic interactions between fibers through the coarse-grained simulation of the matrix fluid. Correctly simulating hydrodynamic interactions and accounting for fluid forces on the microstructure is required to correctly model the shear-induced aggregation process. We are able to determine stresses, viscosity, and fiber forces while simulating the evolution of a model fiber system undergoing shear flow. Fiber-fiber contact interactions are approximated by combinations of common pairwise forces, allowing the exploration of interaction-influenced fiber behaviors such as aggregation and bundling. We are then able to quantify aggregate structure and effective volume fraction for a range of relevant system and fiber-fiber interaction parameters. Our simulations have demonstrated several aggregate types dependent on system parameters such as shear rate, short-range attractive forces, and a resistance to relative rotation while in contact. A resistance to relative rotation at fiber-fiber contact points has been found to strongly contribute to an increased angle between neighboring aggregated fibers and therefore an increase in average aggregate volume fraction. This increase in aggregate volume fraction is strongly correlated with a significant enhancement of system viscosity, leading us to hypothesize that controlling the resistance to relative rotation during manufacturing processes is important when optimizing for desired composite material characteristics.

  20. On the dynamics of magnetorotational turbulent stresses

    NASA Astrophysics Data System (ADS)

    Ogilvie, G. I.

    2003-04-01

    The turbulent stresses that lead to angular momentum transport in accretion discs have often been treated as resulting from an isotropic effective viscosity, related to the pressure through the alpha parametrization of Shakura and Sunyaev. This simple approach may be adequate for the simplest aspects of accretion disc theory, and was necessitated historically by an incomplete understanding of the origin of the turbulence. More recently, Balbus and Hawley have shown that the magnetorotational instability provides a robust mechanism of generating turbulent Reynolds and Maxwell stresses in sufficiently ionized discs. The alpha viscosity model fails to describe numerous aspects of this process. The present paper introduces a new analytical model that aims to represent more faithfully the dynamics of magnetorotational turbulent stresses and bridge the gap between analytical studies and numerical simulations. Covariant evolutionary equations for the mean Reynolds and Maxwell tensors are presented, which correctly include the linear interaction with the mean flow. Non-linear and dissipative effects, in the absence of an imposed magnetic flux and in the limit of large Reynolds number and magnetic Reynolds number, are modelled through five non-linear terms that represent known physical processes and are strongly constrained by symmetry properties and dimensional considerations. The resulting model explains the development of statistically steady, anisotropic turbulent stresses in the shearing sheet, a local representation of a differentially rotating disc, in agreement with numerical simulations. It also predicts that purely hydrodynamic turbulence is not sustained in a flow that adequately satisfies Rayleigh's stability criterion. The model is usually formally hyperbolic and therefore `causal', and guarantees the realizability of the stress tensors. It should be particularly useful in understanding the dynamics of warped, eccentric and tidally distorted discs, non

  1. Experimental research of mechanical behavior of porcine brain tissue under rotational shear stress.

    PubMed

    Li, Gang; Zhang, Jianhua; Wang, Kan; Wang, Mingyu; Gao, Changqing; Ma, Chao

    2016-04-01

    The objective of this paper is to investigate mechanical behavior of porcine brain tissue with a series of rotational shear stress control experiments. To this end, several experiments including stress sweep tests, frequency sweep tests and quasi-static creep tests were designed and conducted with a standard rheometer (HAAKE RheoStress6000). The effects of the loading stress rates to mechanical properties of brain tissue were also studied in stress sweep tests. The results of stress sweep tests performed on the same brain showed that brain tissue had an obvious regional inhomogeneity and the mechanical damage occurred at the rotational shear stress of 10-15Pa. The experimental data from three different loading stress rates demonstrated that the mechanical behavior of porcine brain tissue was loading stress rate dependent. With the decrease of loading stress rate, a stiffer mechanical characteristic of brain tissue was observed and the occurrence of mechanical damage can be delayed to a higher stress. From the results of frequency sweep tests we found that brain tissue had almost completely elastic properties at high frequency area. The nonlinear creep response under the rotational shear stress of 1, 3, 5, 7 and 9Pa was shown in results of creep tests. A new nonlinear viscoelastic solid model was proposed for creep tests and matched well with the test data. Considering the regional differences, loading stress rates and test conditions effects, loss tangent tan δ in porcine brain tissue showed a high uniformity of 0.25-0.45. PMID:26735181

  2. Shear Modulus of the Lower Leg Muscles in Patients with Medial Tibial Stress Syndrome.

    PubMed

    Akiyama, Kei; Akagi, Ryota; Hirayama, Kuniaki; Hirose, Norikazu; Takahashi, Hideyuki; Fukubayshi, Toru

    2016-08-01

    This study aimed to investigate the in vivo kinematics of shear modulus of the lower leg muscles in patients with medial tibial stress syndrome (MTSS). The study population included 46 limbs with MTSS and 40 healthy limbs. The shear modulus of the medial head of the gastrocnemius, lateral head of the gastrocnemius, soleus, peroneus longus and tibialis anterior muscles were measured using shear wave ultrasound elastography. As a result, the shear modulus of the lower leg muscles was significantly greater in patients with MTSS than in healthy patients (p < 0.01). Based on the differences in shear modulus of lower leg muscles between the patients with MTSS and healthy patients, the measurements obtained via shear wave ultrasound elastography could be used to evaluate risk factors of MTSS. PMID:27129903

  3. Surface morphology of platelet adhesion influenced by activators, inhibitors and shear stress

    NASA Astrophysics Data System (ADS)

    Watson, Melanie Groan

    Platelet activation involves multiple events, one of which is the generation and release of nitric oxide (NO), a platelet aggregation inhibitor. Platelets simultaneously send and receive various agents that promote a positive and negative feedback control system during hemostasis. Although the purpose of platelet-derived NO is not fully understood, NO is known to inhibit platelet recruitment. NO's relatively large diffusion coefficient allows it to diffuse more rapidly than platelet agonists. It may thus be able to inhibit recruitment of platelets near the periphery of a growing thrombus before agonists have substantially accumulated in those regions. Results from two studies in our laboratory differed in the extent to which platelet-derived NO decreased platelet adhesion. Frilot studied the effect of L-arginine (L-A) and NG-Methyl-L-arginine acetate salt (L-NMMA) on platelet adhesion to collagen under static conditions in a Petri dish. Eshaq examined the percent coverage on collagen-coated and fibrinogen-coated microchannels under shear conditions with different levels of L-A and Adenosine Diphosphate (ADP). Frilot's results showed no effect of either L-A or L-NMMA on surface coverage, thrombus size or serotonin release, while Eshaq's results showed a decrease in surface coverage with increased levels of L-A. A possible explanation for these contrasting results is that platelet-derived NO may be more important under flow conditions than under static conditions. For this project, the effects of L-A. ADP and L-NMMA on platelet adhesion were studied at varying shear stresses on protein-coated glass slides. The surface exposed to platelet-rich-plasma in combination with each chemical solution was observed under AFM, FE-SEM and fluorescence microscopy. Quantitative and qualitative comparisons of images obtained with these techniques confirmed the presence of platelets on the protein coatings. AFM images of fibrinogen and collagen-coated slides presented characteristic

  4. Analysis of shearing stress in the limited durability of bovine pericardium used as a biomaterial.

    PubMed

    Carrera San Martin, A; García Paez, J M; García Sestafe, J V; Herrero, E J; Navidad, R; Cordón, A; Castillo-Olivares, J L

    1998-02-01

    The objective of the study was to determine the shearing stress exerted by the suture thread under conditions of normal working stress. Thirty-six samples of calf pericardium, similar to that employed in the manufacture of bioprosthetic cardiac valve leaflets, were subjected to tensile testing. Prior to the trial, a continuous suture was sewn in the central zone of each sample, at a 45 degrees angle to the longest axis of the sample, using commercially-available threads (silk, Gore-Tex, Surgilene and nylon). Application of the Mohr circle for combined wear revealed that the shearing stress ranged between 2.68-fold greater (for samples sewn with silk) and 5.48-fold greater (for samples sewn with nylon) than the working tensile stress in the region of the suture. It is concluded that the shearing stress is responsible for the limited durability of sutured samples of calf pericardium prepared to simulate bioprosthetic cardiac valve leaflets. PMID:15348911

  5. A new technique for the measurement of surface shear stress vectors using liquid crystal coatings

    NASA Technical Reports Server (NTRS)

    Reda, Daniel C.; Muratore, J. J., Jr.

    1994-01-01

    Research has recently shown that liquid crystal coating (LCC) color-change response to shear depends on both shear stress magnitude and direction. Additional research was thus conducted to extend the LCC method from a flow-visualization tool to a surface shear stress vector measurement technique. A shear-sensitive LCC was applied to a planar test surface and illuminated by white light from the normal direction. A fiber optic probe was used to capture light scattered by the LCC from a point on the centerline of a turbulent, tangential-jet flow. Both the relative shear stress magnitude and the relative in-plane view angle between the sensor and the centerline shear vector were systematically varied. A spectrophotometer was used to obtain scattered-light spectra which were used to quantify the LCC color (dominant wavelength) as a function of shear stress magnitude and direction. At any fixed shear stress magnitude, the minimum dominant wavelength was measured when the shear vector was aligned with and directed away from the observer; changes in the relative in-plane view angle to either side of this vector/observer aligned position resulted in symmetric Gaussian increases in measured dominant wavelength. Based on these results, a vector measurement methodology, involving multiple oblique-view observations of the test surface, was formulated. Under present test conditions, the measurement resolution of this technique was found to be +/- 1 deg for vector orientations and +/- 5% for vector magnitudes. An approach t o extend the present methodology to full-surface applications is proposed.

  6. Evaluations of wall shear stress in the standpipe of a circulating fluidized bed

    SciTech Connect

    Monazam, E.R.; Shadle, L.J.

    2008-05-13

    Shear stress was obtained in the standpipe of a Circulating Fluidized Bed (CFB) for a light cork particles under a variety of flow conditions. The shear stress data were estimated using incremental gas phase pressure drop readings, and an estimate of the bed height to predict the hydrostatic pressure drop [(dp/dy) = ρs (1-ε) g+4τsw/D]. In addition, we have also obtained data on aeration rate in the standpipe, particle circulation rate and riser gas flow rate. Analysis of the results using a one-dimensional momentum equation reveal that the observed forced per unit area may be attributed to wall friction. The resulting shear stress demonstrates that as the aeration air in the standpipe was increased the shear at the wall was decreased. An attempt was made to model solids friction factor as a function of particle velocity and it was compared to the other literature correlations.

  7. Numerical study of shear stress distribution at sand ripple surface in wind tunnel flow

    NASA Astrophysics Data System (ADS)

    Bar, Nitsan; Elperin, Tov; Katra, Itzhak; Yizhaq, Hezi

    2016-06-01

    The mechanism responsible for the formation and sustainability of sand ripples sheared by a uniform air flow is not well understood, despite the significant attention that has been given to it ever since the pioneering studies of Bagnold (1941). In this study we explore ANSYS Fluent simulations of fine-scale turbulent flow structure in the vicinity of 2D sand ripples with particular emphasis on shear stress distribution at the sand bed. The flow parameters in the simulations were pertinent to the wind tunnel experiments for studying sand ripples formation. The simulations show that the shear stress at the crest is about 2.5 times larger than the shear stress at the trough and that in most of the simulations a separation bubble has been developed at the lee slope. In contrast to wind tunnel experiments the simulations show that ripples will be flattened at wind speed of 9 m/s as shear stress at the ripples surface exceeds the fluid threshold. This discrepancy between the calculations and real wind tunnel measurements are due to the important role of the saltation layer on the decrease of the shear stress at the surface. Without this effect ripples cannot grow higher and will be diminished at quite moderate winds.

  8. Effect of cell size and shear stress on bacterium growth rate

    NASA Astrophysics Data System (ADS)

    Fadlallah, Hadi; Jarrahi, Mojtaba; Herbert, Éric; Peerhossaini, Hassan; PEF Team

    2015-11-01

    Effect of shear stress on the growth rate of Synechocystis and Chlamydomonas cells is studied. An experimental setup was prepared to monitor the growth rate of the microorganisms versus the shear rate inside a clean room, under atmospheric pressure and 20 °C temperature. Digital magnetic agitators are placed inside a closed chamber provided with airflow, under a continuous uniform light intensity over 4 weeks. In order to study the effect of shear stress on the growth rate, different frequencies of agitation are tested, 2 vessels filled with 150 ml of each specie were placed on different agitating system at the desired frequency. The growth rate is monitored daily by measuring the optical density and then correlate it to the cellular concentration. The PH was adjusted to 7 in order to maintain the photosynthetic activity. Furthermore, to measure the shear stress distribution, the flow velocity field was measured using PIV. Zones of high and low shear stress were identified. Results show that the growth rate is independent of the shear stress magnitude, mostly for Synechocystis, and with lower independency for Chlamydomonas depending on the cell size for each species.

  9. Orbital fluid shear stress promotes osteoblast metabolism, proliferation and alkaline phosphates activity in vitro

    SciTech Connect

    Aisha, M.D.; Nor-Ashikin, M.N.K.; Sharaniza, A.B.R.; Nawawi, H.; Froemming, G.R.A.

    2015-09-10

    Prolonged disuse of the musculoskeletal system is associated with reduced mechanical loading and lack of anabolic stimulus. As a form of mechanical signal, the multidirectional orbital fluid shear stress transmits anabolic signal to bone forming cells in promoting cell differentiation, metabolism and proliferation. Signals are channeled through the cytoskeleton framework, directly modifying gene and protein expression. For that reason, we aimed to study the organization of Normal Human Osteoblast (NHOst) cytoskeleton with regards to orbital fluid shear (OFS) stress. Of special interest were the consequences of cytoskeletal reorganization on NHOst metabolism, proliferation, and osteogenic functional markers. Cells stimulated at 250 RPM in a shaking incubator resulted in the rearrangement of actin and tubulin fibers after 72 h. Orbital shear stress increased NHOst mitochondrial metabolism and proliferation, simultaneously preventing apoptosis. The ratio of RANKL/OPG was reduced, suggesting that orbital shear stress has the potential to inhibit osteoclastogenesis and osteoclast activity. Increase in ALP activity and OCN protein production suggests that stimulation retained osteoblast function. Shear stress possibly generated through actin seemed to hold an anabolic response as osteoblast metabolism and functional markers were enhanced. We hypothesize that by applying orbital shear stress with suitable magnitude and duration as a non-drug anabolic treatment can help improve bone regeneration in prolonged disuse cases. - Highlights: • OFS stress transmits anabolic signals to osteoblasts. • Actin and tubulin fibers are rearranged under OFS stress. • OFS stress increases mitochondrial metabolism and proliferation. • Reduced RANKL/OPG ratio in response to OFS inhibits osteoclastogenesis. • OFS stress prevents apoptosis and stimulates ALP and OCN.

  10. Shear flow and drift wave turbulence dynamics in a cylindrical plasma device

    SciTech Connect

    Yan, Z.; Tynan, G. R.; Holland, C.; Xu, M.; Mueller, S. H.; Yu, J. H.

    2010-03-15

    The experimental observations of the dynamics of the coupled drift wave turbulence (DWT)/sheared zonal flow (ZF) system in a cylindrical plasma device using a combination of Langmuir probe and fast-framing imaging measurements are reported. The results show the presence of an azimuthal ZF that exhibits low frequency (approx250 Hz) fluctuations. The envelope of the higher frequency (above 5 kHz) floating potential fluctuations associated with the DWT, the density gradient, and the turbulent radial particle flux are all modulated out of phase with the strength of the ZF. The divergence of the turbulent Reynolds stress is also modulated at the same slow time scale in a phase-coherent manner consistent with a turbulent-driven shear flow sustained against the collisional and viscous damping. The radial turbulence correlation length and cross-field particle transport are reduced during periods of strong flow shear. The results are qualitatively consistent with theoretical expectations for coupled DWT-ZF dynamics.

  11. PEG-albumin supraplasma expansion is due to increased vessel wall shear stress induced by blood viscosity shear thinning

    PubMed Central

    Sriram, Krishna; Tsai, Amy G.; Cabrales, Pedro; Meng, Fantao; Acharya, Seetharama A.; Tartakovsky, Daniel M.

    2012-01-01

    We studied the extreme hemodilution to a hematocrit of 11% induced by three plasma expanders: polyethylene glycol (PEG)-conjugated albumin (PEG-Alb), 6% 70-kDa dextran, and 6% 500-kDa dextran. The experimental component of our study relied on microelectrodes and cardiac output to measure both the rheological properties of plasma-expander blood mixtures and nitric oxide (NO) bioavailability in vessel walls. The modeling component consisted of an analysis of the distribution of wall shear stress (WSS) in the microvessels. Our experiments demonstrated that plasma expansion with PEG-Alb caused a state of supraperfusion with cardiac output 40% above baseline, significantly increased NO vessel wall bioavailability, and lowered peripheral vascular resistance. We attributed this behavior to the shear thinning nature of blood and PEG-Alb mixtures. To substantiate this hypothesis, we developed a mathematical model of non-Newtonian blood flow in a vessel. Our model used the Quemada rheological constitutive relationship to express blood viscosity in terms of both hematocrit and shear rate. The model revealed that the net effect of the hemodilution induced by relatively low-viscosity shear thinning PEG-Alb plasma expanders is to reduce overall blood viscosity and to increase the WSS, thus intensifying endothelial NO production. These changes act synergistically, significantly increasing cardiac output and perfusion due to lowered overall peripheral vascular resistance. PMID:22505638

  12. Plasma functionalization of polycarbonaturethane to improve endothelialization--Effect of shear stress as a critical factor for biocompatibility control.

    PubMed

    Lukas, Karin; Thomas, Ulrich; Gessner, André; Wehner, Daniel; Schmid, Thomas; Schmid, Christof; Lehle, Karla

    2016-04-01

    Medical devices made of polycarbonaturethane (PCU) combine excellent mechanical properties and little biological degradation, but restricted hemocompatibility. Modifications of PCU might reduce platelet adhesion and promote stable endothelialization. PCU was modified using gas plasma treatment, binding of hydrogels, and coupling of cell-active molecules (modified heparin, anti-thrombin III (ATIII), argatroban, fibronectin, laminin-nonapeptide, peptides with integrin-binding arginine-glycine-aspartic acid (RGD) motif). Biocompatibility was verified with static and dynamic cell culture techniques. Blinded analysis focused on improvement in endothelial cell (EC) adhesion/proliferation, anti-thrombogenicity, reproducible manufacturing process, and shear stress tolerance of ECs. EC adhesion and antithrombogenicity were achieved with 9/35 modifications. Additionally, 6/9 stimulated EC proliferation and 3/6 modification processes were highly reproducible for endothelialization. The latter modifications comprised immobilization of ATIII (A), polyethyleneglycole-diamine-hydrogel (E) and polyethylenimine-hydrogel connected with modified heparin (IH). Under sheer stress, only the IH modification improved EC adhesion within the graft. However, ECs did not arrange in flow direction and cell anchorage was restricted. Despite large variation in surface modification chemistry and improved EC adhesion under static culture conditions, additional introduction of shear stress foiled promising preliminary data. Therefore, biocompatibility testing required not only static tests but also usage of physiological conditions such as shear stress in the case of vascular grafts. PMID:26762398

  13. Quantitative Assessment of Wall Shear Stress in an Aortic Coarctation - Impact of Virtual Interventions

    NASA Astrophysics Data System (ADS)

    Karlsson, Matts; Andersson, Magnus; Lantz, Jonas

    2014-11-01

    Turbulent and wall impinging blood flow causes abnormal shear forces onto the lumen and may play an important role in the pathogenesis of numerous cardiovascular diseases. In the present study, wall shear stress (WSS) and related flow parameters were studied in a pre-treated aortic coarctation (CoA) as well as after several virtual interventions using computational fluid dynamics (CFD). Patient-specific geometry and flow conditions were derived from magnetic resonance imaging (MRI) data. Finite element analysis was performed to acquire six different dilated CoAs. The unsteady pulsatile flow was resolved by large eddy simulation (LES) including non-Newtonian blood rheology. Pre-intervention, the presence of jet flow wall impingement caused an elevated WSS zone, with a distal region of low and oscillatory WSS. After intervention, cases with a more favorable centralized jet showed reduced high WSS values at the opposed wall. Despite significant turbulence reduction post-treatment, enhanced regions of low and oscillatory WSS were observed for all cases. This numerical method has demonstrated the morphological impact on WSS distribution in an CoA. With the predictability and validation capabilities of a combined CFD/MRI approach, a step towards patient-specific intervention planning is taken.

  14. Dynamics of non-Brownian fiber suspensions under periodic shear.

    PubMed

    Franceschini, Alexandre; Filippidi, Emmanouela; Guazzelli, Elisabeth; Pine, David J

    2014-09-21

    We report experiments studying the dynamics of dense non-Brownian fiber suspensions subjected to periodic oscillatory shear. We find that periodic shear initially causes fibers to collide and to undergo irreversible diffusion. As time progresses, the fibers tend to orient in the vorticity direction while the number of collisions decreases. Ultimately, the system goes to one of two steady states: an absorbing steady state, where collisions cease and the fibers undergo reversible trajectories; an active state, where fibers continue to collide causing them to diffuse and undergo irreversible trajectories. Collisions between fibers can be characterized by an effective volume fraction Φ with a critical volume fraction Φc that separates absorbing from active (diffusing) steady states. The effective volume fraction Φ depends on the mean fiber orientation and thus decreases in time as fibers progressively orient under periodic shear. In the limit that the temporal evolution of Φ is slow compared to the activity relaxation time τ, all the data for all strain amplitudes and all concentrations can be scaled onto a single master curve with a functional dependence well-described by t(-β/ν)R(e(-t)R), where tR is the rescaled time. As Φ → Φc, τ diverges. Therefore, for experiments in which Φ(t) starts above Φc but goes to a steady state below Φc, departures from scaling are observed for Φ very near Φc. The critical exponents are measured to be β = 0.84 ± 0.04 and ν = 1.1 ± 0.1, which is consistent with the Manna universality class for directed percolation. PMID:25068577

  15. Shear wave transducer for stress measurements in boreholes

    DOEpatents

    Mao, Nai-Hsien

    1987-01-01

    A technique and apparatus for estimating in situ stresses by measuring stress-induced velocity anisotropy around a borehole. Two sets each of radially and tangentially polarized transducers are placed inside the hole with displacement directions either parallel or perpendicular to the principal stress directions. With this configuration, relative travel times are measured by both a pulsed phase-locked loop technique and a cross correlation of digitized waveforms. The biaxial velocity data is used to back-calculate the applied stress.

  16. The environment shear stress field for the 1976 Tangshan earthquake sequence

    NASA Astrophysics Data System (ADS)

    Chen, Pei-Shan; Xiao, Lei; Bai, Tong-Xia; Wang, Xi-Li

    1994-11-01

    The environment shear stress of Tangshan main earthquake and 38 great aftershocks have been calculated by the acceleration data of Tangshan earthquake sequence. The environment shear stress for 52 smaller aftershocks from July of 1982 to July of 1984 have also been calculated by use of the digital data of the Sino-American cooperation recorded by the instrumental arrays in Tangshan. The results represent that the environment shear stress τ0 values have a weak dependence on the seismic moment, only the small and moderate earthquakes will be able to occur in the region with smaller τ0 value and the large earthquakes are only in the region with greater τ0 value. The peak acceleration, velocity and displacement will be larger for the earthquakes occurred in the region with greater τ0 value, Therefore, the measurement of environment shear stress τ0 value for the significant region will play an important role in earthquake prediction and engineering shock-proof. The environment shear stress values for the great aftershocks occurred in the two ends of the main fault are often higher than that for the main shock. This case may represent the stress concentration in the two ends of the fault. This phenomenon provides the references for the place where the great aftershock will occur.

  17. Change in properties of the glycocalyx affects the shear rate and stress distribution on endothelial cells.

    PubMed

    Wang, Wen

    2007-06-01

    The endothelial glycocalyx mediates interactions between the blood flow and the endothelium. This study aims to evaluate, quantitatively, effects of structural change of the glycocalyx on stress distribution and shear rate on endothelial cells. In the study, the endothelial glycocalyx is modeled as a surface layer of fiber matrix and when exposed to laminar shear flow, the matrix deforms. Fluid velocity and stress distribution inside the matrix and on cell membranes are studied based on a binary mixture theory. Parameters, such as the height and porosity of the matrix and the drag coefficient between fluid and matrix fibrils, are based on available data and estimation from experiments. Simple theoretical solutions are achieved for fluid velocity and stress distribution in the surface matrix. Degradation of the matrix, e.g., by enzyme digestion, is represented by reductions in the volume fraction of fibrils, height, and drag coefficient. From a force balance, total stress on endothelial surface remains constant regardless of structural alteration of the glycocalyx. However, the stress that is transmitted to endothelial cells by direct "pulling" of fiber branches of the glycocalyx is reduced significantly. Fluid shear rate at the cell membrane, on the other hand, increases. The study gives quantitative insight into the effect of the structural change of the glycocalyx on the shear rate and pulling stress on the endothelium. Results can be used to interpret experiments on effects of the glycocalyx in shear induced endothelial responses. PMID:17536899

  18. Endothelial cell and model membranes respond to shear stress by rapidly decreasing the order of their lipid phases.

    PubMed

    Yamamoto, Kimiko; Ando, Joji

    2013-03-01

    Endothelial cells (ECs) sense shear stress and transduce blood flow information into functional responses that play important roles in vascular homeostasis and pathophysiology. A unique feature of shear-stress-sensing is the involvement of many different types of membrane-bound molecules, including receptors, ion channels and adhesion proteins, but the mechanisms remain unknown. Because cell membrane properties affect the activities of membrane-bound proteins, shear stress might activate various membrane-bound molecules by altering the physical properties of EC membranes. To determine how shear stress influences the cell membrane, cultured human pulmonary artery ECs were exposed to shear stress and examined for changes in membrane lipid order and fluidity by Laurdan two-photon imaging and FRAP measurements. Upon shear stress stimulation, the lipid order of EC membranes rapidly decreased in an intensity-dependent manner, and caveolar membrane domains changed from the liquid-ordered state to the liquid-disordered state. Notably, a similar decrease in lipid order occurred when the artificial membranes of giant unilamellar vesicles were exposed to shear stress, suggesting that this is a physical phenomenon. Membrane fluidity increased over the entire EC membranes in response to shear stress. Addition of cholesterol to ECs abolished the effects of shear stress on membrane lipid order and fluidity and markedly suppressed ATP release, which is a well-known EC response to shear stress and is involved in shear-stress Ca(2+) signaling. These findings indicate that EC membranes directly respond to shear stress by rapidly decreasing their lipid phase order and increasing their fluidity; these changes could be linked to shear-stress-sensing and response mechanisms. PMID:23378020

  19. Dynamic and Thermal Turbulent Time Scale Modelling for Homogeneous Shear Flows

    NASA Technical Reports Server (NTRS)

    Schwab, John R.; Lakshminarayana, Budugur

    1994-01-01

    A new turbulence model, based upon dynamic and thermal turbulent time scale transport equations, is developed and applied to homogeneous shear flows with constant velocity and temperature gradients. The new model comprises transport equations for k, the turbulent kinetic energy; tau, the dynamic time scale; k(sub theta), the fluctuating temperature variance; and tau(sub theta), the thermal time scale. It offers conceptually parallel modeling of the dynamic and thermal turbulence at the two equation level, and eliminates the customary prescription of an empirical turbulent Prandtl number, Pr(sub t), thus permitting a more generalized prediction capability for turbulent heat transfer in complex flows and geometries. The new model also incorporates constitutive relations, based upon invariant theory, that allow the effects of nonequilibrium to modify the primary coefficients for the turbulent shear stress and heat flux. Predictions of the new model, along with those from two other similar models, are compared with experimental data for decaying homogeneous dynamic and thermal turbulence, homogeneous turbulence with constant temperature gradient, and homogeneous turbulence with constant temperature gradient and constant velocity gradient. The new model offers improvement in agreement with the data for most cases considered in this work, although it was no better than the other models for several cases where all the models performed poorly.

  20. Comminution of Ceramic Materials Under High-Shear Dynamic Compaction

    NASA Astrophysics Data System (ADS)

    Homel, Michael; Loiseau, Jason; Higgins, Andrew; Herbold, Eric; Hogan, Jamie

    The post-failure ``granular flow'' response of high-strength lightweight ceramics has important implications on the materials' effectiveness for ballistic protection. We study the dynamic compaction and shear flow of ceramic fragments and powders using computational and experimental analysis of a collapsing thick-walled cylinder geometry. Using newly developed tools for mesoscale simulation of brittle materials, we study the effect of fracture, comminution, shear-enhanced dilatation, and frictional contact on the continuum compaction response. Simulations are directly validated through particle Doppler velocimetry measurements at the inner surface of the cylindrical powder bed. We characterize the size distribution and morphologies of the initial and compacted material fragments to both validate the computational model and to elucidate the dominant failure processes. A portion of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC. LLNL-ABS-678862.

  1. Magnetic field effects on shear and normal stresses in magnetorheological finishing.

    PubMed

    Lambropoulos, John C; Miao, Chunlin; Jacobs, Stephen D

    2010-09-13

    We use a recent experimental technique to measure in situ shear and normal stresses during magnetorheological finishing (MRF) of a borosilicate glass over a range of magnetic fields. At low fields shear stresses increase with magnetic field, but become field-independent at higher magnetic fields. Micromechanical models of formation of magnetic particle chains suggest a complex behavior of magnetorheological (MR) fluids that combines fluid- and solid-like responses. We discuss the hypothesis that, at higher fields, slip occurs between magnetic particle chains and the immersed glass part, while the normal stress is governed by the MRF ribbon elasticity. PMID:20940866

  2. Suppression of endothelial t-PA expression by prolonged high laminar shear stress

    SciTech Connect

    Ulfhammer, Erik; Carlstroem, Maria; Bergh, Niklas; Larsson, Pia; Karlsson, Lena; Jern, Sverker

    2009-02-06

    Primary hypertension is associated with an impaired capacity for acute release of endothelial tissue-type plasminogen activator (t-PA), which is an important local protective response to prevent thrombus extension. As hypertensive vascular remodeling potentially results in increased vascular wall shear stress, we investigated the impact of shear on regulation of t-PA. Cultured human endothelial cells were exposed to low ({<=}1.5 dyn/cm{sup 2}) or high (25 dyn/cm{sup 2}) laminar shear stress for up to 48 h in two different experimental models. Using real-time RT-PCR and ELISA, shear stress was observed to time and magnitude-dependently suppress t-PA transcript and protein secretion to approximately 30% of basal levels. Mechanistic experiments revealed reduced nuclear protein binding to the t-PA specific CRE element (EMSA) and an almost completely abrogated shear response with pharmacologic JNK inhibition. We conclude that prolonged high laminar shear stress suppresses endothelial t-PA expression and may therefore contribute to the enhanced risk of arterial thrombosis in hypertensive disease.

  3. Yield shear stress model of magnetorheological fluids based on exponential distribution

    NASA Astrophysics Data System (ADS)

    Guo, Chu-wen; Chen, Fei; Meng, Qing-rui; Dong, Zi-xin

    2014-06-01

    The magnetic chain model that considers the interaction between particles and the external magnetic field in a magnetorheological fluid has been widely accepted. Based on the chain model, a yield shear stress model of magnetorheological fluids was proposed by introducing the exponential distribution to describe the distribution of angles between the direction of magnetic field and the chain formed by magnetic particles. The main influencing factors were considered in the model, such as magnetic flux density, intensity of magnetic field, particle size, volume fraction of particles, the angle of magnetic chain, and so on. The effect of magnetic flux density on the yield shear stress was discussed. The yield stress of aqueous Fe3O4 magnetreological fluids with volume fraction of 7.6% and 16.2% were measured by a device designed by ourselves. The results indicate that the proposed model can be used for calculation of yield shear stress with acceptable errors.

  4. Dynamic shear of granular material under variable gravity conditions

    NASA Technical Reports Server (NTRS)

    White, B. R.; Klein, S. P.

    1988-01-01

    This paper describes some experiments with granular materials which recently have been conducted aboard the NASA KC-135 aircraft during variable gravity maneuvers. The main experimental apparatus consisted of a small drum containing granular material which was rotated slowly while the angle assumed by the slip surface with respect to the horizontal was observed and recorded photographically. Conventional wisdom has held that this 'dynamic angle of response' was a material constant, independent of (among other things) gravitational level. The results presented here are quite contrary, suggesting instead an angle that varies with the reciprocal of the square root of gravity. This finding may have important consequences on the understanding of many active processes in Planetary Geology involving granular materials and may provide qualitative confirmation of some of the theoretical predictions of modern models of granular shear flows.

  5. Non-Newtonian behavior and molecular structure of Cooee bitumen under shear flow: A non-equilibrium molecular dynamics study

    NASA Astrophysics Data System (ADS)

    Lemarchand, Claire A.; Bailey, Nicholas P.; Todd, Billy D.; Daivis, Peter J.; Hansen, Jesper S.

    2015-06-01

    The rheology and molecular structure of a model bitumen (Cooee bitumen) under shear are investigated in the non-Newtonian regime using non-equilibrium molecular dynamics simulations. The shear viscosity, normal stress differences, and pressure of the bitumen mixture are computed at different shear rates and different temperatures. The model bitumen is shown to be a shear-thinning fluid at all temperatures. In addition, the Cooee model is able to reproduce experimental results showing the formation of nanoaggregates composed of stacks of flat aromatic molecules in bitumen. These nanoaggregates are immersed in a solvent of saturated hydrocarbon molecules. At a fixed temperature, the shear-shinning behavior is related not only to the inter- and intramolecular alignments of the solvent molecules but also to the decrease of the average size of the nanoaggregates at high shear rates. The variation of the viscosity with temperature at different shear rates is also related to the size and relative composition of the nanoaggregates. The slight anisotropy of the whole sample due to the nanoaggregates is considered and quantified. Finally, the position of bitumen mixtures in the broad literature of complex systems such as colloidal suspensions, polymer solutions, and associating polymer networks is discussed.

  6. Analysis of shear stress distribution in pushout process of fiber-reinforced ceramics

    SciTech Connect

    Honda, Kouichi; Kagawa, Yutaka

    1995-04-01

    The interfacial shear stress distribution of a thin specimen of SiC fiber-reinforced glass matrix composite (fiber volume fraction of 0.1, 0.5, and 0.7) during a fiber pushout process was subjected to finite element analysis using a three concentric axisymmetrical model which consisted of fiber, matrix, and composite. A stress criterion was used to determine interface debonding. Effects of thermally-induced stress and a post debond sliding process at the interface were also included in the analysis. The analytical result showed that shear stress near the specimen surface was introduced during the specimen preparation process. Before the interfacial debonding, the distribution of shear stress during the pushout test was affected by the existence of thermally-induced stress in the specimen. The interfacial shear debonding initiated {approximately}30 {mu}m below the pushing surface and the sliding at the debonded interface proceeded in the direction of both the pushing surface and back surface from the peak shear position; the debonding from the back surface initiated just before the complete debonding of the interface. The pushout load-displacement curve near the origin was straight, however, after the existence of interface sliding at the debonded interface, the curve exhibited non-linearity with the increase in applied load up to the complete debonding at the interface. This debonding process was essentially independent of the fiber volume fraction. The results indicate that the total of thermally-induced stress in the specimen and shear stress distribution generated by applied load are important for the initiation of debonding and the frictional sliding process of the thin specimen pushout test.

  7. Nanoscale patterning of extracellular matrix alters endothelial function under shear stress

    PubMed Central

    Nakayama, Karina H.; Surya, Vinay N.; Gole, Monica; Walker, Travis; Yang, Weiguang; Lai, Edwina S.; Ostrowski, Maggie; Fuller, Gerald G.; Dunn, Alexander R.; Huang, Ngan F.

    2016-01-01

    The role of nanotopographical extracellular matrix (ECM) cues on vascular endothelial cell (EC) organization and function is not well-understood, despite the composition of nano- to micro-scale fibrillar ECMs within blood vessels. Instead, the predominant modulator of EC organization and function is traditionally thought to be hemodynamic shear stress, in which uniform shear stress induces parallel-alignment of ECs with anti-inflammatory function, whereas disturbed flow induce pro-inflammatory cells in disorganized configuration. Since shear stress acts on ECs by applying a mechanical force concomitant with inducing spatial patterning of the cells, we sought to decouple the effects of shear stress using parallel-aligned nanofibrillar collagen films that induce parallel EC alignment prior to stimulation with disturbed flow resulting from spatial wall shear stress gradients. Using real time live-cell imaging, we tracked the alignment, migration trajectories, proliferation, and anti-inflammatory behavior of ECs when they were cultured on parallel-aligned or randomly oriented nanofibrillar films. Intriguingly, ECs cultured on aligned nanofibrillar films remained well-aligned and migrated predominantly along the direction of aligned nanofibrils, despite exposure to shear stress orthogonal to the direction of the aligned nanofibrils. Furthermore, in stark contrast to ECs cultured on randomly oriented films, ECs on aligned nanofibrillar films exposed to disturbed flow had significantly reduced inflammation and proliferation, while maintaining intact intercellular junctions. This work reveals fundamental insights into the importance of nanoscale ECM interactions in the maintenance of endothelial function. Importantly, it provides new insight into how ECs respond to opposing cues derived from nanotopography and mechanical shear force, and has strong implications in the design of polymeric conduits and bioengineered tissues. PMID:26670737

  8. Localized shear deformation and softening of bulk metallic glass: stress or temperature driven?

    PubMed Central

    Ketov, S. V.; Louzguine-Luzgin, D. V.

    2013-01-01

    Metallic glasses due to their unique combination of physical and chemical properties have a great potential in various applications: materials for construction, medical, MEMs devices and so on. The deformation mechanism in metallic glasses is very much different from that in conventional crystalline materials and not yet fully understood. Here we are trying to find out what drives shear deformation in metallic glasses. The compression experiments of the bulk metallic glassy (BMG) samples coated with tin, Rose metal and indium were performed. There were no melting sites of the coating observed near individual shear bands. Melting occurred only near fracture surface, near microcracks and in the places of shear band concentrations. The results indicate that shear banding is rather a stress driven process while the temperature rise that was observed takes place due to friction forces in the viscous supercooled liquid thin layer in the shear bands. PMID:24100784

  9. Nature's rheologists: Lymphatic endothelial cells control migration in response to shear stress

    NASA Astrophysics Data System (ADS)

    Fuller, Gerald; Dunn, Alex; Surya, Vinay

    2015-03-01

    Endothelial cells (ECs) line the inner surface of blood and lymphatic vessels and are sensitive to fluid flow as part of their physiological function. EC organization, migration and vessel development are profoundly influenced by shear stresses, with important implications in cardiovascular disease and tumor metastasis. How ECs sense fluid flow is a central and unanswered question in cardiovascular biology. We developed a high-throughput live-cell flow chamber that models the gradients in wall shear stress experienced by ECs in vivo. Live-cell imaging allows us to probe cellular responses to flow, most notably EC migration, which has a key role in vessel remodeling. We find that most EC subtypes, including ECs from the venous, arterial, and microvascular systems, migrate in the flow direction. In contrast, human lymphatic microvascular ECs (hLMVECs) migrate against flow and up spatial gradients in wall shear stress. Further experiments reveal that hLMVECs are sensitive to the magnitude, direction, and the local spatial gradients in wall shear stress. Lastly, recent efforts have aimed to link this directional migration to spatial gradients in cell-mediated small molecule emission that may be linked to the gradient in wall shear stress.

  10. Estimation of shear stress in counter-current gas-liquid annular two-phase flow

    NASA Astrophysics Data System (ADS)

    Abe, Yutaka; Akimoto, Hajime; Murao, Yoshio

    1991-01-01

    The accuracy of the correlations of the friction factor is important for the counter-current flow (CCF) analysis with two-fluid model. However, existing two fluid model codes use the correlations of friction factors for co-current flow or correlation developed based on the assumption of no wall shear stress. The assessment calculation for two fluid model code with those existing correlations of friction factors shows the falling water flow rate is overestimated. Analytical model is developed to calculate the shear stress distribution in water film at CCF in order to get the information on the shear stress at the interface and the wall. The analytical results with the analysis model and Bharathan's CCF data shows that the wall shear stress acting on the falling water film is almost the same order as the interfacial shear stress and the correlations for co-current flow cannot be applied to the counter-current flow. Tentative correlations of the interfacial and the wall friction factors are developed based on the results of the present study.

  11. Cultivation of shear stress sensitive microorganisms in disposable bag reactor systems.

    PubMed

    Jonczyk, Patrick; Takenberg, Meike; Hartwig, Steffen; Beutel, Sascha; Berger, Ralf G; Scheper, Thomas

    2013-09-20

    Technical scale (≥5l) cultivations of shear stress sensitive microorganisms are often difficult to perform, as common bioreactors are usually designed to maximize the oxygen input into the culture medium. This is achieved by mechanical stirrers, causing high shear stress. Examples for shear stress sensitive microorganisms, for which no specific cultivation systems exist, are many anaerobic bacteria and fungi, such as basidiomycetes. In this work a disposable bag bioreactor developed for cultivation of mammalian cells was investigated to evaluate its potential to cultivate shear stress sensitive anaerobic Eubacterium ramulus and shear stress sensitive basidiomycetes Flammulina velutipes and Pleurotus sapidus. All cultivations were compared with conventional stainless steel stirred tank reactors (STR) cultivations. Good growth of all investigated microorganisms cultivated in the bag reactor was found. E. ramulus showed growth rates of μ=0.56 h⁻¹ (bag) and μ=0.53 h⁻¹ (STR). Differences concerning morphology, enzymatic activities and growth in fungal cultivations were observed. In the bag reactor growth in form of small, independent pellets was observed while STR cultivations showed intense aggregation. F. velutipes reached higher biomass concentrations (21.2 g l⁻¹ DCW vs. 16.8 g l⁻¹ DCW) and up to 2-fold higher peptidolytic activities in comparison to cell cultivation in stirred tank reactors. PMID:23892193

  12. Bi-crystallographic lattice structure directs grain boundary motion under shear stress

    PubMed Central

    Wan, Liang; Han, Weizhong; Chen, Kai

    2015-01-01

    Shear stress driven grain boundary (GB) migration was found to be a ubiquitous phenomenon in small grained polycrystalline materials. Here we show that the GB displacement shift complete (DSC) dislocation mechanism for GB shear coupled migration is still functioning even if the geometry orientation of the GBs deviates a few degrees from the appropriate coincidence site lattice (CSL) GBs. It means that any large angle GB can have a considerable chance to be such a “CSL-related GB” for which the shear coupled GB migration motion can happen by the GB DSC dislocation mechanism. We conclude that the CSL-DSC bi-crystallographic lattice structure in GB is the main reason that GB can migrate under shear stress. PMID:26304553

  13. Bi-crystallographic lattice structure directs grain boundary motion under shear stress

    NASA Astrophysics Data System (ADS)

    Wan, Liang; Han, Weizhong; Chen, Kai

    2015-08-01

    Shear stress driven grain boundary (GB) migration was found to be a ubiquitous phenomenon in small grained polycrystalline materials. Here we show that the GB displacement shift complete (DSC) dislocation mechanism for GB shear coupled migration is still functioning even if the geometry orientation of the GBs deviates a few degrees from the appropriate coincidence site lattice (CSL) GBs. It means that any large angle GB can have a considerable chance to be such a “CSL-related GB” for which the shear coupled GB migration motion can happen by the GB DSC dislocation mechanism. We conclude that the CSL-DSC bi-crystallographic lattice structure in GB is the main reason that GB can migrate under shear stress.

  14. Heart rate reduction with ivabradine promotes shear stress-dependent anti-inflammatory mechanisms in arteries.

    PubMed

    Luong, Le; Duckles, Hayley; Schenkel, Torsten; Mahmoud, Marwa; Tremoleda, Jordi L; Wylezinska-Arridge, Marzena; Ali, Majid; Bowden, Neil P; Villa-Uriol, Mari-Cruz; van der Heiden, Kim; Xing, Ruoyu; Gijsen, Frank J; Wentzel, Jolanda; Lawrie, Allan; Feng, Shuang; Arnold, Nadine; Gsell, Willy; Lungu, Angela; Hose, Rodney; Spencer, Tim; Halliday, Ian; Ridger, Victoria; Evans, Paul C

    2016-07-01

    Blood flow generates wall shear stress (WSS) which alters endothelial cell (EC) function. Low WSS promotes vascular inflammation and atherosclerosis whereas high uniform WSS is protective. Ivabradine decreases heart rate leading to altered haemodynamics. Besides its cardio-protective effects, ivabradine protects arteries from inflammation and atherosclerosis via unknown mechanisms. We hypothesised that ivabradine protects arteries by increasing WSS to reduce vascular inflammation. Hypercholesterolaemic mice were treated with ivabradine for seven weeks in drinking water or remained untreated as a control. En face immunostaining demonstrated that treatment with ivabradine reduced the expression of pro-inflammatory VCAM-1 (p<0.01) and enhanced the expression of anti-inflammatory eNOS (p<0.01) at the inner curvature of the aorta. We concluded that ivabradine alters EC physiology indirectly via modulation of flow because treatment with ivabradine had no effect in ligated carotid arteries in vivo, and did not influence the basal or TNFα-induced expression of inflammatory (VCAM-1, MCP-1) or protective (eNOS, HMOX1, KLF2, KLF4) genes in cultured EC. We therefore considered whether ivabradine can alter WSS which is a regulator of EC inflammatory activation. Computational fluid dynamics demonstrated that ivabradine treatment reduced heart rate by 20 % and enhanced WSS in the aorta. In conclusion, ivabradine treatment altered haemodynamics in the murine aorta by increasing the magnitude of shear stress. This was accompanied by induction of eNOS and suppression of VCAM-1, whereas ivabradine did not alter EC that could not respond to flow. Thus ivabradine protects arteries by altering local mechanical conditions to trigger an anti-inflammatory response. PMID:27075869

  15. Endothelial mitochondria regulate the intracellular Ca2+ response to fluid shear stress.

    PubMed

    Scheitlin, Christopher G; Julian, Justin A; Shanmughapriya, Santhanam; Madesh, Muniswamy; Tsoukias, Nikolaos M; Alevriadou, B Rita

    2016-03-15

    Shear stress is known to stimulate an intracellular free calcium concentration ([Ca(2+)]i) response in vascular endothelial cells (ECs). [Ca(2+)]i is a key second messenger for signaling that leads to vasodilation and EC survival. Although it is accepted that the shear-induced [Ca(2+)]i response is, in part, due to Ca(2+) release from the endoplasmic reticulum (ER), the role of mitochondria (second largest Ca(2+) store) is unknown. We hypothesized that the mitochondria play a role in regulating [Ca(2+)]i in sheared ECs. Cultured ECs, loaded with a Ca(2+)-sensitive fluorophore, were exposed to physiological levels of shear stress. Shear stress elicited [Ca(2+)]i transients in a percentage of cells with a fraction of them displaying oscillations. Peak magnitudes, percentage of oscillating ECs, and oscillation frequencies depended on the shear level. [Ca(2+)]i transients/oscillations were present when experiments were conducted in Ca(2+)-free solution (plus lanthanum) but absent when ECs were treated with a phospholipase C inhibitor, suggesting that the ER inositol 1,4,5-trisphosphate receptor is responsible for the [Ca(2+)]i response. Either a mitochondrial uncoupler or an electron transport chain inhibitor, but not a mitochondrial ATP synthase inhibitor, prevented the occurrence of transients and especially inhibited the oscillations. Knockdown of the mitochondrial Ca(2+) uniporter also inhibited the shear-induced [Ca(2+)]i transients/oscillations compared with controls. Hence, EC mitochondria, through Ca(2+) uptake/release, regulate the temporal profile of shear-induced ER Ca(2+) release. [Ca(2+)]i oscillation frequencies detected were within the range for activation of mechanoresponsive kinases and transcription factors, suggesting that dysfunctional EC mitochondria may contribute to cardiovascular disease by deregulating the shear-induced [Ca(2+)]i response. PMID:26739489

  16. The impact of static stress change, dynamic stress change, and the background stress on aftershock focal mechanisms

    USGS Publications Warehouse

    Hardebeck, Jeanne L.

    2014-01-01

    The focal mechanisms of earthquakes in Southern California before and after four M ≥ 6.7 main shocks provide insight into how fault systems respond to stress and changes in stress. The main shock static stress changes have two observed impacts on the seismicity: changing the focal mechanisms in a given location to favor those aligned with the static stress change and changing the spatial distribution of seismicity to favor locations where the static stress change aligns with the background stress. The aftershock focal mechanisms are significantly aligned with the static stress changes for absolute stress changes of ≥ 0.02 MPa, for up to ~20 years following the main shock. The dynamic stress changes have similar, although smaller, effects on the local focal mechanisms and the spatial seismicity distribution. Dynamic stress effects are best observed at long periods (30–60 s) and for metrics based on repeated stress cycling in the same direction. This implies that dynamic triggering operates, at least in part, through cyclic shear stress loading in the direction of fault slip. The background stress also strongly controls both the preshock and aftershock mechanisms. While most aftershock mechanisms are well oriented in the background stress field, 10% of aftershocks are identified as poorly oriented outliers, which may indicate limited heterogeneity in the postmain shock stress field. The fault plane orientations of the outliers are well oriented in the background stress, while their slip directions are not, implying that the background stress restricts the distribution of available fault planes.

  17. Influence of thickness and permeability of endothelial surface layer on transmission of shear stress in capillaries

    NASA Astrophysics Data System (ADS)

    Zhang, SongPeng; Zhang, XiangJun; Tian, Yu; Meng, YongGang; Lipowsky, Herbert

    2015-07-01

    The molecular coating on the surface of microvascular endothelium has been identified as a barrier to transvascular exchange of solutes. With a thickness of hundreds of nanometers, this endothelial surface layer (ESL) has been treated as a porous domain within which fluid shear stresses are dissipated and transmitted to the solid matrix to initiate mechanotransduction events. The present study aims to examine the effects of the ESL thickness and permeability on the transmission of shear stress throughout the ESL. Our results indicate that fluid shear stresses rapidly decrease to insignificant levels within a thin transition layer near the outer boundary of the ESL with a thickness on the order of ten nanometers. The thickness of the transition zone between free fluid and the porous layer was found to be proportional to the square root of the Darcy permeability. As the permeability is reduced ten-fold, the interfacial fluid and solid matrix shear stress gradients increase exponentially two-fold. While the interfacial fluid shear stress is positively related to the ESL thickness, the transmitted matrix stress is reduced by about 50% as the ESL thickness is decreased from 500 to 100 nm, which may occur under pathological conditions. Thus, thickness and permeability of the ESL are two main factors that determine flow features and the apportionment of shear stresses between the fluid and solid phases of the ESL. These results may shed light on the mechanisms of force transmission through the ESL and the pathological events caused by alterations in thickness and permeability of the ESL.

  18. Migration arising from gradients in shear stress: Particle distributions in Poiseuille flow

    NASA Technical Reports Server (NTRS)

    Leighton, D. T., Jr.

    1988-01-01

    Experimental evidence for the existence of shear induced migration processes is reviewed and the mechanism by Leighton and Acrivos (1987b) is described in detail. The proposed mechanism is shown to lead to the existence of an additional shear induced migration in the presence of gradients in shear stress such as would be found in Poiseuille flow, and which may be used to predict the amplitude of the observed short-term viscosity increase. The concentration and velocity profiles which result from such a migration are discussed in detail and are compared to the experimental observations of Karnis, Goldsmith and Mason (1966).

  19. Rheological regional properties of brain tissue studied under cyclic creep/ recovery shear stresses

    NASA Astrophysics Data System (ADS)

    Boudjema, F.; Lounis, M.; Khelidj, B.; Bessai, N.

    2015-04-01

    The rheological properties of brain tissue were studied by repeated creep-recovery shear tests under static conditions for different regions. Corpus callosum CC, Thalamus Th and Corona radiata CR. Non-linear viscoelastic model was also proposed to characterize the transient/steady states of shear creep results. From the creep-recovery data it was obvious that the brain tissues show high regional anisotropy. However. the both samples exhibit fluid viscoelastic properties in the first shear stress cycle of 100 Pa, while this behaviour evolutes to solid viscoelastic with cyclic effect.

  20. Shear veins observed within anisotropic fabric at high angles to the maximum compressive stress

    NASA Astrophysics Data System (ADS)

    Fagereng, Åke; Remitti, Francesca; Sibson, Richard H.

    2010-07-01

    Some faults seem to slip at unusually high angles (>45°) relative to the orientation of the greatest principal compressive stress. This implies that these faults are extremely weak compared with the surrounding rock. Laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre-existing frictionally weak surface, weakening from chemical reactions, elevated fluid pressure or dissolution-precipitation creep. Here we describe shear veins within the Chrystalls Beach accretionary mélange, New Zealand. The mélange is a highly sheared assemblage of relatively competent rock within a cleaved, anisotropic mudstone matrix. The orientation of the shear veins-compared with the direction of hydrothermal extension veins that formed contemporaneously-indicates that they were active at an angle of 80°+/-5° to the greatest principal compressive stress. We show that the shear veins developed incrementally along the cleavage planes of the matrix. Thus, we suggest that episodic slip was facilitated by the anisotropic internal fabric, in a fluid-overpressured, heterogeneous shear zone. A similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings. We therefore conclude that incremental slip along a pre-existing planar fabric, coupled to high fluid pressure and dissolution-precipitation creep, may explain active slip on severely misoriented faults.

  1. Effects of fluid shear stress on polyelectrolyte multilayers by neutron scattering studies.

    PubMed

    Singh, Saurabh; Junghans, Ann; Watkins, Erik; Kapoor, Yash; Toomey, Ryan; Majewski, Jaroslaw

    2015-03-10

    The structure of layer-by-layer (LbL) deposited nanofilm coatings consists of alternating polyethylenimine (PEI) and polystyrenesulfonate (PSS) films deposited on a single crystal quartz substrate. LbL-deposited nanofilms were investigated by neutron reflectomery (NR) in contact with water in the static and fluid shear stress conditions. The fluid shear stress was applied through a laminar flow of the liquid parallel to the quartz/polymer interface in a custom-built solid-liquid interface cell. The scattering length density profiles obtained from NR results of these polyelectrolyte multilayers (PEM), measured under different shear conditions, showed proportional decrease of volume fraction of water hydrating the polymers. For the highest shear rate applied (ca. 6800 s(-1)) the water volume fraction decreased by approximately 7%. The decrease of the volume fraction of water was homogeneous through the thickness of the film. Since there were not any significant changes in the total polymer thickness, it resulted in negative osmotic pressures in the film. The PEM films were compared with the behavior of thin films of thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) deposited via spin-coating. The PEM and pNIPAM differ in their interactions with water molecules, and they showed opposite behaviors under the fluid shear stress. In both cases the polymer hydration was reversible upon the restoration of static conditions. A theoretical explanation is given to explain this difference in the effect of shear on hydration of polymeric thin films. PMID:25689755

  2. 4-Dimensional light-sheet microscopy to elucidate shear stress modulation of cardiac trabeculation

    PubMed Central

    Lee, Juhyun; Fei, Peng; Packard, René R. Sevag; Kang, Hanul; Xu, Hao; Baek, Kyung In; Jen, Nelson; Chen, Junjie; Yen, Hilary; Chi, Neil C.; Ho, Chih-Ming; Hsiai, Tzung K.

    2016-01-01

    Hemodynamic shear forces are intimately linked with cardiac development, during which trabeculae form a network of branching outgrowths from the myocardium. Mutations that alter Notch signaling also result in trabeculation defects. Here, we assessed whether shear stress modulates trabeculation to influence contractile function. Specifically, we acquired 4D (3D + time) images with light sheets by selective plane illumination microscopy (SPIM) for rapid scanning and deep axial penetration during zebrafish morphogenesis. Reduction of blood viscosity via gata1a morpholino oligonucleotides (MO) reduced shear stress, resulting in downregulation of Notch signaling and attenuation of trabeculation. Arrest of cardiomyocyte contraction either by troponin T type 2a (tnnt2a) MO or in weak atriumm58 (wea) mutants resulted in reduced shear stress and downregulation of Notch signaling and trabeculation. Integrating 4D SPIM imaging with synchronization algorithm demonstrated that coinjection of neuregulin1 mRNA with gata1 MO rescued trabeculation to restore contractile function in association with upregulation of Notch-related genes. Crossbreeding of Tg(flk:mCherry) fish, which allows visualization of the vascular system with the Tg(tp1:gfp) Notch reporter line, revealed that shear stress–mediated Notch activation localizes to the endocardium. Deleting endocardium via the clochesk4 mutants downregulated Notch signaling, resulting in nontrabeculated ventricle. Subjecting endothelial cells to pulsatile flow in the presence of the ADAM10 inhibitor corroborated shear stress–activated Notch signaling to modulate trabeculation. PMID:27018592

  3. Effects of fluid shear stress on polyelectrolyte multilayers by neutron scattering studies

    SciTech Connect

    Singh, Saurabh; Junghans, Ann; Watkins, Erik; Kapoor, Yash; Toomey, Ryan; Majewski, Jaroslaw

    2015-02-17

    The structure of layer-by-layer (LbL) deposited nanofilm coatings consists of alternating polyethylenimine (PEI) and polystyrenesulfonate (PSS) films deposited on a single crystal quartz substrate. LbL-deposited nanofilms were investigated by neutron reflectomery (NR) in contact with water in the static and fluid shear stress conditions. The fluid shear stress was applied through a laminar flow of the liquid parallel to the quartz/polymer interface in a custom-built solid–liquid interface cell. The scattering length density profiles obtained from NR results of these polyelectrolyte multilayers (PEM), measured under different shear conditions, showed proportional decrease of volume fraction of water hydrating the polymers. For the highest shear rate applied (ca. 6800 s–1) the water volume fraction decreased by approximately 7%. The decrease of the volume fraction of water was homogeneous through the thickness of the film. Since there were not any significant changes in the total polymer thickness, it resulted in negative osmotic pressures in the film. The PEM films were compared with the behavior of thin films of thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) deposited via spin-coating. The PEM and pNIPAM differ in their interactions with water molecules, and they showed opposite behaviors under the fluid shear stress. In both cases the polymer hydration was reversible upon the restoration of static conditions. Furthermore, a theoretical explanation is given to explain this difference in the effect of shear on hydration of polymeric thin films.

  4. Wall shear stress as a stimulus for carotid atherosclerotic plaque progression: An MRI-based CFD pilot study

    NASA Astrophysics Data System (ADS)

    Canton, Gador; Chiu, Bernard; Hatsukami, Tom; Kerwin, William; Yuan, Chun

    2010-11-01

    The aim of this study was to explore the hypothesis that intra-plaque hemorrhage, a feature associated with adverse outcomes and atherosclerotic plaque progression and destabilization, is more likely to occur in plaques with elevated levels of wall shear stress (WSS). We used multi-sequence in-vivo magnetic resonance imaging (MRI) to characterize ten human carotid atherosclerotic plaques and an MRI-based computational fluid dynamics (CFD) model to solve the equations governing the blood flow. Hemorrhage was detected within the necrotic core (intra-plaque hemorrhage) in five of these ten cases. WSS data were extracted from the results of the CFD simulations to compare patterns between the cases with and without hemorrhage. We computed the mean value of the WSS (for each time point of the cardiac cycle) at the region where a necrotic core was detected. The results from this pilot study indicate a possible link between the presence of hemorrhage within a lipid-rich necrotic core in human carotid atherosclerotic plaques and elevated levels of shear stress force acting on the luminal surface. Thus, elevated wall shear stress may be used as a high risk feature in advanced carotid atherosclerotic plaques.

  5. On the influence of dynamic stress variations on strain accumulation in fault zones

    NASA Astrophysics Data System (ADS)

    Grigoriev, A. S.; Shilko, E. V.; Astafurov, S. V.; Dimaki, A. V.; Vysotsky, E. M.; Psakhie, S. G.

    2015-10-01

    In this paper, a numerical study of the influence of the stress state of interface of the block medium structural elements on the deformation response of interface to the dynamic impacts. It is shown that the basic characteristics of the stress state determining the deformation response of the interface are the values of shear stress and mean stress. It is found that the dependence of the irreversible displacement at the interface zone initiated by dynamic impact on the reduced shear stress is described by the logistic function. Herewith, the influence of the mean stress and dynamic impact energy on the value of displacement initiated by dynamic impact can be taken into account by dependence of the logistic function numerator on these parameters.

  6. Steady and Dynamic Shear Rheological Properties of Buckwheat Starch-galactomannan Mixtures

    PubMed Central

    Choi, Dong Won; Chang, Yoon Hyuk

    2012-01-01

    This study investigated the effects of galacomannans (guar gum, tara gum, and locust bean gum) on the rheological properties of buckwheat starch pastes under steady and dynamic shear conditions. The power law and Casson models were applied to describe the flow behavior of the buckwheat starch and galactomannan mixtures. The values of the apparent viscosity (ηa,100), consistency index (K), and yield stress (σoc) for buckwheat starch-galactomannan mixtures were significantly greater than those for the control, indicating that there was a high synergism of the starch with galactomannans. The magnitudes of storage modulus (G′) and loss modulus (G″) for the starch-galactomannan mixtures increased with increasing frequency (ω). The dynamic moduli (G′, G″), and complex viscosity (η*) for the buckwheat starch-galactomannan mixtures were significantly higher than those for the control. PMID:24471083

  7. Shear viscosity for dense plasmas by equilibrium molecular dynamics in asymmetric Yukawa ionic mixtures.

    PubMed

    Haxhimali, Tomorr; Rudd, Robert E; Cabot, William H; Graziani, Frank R

    2015-11-01

    We present molecular dynamics (MD) calculations of shear viscosity for asymmetric mixed plasma for thermodynamic conditions relevant to astrophysical and inertial confinement fusion plasmas. Specifically, we consider mixtures of deuterium and argon at temperatures of 100-500 eV and a number density of 10^{25} ions/cc. The motion of 30,000-120,000 ions is simulated in which the ions interact via the Yukawa (screened Coulomb) potential. The electric field of the electrons is included in this effective interaction; the electrons are not simulated explicitly. Shear viscosity is calculated using the Green-Kubo approach with an integral of the shear stress autocorrelation function, a quantity calculated in the equilibrium MD simulations. We systematically study different mixtures through a series of simulations with increasing fraction of the minority high-Z element (Ar) in the D-Ar plasma mixture. In the more weakly coupled plasmas, at 500 eV and low Ar fractions, results from MD compare very well with Chapman-Enskog kinetic results. In the more strongly coupled plasmas, the kinetic theory does not agree well with the MD results. We develop a simple model that interpolates between classical kinetic theories at weak coupling and the Murillo Yukawa viscosity model at higher coupling. This hybrid kinetics-MD viscosity model agrees well with the MD results over the conditions simulated, ranging from moderately weakly coupled to moderately strongly coupled asymmetric plasma mixtures. PMID:26651805

  8. Study of shear viscosity for dense plasmas by equilibrium molecular dynamics in asymmetric Yukawa ionic mixtures

    NASA Astrophysics Data System (ADS)

    Haxhimali, Tomorr; Rudd, Robert; Cabot, William; Graziani, Frank

    2015-11-01

    We present molecular dynamics (MD) calculations of shear viscosity for asymmetric mixed plasma for thermodynamic conditions relevant to astrophysical and Inertial Confinement Fusion plasmas. Specifically, we consider mixtures of deuterium and argon at temperatures of 100-500 eV and a number density of 1025 ions/cc. The motion of 30000-120000 ions is simulated in which the ions interact via the Yukawa (screened Coulomb) potential. The electric field of the electrons is included in this effective interaction. Shear viscosity is calculated using the Green-Kubo approach with an integral of the shear stress autocorrelation function, a quantity calculated in the equilibrium MD simulations. We study different mixtures with increasing fraction of the minority high-Z element (Ar) in the D-Ar plasma mixture. In the more weakly coupled plasmas, at 500 eV and low Ar fractions, results from MD compare very well with Chapman-Enskog kinetic results. We introduce a model that interpolates between a screened-plasma kinetic theory at weak coupling and the Murillo Yukawa viscosity model at higher coupling. This hybrid kinetics-MD viscosity model agrees well with the MD results over the conditions simulated. This work was performed under the auspices of the US Dept. of Energy by Lawrence Livermore National Security, LLC under Contract DE-AC52-07NA27344.

  9. Shear viscosity for dense plasmas by equilibrium molecular dynamics in asymmetric Yukawa ionic mixtures

    NASA Astrophysics Data System (ADS)

    Haxhimali, Tomorr; Rudd, Robert E.; Cabot, William H.; Graziani, Frank R.

    2015-11-01

    We present molecular dynamics (MD) calculations of shear viscosity for asymmetric mixed plasma for thermodynamic conditions relevant to astrophysical and inertial confinement fusion plasmas. Specifically, we consider mixtures of deuterium and argon at temperatures of 100-500 eV and a number density of 1025 ions/cc. The motion of 30 000-120 000 ions is simulated in which the ions interact via the Yukawa (screened Coulomb) potential. The electric field of the electrons is included in this effective interaction; the electrons are not simulated explicitly. Shear viscosity is calculated using the Green-Kubo approach with an integral of the shear stress autocorrelation function, a quantity calculated in the equilibrium MD simulations. We systematically study different mixtures through a series of simulations with increasing fraction of the minority high-Z element (Ar) in the D-Ar plasma mixture. In the more weakly coupled plasmas, at 500 eV and low Ar fractions, results from MD compare very well with Chapman-Enskog kinetic results. In the more strongly coupled plasmas, the kinetic theory does not agree well with the MD results. We develop a simple model that interpolates between classical kinetic theories at weak coupling and the Murillo Yukawa viscosity model at higher coupling. This hybrid kinetics-MD viscosity model agrees well with the MD results over the conditions simulated, ranging from moderately weakly coupled to moderately strongly coupled asymmetric plasma mixtures.

  10. The Effects of Hemodynamic Shear Stress on Stemness of Acute Myelogenous Leukemia (AML)

    NASA Astrophysics Data System (ADS)

    Raddatz, Andrew; Triantafillu, Ursula; Kim, Yonghyun (John)

    2015-11-01

    Cancer stem cells (CSCs) have recently been identified as the root cause of tumors generated from cancer cell populations. This is because these CSCs are drug-resistant and have the ability to self-renew and differentiate. Current methods of culturing CSCs require much time and money, so cancer cell culture protocols, which maximize yield of CSCs are needed. It was hypothesized that the quantity of Acute myelogenous leukemia stem cells (LSCs) would increase after applying shear stress to the leukemia cells based on previous studies with breast cancer in bioreactors. The shear stress was applied by pumping the cells through narrow tubing to mimic the in vivo bloodstream environment. In support of the hypothesis, shear stress was found to increase the amount of LSCs in a given leukemia population. This work was supported by NSF REU Site Award 1358991.

  11. An integrated temperature-compensated flexible shear-stress sensor microarray with concentrated leading-wire.

    PubMed

    Tang, Jian; Liu, Wu; Zhang, Weiping; Sun, Yongming; Chen, Honghai

    2016-02-01

    Flexible shear stress sensor is quite important for characterizing curved surface flows. In this work, a novel integrated shear stress sensor microarray is designed with twenty parallel channels, which share the concentrated leading-wire to transmit the ground signal. Electrical pads in rows are easily connected to the circuits with two separate Wheatstone bridges and constant-temperature-difference mode operation is provided for the hot-wires. Temperature crosstalk between adjacent hot-wires is prevented well and the effectiveness of the temperature compensated circuits is verified. Relatively large output response is obtained as the shear stress varies and the sensitivity of the sensors is measured about 0.086 V(2)/Pa(1/3) with nonlinearity lower than 1%, revealing high performance characteristic of the sensors. PMID:26931882

  12. On investigating wall shear stress in two-dimensional plane turbulent wall jets

    NASA Astrophysics Data System (ADS)

    Mehdi, Faraz; Johansson, Gunnar; White, Christopher; Naughton, Jonathan

    2012-11-01

    Mehdi & White [Exp Fluids 50:43-51(2011)] presented a full momentum integral based method for determining wall shear stress in zero pressure gradient turbulent boundary layers. They utilized the boundary conditions at the wall and at the outer edge of the boundary layer. A more generalized expression is presented here that uses just one boundary condition at the wall. The method is mathematically exact and has an advantage of having no explicit streamwise gradient terms. It is successfully applied to two different experimental plane turbulent wall jet datasets for which independent estimates of wall shear stress were known. Complications owing to experimental inaccuracies in determining wall shear stress from the proposed method are also discussed.

  13. On determining wall shear stress in spatially developing two-dimensional wall-bounded flows

    NASA Astrophysics Data System (ADS)

    Mehdi, Faraz; Johansson, T. Gunnar; White, Christopher M.; Naughton, Jonathan W.

    2014-01-01

    A full momentum integral-based method for determining wall shear stress is presented. The method is mathematically exact and has the advantage of having no explicit streamwise gradient terms. It is applicable for flows that change rapidly in the streamwise direction and, in particular, to flows with ill-defined outer boundary conditions or when the measurement grid does not extend over the whole boundary layer thickness. The method is applied to two different experimental plane turbulent wall jet data sets for which independent estimates of wall shear stress were known, and the different results compare favorably. Complications owing to experimental limitations and measurement error in determining wall shear stress from the proposed method are presented, and mitigating strategies are described.

  14. An integrated temperature-compensated flexible shear-stress sensor microarray with concentrated leading-wire

    NASA Astrophysics Data System (ADS)

    Tang, Jian; Liu, Wu; Zhang, Weiping; Sun, Yongming; Chen, Honghai

    2016-02-01

    Flexible shear stress sensor is quite important for characterizing curved surface flows. In this work, a novel integrated shear stress sensor microarray is designed with twenty parallel channels, which share the concentrated leading-wire to transmit the ground signal. Electrical pads in rows are easily connected to the circuits with two separate Wheatstone bridges and constant-temperature-difference mode operation is provided for the hot-wires. Temperature crosstalk between adjacent hot-wires is prevented well and the effectiveness of the temperature compensated circuits is verified. Relatively large output response is obtained as the shear stress varies and the sensitivity of the sensors is measured about 0.086 V2/Pa1/3 with nonlinearity lower than 1%, revealing high performance characteristic of the sensors.

  15. On pressure-shear plate impact for studying the kinetics of stress-induced phase transformations

    NASA Astrophysics Data System (ADS)

    Escobar, Joanne C.; Clifton, Rodney J.

    1992-07-01

    Pressure-shear plate impact experiments are proposed for studying the kinetics of stress-induced phase transformations. The purpose of this paper is to determine loading conditions and specimen orientations which can be expected to activate a single habit plane variant parallel to the impact plane, thereby simplifying the study of the kinetics of the transformation through monitoring the wave profiles associated with the propagating phase boundary. The Wechsler Lieberman-Read phenomenological theory was used to determine habit plane indices and directions of shape deformation for a Cu-Al-Ni shape memory alloy which undergoes a martensitic phase transformation under stress. Elastic waves generated by pressure-shear impact were analyzed for wave propagation in the direction of the normal to a habit plane. A critical resolved shear stress criterion was used to predict variants which are expected to be activated for a range of impact velocities and relative magnitudes of the normal and transverse components of the impact velocity.

  16. Development of a novel bioreactor to apply shear stress and tensile strain simultaneously to cell monolayers

    NASA Astrophysics Data System (ADS)

    Breen, Liam T.; McHugh, Peter E.; McCormack, Brendan A.; Muir, Gordon; Quinlan, Nathan J.; Heraty, Kevin B.; Murphy, Bruce P.

    2006-10-01

    To date many bioreactor experiments have investigated the cellular response to isolated in vitro forces. However, in vivo, wall shear stress (WSS) and tensile hoop strain (THS) coexist. This article describes the techniques used to build and validate a novel vascular tissue bioreactor, which is capable of applying simultaneous wall shear stress and tensile stretch to multiple cellular substrates. The bioreactor design presented here combines a cone and plate rheometer with flexible substrates. Using such a combination, the bioreactor is capable of applying a large range of pulsatile wall shear stress (-30to+30dyn/cm2) and tensile hoop strain (0%-12%). The WSS and THS applied to the cellular substrates were validated and calibrated. In particular, curves were produced that related the desired WSS to the bioreactor control parameters. The bioreactor was shown to be biocompatible and noncytotoxic and suitable for cellular mechanical loading studies in physiological condition, i.e., under simultaneous WSS and THS conditions.

  17. Theory and Practice of Shear/Stress Strain Gage Hygrometry

    NASA Technical Reports Server (NTRS)

    Shams, Qamar A.; Fenner, Ralph L.

    2006-01-01

    Mechanical hygrometry has progressed during the last several decades from crude hygroscopes to state-of-the art strain-gage sensors. The strain-gage devices vary from different metallic beams to strain-gage sensors using cellulose crystallite elements, held in full shear restraint. This old technique is still in use but several companies are now actively pursuing development of MEMS miniaturized humidity sensors. These new sensors use polyimide thin film for water vapor adsorption and desorption. This paper will provide overview about modern humidity sensors.

  18. Estimation of the bed shear stress in vegetated and bare channels with smooth beds

    NASA Astrophysics Data System (ADS)

    Yang, Judy Q.; Kerger, Francois; Nepf, Heidi M.

    2015-05-01

    The shear stress at the bed of a channel influences important benthic processes such as sediment transport. Several methods exist to estimate the bed shear stress in bare channels without vegetation, but most of these are not appropriate for vegetated channels due to the impact of vegetation on the velocity profile and turbulence production. This study proposes a new model to estimate the bed shear stress in both vegetated and bare channels with smooth beds. The model, which is supported by measurements, indicates that for both bare and vegetated channels with smooth beds, within a viscous sublayer at the bed, the viscous stress decreases linearly with increasing distance from the bed, resulting in a parabolic velocity profile at the bed. For bare channels, the model describes the velocity profile in the overlap region of the Law of the Wall. For emergent canopies of sufficient density (frontal area per unit canopy volume a≥4.3 m-1), the thickness of the linear-stress layer is set by the stem diameter, leading to a simple estimate for bed shear stress.

  19. Fluid Shear Stress Regulates the Invasive Potential of Glioma Cells via Modulation of Migratory Activity and Matrix Metalloproteinase Expression

    PubMed Central

    Qazi, Henry; Shi, Zhong-Dong; Tarbell, John M.

    2011-01-01

    Background Glioma cells are exposed to elevated interstitial fluid flow during the onset of angiogenesis, at the tumor periphery while invading normal parenchyma, within white matter tracts, and during vascular normalization therapy. Glioma cell lines that have been exposed to fluid flow forces in vivo have much lower invasive potentials than in vitro cell motility assays without flow would indicate. Methodology/Principal Findings A 3D Modified Boyden chamber (Darcy flow through collagen/cell suspension) model was designed to mimic the fluid dynamic microenvironment to study the effects of fluid shear stress on the migratory activity of glioma cells. Novel methods for gel compaction and isolation of chemotactic migration from flow stimulation were utilized for three glioma cell lines: U87, CNS-1, and U251. All physiologic levels of fluid shear stress suppressed the migratory activity of U87 and CNS-1 cell lines. U251 motility remained unaltered within the 3D interstitial flow model. Matrix Metalloproteinase (MMP) inhibition experiments and assays demonstrated that the glioma cells depended on MMP activity to invade, and suppression in motility correlated with downregulation of MMP-1 and MMP-2 levels. This was confirmed by RT-PCR and with the aid of MMP-1 and MMP-2 shRNA constructs. Conclusions/Significance Fluid shear stress in the tumor microenvironment may explain reduced glioma invasion through modulation of cell motility and MMP levels. The flow-induced migration trends were consistent with reported invasive potentials of implanted gliomas. The models developed for this study imply that flow-modulated motility involves mechanotransduction of fluid shear stress affecting MMP activation and expression. These models should be useful for the continued study of interstitial flow effects on processes that affect tumor progression. PMID:21637818

  20. Atrial Fibrillation Pacing Decreases Intravascular Shear Stress in a New Zealand White Rabbit Model: Implications in Endothelial Function

    PubMed Central

    Jen, Nelson; Yu, Fei; Lee, Juhyun; Wasmund, Steve; Dai, Xiaohu; Chen, Christina; Chawareeyawong, Pai; Yang, Yongmo; Li, Rongsong; Hamdan, Mohamed H.; Hsiai, Tzung

    2012-01-01

    Atrial fibrillation (AF) is characterized by multiple rapid and irregular atrial depolarization leading to rapid ventricular responses exceeding 100 beats per minute (bpm). We hypothesized that rapid and irregular pacing reduced intravascular shear stress (ISS) with implication to modulating endothelial responses. To simulate AF, we paced the left atrial appendage of New Zealand White (NZW) rabbits (n=4) at rapid and irregular intervals. Surface electrical cardiograms (ECG) were recorded for atrial and ventricular rhythm, and intravascular convective heat transfer was measured by micro thermal sensors, from which ISS was inferred. Rapid and irregular pacing decreased arterial systolic and diastolic pressures (baseline: 99/75 mmHg; rapid regular pacing: 92/73; rapid irregular pacing: 90/68; P < 0.001, n=4), temporal gradients (∂τ/∂t from 1275 ± 80 to 1056 ± 180 dyne/cm2·s), and reduced ISS (from baseline at 32.0 ± 2.4 to 22.7 ± 3.5 dyne/cm2). Computational fluid dynamics (CFD) code demonstrated that experimentally inferred ISS provided a close approximation to the computed wall shear stress (WSS) at a given catheter to vessel diameter ratio, shear stress range, and catheter position. In an in vitro flow system in which time-averaged shear stress was maintained at τavg=23 ±4 dyn·cm−2·s−1, we further demonstrated that rapid pulse rates at 150 bpm down-regulated endothelial nitric oxide (NO), promoted superoxide (O2·−) production, and increased monocyte binding to endothelial cells. These findings suggest that rapid pacing reduces ISS and ∂τ/∂t, and rapid pulse rates modulate endothelial responses. PMID:22983703

  1. Interfacial shear stress between a single-walled carbon nanotube and a gold surface after different physical treatments.

    PubMed

    Pan, Huiyan; Wu, Yu-Chiao; Adams, George G; McGruer, Nicol E

    2015-06-01

    The interfacial shear stress between gold and dielectrophoretically assembled single-walled carbon nanotubes can be increased by annealing in N2, by e-beam irradiation, or by e-beam deposition of carbon. For the first time this increase has been measured, using a technique developed by this group that is based on NEMS cantilever measurements combined with modeling. Annealing increases the shear stress by more than a factor of 3 over its value of 87MPa for untreated gold surfaces, while e-beam irradiation increases the shear stress by more than a factor of 2 and carbon deposition increases the shear stress by a smaller amount. PMID:25700215

  2. Gyrokinetic simulation of momentum transport with residual stress from diamagnetic level velocity shears

    NASA Astrophysics Data System (ADS)

    Waltz, R. E.; Staebler, G. M.; Solomon, W. M.

    2011-04-01

    Residual stress refers to the remaining toroidal angular momentum (TAM) flux (divided by major radius) when the shear in the equilibrium fluid toroidal velocity (and the velocity itself) vanishes. Previously [Waltz et al., Phys. Plasmas 14, 122507 (2007); errata 16, 079902 (2009)], we demonstrated with GYRO [Candy and Waltz, J. Comp. Phys. 186, 545 (2003)] gyrokinetic simulations that TAM pinching from (ion pressure gradient supported or diamagnetic level) equilibrium E ×B velocity shear could provide some of the residual stress needed to support spontaneous toroidal rotation against normal diffusive loss. Here we show that diamagnetic level shear in the intrinsic drift wave velocities (or "profile shear" in the ion and electron density and temperature gradients) provides a comparable residual stress. The individual signed contributions of these small (rho-star level) E ×B and profile velocity shear rates to the turbulence level and (rho-star squared) ion energy transport stabilization are additive if the rates are of the same sign. However because of the additive stabilization effect, the contributions to the small (rho-star cubed) residual stress is not always simply additive. If the rates differ in sign, the residual stress from one can buck out that from the other (and in some cases reduce the stabilization.) The residual stress from these diamagnetic velocity shear rates is quantified by the ratio of TAM flow to ion energy (power) flow (M/P) in a global GYRO core simulation of a "null" toroidal rotation DIII-D [Mahdavi and Luxon, Fusion Sci. Technol. 48, 2 (2005)] discharge by matching M/P profiles within experimental uncertainty. Comparison of global GYRO (ion and electron energy as well as particle) transport flow balance simulations of TAM transport flow in a high-rotation DIII-D L-mode quantifies and isolates the E ×B shear and parallel velocity (Coriolis force) pinching components from the larger "diffusive" parallel velocity shear driven component and

  3. Interface shear stresses during ambulation with a below-knee prosthetic limb

    PubMed Central

    Sanders, Joan E.; Daly, Colin H.; Burgess, Ernest M.

    2015-01-01

    Shear stresses on a residual limb in a prosthetic socket are considered clinically to contribute to tissue breakdown in below-knee amputees. When applied simultaneously with normal stresses, they can cause injury within the skin or can generate an abrasion on the surface. To gain insight into shear stresses and parameters that affect them, interface stresses were recorded on below-knee amputee subjects during walking trials. On the tibial flares, resultant shear ranged from 5.6 kPa to 39.0 kPa, while on the posterior surface it ranged from 5.0 kPa to 40.7 kPa. During stance phase, anterior resultant shears on a socket were directed toward the apex while posterior resultant shears were directed downward approximately perpendicular to the ground. Waveform shapes were usually double-peaked, with the first peak at 25% to 40% into stance phase and the second peak at 65% to 85% into stance. Application of these results to residual limb tissue mechanics and prosthetic design is discussed. PMID:1432723

  4. Shear-stress fluctuations in self-assembled transient elastic networks

    NASA Astrophysics Data System (ADS)

    Wittmer, J. P.; Kriuchevskyi, I.; Cavallo, A.; Xu, H.; Baschnagel, J.

    2016-06-01

    Focusing on shear-stress fluctuations, we investigate numerically a simple generic model for self-assembled transient networks formed by repulsive beads reversibly bridged by ideal springs. With Δ t being the sampling time and t(f ) ˜1 /f the Maxwell relaxation time (set by the spring recombination frequency f ), the dimensionless parameter Δ x =Δ t /t(f ) is systematically scanned from the liquid limit (Δ x ≫1 ) to the solid limit (Δ x ≪1 ) where the network topology is quenched and an ensemble average over m -independent configurations is required. Generalizing previous work on permanent networks, it is shown that the shear-stress relaxation modulus G (t ) may be efficiently determined for all Δ x using the simple-average expression G (t ) =μA-h (t ) with μA=G (0 ) characterizing the canonical-affine shear transformation of the system at t =0 and h (t ) the (rescaled) mean-square displacement of the instantaneous shear stress as a function of time t . This relation is compared to the standard expression G (t ) =c ˜(t ) using the (rescaled) shear-stress autocorrelation function c ˜(t ) . Lower bounds for the m configurations required by both relations are given.

  5. Shear-stress fluctuations in self-assembled transient elastic networks.

    PubMed

    Wittmer, J P; Kriuchevskyi, I; Cavallo, A; Xu, H; Baschnagel, J

    2016-06-01

    Focusing on shear-stress fluctuations, we investigate numerically a simple generic model for self-assembled transient networks formed by repulsive beads reversibly bridged by ideal springs. With Δt being the sampling time and t_{☆}(f)∼1/f the Maxwell relaxation time (set by the spring recombination frequency f), the dimensionless parameter Δx=Δt/t_{☆}(f) is systematically scanned from the liquid limit (Δx≫1) to the solid limit (Δx≪1) where the network topology is quenched and an ensemble average over m-independent configurations is required. Generalizing previous work on permanent networks, it is shown that the shear-stress relaxation modulus G(t) may be efficiently determined for all Δx using the simple-average expression G(t)=μ_{A}-h(t) with μ_{A}=G(0) characterizing the canonical-affine shear transformation of the system at t=0 and h(t) the (rescaled) mean-square displacement of the instantaneous shear stress as a function of time t. This relation is compared to the standard expression G(t)=c[over ̃](t) using the (rescaled) shear-stress autocorrelation function c[over ̃](t). Lower bounds for the m configurations required by both relations are given. PMID:27415324

  6. Dynamics of A Vortex Pair In Shear Flows

    NASA Astrophysics Data System (ADS)

    Gorshkov, K. A.; Soustova, I. A.

    The dynamics (more specifically, stability) of infinite sequences of such vortex pairs that initially have the form of a Karman street in a jet-type shear flow is investigated Such a geometry is most directly related to the wake behind a streamlined cylinder We assume that the characteristic scale of velocity variation in a shear flow is much larger than the sizes of vortex spots. This means that each vortex spot during its evolution is always in a flow with an almost linear velocity distribution and first of all undergoes general drift and weak (primarily elliptic due to its linear profile of the flow veloc- ity) deformation of its shape. In this formulation the problem can be solved using a previously developed perturbation theory for hydrodynamic vortices. It is shown that variations of the parameters of the chess structure and the value of the velocity of the jet are interdependent. For instance, at short times when the jet velocity is relative large , all vortices remain almost on the same line while in the late wake, when the jet slows down, the vortices form the chess structure.. Such vortex sheets were indeed observed in [[ Spedding G. R., Browand F. K., Fincham A. M. Turbulence, similar- ity scaling and vortex geometry in the wake of a towed sphere in a stably stratified fluid. J. Fluid Mech., 1996, v.31410]. Note that possible initial, short-lived large-scale perturbations were not realised under the experimental conditions of. , presumably because of the limited size of the tank. This in itself implies a concentration of flow in horizontal plane. Still, to make a positive conclusion, a more thorough analysis is needed including study of the effect of the vortex sheet onto the jet flow.

  7. Averaging interval selection for the calculation of Reynolds shear stress for studies of boundary layer turbulence.

    NASA Astrophysics Data System (ADS)

    Lee, Zoe; Baas, Andreas

    2013-04-01

    It is widely recognised that boundary layer turbulence plays an important role in sediment transport dynamics in aeolian environments. Improvements in the design and affordability of ultrasonic anemometers have provided significant contributions to studies of aeolian turbulence, by facilitating high frequency monitoring of three dimensional wind velocities. Consequently, research has moved beyond studies of mean airflow properties, to investigations into quasi-instantaneous turbulent fluctuations at high spatio-temporal scales. To fully understand, how temporal fluctuations in shear stress drive wind erosivity and sediment transport, research into the best practice for calculating shear stress is necessary. This paper builds upon work published by Lee and Baas (2012) on the influence of streamline correction techniques on Reynolds shear stress, by investigating the time-averaging interval used in the calculation. Concerns relating to the selection of appropriate averaging intervals for turbulence research, where the data are typically non-stationary at all timescales, are well documented in the literature (e.g. Treviño and Andreas, 2000). For example, Finnigan et al. (2003) found that underestimating the required averaging interval can lead to a reduction in the calculated momentum flux, as contributions from turbulent eddies longer than the averaging interval are lost. To avoid the risk of underestimating fluxes, researchers have typically used the total measurement duration as a single averaging period. For non-stationary data, however, using the whole measurement run as a single block average is inadequate for defining turbulent fluctuations. The data presented in this paper were collected in a field study of boundary layer turbulence conducted at Tramore beach near Rosapenna, County Donegal, Ireland. High-frequency (50 Hz) 3D wind velocity measurements were collected using ultrasonic anemometry at thirteen different heights between 0.11 and 1.62 metres above

  8. Vascular endothelial cell membranes differentiate between stretch and shear stress through transitions in their lipid phases.

    PubMed

    Yamamoto, Kimiko; Ando, Joji

    2015-10-01

    Vascular endothelial cells (ECs) respond to the hemodynamic forces stretch and shear stress by altering their morphology, functions, and gene expression. However, how they sense and differentiate between these two forces has remained unknown. Here we report that the plasma membrane itself differentiates between stretch and shear stress by undergoing transitions in its lipid phases. Uniaxial stretching and hypotonic swelling increased the lipid order of human pulmonary artery EC plasma membranes, thereby causing a transition from the liquid-disordered phase to the liquid-ordered phase in some areas, along with a decrease in membrane fluidity. In contrast, shear stress decreased the membrane lipid order and increased membrane fluidity. A similar increase in lipid order occurred when the artificial lipid bilayer membranes of giant unilamellar vesicles were stretched by hypotonic swelling, indicating that this is a physical phenomenon. The cholesterol content of EC plasma membranes significantly increased in response to stretch but clearly decreased in response to shear stress. Blocking these changes in the membrane lipid order by depleting membrane cholesterol with methyl-β-cyclodextrin or by adding cholesterol resulted in a marked inhibition of the EC response specific to stretch and shear stress, i.e., phosphorylation of PDGF receptors and phosphorylation of VEGF receptors, respectively. These findings indicate that EC plasma membranes differently respond to stretch and shear stress by changing their lipid order, fluidity, and cholesterol content in opposite directions and that these changes in membrane physical properties are involved in the mechanotransduction that activates membrane receptors specific to each force. PMID:26297225

  9. Critical shear stress for erosion of cohesive soils subjected to temperatures typical of wildfires

    USGS Publications Warehouse

    Moody, J.A.; Dungan, Smith J.; Ragan, B.W.

    2005-01-01

    [1] Increased erosion is a well-known response after wildfire. To predict and to model erosion on a landscape scale requires knowledge of the critical shear stress for the initiation of motion of soil particles. As this soil property is temperature-dependent, a quantitative relation between critical shear stress and the temperatures to which the soils have been subjected during a wildfire is required. In this study the critical shear stress was measured in a recirculating flume using samples of forest soil exposed to different temperatures (40??-550??C) for 1 hour. Results were obtained for four replicates of soils derived from three different types of parent material (granitic bedrock, sandstone, and volcanic tuffs). In general, the relation between critical shear stress and temperature can be separated into three different temperature ranges (275??C), which are similar to those for water repellency and temperature. The critical shear stress was most variable (1.0-2.0 N m-2) for temperatures 2.0 N m-2) between 175?? and 275??C, and was essentially constant (0.5-0.8 N m-2) for temperatures >275??C. The changes in critical shear stress with temperature were found to be essentially independent of soil type and suggest that erosion processes in burned watersheds can be modeled more simply than erosion processes in unburned watersheds. Wildfire reduces the spatial variability of soil erodibility associated with unburned watersheds by eliminating the complex effects of vegetation in protecting soils and by reducing the range of cohesion associated with different types of unburned soils. Our results indicate that modeling the erosional response after a wildfire depends primarily on determining the spatial distribution of the maximum soil temperatures that were reached during the wildfire. Copyright 2005 by the American Geophysical Union.

  10. Significance of electrically induced shear stress in drainage of thin aqueous films.

    PubMed

    Ketelaar, Christiaan; Ajaev, Vladimir S

    2015-05-01

    We develop a novel model of drainage of microscale thin aqueous film separating a gas bubble and a solid wall. In contrast to previous studies, the electrostatic effects are accounted for not only in the normal but also in the shear stress balance at the liquid-gas interface. We show that the action of the tangential component of the electric field leads to potentially strong spatially variable shear stress at the deforming charged interface. This previously overlooked effect turns out to be essential for correctly estimating the long-time drainage rates. Comparison of time-dependent fluid interface shapes predicted by our model with the experimental data is discussed. PMID:26066179

  11. Characteristics of arterial wall shear stress which cause endothelium-dependent vasodilatation in the anaesthetized dog

    PubMed Central

    Snow, H M; Markos, F; O'Regan, D; Pollock, K

    2001-01-01

    The effects of changes in the mean and amplitude of arterial wall shear stress on endothelium-dependent arterial dilatation of the iliac artery of the anaesthetized dog were examined. Changes in the mean and amplitude of blood flow and wall shear stress were brought about by varying local peripheral resistance and stroke volume using a distal infusion of acetylcholine and the stimulation of the left ansa subclavia. Changes in the diameter of a segment of the iliac artery with the endothelium intact, relative to a segment with no endothelium, were used as an index of the release of nitric oxide. The increase in mean blood flow was from 84 ± 12 to 527 ± 53 ml min−1 and in amplitude was from 365 ± 18 to 695 ± 38 ml min−1 (means ±s.e.m.). The increase in mean wall shear stress was from 1.78 ± 0.30 to 7.66 ± 1.01 N m−2 and in amplitude was from 7.37 ± 0.46 to 13.9 ± 2.00 N m−2 (means ±s.e.m.). Increases in mean shear stress caused an increase in the diameter only of the section of artery with endothelium; the slope of the relationship was 0.064 ± 0.006 mm N−1 m2 (mean ±s.e.m., P < 0.001); changes in the amplitude of shear stress did not cause an increase in diameter. Changes in both the mean and amplitude of shear stress had no significant effect on the diameter of the section of artery with no endothelium. These findings coupled with the known anti-atheroma effects of nitric oxide and the effect of shear stress on cell adhesion and platelet aggregation offer a possible explanation for the disposition of atheroma in those parts of the arterial system which have low mean and high amplitude of wall shear stress. PMID:11251063

  12. Evolution of shear banding flows in metallic glasses characterized by molecular dynamics

    NASA Astrophysics Data System (ADS)

    Yao, Li; Luan, Yingwei

    2016-06-01

    To reveal the evolution of shear banding flows, one-dimensional nanostructure metallic glass composites have been studied with molecular dynamics. The inherent size determines the initial thickness of shear bands, and the subsequent broadening can be restricted to some extent. The vortex-like flows evoke the atomic motion perpendicular to the shear plane, which accelerates the interatomic diffusion. The reduction of local strain rate causes the flow softening for monolithic Cu-Zr glass, but the participation of Cu-atoms in the shear banding flow gradually leads to the shear hardening for the composites.

  13. A sliding plate microgap rheometer for the simultaneous measurement of shear stress and first normal stress difference.

    PubMed

    Baik, Seung Jae; Moldenaers, Paula; Clasen, Christian

    2011-03-01

    A new generation of the "flexure-based microgap rheometer" (the N-FMR) has been developed which is also capable of measuring, in addition to the shear stress, the first normal stress difference of micrometer thin fluid films. This microgap rheometer with a translation system based on compound spring flexures measures the rheological properties of microliter samples of complex fluids confined in a plane couette configuration with gap distances of h = 1-400 μm up to shear rates of γ = 3000 s(-1). Feed back loop controlled precise positioning of the shearing surfaces with response times <1 ms enables to control the parallelism within 1.5 μrad and to maintain the gap distance within 20 nm. This precise gap control minimizes squeeze flow effects and allows therefore to measure the first normal stress difference N(1) of the thin film down to a micrometer gap distance, with a lower limit of N(1)/γ = 9.375×10(-11) η/h(2) that depends on the shear viscosity η and the squared inverse gap. Structural development of complex fluids in the confinement can be visualized by using a beam splitter on the shearing surface and a long working distance microscope. In summary, this new instrument allows to investigate the confinement dependent rheological and morphological evolution of micrometer thin films. PMID:21456802

  14. An investigation of stress wave propagation in a shear deformable nanobeam based on modified couple stress theory

    NASA Astrophysics Data System (ADS)

    Akbarzadeh Khorshidi, Majid; Shariati, Mahmoud

    2016-04-01

    This paper presents a new investigation for propagation of stress wave in a nanobeam based on modified couple stress theory. Using Euler-Bernoulli beam theory, Timoshenko beam theory, and Reddy beam theory, the effect of shear deformation is investigated. This nonclassical model contains a material length scale parameter to capture the size effect and the Poisson effect is incorporated in the current model. Governing equations of motion are obtained by Hamilton's principle and solved explicitly. This solution leads to obtain two phase velocities for shear deformable beams in different directions. Effects of shear deformation, material length scale parameter, and Poisson's ratio on the behavior of these phase velocities are investigated and discussed. The results also show a dual behavior for phase velocities against Poisson's ratio.

  15. Stress-structure relation in dense colloidal melts under forward and instantaneous reversal of the shear.

    PubMed

    Bhattacharjee, Amit Kumar

    2015-07-28

    A dense supercooled colloidal melt in forward shear from a quiescent state shows overshoot in the shear stress at 10% strain with an unchanged fluid structure at equal stress before and after overshoot. In addition, we find an overshoot in the normal stress with a monotonic increase in the osmotic pressure at an identical strain. The first and second normal stresses become comparable in magnitude and opposite in sign. A functional dependence of the steady state stress and osmotic pressure with the Péclet number demonstrates the signature of crossover between Newtonian and nearly-Newtonian regimes. Moreover, instantaneous shear reversal from a steady state exhibiting the Bauschinger effect, where a strong history dependence is observed depending on the time of the flow reversal. The distribution of the particulate stress and osmotic pressure at the point of the flow reversal is shown to be a signature of the subsequent response. We link the history dependence of the stress-strain curves to changes in the fluid structure measured through the angular components of the radial distribution function. A uniform compression in the transition from forward to reversed flowing states is found. PMID:26082951

  16. Proteomic analysis of Staphylococcus aureus biofilm cells grown under physiologically relevant fluid shear stress conditions

    PubMed Central

    2014-01-01

    Background The biofilm forming bacterium Staphylococcus aureus is responsible for maladies ranging from severe skin infection to major diseases such as bacteremia, endocarditis and osteomyelitis. A flow displacement system was used to grow S. aureus biofilms in four physiologically relevant fluid shear rates (50, 100, 500 and 1000 s-1) to identify proteins that are associated with biofilm. Results Global protein expressions from the membrane and cytosolic fractions of S. aureus biofilm cells grown under the above shear rate conditions are reported. Sixteen proteins in the membrane-enriched fraction and eight proteins in the cytosolic fraction showed significantly altered expression (p < 0.05) under increasing fluid shear. These 24 proteins were identified using nano-LC-ESI-MS/MS. They were found to be associated with various metabolic functions such as glycolysis / TCA pathways, protein synthesis and stress tolerance. Increased fluid shear stress did not influence the expression of two important surface binding proteins: fibronectin-binding and collagen-binding proteins. Conclusions The reported data suggest that while the general metabolic function of the sessile bacteria is minimal under high fluid shear stress conditions, they seem to retain the binding capacity to initiate new infections. PMID:24855455

  17. Perturbation of the yield-stress rheology of polymer thin films by nonlinear shear ultrasound.

    PubMed

    Léopoldès, J; Conrad, G; Jia, X

    2015-01-01

    We investigate the nonlinear response of macromolecular thin films subjected to high-amplitude ultrasonic shear oscillation using a sphere-plane contact geometry. At a film thickness comparable to the radius of gyration, we observe the rheological properties intermediate between bulk and boundary nonlinear regimes. As the driving amplitude is increased, these films progressively exhibit oscillatory linear, microslip, and full slip regimes, which can be explained by the modified Coulomb friction law. At highest oscillation amplitudes, the interfacial adhesive failure takes place, being accompanied by a dewettinglike pattern. Moreover, the steady state sliding is investigated in thicker films with imposed shear stresses beyond the yield point. We find that applying high-amplitude shear ultrasound affects not only the yielding threshold but also the sliding velocity at a given shear load. A possible mechanism for the latter effect is discussed. PMID:25679626

  18. Perturbation of the yield-stress rheology of polymer thin films by nonlinear shear ultrasound

    NASA Astrophysics Data System (ADS)

    Léopoldès, J.; Conrad, G.; Jia, X.

    2015-01-01

    We investigate the nonlinear response of macromolecular thin films subjected to high-amplitude ultrasonic shear oscillation using a sphere-plane contact geometry. At a film thickness comparable to the radius of gyration, we observe the rheological properties intermediate between bulk and boundary nonlinear regimes. As the driving amplitude is increased, these films progressively exhibit oscillatory linear, microslip, and full slip regimes, which can be explained by the modified Coulomb friction law. At highest oscillation amplitudes, the interfacial adhesive failure takes place, being accompanied by a dewettinglike pattern. Moreover, the steady state sliding is investigated in thicker films with imposed shear stresses beyond the yield point. We find that applying high-amplitude shear ultrasound affects not only the yielding threshold but also the sliding velocity at a given shear load. A possible mechanism for the latter effect is discussed.

  19. The Behavior Under Shearing Stress of Duralumin Strip with Round, Flanged Holes

    NASA Technical Reports Server (NTRS)

    Schussler, Karl

    1934-01-01

    This report presents the results of an investigation to determine the behavior of dural strip with flanged holes in the center when subjected to shear stresses. They buckle under a certain load just as a flat sheet. There is one optimum hole spacing and a corresponding buckling load in shear for each sheet width, sheet thickness, and flange form. Comparison with non-flanged sheets revealed a marked increase of buckling load in shear due to the flanging and a slightly greater displacement. Strips were clamped between two stationary end rails and one sliding center rail at which the shear is applied. The force was measured with a tension stirrup up to 20 tons and a compression dynamometer up to 10 tons. The displacement was recorded with the Zeiss dial gauge. The following were investigated: 1) effect of strip width; 2) strip thickness; 3) diameter of flanging; 4) depth of flanging; 5) and hole distance.

  20. Brownian dynamics simulation of orientational behavior, flow-induced structure, and rheological properties of a suspension of oblate spheroid particles under simple shear

    NASA Astrophysics Data System (ADS)

    Yamamoto, Takehiro; Suga, Takanori; Mori, Noriyasu

    2005-08-01

    Brownian dynamics (BD) simulations were carried out for suspensions of oblate spheroid particles interacting via the Gay-Berne (GB) potential. The oblate spheroid particles were applied as a model of disc-like particles and the system of suspension of the particles was considered. Numerically analyzed were both the change in phase with the number density of the particles at equilibrium state and the behavior of the particles in simple shear flows. The system changed from isotropic phase to nematic one with increasing the particle concentration. In the simulation of shear flows, the shear was imposed upon the systems in nematic phase at equilibrium. The systems exhibited various motions of the director depending on the shear rate, e.g. the continuous rotation of director at low shear rates, the wagging at moderate shear rates, and the flow aligning at high shear rates. Temporal change in inner structure of suspensions was also analyzed and collapse of initial particle configurations due to shear was found. Moreover, rheological properties of the suspension were investigated. The numerical simulation predicted the shear-thinning in viscosity, negative first normal stress difference, and positive second normal stress difference, and these results qualitatively agreed with the predictions using a constitutive equation for discotic nematics. The present study proved that the BD simulation using spheroid particles interacting via the GB potential is an effective approach for investigating the flow behavior and flow-induced structure of suspensions of disklike particles at a particulate level.

  1. Brownian dynamics simulation of orientational behavior, flow-induced structure, and rheological properties of a suspension of oblate spheroid particles under simple shear.

    PubMed

    Yamamoto, Takehiro; Suga, Takanori; Mori, Noriyasu

    2005-08-01

    Brownian dynamics (BD) simulations were carried out for suspensions of oblate spheroid particles interacting via the Gay-Berne (GB) potential. The oblate spheroid particles were applied as a model of disc-like particles and the system of suspension of the particles was considered. Numerically analyzed were both the change in phase with the number density of the particles at equilibrium state and the behavior of the particles in simple shear flows. The system changed from isotropic phase to nematic one with increasing the particle concentration. In the simulation of shear flows, the shear was imposed upon the systems in nematic phase at equilibrium. The systems exhibited various motions of the director depending on the shear rate, e.g. the continuous rotation of director at low shear rates, the wagging at moderate shear rates, and the flow aligning at high shear rates. Temporal change in inner structure of suspensions was also analyzed and collapse of initial particle configurations due to shear was found. Moreover, rheological properties of the suspension were investigated. The numerical simulation predicted the shear-thinning in viscosity, negative first normal stress difference, and positive second normal stress difference, and these results qualitatively agreed with the predictions using a constitutive equation for discotic nematics. The present study proved that the BD simulation using spheroid particles interacting via the GB potential is an effective approach for investigating the flow behavior and flow-induced structure of suspensions of disklike particles at a particulate level. PMID:16196575

  2. Three-dimensional macro-scale assessment of regional and temporal wall shear stress characteristics on aortic valve leaflets.

    PubMed

    Cao, K; Bukač, M; Sucosky, P

    2016-01-01

    The aortic valve (AV) achieves unidirectional blood flow between the left ventricle and the aorta. Although hemodynamic stresses have been shown to regulate valvular biology, the native wall shear stress (WSS) experienced by AV leaflets remains largely unknown. The objective of this study was to quantify computationally the macro-scale leaflet WSS environment using fluid-structure interaction modeling. An arbitrary Lagrangian-Eulerian approach was implemented to predict valvular flow and leaflet dynamics in a three-dimensional AV geometry subjected to physiologic transvalvular pressure. Local WSS characteristics were quantified in terms of temporal shear magnitude (TSM), oscillatory shear index (OSI) and temporal shear gradient (TSG). The dominant radial WSS predicted on the leaflets exhibited high amplitude and unidirectionality on the ventricularis (TSM>7.50 dyn/cm(2), OSI < 0.17, TSG>325.54 dyn/cm(2) s) but low amplitude and bidirectionality on the fibrosa (TSM < 2.73 dyn/cm(2), OSI>0.38, TSG < 191.17 dyn/cm(2) s). The radial WSS component computed in the leaflet base, belly and tip demonstrated strong regional variability (ventricularis TSM: 7.50-22.32 dyn/cm(2), fibrosa TSM: 1.26-2.73 dyn/cm(2)). While the circumferential WSS exhibited similar spatially dependent magnitude (ventricularis TSM: 1.41-3.40 dyn/cm(2), fibrosa TSM: 0.42-0.76 dyn/cm(2)) and side-specific amplitude (ventricularis TSG: 101.73-184.43 dyn/cm(2) s, fibrosa TSG: 41.92-54.10 dyn/cm(2) s), its temporal variations were consistently bidirectional (OSI>0.25). This study provides new insights into the role played by leaflet-blood flow interactions in valvular function and critical hemodynamic stress data for the assessment of the hemodynamic theory of AV disease. PMID:26155915

  3. Estimates of Shear Stress and Measurements of Water Levels in the Lower Fox River near Green Bay, Wisconsin

    USGS Publications Warehouse

    Westenbroek, Stephen M.

    2006-01-01

    Turbulent shear stress in the boundary layer of a natural river system largely controls the deposition and resuspension of sediment, as well as the longevity and effectiveness of granular-material caps used to cover and isolate contaminated sediments. This report documents measurements and calculations made in order to estimate shear stress and shear velocity on the Lower Fox River, Wisconsin. Velocity profiles were generated using an acoustic Doppler current profiler (ADCP) mounted on a moored vessel. This method of data collection yielded 158 velocity profiles on the Lower Fox River between June 2003 and November 2004. Of these profiles, 109 were classified as valid and were used to estimate the bottom shear stress and velocity using log-profile and turbulent kinetic energy methods. Estimated shear stress ranged from 0.09 to 10.8 dynes per centimeter squared. Estimated coefficients of friction ranged from 0.001 to 0.025. This report describes both the field and data-analysis methods used to estimate shear-stress parameters for the Lower Fox River. Summaries of the estimated values for bottom shear stress, shear velocity, and coefficient of friction are presented. Confidence intervals about the shear-stress estimates are provided.

  4. Evaluation of Transverse Thermal Stresses in Composite Plates Based on First-Order Shear Deformation Theory

    NASA Technical Reports Server (NTRS)

    Rolfes, R.; Noor, A. K.; Sparr, H.

    1998-01-01

    A postprocessing procedure is presented for the evaluation of the transverse thermal stresses in laminated plates. The analytical formulation is based on the first-order shear deformation theory and the plate is discretized by using a single-field displacement finite element model. The procedure is based on neglecting the derivatives of the in-plane forces and the twisting moments, as well as the mixed derivatives of the bending moments, with respect to the in-plane coordinates. The calculated transverse shear stiffnesses reflect the actual stacking sequence of the composite plate. The distributions of the transverse stresses through-the-thickness are evaluated by using only the transverse shear forces and the thermal effects resulting from the finite element analysis. The procedure is implemented into a postprocessing routine which can be easily incorporated into existing commercial finite element codes. Numerical results are presented for four- and ten-layer cross-ply laminates subjected to mechanical and thermal loads.

  5. Cloning the Gravity and Shear Stress Related Genes from MG-63 Cells by Subtracting Hybridization

    NASA Astrophysics Data System (ADS)

    Zhang, Shu; Dai, Zhong-quan; Wang, Bing; Cao, Xin-sheng; Li, Ying-hui; Sun, Xi-qing

    2008-06-01

    Background The purpose of the present study was to clone the gravity and shear stress related genes from osteoblast-like human osteosarcoma MG-63 cells by subtractive hybridization. Method MG-63 cells were divided into two groups (1G group and simulated microgravity group). After cultured for 60 h in two different gravitational environments, two groups of MG-63 cells were treated with 1.5Pa fluid shear stress (FSS) for 60 min, respectively. The total RNA in cells was isolated. The gravity and shear stress related genes were cloned by subtractive hybridization. Result 200 clones were gained. 30 positive clones were selected using PCR method based on the primers of vector and sequenced. The obtained sequences were analyzed by blast. changes of 17 sequences were confirmed by RT-PCR and these genes are related to cell proliferation, cell differentiation, protein synthesis, signal transduction and apoptosis. 5 unknown genes related to gravity and shear stress were found. Conclusion In this part of our study, our result indicates that simulated microgravity may change the activities of MG-63 cells by inducing the functional alterations of specific genes.

  6. POLYACRYLAMIDE EFFECTS ON CRITICAL SHEAR STRESS AND RILL ERODIBILITY FOR A RANGE OF SOILS

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The effect of PAM application rate on critical shear stress and erosion was determined for 7 different soils, with a wide range of textural and chemical properties. At least four PAM application rates were tested on each of the soils, ranging from 0.1 to 80 kg ha-1. Flow rates ranging from 4 to 56 L...

  7. Assessment of shear stress related parameters in the carotid bifurcation using mouse-specific FSI simulations.

    PubMed

    De Wilde, David; Trachet, Bram; Debusschere, Nic; Iannaccone, Francesco; Swillens, Abigail; Degroote, Joris; Vierendeels, Jan; De Meyer, Guido R Y; Segers, Patrick

    2016-07-26

    The ApoE(-)(/)(-) mouse is a common small animal model to study atherosclerosis, an inflammatory disease of the large and medium sized arteries such as the carotid artery. It is generally accepted that the wall shear stress, induced by the blood flow, plays a key role in the onset of this disease. Wall shear stress, however, is difficult to derive from direct in vivo measurements, particularly in mice. In this study, we integrated in vivo imaging (micro-Computed Tomography-µCT and ultrasound) and fluid-structure interaction (FSI) modeling for the mouse-specific assessment of carotid hemodynamics and wall shear stress. Results were provided for 8 carotid bifurcations of 4 ApoE(-)(/)(-) mice. We demonstrated that accounting for the carotid elasticity leads to more realistic flow waveforms over the complete domain of the model due to volume buffering capacity in systole. The 8 simulated cases showed fairly consistent spatial distribution maps of time-averaged wall shear stress (TAWSS) and relative residence time (RRT). Zones with reduced TAWSS and elevated RRT, potential indicators of atherosclerosis-prone regions, were located mainly at the outer sinus of the external carotid artery. In contrast to human carotid hemodynamics, no flow recirculation could be observed in the carotid bifurcation region. PMID:26655592

  8. Oscillatory motion based measurement method and sensor for measuring wall shear stress due to fluid flow

    DOEpatents

    Armstrong, William D.; Naughton, Jonathan; Lindberg, William R.

    2008-09-02

    A shear stress sensor for measuring fluid wall shear stress on a test surface is provided. The wall shear stress sensor is comprised of an active sensing surface and a sensor body. An elastic mechanism mounted between the active sensing surface and the sensor body allows movement between the active sensing surface and the sensor body. A driving mechanism forces the shear stress sensor to oscillate. A measuring mechanism measures displacement of the active sensing surface relative to the sensor body. The sensor may be operated under periodic excitation where changes in the nature of the fluid properties or the fluid flow over the sensor measurably changes the amplitude or phase of the motion of the active sensing surface, or changes the force and power required from a control system in order to maintain constant motion. The device may be operated under non-periodic excitation where changes in the nature of the fluid properties or the fluid flow over the sensor change the transient motion of the active sensor surface or change the force and power required from a control system to maintain a specified transient motion of the active sensor surface.

  9. Direct Measurement Sensor of the Boundary Shear Stress in Fluid Flow

    NASA Technical Reports Server (NTRS)

    Badescu, Mircea; Bao, Xiaoqi; Bar-Cohen, Yoseph; Chang, Zensheu; Kerenyi, Kornel; Lih, Shyh-Shiuh; Sherrit, Stewart; Trease, Brian P.; Widholm, Scott

    2010-01-01

    The flow fields and boundary erosion that are associated with scour at bridge piers are very complex. Direct measurement of the boundary shear stress and boundary pressure fluctuations in experimental scour research has always been a challenge and high spatial resolution and fidelity have been almost impossible. Most researchers have applied an indirect process to determine shear stress using precise measured velocity profiles. Laser Doppler Anemometry and Particle Image Velocimetry are common techniques used to accurately measure velocity profiles. These methods are based on theoretical assumptions to estimate boundary shear stress. In addition, available turbulence models cannot very well account for the effect of bed roughness which is fundamentally important for any CFD simulation. The authors have taken on the challenge to advance the magnitude level to which direct measurements of the shear stress in water flow can be performed. This paper covered the challenges and the efforts to develop a higher accuracy and small spatial resolution sensor. Also, preliminary sensor designs and test results are presented.

  10. Microflow-induced shear stress on biomaterial wall by ultrasound-induced encapsulated microbubble oscillation

    NASA Astrophysics Data System (ADS)

    Hu, Ji-Wen; Qian, Sheng-You; Sun, Jia-Na; Lü, Yun-Bin; Hu, Ping

    2015-09-01

    A model of an ultrasound-driven encapsulated microbubble (EMB) oscillation near biomaterial wall is presented and used for describing the microflow-induced shear stress on the wall by means of a numerical method. The characteristic of the model lies in the explicit treatment of different types of wall for the EMB responses. The simulation results show that the radius-time change trends obtained by our model are consistent with the existing models and experimental results. In addition, the effect of the elastic wall on the acoustic EMB response is stronger than that of the rigid wall, and the shear stress on the elastic wall is larger than that of the rigid wall. The closer the EMB to the wall, the greater the shear stress on the wall. The substantial shear stress on the wall surface occurs inside a circular zone with a radius about two-thirds of the bubble radius. This paper may be of interest in the study of potential damage mechanisms to the microvessel for drug and gene delivery due to sonoporation. Projects supported by the National Natural Science Foundation of China (Grant Nos. 11174077 and 11474090), the Natural Science Foundation of Hunan Province, China (Grant No. 13JJ3076), the Science Research Program of Education Department of Hunan Province, China (Grant No. 14A127), and the Doctoral Fund of University of South China (Grant No. 2011XQD46).

  11. Development of a MEMS dual-axis differential capacitance floating element shear stress sensor

    SciTech Connect

    Barnard, Casey; Griffin, Benjamin

    2015-09-01

    A single-axis MEMS wall shear stress sensor with differential capacitive transduction method is produced. Using a synchronous modulation and demodulation interface circuit, the system is capable of making real time measurements of both mean and fluctuating wall shear stress. A sensitivity of 3.44 mV/Pa is achieved, with linearity in response demonstrated up to testing limit of 2 Pa. Minimum detectable signals of 340 μPa at 100 Hz and 120 μPa at 1 kHz are indicated, with a resonance of 3.5 kHz. Multiple full scale wind tunnel tests are performed, producing spectral measurements of turbulent boundary layers in wind speeds ranging up to 0.5 Ma (18 Pa of mean wall shear stress). The compact packaging allows for minimally invasive installation, and has proven relatively robust over multiple testing events. Temperature sensitivity, likely due to poor CTE matching of packaged materials, is an ongoing concern being addressed. These successes are being directly leveraged into a development plan for a dual-axis wall shear stress sensor, capable of producing true vector estimates at the wall.

  12. Periodontal Treatment Elevates Carotid Wall Shear Stress in the Medium Term.

    PubMed

    Carallo, Claudio; Franceschi, Maria Serena De; Tripolino, Cesare; Iovane, Claudio; Catalano, Serena; Giudice, Amerigo; Crispino, Antonio; Figliuzzi, Michele; Irace, Concetta; Fortunato, Leonzio; Gnasso, Agostino

    2015-10-01

    Periodontal disease is associated with endothelial dysfunction of the brachial artery and hemodynamic alterations of the common carotid artery. Periodontal therapy improves endothelial function. It is not known if it is able also to improve the hemodynamics of the carotid artery. The aim of the current study was to evaluate the efficacy of 2 different periodontal treatments on carotid hemodynamics: scaling and root planing (SRP) alone or together with low-level laser therapy (LLLT). Forty patients were recruited and randomly treated with SRP (n = 20) or SRP + LLLT (n = 20). Periodontal indices (plaque, gingival, and probing depth indices) were measured before and 5 months after treatment. Blood viscosity, common carotid wall shear stress, circumferential wall tension, and Peterson elastic modulus were evaluated before, soon after and 5 months after treatment. It was found that the periodontal indices improved in both groups, but significantly more so for SRP + LLLT than for SRP (decrease in gingival index 69.3% versus 45.4%, respectively, P = 0.04). In the SRP + LLLT group, after a transient reduction by 5% immediately after therapy, shear stress increased by 11% after 5 months. In SRP only group, however, shear stress variations were less marked. No significant changes were found for the other hemodynamic parameters in either of the groups. Periodontal disease treatment by SRP + LLLT can therefore be said to improve common carotid wall shear stress. This suggests a possible mechanism by which the treatment of periodontal disease has beneficial effects on the cardiovascular system. PMID:26496285

  13. Cell-matrix adhesion characterization using multiple shear stress zones in single stepwise microchannel

    NASA Astrophysics Data System (ADS)

    Kim, Min-Ji; Doh, Il; Bae, Gab-Yong; Cha, Hyuk-Jin; Cho, Young-Ho

    2014-08-01

    This paper presents a cell chip capable to characterize cell-matrix adhesion by monitoring cell detachment rate. The proposed cell chip can supply multiple levels of shear stress in single stepwise microchannel. As epithelial-mesenchymal transition (EMT), one of hallmarks of cancer metastasis is closely associated to the interaction with extracelluar matrix (ECM), we took advantage of two lung cancer cell models with different adhesion properties to ECM depending their epithelial or mesenchymal properties, including the pair of lung cancer cells with (A549sh) or without E-cadherin expression (A549sh-Ecad), which would be optimal model to examine the alteration of adhesion properties after EMT induction. The cell-matrix adhesion resisting to shear stress appeared to be remarkably differed between lung cancer cells. The detachment rate of epithelial-like H358 and mesenchymal-like H460 cells was 53%-80% and 25%-66% in the shear stress range of 34-60 dyn/cm2, respectively. A549sh-Ecad cells exhibits lower detachment rate (5%-9%) compared to A549sh cells (14%-40%). By direct comparison of adhesion between A549sh and A549sh-Ecad, we demonstrated that A549shE-cad to mimic EMT were more favorable to the ECM attachment under the various levels of shear stress. The present method can be applied to quantitative analysis of tumor cell-ECM adhesion.

  14. Application of multiple levels of fluid shear stress to endothelial cells plated on polyacrylamide gels†

    PubMed Central

    Galie, P. A.; van Oosten, A.; Chen, C. S.

    2015-01-01

    Measurements of endothelial cell response to fluid shear stress have previously been performed on unphysiologically rigid substrates. We describe the design and implementation of a microfluidic device that applies discrete levels of shear stress to cells plated on hydrogel-based substrates of physiologicallyrelevant stiffness. The setup allows for measurements of cell morphology and inflammatory response to the combined stimuli, and identifies mechanisms by which vascular stiffening leads to pathological responses to blood flow. We found that the magnitude of shear stress required to affect endothelial cell morphology and inflammatory response depended on substrate stiffness. Endothelial cells on 100 Pa substrates demonstrate a greater increase in cell area and cortical stiffness and decrease in NF-κB nuclear translocation in response to TNF-α treatment compared to controls than cells plated on 10 kPa substrates. The response of endothelial cells on soft substrates to shear stress depends on the presence of hyaluronan (HA). These results emphasize the importance of substrate stiffness on endothelial function, and elucidate a means by which vascular stiffening in aging and disease can impact the endothelium. PMID:25573790

  15. The effect of roughness elements on wind erosion: The importance of surface shear stress distribution

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Representation of surface roughness effects on aeolian sediment transport is a key source of uncertainty in wind erosion models. Drag partitioning schemes are used to account for roughness by scaling the soil entrainment threshold by the ratio of shear stress on roughness elements to that on the veg...

  16. Arterial intimal-medial permeability and coevolving structural responses to defined shear-stress exposures.

    PubMed

    Fry, Donald L

    2002-12-01

    The purpose of this research was to examine the evolution of arterial shear stress-induced intimal albumin permeability and coevolving structural responses in swine arteries. Uniform laminar shear-stress responses were compared with those of a simulated "flow separation" stress field. These fields were created using specially designed flow-configuring devices in an experimentally controlled, metabolically supported, ex vivo thoracoabdominal aorta preparation. The Evans blue dye-albumin complex (EBD-alb) permeability patterns that evolved were measured by a reflectometric method. The corresponding tissue structural responses were evaluated by histological, immunostaining, and ultrastructural microscopic techniques. It was shown that when a previously in vivo-adapted artery is challenged by a new mechanochemical environment, it undergoes a sequence of adaptive processes over the ensuing 95 h. Intimal regions of laminar shear-stress exposure ( approximately 16 dyn/cm(2)) responded initially (23 h) with an increase in permeability. With continued stress exposure, intimal-medial structural changes ensued that restored the artery to a physiologically normal permeability. Over this same period, adjacent endothelial regions exposed to simulated flow separation stress fields ( approximately 0.03-0.27 dyn/cm(2)) developed early and progressively increasing permeability. This was associated with formation of local intimal edema, loss of intimal matrix material, and development of distinctively raised, gelatinous-appearing intimal lesions having a potentially preatheromatous architecture. PMID:12388284

  17. A coupled global-local shell model with continuous interlaminar shear stresses

    NASA Astrophysics Data System (ADS)

    Gruttmann, F.; Wagner, W.; Knust, G.

    2016-02-01

    In this paper layered composite shells subjected to static loading are considered. The theory is based on a multi-field functional, where the associated Euler-Lagrange equations include besides the global shell equations formulated in stress resultants, the local in-plane equilibrium in terms of stresses and a constraint which enforces the correct shape of warping through the thickness. Within a four-node element the warping displacements are interpolated with layerwise cubic functions in thickness direction and constant shape throughout the element reference surface. Elimination of stress, warping and Lagrange parameters on element level leads to a mixed hybrid shell element with 5 or 6 nodal degrees of freedom. The implementation in a finite element program is simple. The computed interlaminar shear stresses are automatically continuous at the layer boundaries. Also the stress boundary conditions at the outer surfaces are fulfilled and the integrals of the shear stresses coincide exactly with the independently interpolated shear forces without introduction of further constraints. The essential feature of the element formulation is the fact that it leads to usual shell degrees of freedom, which allows application of standard boundary or symmetry conditions and computation of shell structures with intersections.

  18. Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

    NASA Astrophysics Data System (ADS)

    Matsumoto, Satoshi; Nakao, Shigeru; Ohkura, Takahiro; Miyazaki, Masahiro; Shimizu, Hiroshi; Abe, Yuki; Inoue, Hiroyuki; Nakamoto, Manami; Yoshikawa, Shin; Yamashita, Yusuke

    2015-10-01

    We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes. We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field. Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area. The minimum principal compression stress ( σ3), with its near north-south trend, is dominant throughout the entire region. However, the σ 3 axes around the shear zone are rotated normal to the zone. This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data. Conversely, the stress field of the normal fault is dominant in the Beppu-Shimabara area, which is located in the central part of the island. This result and the direction of σ3 are consistent with the formation of a graben structure in the area.

  19. Some constraints on levels of shear stress in the crust from observations and theory.

    USGS Publications Warehouse

    McGarr, A.

    1980-01-01

    In situ stress determinations in North America, southern Africa, and Australia indicate that on the average the maximum shear stress increases linearly with depth to at least 5.1 km measured in soft rock, such as shale and sandstone, and to 3.7 km in hard rock, including granite and quartzite. Regression lines fitted to the data yield gradients of 3.8 MPa/km and 6.6 MPa/km for soft and hard rock, respectively. Generally, the maximum shear stress in compressional states of stress for which the least principal stress is oriented near vertically is substantially greater than in extensional stress regimes, with the greatest principal stress in a vertical direction. The equations of equilibrium and compatibility can be used to provide functional constrains on the state of stress. If the stress is assumed to vary only with depth z in a given region, then all nonzero components must have the form A + Bz, where A and B are constants which generally differ for the various components. - Author

  20. Quantification of Shear Deformations and Corresponding Stresses in the Biaxially Tested Human Myocardium.

    PubMed

    Sommer, Gerhard; Haspinger, Daniel Ch; Andrä, Michaela; Sacherer, Michael; Viertler, Christian; Regitnig, Peter; Holzapfel, Gerhard A

    2015-10-01

    One goal of cardiac research is to perform numerical simulations to describe/reproduce the mechanoelectrical function of the human myocardium in health and disease. Such simulations are based on a complex combination of mathematical models describing the passive mechanical behavior of the myocardium and its electrophysiology, i.e., the activation of cardiac muscle cells. The problem in developing adequate constitutive models is the shortage of experimental data suitable for detailed parameter estimation in specific functional forms. A combination of shear and biaxial extension tests with different loading protocols on different specimen orientations is necessary to capture adequately the direction-dependent (orthotropic) response of the myocardium. In most experimental animal studies, where planar biaxial extension tests on the myocardium have been conducted, the generated shear stresses were neither considered nor discussed. Hence, in this study a method is presented which allows the quantification of shear deformations and related stresses. It demonstrates an approach for experimenters as to how the generation of these shear stresses can be minimized during mechanical testing. Experimental results on 14 passive human myocardial specimens, obtained from nine human hearts, show the efficiency of this newly developed method. Moreover, the influence of the clamping technique of the specimen, i.e., the load transmission between the testing device and the tissue, on the stress response is determined by testing an isotropic material (Latex). We identified that the force transmission between the testing device and the specimen by means of hooks and cords does not influence the performed experiments. We further showed that in-plane shear stresses definitely exist in biaxially tested human ventricular myocardium, but can be reduced to a minimum by preparing the specimens in an appropriate manner. Moreover, we showed whether shear stresses can be neglected when performing

  1. Identification of two novel shear stress responsive elements in rat angiotensin I converting enzyme promoter.

    PubMed

    Miyakawa, Ayumi Aurea; de Lourdes Junqueira, Maria; Krieger, José Eduardo

    2004-04-13

    Mechanical forces contribute to maintenance of cardiovascular homeostasis via the control of release and production of vasoactive substances. We demonstrated previously that shear stress decreases rat ACE activity and expression. Using a reporter gene approach and mutagenesis, we show now that the classic shear stress responsive element or SSRE (GAGACC) contained within 1,274 bp of this promoter is not functional in response to shear stress (15 dyn/cm2, 18 h) [for the wild-type ACE promoter (WLuc), static control (C) = 107 +/- 6.5%, shear stress (SS) = 65.9 +/- 9.4%, n = 8; for the promoter with the classic SSRE mutated (WSS-mut), C = 100 +/- 8.2%, SS = 60.2 +/- 5.2%, n = 10, respectively]. Analysis of progressive deletion mutants unraveled a 57-bp fragment, position -251 to -195, from the transcription start site, containing functional SSRE (for WLuc, C = 107 +/- 6.5%, SS = 65.9 +/- 9.4%, n = 8; for 378, C = 100 +/- 6.4%, SS = 60.4 +/- 4.3%, n = 11; for 251, C = 99.7 +/- 2.6%, SS = 63.2 +/- 5.5%, n = 7; for 194, C = 104.6 +/- 8.1%, SS = 92.4 +/- 6.9%, n = 9). This fragment responded to shear stress even in the context of a heterologous promoter. Finally, functional analysis of mutated candidate regulatory elements identified by gel shift, DNase I footprint, and conservation of aligned sequences revealed that only the double mutant (Barbie/GAGA-mut) but not isolated disruption of the Barbie (WBarbie-mut) or the GAGA (WGAGA-mut) prevented the shear-stress-induced response (for Barbie/GAGA-mut, C = 97.9 +/- 5%, SS = 99.4 +/- 7.2%, n = 6; for WBarbie-mut, C = 106.1 +/- 8.6%, SS = 65.9 +/- 9.4%, n = 6; for WGAGA-mut, C = 100.1 +/- 2.9%, SS = 66.7 +/- 1.6, n = 6;). Taken together, these data provide direct evidence for the new role of Barbie and GAGA boxes in mediating the shear-stress-induced downregulation of rat ACE expression and demonstrate that the classic SSRE (GAGACC) is not functional under the experimental conditions tested. PMID:14872008

  2. A simple model to understand the effect of membrane shear elasticity and stress-free shape on the motion of red blood cells in shear flow.

    PubMed

    Dupire, Jules; Abkarian, Manouk; Viallat, Annie

    2015-11-14

    An analytical model was proposed by Keller and Skalak in 1982 to understand the motion of red blood cells in shear flow. The cell was described as a fluid ellipsoid of fixed shape. This model was extended in 2007 to introduce shear elasticity of the red blood cell membrane. Here, this model is further extended to take into account that the cell discoid shape physiologically observed is not a stress-free shape. The model shows that spheroid stress-free shapes allow us to fit the experimental data with the values of shear elasticity typical to that found with micropipette and optical tweezer experiments. In the range of moderate shear rates (for which RBCs keep their discoid shape) this model enables us to quantitatively determine (i) an effective cell viscosity, which combines membrane and hemoglobin viscosities and (ii) an effective shear modulus of the membrane that combines the shear modulus and the stress-free shape. This model can also be used to determine RBC mechanical parameters not only in the tanktreading regime when cells are suspended in medium of high viscosity but also in the tumbling regime characteristic of cells suspended in media of low viscosity. In this regime, a transition is predicted between a rigid-like tumbling motion and a fluid-like tumbling motion above a critical shear rate, which is directly related to the mechanical parameters of the cell. PMID:26352875

  3. Using a co-culture microsystem for cell migration under fluid shear stress.

    PubMed

    Yeh, Chia-Hsien; Tsai, Shen-Hsing; Wu, Li-Wha; Lin, Yu-Cheng

    2011-08-01

    We have successfully developed a microsystem to co-cultivate two types of cells with a minimum defined gap of 50 μm, and to quantitatively study the impact of fluid shear stress on the mutual influence of cell migration velocity and distance. We used the hydrostatic pressure to seed two different cells, endothelial cells (ECs) and smooth muscle cells (SMCs), on opposite sides of various gap sizes (500 μm, 200 μm, 100 μm, and 50 μm). After cultivating the cells for 12 h and peeling the co-culture microchip from the culture dish, we studied the impacts of gap size on the migration of either cell type in the absence or presence of fluid shear stress (7 dyne cm(-2) and 12 dyne cm(-2)) influence. We found that both gap size and shear stress have profound influence on cell migration. Smaller gap sizes (100 μm and 50 μm) significantly enhanced cell migration, suggesting a requirement of an effective concentration of released factor(s) by either cell type in the gap region. Flow-induced shear stress delayed the migration onset of either cell type in a dose-dependent manner regardless of the gap size. Moreover, shear stress-induced decrease of cell migration becomes evident when the gap size was 500 μm. We have developed a co-culture microsystem for two kinds of cells and overcome the conventional difficulties in observation and mixed culture, and it would have more application for bio-manipulation and tissue repair engineering. PMID:21695290

  4. Sox18 preserves the pulmonary endothelial barrier under conditions of increased shear stress.

    PubMed

    Gross, Christine M; Aggarwal, Saurabh; Kumar, Sanjiv; Tian, Jing; Kasa, Anita; Bogatcheva, Natalia; Datar, Sanjeev A; Verin, Alexander D; Fineman, Jeffrey R; Black, Stephen M

    2014-11-01

    Shear stress secondary to increased pulmonary blood flow (PBF) is elevated in some children born with congenital cardiac abnormalities. However, the majority of these patients do not develop pulmonary edema, despite high levels of permeability inducing factors. Previous studies have suggested that laminar fluid shear stress can enhance pulmonary vascular barrier integrity. However, little is known about the mechanisms by which this occurs. Using microarray analysis, we have previously shown that Sox18, a transcription factor involved in blood vessel development and endothelial barrier integrity, is up-regulated in an ovine model of congenital heart disease with increased PBF (shunt). By subjecting ovine pulmonary arterial endothelial cells (PAEC) to laminar flow (20 dyn/cm(2) ), we identified an increase in trans-endothelial resistance (TER) across the PAEC monolayer that correlated with an increase in Sox18 expression. Further, the TER was also enhanced when Sox18 was over-expressed and attenuated when Sox18 expression was reduced, suggesting that Sox18 maintains the endothelial barrier integrity in response to shear stress. Further, we found that shear stress up-regulates the cellular tight junction protein, Claudin-5, in a Sox18 dependent manner, and Claudin-5 depletion abolished the Sox18 mediated increase in TER in response to shear stress. Finally, utilizing peripheral lung tissue of 4 week old shunt lambs with increased PBF, we found that both Sox18 and Claudin-5 mRNA and protein levels were elevated. In conclusion, these novel findings suggest that increased laminar flow protects endothelial barrier function via Sox18 dependent up-regulation of Claudin-5 expression. PMID:24677020

  5. Sox18 preserves the pulmonary endothelial barrier under conditions of increased shear stress

    PubMed Central

    Gross, Christine M.; Aggarwal, Saurabh; Kumar, Sanjiv; Tian, Jing; Kasa, Anita; Bogatcheva, Natalia; Datar, Sanjeev A.; Verin, Alexander D.; Fineman, Jeffrey R.; Black, Stephen M.

    2014-01-01

    Shear stress secondary to increased pulmonary blood flow (PBF) is elevated in some children born with congenital cardiac abnormalities. However, the majority of these patients do not develop pulmonary edema, despite high levels of permeability inducing factors. Previous studies have suggested that laminar fluid shear stress can enhance pulmonary vascular barrier integrity. However, little is known about the mechanisms by which this occurs. Using microarray analysis, we have previously shown that Sox18, a transcription factor involved in blood vessel development and endothelial barrier integrity, is up-regulated in an ovine model of congenital heart disease with increased PBF (shunt). By subjecting ovine pulmonary arterial endothelial cells (PAEC) to laminar flow (20 dyn/cm2), we identified an increase in trans-endothelial resistance (TER) across the PAEC monolayer that correlated with an increase in Sox18 expression. Further, the TER was also enhanced when Sox18 was over-expressed and attenuated when Sox18 expression was reduced, suggesting that Sox18 maintains the endothelial barrier integrity in response to shear stress. Further, we found that shear stress up-regulates the cellular tight junction protein, Claudin-5, in a Sox18 dependent manner, and Claudin-5 depletion abolished the Sox18 mediated increase in TER in response to shear stress. Finally, utilizing peripheral lung tissue of 4 week old shunt lambs with increased PBF, we found that both Sox18 and Claudin-5 mRNA and protein levels were elevated. In conclusion, these novel findings suggest that increased laminar flow protects endothelial barrier function via Sox18 dependent up-regulation of Claudin-5 expression. PMID:24677020

  6. Flow separation and shear stress over angle-of-repose bed forms: A numerical investigation

    NASA Astrophysics Data System (ADS)

    Lefebvre, Alice; Paarlberg, Andries J.; Winter, Christian

    2014-02-01

    Large asymmetric bed forms commonly develop in rivers. The turbulence associated with flow separation that develops over their steep lee side is responsible for the form shear stress which can represent a substantial part of total shear stress in rivers. This paper uses the Delft3D modeling system to investigate the effects of bed form geometry and forcing conditions on flow separation length and associated turbulence, and bed form shear stress over angle-of-repose (30° lee side angle) bed forms. The model was validated with lab measurements that showed sufficient agreement to be used for a systematic analysis. The influence of flow velocity, bed roughness, relative height (bed form height/water depth), and aspect ratio (bed form height/length) on the variations of the normalized length of the flow separation zone, the extent of the wake region (where the turbulent kinetic energy (TKE) was more than 70% of the maximum TKE), the average TKE within the wake region and the form shear stress were investigated. Form shear stress was found not to scale with the size of the flow separation zone but to be related to the product of the normalized extent of the wake region (extent of the wake region/extent of water body above the bed form) and the average TKE within the wake region. The results add to understanding of the hydrodynamics of bed forms and may be used for the development of better parameterizations of small-scale processes for application in large-scale studies.

  7. Open and closed shear-walls in high-rise structural systems: Static and dynamic analysis

    NASA Astrophysics Data System (ADS)

    Carpinteri, Alberto; Lacidogna, Giuseppe; Nitti, Giuseppe

    2016-06-01

    In the present paper, a General Algorithm is applied to the analysis of high-rise structures. This algorithm is to be used as a calculation tool in preliminary design; it allows to define the interaction between closed and open, straight or curved shear-walls, and the forces exchanged in structures subject to mainly horizontal loads. The analysis can be performed in both static and dynamic regimes, the mode shapes and the natural frequencies being assessed. This general formulation allows analyses of high-rise structures by taking into account the torsional rigidity and the warping deformations of the elements composing the building without gross simplifications. In thisway it is possible to model the structure as a single equivalent cantilever, thus minimising the degrees of freedom of the system, and consequently the calculation time. Finally, potentials of the method proposed are demonstrated by a numerical example which emphasizes the link between global displacements and stresses in the elements composing the structure.

  8. The shear response of copper bicrystals with Σ11 symmetric and asymmetric tilt grain boundaries by molecular dynamics simulation

    NASA Astrophysics Data System (ADS)

    Zhang, Liang; Lu, Cheng; Tieu, Kiet; Zhao, Xing; Pei, Linqing

    2015-04-01

    Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11<1 1 0> tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination along the boundary plane. A non-planar structure with dissociated intrinsic stacking faults was prevalent in Σ11 asymmetric GBs of Cu. This type of structure can significantly increase the ductility of bicrystal models under shear deformation. A grain boundary can be a source of dislocation and migrate itself at different stress levels. The intrinsic free volume involved in the grain boundary area was correlated with dislocation nucleation and GB sliding, while the dislocation nucleation mechanism can be different for a grain boundary due to its different equilibrium structures.Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11<1 1 0> tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination

  9. Imaging deformation of adherent cells due to shear stress using quantitative phase imaging.

    PubMed

    Eldridge, Will J; Sheinfeld, Adi; Rinehart, Matthew T; Wax, Adam

    2016-01-15

    We present a platform for detecting cellular deformations from mechanical stimuli, such as fluid shear stress, using rapid quantitative phase imaging. Rapid quantitative phase imaging was used to analyze changes in the optical path length of adherent skin cancer cells during mechanical displacement. Both the whole-cell phase displacement and the resultant shift of the cellular center of mass were calculated over the duration of the stimulus. Whole-cell phase displacement images were found to match expectation. Furthermore, center-of-mass shifts of adherent cells were found to resemble that of a one-dimensional Kelvin-Voigt (KV) viscoelastic solid. Cellular steady-state displacements from step fluid shear stimuli were found to be linearly related to the shear stress. Shear stiffness constants for cells exposed to a cytoskeletal disrupting toxin were found to be significantly lower than unexposed cells. This novel technique allows for elastographic analysis of whole-cell effective shear stiffness without the use of an exogenous force applicator, a specialized culture substrate, or tracking net perimeter movement of the cell. PMID:26766712

  10. Deep heterogeneity of the stress state in the horizontal shear zones

    NASA Astrophysics Data System (ADS)

    Rebetsky, Yu. L.; Mikhailova, A. V.

    2014-11-01

    The formation structures of brittle destruction in a rock layer above an active strike-slip fault in the crystalline basement is studied. The problem is analyzed from the standpoint of loading history, when after the stage of pure gravitational loading, an additional strain state of uniform horizontal shear of both the layer and underlying basement develops, which is further followed by a vertically nonuniform shear caused by the activation of the deep fault. For the studied object, irreversible fracture deformations on macro- and microlevels arise as early as the initial stage of loading under the action of gravitational stresses. These deformations continue evolving on the megascopic level in the course of horizontal shearing that is quasi-uniform both along the depth and laterally. The final formation of the structural ensemble occurs after a long stage of horizontal displacement of the blocks of the crystalline basement—the stage of localized shear. The theoretical analysis of the evolution of the stress state and morphology of the failure structures established the presence of numerous fractures with the normal dip-slip components in the intermediate-depth part of the rock mass, which are formed at the stages of uniform and localized horizontal shearing. The fractures with a strike-slip component mainly arise in the upper and near-axial deep parts of the section.

  11. Basal shear stress under alpine glaciers: Insights from experiments using the iSOSIA and Elmer/ICE models

    NASA Astrophysics Data System (ADS)

    Brædstrup, C. F.; Egholm, D. L.; Ugelvig, S. V.; Pedersen, V. K.

    2015-10-01

    Shear stress at the base of glaciers controls basal sliding and is therefore immensely important for glacial erosion and landscape evolution in arctic and high-altitude areas. However, the inaccessible nature of glacial beds complicates empirical studies of basal shear stress, and little is therefore known of its spatial and temporal distribution. In this study we seek to improve our understanding of basal shear stress using a higher-order numerical ice model (iSOSIA). In order to test the validity of the higher-order model, we first compare the detailed distribution of basal shear stress in iSOSIA and in a three-dimensional full-Stokes model (Elmer/ICE). We find that iSOSIA and Elmer/ICE predict similar first-order stress and velocity patterns, and that differences are restricted to local variations over length-scales on the order of the grid resolution. In addition, we find that subglacial shear stress is relatively uniform and insensitive to suble changes in local topographic relief. Following these initial stress benchmark experiments, we use iSOSIA to investigate changes in basal shear stress as a result of landscape evolution by glacial erosion. The experiments with landscape evolution show that subglacial shear stress decreases as glacial erosion transforms preglacial V-shaped valleys into U-shaped troughs. These findings support the hypothesis that glacial erosion is most efficient in the early stages of glacial landscape development.

  12. A pressure and shear sensor system for stress measurement at lower limb residuum/socket interface.

    PubMed

    Laszczak, P; McGrath, M; Tang, J; Gao, J; Jiang, L; Bader, D L; Moser, D; Zahedi, S

    2016-07-01

    A sensor system for measurement of pressure and shear at the lower limb residuum/socket interface is described. The system comprises of a flexible sensor unit and a data acquisition unit with wireless data transmission capability. Static and dynamic performance of the sensor system was characterised using a mechanical test machine. The static calibration results suggest that the developed sensor system presents high linearity (linearity error ≤ 3.8%) and resolution (0.9 kPa for pressure and 0.2 kPa for shear). Dynamic characterisation of the sensor system shows hysteresis error of approximately 15% for pressure and 8% for shear. Subsequently, a pilot amputee walking test was conducted. Three sensors were placed at the residuum/socket interface of a knee disarticulation amputee and simultaneous measurements were obtained during pilot amputee walking test. The pressure and shear peak values as well as their temporal profiles are presented and discussed. In particular, peak pressure and shear of approximately 58 kPa and 27 kPa, respectively, were recorded. Their temporal profiles also provide dynamic coupling information at this critical residuum/socket interface. These preliminary amputee test results suggest strong potential of the developed sensor system for exploitation as an assistive technology to facilitate socket design, socket fit and effective monitoring of lower limb residuum health. PMID:27118308

  13. Mean wall shear stress in the femoral arterial bifurcation is low and independent of age at rest.

    PubMed

    Kornet, L; Hoeks, A P; Lambregts, J; Reneman, R S

    2000-01-01

    In elastic arteries, mean wall shear stress appears to be close to 1. 5 Pa, the value predicted by the theory of minimal energy loss. This finding in elastic arteries does not necessarily represent the situation in muscular arteries. Elastic arteries have to store potential energy, while muscular arteries have mainly a conductive function. Therefore, we determined wall shear stress and its age dependency in the common and superficial femoral arteries, 2-3 cm from the flow divider in 54 presumed healthy volunteers between 21 and 74 years of age, using a non-invasive ultrasound system. Prior to the study, the reliability of this system was determined in terms of intrasubject variation. Mean wall shear stress was significantly lower in the common femoral artery (0.35 +/- 0.18 Pa) than in the superficial femoral artery (0.49 +/- 0.15 Pa). In all age categories, peak systolic wall shear stress and the maximal cyclic change in wall shear stress were not significantly different in the common and the superficial femoral arteries. Peak systolic wall shear stress in the common and the superficial femoral arteries was not significantly different from the value previously determined in the common carotid artery, but mean wall shear stress was lower in the common and superficial femoral arteries than in the common carotid artery by a factor of 2-4. In both the common and the superficial femoral arteries, mean, peak systolic and maximal cyclic change in wall shear stress did not change significantly with age, nor did diameter. We conclude that, as compared to elastic arteries, mean wall shear stress is low in the conductive arteries of a resting leg, due to backflow during the first part of the diastolic phase of the cardiac cycle and the absence of flow during the rest of the diastolic phase. Mean wall shear stress is lower in the common than in the superficial femoral artery due to additional reflections from the deep femoral artery. PMID:10754396

  14. IQ Domain GTPase-Activating Protein 1 is Involved in Shear Stress-Induced Progenitor-Derived Endothelial Cell Alignment

    PubMed Central

    Rami, Lila; Auguste, Patrick; Thebaud, Noélie B.; Bareille, Reine; Daculsi, Richard; Ripoche, Jean; Bordenave, Laurence

    2013-01-01

    Shear stress is one of mechanical constraints which are exerted by blood flow on endothelial cells (ECs). To adapt to shear stress, ECs align in the direction of flow through adherens junction (AJ) remodeling. However, mechanisms regulating ECs alignment under shear stress are poorly understood. The scaffold protein IQ domain GTPase activating protein 1 (IQGAP1) is a scaffold protein which couples cell signaling to the actin and microtubule cytoskeletons and is involved in cell migration and adhesion. IQGAP1 also plays a role in AJ organization in epithelial cells. In this study, we investigated the potential IQGAP1 involvement in the endothelial cells alignment under shear stress. Progenitor-derived endothelial cells (PDECs), transfected (or not) with IQGAP1 small interfering RNA, were exposed to a laminar shear stress (1.2 N/m2) and AJ proteins (VE-cadherin and β-catenin) and IQGAP1 were labeled by immunofluorescence. We show that IQGAP1 is essential for ECs alignment under shear stress. We studied the role of IQGAP1 in AJs remodeling of PDECs exposed to shear stress by studying cell localization and IQGAP1 interactions with VE-cadherin and β-catenin by immunofluorescence and Proximity Ligation Assays. In static conditions, IQGAP1 interacts with VE-cadherin but not with β-catenin at the cell membrane. Under shear stress, IQGAP1 lost its interaction from VE-cadherin to β-catenin. This “switch” was concomitant with the loss of β-catenin/VE-cadherin interaction at the cell membrane. This work shows that IQGAP1 is essential to ECs alignment under shear stress and that AJ remodeling represents one of the mechanisms involved. These results provide a new approach to understand ECs alignment under to shear stress. PMID:24278215

  15. Molecular shear heating and vortex dynamics in thermostatted two dimensional Yukawa liquids

    NASA Astrophysics Data System (ADS)

    Gupta, Akanksha; Ganesh, Rajaraman; Joy, Ashwin

    2016-07-01

    It is well known that two-dimensional macroscale shear flows are susceptible to instabilities leading to macroscale vortical structures. The linear and nonlinear fate of such a macroscale flow in a strongly coupled medium is a fundamental problem. A popular example of a strongly coupled medium is a dusty plasma, often modelled as a Yukawa liquid. Recently, laboratory experiments and molecular dynamics (MD) studies of shear flows in strongly coupled Yukawa liquids indicated the occurrence of strong molecular shear heating, which is found to reduce the coupling strength exponentially leading to the destruction of macroscale vorticity. To understand the vortex dynamics of strongly coupled molecular fluids undergoing macroscale shear flows and molecular shear heating, MD simulation has been performed, which allows the macroscopic vortex dynamics to evolve, while at the same time "removes" the microscopically generated heat without using the velocity degrees of freedom. We demonstrate that by using a configurational thermostat in a novel way, the microscale heat generated by shear flow can be thermostatted out efficiently without compromising the large scale vortex dynamics. In the present work, using MD simulations, a comparative study of shear flow evolution in Yukawa liquids in the presence and absence of molecular or microscopic heating is presented for a prototype shear flow, namely, Kolmogorov flow.

  16. Finite Element Modeling of Dynamic Shear Rupture Experiments Along Non-Planar Faults

    NASA Astrophysics Data System (ADS)

    Templeton, E. L.; Baudet, A.; Bhat, H. S.; Rice, J. R.

    2004-12-01

    The study of dynamically propagating shear cracks along weak paths like faults is of great interest for the study of earthquakes. We adapted the ABAQUS/Explicit dynamic finite element program to analyze the nucleation and propagation of shear cracks along a non-planar, kinked, weak path corresponding to the one that was used in recent laboratory fracture studies by Rousseau and Rosakis [JGR, 2003]. Their experiments involved impact loading of thin plates of Homalite-100, a photoelastically sensitive brittle polymer, which had been cut along a kinked path and then weakly glued back together everywhere except along a starter notch near the impact site. Under different conditions, propagation speeds were observed in both the sub-Rayleigh and intersonic (supershear) regimes. Strain gage recordings and high speed photography of isochromatic lines (lines of constant difference between the in-plane principal strains) provided characterization of the transient deformation fields associated with the impact and fracture propagation. For the finite element analyses, we implemented a slip-weakening failure model through an option in the ABAQUS program allowing user defined constitutive relations. The analyses of impact loading and of rupture nucleation and propagation were then carried out in the 2D framework of plane stress. In a first set of studies of nucleation and propagation of rupture along a straight fault, we determined after some trial and error an appropriate CFL number, and examined different element types and layouts, finding that the most acceptable results were obtained for low order elements. We used constant strain triangles, arrayed in groups of four to effectively form four-sided elements with corner nodes and one internal node. The studies also showed that to obtain representations of slip rate and shear stress near the propagating rupture tip that were relatively free from numerical oscillations, it was necessary to have element side lengths of order Ro/50

  17. Adaptive response of vascular endothelial cells to an acute increase in shear stress frequency.

    PubMed

    Zhang, Ji; Friedman, Morton H

    2013-09-15

    Local shear stress sensed by arterial endothelial cells is occasionally altered by changes in global hemodynamic parameters, e.g., heart rate and blood flow rate, as a result of normal physiological events, such as exercise. In a recently study (41), we demonstrated that during the adaptive response to increased shear magnitude, porcine endothelial cells exhibited an unique phenotype featuring a transient increase in permeability and the upregulation of a set of anti-inflammatory and antioxidative genes. In the present study, we characterize the adaptive response of these cells to an increase in shear frequency, another important hemodynamic parameter with implications in atherogenesis. Endothelial cells were preconditioned by a basal-level sinusoidal shear stress of 15 ± 15 dyn/cm(2) at 1 Hz, and the frequency was then elevated to 2 Hz. Endothelial permeability increased slowly after the frequency step-up, but the increase was relatively small. Using microarrays, we identified 37 genes that are sensitive to the frequency step-up. The acute increase in shear frequency upregulates a set of cell-cycle regulation and angiogenesis-related genes. The overall adaptive response to the increased frequency is distinctly different from that to a magnitude step-up. However, consistent with the previous study, our data support the notion that endothelial function during an adaptive response is different than that of fully adapted endothelial cells. Our studies may also provide insights into the beneficial effects of exercise on vascular health: transient increases in frequency may facilitate endothelial repair, whereas similar increases in shear magnitude may keep excessive inflammation and oxidative stress at bay. PMID:23851277

  18. Adaptive response of vascular endothelial cells to an acute increase in shear stress frequency

    PubMed Central

    Zhang, Ji

    2013-01-01

    Local shear stress sensed by arterial endothelial cells is occasionally altered by changes in global hemodynamic parameters, e.g., heart rate and blood flow rate, as a result of normal physiological events, such as exercise. In a recently study (41), we demonstrated that during the adaptive response to increased shear magnitude, porcine endothelial cells exhibited an unique phenotype featuring a transient increase in permeability and the upregulation of a set of anti-inflammatory and antioxidative genes. In the present study, we characterize the adaptive response of these cells to an increase in shear frequency, another important hemodynamic parameter with implications in atherogenesis. Endothelial cells were preconditioned by a basal-level sinusoidal shear stress of 15 ± 15 dyn/cm2 at 1 Hz, and the frequency was then elevated to 2 Hz. Endothelial permeability increased slowly after the frequency step-up, but the increase was relatively small. Using microarrays, we identified 37 genes that are sensitive to the frequency step-up. The acute increase in shear frequency upregulates a set of cell-cycle regulation and angiogenesis-related genes. The overall adaptive response to the increased frequency is distinctly different from that to a magnitude step-up. However, consistent with the previous study, our data support the notion that endothelial function during an adaptive response is different than that of fully adapted endothelial cells. Our studies may also provide insights into the beneficial effects of exercise on vascular health: transient increases in frequency may facilitate endothelial repair, whereas similar increases in shear magnitude may keep excessive inflammation and oxidative stress at bay. PMID:23851277

  19. Effects of fluid shear stress on polyelectrolyte multilayers by neutron scattering studies

    DOE PAGESBeta

    Singh, Saurabh; Junghans, Ann; Watkins, Erik; Kapoor, Yash; Toomey, Ryan; Majewski, Jaroslaw

    2015-02-17

    The structure of layer-by-layer (LbL) deposited nanofilm coatings consists of alternating polyethylenimine (PEI) and polystyrenesulfonate (PSS) films deposited on a single crystal quartz substrate. LbL-deposited nanofilms were investigated by neutron reflectomery (NR) in contact with water in the static and fluid shear stress conditions. The fluid shear stress was applied through a laminar flow of the liquid parallel to the quartz/polymer interface in a custom-built solid–liquid interface cell. The scattering length density profiles obtained from NR results of these polyelectrolyte multilayers (PEM), measured under different shear conditions, showed proportional decrease of volume fraction of water hydrating the polymers. For themore » highest shear rate applied (ca. 6800 s–1) the water volume fraction decreased by approximately 7%. The decrease of the volume fraction of water was homogeneous through the thickness of the film. Since there were not any significant changes in the total polymer thickness, it resulted in negative osmotic pressures in the film. The PEM films were compared with the behavior of thin films of thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) deposited via spin-coating. The PEM and pNIPAM differ in their interactions with water molecules, and they showed opposite behaviors under the fluid shear stress. In both cases the polymer hydration was reversible upon the restoration of static conditions. Furthermore, a theoretical explanation is given to explain this difference in the effect of shear on hydration of polymeric thin films.« less

  20. Exploring German Bight coastal morphodynamics based on modelled bed shear stress

    NASA Astrophysics Data System (ADS)

    Kösters, Frank; Winter, Christian

    2014-02-01

    The prediction of large-scale coastal and estuarine morphodynamics requires a sound understanding of the relevant driving processes and forcing factors. Data- and process-based methods and models suffer from limitations when applied individually to investigate these systems and, therefore, a combined approach is needed. The morphodynamics of coastal environments can be assessed in terms of a mean bed elevation range (BER), which is the difference of the lowest to highest seabed elevation occurring within a defined time interval. In this study of the coastal sector of the German Bight, North Sea, the highly variable distribution of observed BER for the period 1984-2006 is correlated to local bed shear stresses based on hindcast simulations with a well-validated high-resolution (typically 1,000 m in coastal settings) process-based numerical model of the North Sea. A significant correlation of the 95th percentile of bed shear stress and BER was found, explaining between 49 % and 60 % of the observed variance of the BER under realistic forcing conditions. The model then was applied to differentiate the effects of three main hydrodynamic drivers, i.e. tides, wind-induced currents, and waves. Large-scale mapping of these model results quantify previous qualitative suggestions: tides act as main drivers of the East Frisian coast, whereas waves are more relevant for the morphodynamics of the German west coast. Tidal currents are the main driver of the very high morphological activity of the tidal channels of the Ems, Weser and Elbe estuaries, the Jade Bay, and tidal inlets between the islands. This also holds for the backbarrier tidal flats of the North Frisian Wadden Sea. The morphodynamics of the foreshore areas of the barrier island systems are mainly wave-driven; in the deeper areas tides, waves and wind-driven currents have a combined effect. The open tidal flats (outer Ems, Neuwerker Watt, Dithmarschen Bight) are affected by a combination of tides, wind

  1. Effect of shear stress on electromagnetic behaviors in superconductor-ferromagnetic bilayer structure

    NASA Astrophysics Data System (ADS)

    Yong, Huadong; Zhao, Meng; Jing, Ze; Zhou, Youhe

    2014-09-01

    In this paper, the electromagnetic response and shielding behaviour of superconductor-ferromagnetic bilayer structure are studied. The magnetomechanical coupling in ferromagnetic materials is also considered. Based on the linear piezomagnetic coupling model and anti-plane shear deformation, the current density and magnetic field in superconducting strip are obtained firstly. The effect of shear stress on the magnetization of strip is discussed. Then, we consider the magnetic cloak for superconductor-ferromagnetic bilayer structure. The magnetic permeability of ferromagnetic material is obtained for perfect cloaking in uniform magnetic field with magnetomechanical coupling in ferromagnet. The simulation results show that the electromagnetic response in superconductors will change by applying the stress only to the ferromagnetic material. In addition, the performance of invisibility of structure for non-uniform field will be affected by mechanical stress. It may provide a method to achieve tunability of superconducting properties with mechanical loadings.

  2. A microfluidic device to apply shear stresses to polarizing ciliated airway epithelium using air flow

    PubMed Central

    Trieu, Dennis; Waddell, Thomas K.; McGuigan, Alison P.

    2014-01-01

    Organization of airway epithelium determines ciliary beat direction and coordination for proper mucociliary clearance. Fluidic shear stresses have the potential to influence ciliary organization. Here, an in vitro fluidic flow system was developed for inducing long-term airflow shear stresses on airway epithelium with a view to influencing epithelial organization. Our system consists of a fluidic device for cell culture, integrated into a humidified airflow circuit. The fluidic device has a modular design and is made from a combination of polystyrene and adhesive components incorporated into a 6-well filter membrane insert. We demonstrate the system operates within physiologically relevant shear and pressure ranges and estimate the shear stress exerted on the epithelial cell layer as a result of air flow using a computational model. For both the bronchial epithelial cell line BEAS2B and primary human tracheal airway epithelial cells, we demonstrate that cells remain viable within the device when exposed to airflow for 24 h and that normal differentiation and cilia formation occurs. Furthermore, we demonstrate the utility of our device for exploring the impact of exposing cells to airflow: our tool enables quantification of cytoskeletal organization, and is compatible with in situ bead assays to assess the orientation of cilia beating. PMID:25553181

  3. Effect of simulated microgravity on osteocytes responding to fluid shear stress

    NASA Astrophysics Data System (ADS)

    Yang, Xiao; Sun, Lian-Wen; Wu, Xin-Tong; Wang, Xiao-Nan; Fan, Yu-Bo

    2013-03-01

    Osteocytes, as most abundant cells and major mechanical sensor in bone, play an important role in the mechanism of microgravity-induced bone loss. The response of osteocytes to fluid flow stress under simulated microgravity was investigated in this study. MLO-Y4, an osteocyte-like cell line, was cultured under simulated microgravity condition for 5 days. Then cells were sheared at 15 dyn/cm2 in flow chamber. After 15 min shear, nitric oxide (NO) was examined by Griess Reagent and prostaglandin E2 (PGE2) by ELISA. After 6 h shear, alkaline phosphatase (ALP) was examined by PNPP, osteocalcin (OC) and procollagen type I N propeptide (PINP) by ELISA. Cells were divided into four groups: CON (1 G with no shear), CON-S (1 G with shear), SM (simulated microgravity with no shear) and SM-S (simulated microgravity with shear). The results showed that (1) NO, ALP activity, OC and PINP increased significantly while PGE2 showed no change in SM compared with CON. (2) NO, PGE2, ALP activity and PINP increased significantly while OC decreased significantly in CON-S compared with CON. (3) NO in SM-S had no significant difference compared to SM, PGE2 and OC increased while ALP activity and PINP decreased significantly in SM-S compared with SM. (4) The increasing amplitude of PGE2 and OC, the decreasing amplitude of ALP activity in SM-S to SM was lower than that in CON-S to CON. In addition, some changes of F-actin cytoskeleton were observed by confocal microscopy. All results indicated that the response induced by fluid shear in osteocytes could be inhibited by simulated microgravity, namely the mechanosensibility of osteocytes decreased under simulated microgravity. This may partly contribute to the mechanism of microgravity-induced osteoporosis and will be helpful to find out effective description.

  4. Paradoxical Effect of Nonphysiological Shear Stress on Platelets and von Willebrand Factor.

    PubMed

    Chen, Zengsheng; Mondal, Nandan K; Ding, Jun; Koenig, Steven C; Slaughter, Mark S; Wu, Zhongjun J

    2016-07-01

    Blood can become hypercoagulable by shear-induced platelet activation and generation of microparticles. It has been reported that nonphysiological shear stress (NPSS) could induce shedding of platelet receptor glycoprotein (GP) Ibα, which may result in an opposite effect to hemostasis. The aim of this study was to investigate the influence of the NPSS on platelets and von Willebrand factor (vWF). Human blood was exposed to two levels of NPSS (25 Pa, 125 Pa) with an exposure time of 0.5 s, generated by using a novel blood-shearing device. Platelet activation (P-selectin expression, GPIIb/IIIa activation and generation of microparticles) and shedding of three platelet receptors (GPIbα, GPVI, GPIIb/IIIa) in sheared blood were quantified using flow cytometry. Aggregation capacity of sheared blood induced by ristocetin and collagen was evaluated using an aggregometer. Shear-induced vWF damage was characterized with Western blotting. Consistent with the published data, the NPSS caused significantly more platelets to become activated with increasing NPSS level. Meanwhile, the NPSS induced the shedding of platelet receptors. The loss of the platelet receptors increased with increasing NPSS level. The aggregation capacity of sheared blood induced by ristocetin and collagen decreased. There was a loss of high molecular weight multimers (HMWMs) of vWF in sheared blood. These results suggest that the NPSS induced a paradoxical effect. More activated platelets increase the risk of thrombosis, while the reduction in platelet receptors and the loss of HMWM-vWF increased the propensity of bleeding. The finding might provide a new perspective to understand thrombosis and acquired bleeding disorder in patients supported with blood contacting medical devices. PMID:26582038

  5. Fatigue fracture of thin plates under tensile and transverse shear stresses

    SciTech Connect

    Viz, M.J.; Zehnder, A.T.; Bamford, J.D.

    1995-12-31

    Crack growth in thin sheets loaded under tension and transverse shear is studied experimentally and the mechanics of such problems are reviewed. A small scale yielding approach is adopted that describes the crack tip fields using a combination if Kirchhoff plate theory and plane stress elasticity. Techniques for calculating the relevant stress intensity factors are presented and validated with results from six test cases. Fatigue crack growth rates are measured using a double-edge notch test specimen configuration loaded in tension and torsion. A geometrically nonlinear finite element computation is used to determine the stress intensity factors as functions of axial load, torque, and crack length.

  6. Effect of Shear Stress on Pseudomonas aeruginosa Isolated from the Cystic Fibrosis Lung

    PubMed Central

    Dingemans, Jozef; Monsieurs, Pieter; Yu, Sung-Huan; Crabbé, Aurélie; Förstner, Konrad U.; Malfroot, Anne

    2016-01-01

    ABSTRACT Chronic colonization of the lungs by Pseudomonas aeruginosa is one of the major causes of morbidity and mortality in cystic fibrosis (CF) patients. To gain insights into the characteristic biofilm phenotype of P. aeruginosa in the CF lungs, mimicking the CF lung environment is critical. We previously showed that growth of the non-CF-adapted P. aeruginosa PAO1 strain in a rotating wall vessel, a device that simulates the low fluid shear (LS) conditions present in the CF lung, leads to the formation of in-suspension, self-aggregating biofilms. In the present study, we determined the phenotypic and transcriptomic changes associated with the growth of a highly adapted, transmissible P. aeruginosa CF strain in artificial sputum medium under LS conditions. Robust self-aggregating biofilms were observed only under LS conditions. Growth under LS conditions resulted in the upregulation of genes involved in stress response, alginate biosynthesis, denitrification, glycine betaine biosynthesis, glycerol metabolism, and cell shape maintenance, while genes involved in phenazine biosynthesis, type VI secretion, and multidrug efflux were downregulated. In addition, a number of small RNAs appeared to be involved in the response to shear stress. Finally, quorum sensing was found to be slightly but significantly affected by shear stress, resulting in higher production of autoinducer molecules during growth under high fluid shear (HS) conditions. In summary, our study revealed a way to modulate the behavior of a highly adapted P. aeruginosa CF strain by means of introducing shear stress, driving it from a biofilm lifestyle to a more planktonic lifestyle. PMID:27486191

  7. Fluid shear stress regulates metalloproteinase-1 and 2 in human periodontal ligament cells: involvement of extracellular signal-regulated kinase (ERK) and P38 signaling pathways.

    PubMed

    Zheng, Lisha; Huang, Yan; Song, Wei; Gong, Xianghui; Liu, Meili; Jia, Xiaolin; Zhou, Gang; Chen, Luoping; Li, Ang; Fan, Yubo

    2012-09-21

    Matrix metalloproteinase (MMP)-1, 2, with their endogenous inhibitors, tissue inhibitor of metalloproteinase (TIMP)-1, 2 are critical for extracellular matrix remodeling in human periodontal ligament (PDL) and their expression are sensitive to mechanical stresses. Shear stress as the main type of mechanical stress in tooth movement is involved in matrix turnover. However, how shear stress regulates MMPs and TIMPs system is still unclear. In this study, we investigated the effect of fluid shear stress on expression of MMP-1, 2 and TIMP-1, 2 in human PDL cells and the possible roles of mitogen-activated protein kinases in this process. Three levels of fluid shear stresses (6, 9 and 12 dyn/cm(2)) were loaded on PDL cells for 2, 4, 8 and 12h. The results indicated that fluid shear stress rearranged cytoskeleton in PDL cells. Fluid shear stress increased expression of MMP-1, 2, TIMP-1 and suppressed TIMP-2 expression. MAP kinases including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38 were activated rapidly by fluid shear stress. The ERK inhibitor blocked fluid shear stress induced MMP-1 expression and P38 inhibitor reduced fluid shear stress stimulated MMP-2 expression. Our study suggested that fluid shear stress involved in PDL remodeling via regulating MMP-1, 2 and TIMP-1, 2 expression. ERK regulated fluid shear stress induced MMP-1 expression and P38 play a role in fluid shear stress induced MMP-2 upregulation. PMID:22863019

  8. Finite Element Simulations of Dynamic Shear Rupture Experiments and Path Selection Along Branched Faults

    NASA Astrophysics Data System (ADS)

    Templeton, E. L.; Baudet, A.; Bhat, H. S.; Dmowska, R.; Rice, J. R.; Rosakis, A. J.; Rousseau, C. E.

    2005-12-01

    The study of dynamically propagating shear cracks along geometrically complex paths is important to understanding the mechanics of earthquakes. Recent laboratory fracture studies of Rousseau and Rosakis examined a branched configuration, analogous to their study of rupture along a bent fault path [Rousseau and Rosakis, JGR, 2003], to enhance understanding of the behavior of a shear rupture approaching the intersection of two paths. Whereas crack motion along a simple bent path is readily explained by means of the energy available to sustain the propagating crack, or through a crack tip stress field criterion, the behavior of multiple paths displays more intricate variations featuring the inability of the crack to extend along secondary paths situated at shallow angles with respect to the initial direction of propagation. Secondary paths located at larger angles, on the extensional side, generally promote simultaneous extension along both paths beyond the junction, in contrast to preferred motion along the straight path, which is favored when secondary paths are situated on the compressional side. The experiments involve impact loading of thin plates of Homalite-100, a photoelastic polymer, which are cut along branched paths and weakly glued back together everywhere except along a starter notch near the impact site. High-speed photography of isochromatic fringe patterns (lines of constant difference between in-plane principal stresses) characterized the transient deformation field associated with the impact and rupture propagation. We adapted the ABAQUS/Explicit dynamic finite element program to analyze the propagation of shear cracks along such branched weakened paths. Two configurations for weakened paths, branches at 35° to the compressional side and the extensional side, were analyzed. We implemented a linear slip-weakening failure model as a user-defined constitutive relation within the ABAQUS program, where weakening could be included in either or both of (1

  9. Determination of the stress conditions of the ductile-to-brittle regime along the Asuke Shear Zone, SW Japan

    NASA Astrophysics Data System (ADS)

    Kanai, Takuto; Takagi, Hideo

    2016-04-01

    The stress conditions of the ductile-to-brittle regime have been assessed along the Asuke Shear Zone (ASZ), which strikes NE-SW in the Cretaceous Ryoke granite terrain in SW Japan. Along the ASZ, pseudotachylyte and mylonitized pseudotachylyte are locally developed together with cataclasite. The simultaneous operation of dislocation creep and grain-size-sensitive creep, as indicated by the coexistence of the Z-maximum and relatively random c-axis lattice preferred orientations as well as the sizes of dynamically recrystallized quartz grains (6.40-7.79 μm) in the mylonitized pseudotachylyte, suggest differential stresses of 110-130 MPa at ∼300 °C. The e-twin morphology, twinning ratio, and distribution of the glide direction on the e-twin plane of the twinned calcite in the amygdules of the pseudotachylyte suggest the stress conditions of the σ1 and σ3 axes trend 228° and 320° and plunge 55° and 1°, respectively, and indicate differential stresses of 40-80 MPa at 150-200 °C. Based on kinematic indicators in the fault rocks, the stress conditions estimated from calcite twins, and the cooling history of the granitic protolith, the ASZ is inferred to have been activated under a stress state that caused sinistral normal movements before and after pseudotachylyte formation at 70-50 Ma.

  10. Basal shear stress under alpine glaciers: insights from experiments using the iSOSIA and Elmer/Ice models

    NASA Astrophysics Data System (ADS)

    Brædstrup, C. F.; Egholm, D. L.; Ugelvig, S. V.; Pedersen, V. K.

    2016-02-01

    Shear stress at the base of glaciers exerts a significant control on basal sliding and hence also glacial erosion in arctic and high-altitude areas. However, the inaccessible nature of glacial beds complicates empirical studies of basal shear stress, and little is therefore known of its spatial and temporal distribution. In this study we seek to improve our understanding of basal shear stress using a higher-order numerical ice model (iSOSIA). In order to test the validity of the higher-order model, we first compare the detailed distribution of basal shear stress in iSOSIA and in a three-dimensional full-Stokes model (Elmer/Ice). We find that iSOSIA and Elmer/Ice predict similar first-order stress and velocity patterns, and that differences are restricted to local variations at length scales of the order of the grid resolution. In addition, we find that subglacial shear stress is relatively uniform and insensitive to subtle changes in local topographic relief. Following the initial comparison studies, we use iSOSIA to investigate changes in basal shear stress as a result of landscape evolution by glacial erosion. The experiments with landscape evolution show that subglacial shear stress decreases as glacial erosion transforms preglacial V-shaped valleys into U-shaped troughs. These findings support the hypothesis that glacial erosion is most efficient in the early stages of glacial landscape development.

  11. Cell shape-dependent shear stress on adherent cells in a micro-physiologic system as revealed by FEM.

    PubMed

    Pfister, C; Bozsak, C; Wolf, P; Demmel, F; Brischwein, M

    2015-05-01

    Flow-induced shear stress on adherent cells leads to biochemical signaling and mechanical responses of the cells. To determine the flow-induced shear stress on adherent cells cultured in a micro-scaled reaction chamber, we developed a suitable finite element method model. The influence of the most important parameters-cell shape, cell density, shear modulus and fluid velocity-was investigated. Notably, the cell shape strongly influences the resulting shear stress. Long and smooth cells undergo lower shear stress than more rounded cells. Changes in the curvature of the cells lead to stress peaks and single cells experience higher shear stress values than cells of a confluent monolayer. The computational results of the fluid flow simulation were validated experimentally. We also analyzed the influence of flow-induced shear stress on the metabolic activity and shape of L929, a mouse fibroblast cell line, experimentally. The results indicate that threshold stress values for continuous flow conditions cannot be transferred to quasi static flow conditions interrupted by short fluid exchange events. PMID:25856467

  12. Nonlinear size-dependent dynamics of an imperfect shear deformable microplate

    NASA Astrophysics Data System (ADS)

    Farokhi, Hamed; Ghayesh, Mergen H.

    2016-01-01

    The nonlinear size-dependent dynamics of a geometrically imperfect shear deformable microplate is investigated based on the modified couple stress theory. An initial imperfection in the out-of-plane direction is taken into account. The third-order shear deformation theory is employed to model the system by retaining in-plane displacements and inertia. The governing equations of motion of the system are obtained employing an energy method through use of the Lagrange equations, which upon employing an assumed-mode technique, yields a set of second-order nonlinear ordinary differential equations with coupled terms. A high-dimensional discretised system is considered and made double-dimensional via a change of variables in order to obtain a set of first-order nonlinear ordinary differential equations. The resulting equations are solved using a direct time-integration technique, resulting in time-dependent generalised coordinates for the in-plane and out-of-plane displacements and two rotations. From these generalised coordinates, phase-plane portraits and fast Fourier transforms (FFTs) are also obtained. Moreover, the frequency-response and force-response curves of the system are obtained using a continuation technique; stability analysis is conducted via the Floquet theory. The effect of the initial imperfection as well as the length-scale parameter on the system response is also examined.

  13. Velocity and stress autocorrelation decay in isothermal dissipative particle dynamics

    NASA Astrophysics Data System (ADS)

    Chaudhri, Anuj; Lukes, Jennifer R.

    2010-02-01

    The velocity and stress autocorrelation decay in a dissipative particle dynamics ideal fluid model is analyzed in this paper. The autocorrelation functions are calculated at three different friction parameters and three different time steps using the well-known Groot/Warren algorithm and newer algorithms including self-consistent leap-frog, self-consistent velocity Verlet and Shardlow first and second order integrators. At low friction values, the velocity autocorrelation function decays exponentially at short times, shows slower-than exponential decay at intermediate times, and approaches zero at long times for all five integrators. As friction value increases, the deviation from exponential behavior occurs earlier and is more pronounced. At small time steps, all the integrators give identical decay profiles. As time step increases, there are qualitative and quantitative differences between the integrators. The stress correlation behavior is markedly different for the algorithms. The self-consistent velocity Verlet and the Shardlow algorithms show very similar stress autocorrelation decay with change in friction parameter, whereas the Groot/Warren and leap-frog schemes show variations at higher friction factors. Diffusion coefficients and shear viscosities are calculated using Green-Kubo integration of the velocity and stress autocorrelation functions. The diffusion coefficients match well-known theoretical results at low friction limits. Although the stress autocorrelation function is different for each integrator, fluctuates rapidly, and gives poor statistics for most of the cases, the calculated shear viscosities still fall within range of theoretical predictions and nonequilibrium studies.

  14. Dynamic localization and shear-induced hopping of particles: A way to understand the rheology of dense colloidal dispersions

    SciTech Connect

    Jiang, Tianying; Zukoski, Charles F.

    2014-09-01

    For decades, attempts have been made to understand the formation of colloidal glasses and gels by linking suspension mechanics to particle properties where details of size, shape, and spatial dependencies of pair potentials present a bewildering array of variables that can be manipulated to achieve observed properties. Despite the range of variables that control suspension properties, one consistent observation is the remarkably similarity of flow properties observed as particle properties are varied. Understanding the underlying origins of the commonality in those behaviors (e.g., shear-thinning with increasing stress, diverging zero shear rate viscosity with increasing volume fraction, development of a dynamic yield stress plateau with increases in volume faction or strength of attraction, development of two characteristic relaxation times probed in linear viscoelasticity, the creation of a rubbery plateau modulus at high strain frequencies, and shear-thickening) remains a challenge. Recently, naïve mode coupling and dynamic localization theories have been developed to capture collective behavior giving rise to formation of colloidal glasses and gels. This approach characterizes suspension mechanics of strongly interacting particles in terms of sluggish long-range particle diffusion modulated by varying particle interactions and volume fraction. These theories capture the scaling of the modulus with the volume fraction and strength of interparticle attraction, the frequency dependence of the moduli at the onset of the gel/glass transition, together with the divergence of the zero shear rate viscosity and cessation of diffusivity for hard sphere systems as close packing is approached. In this study, we explore the generality of the predictions of dynamic localization theory for systems of particles composed of bimodal particle size distributions experiencing weak interactions. We find that the mechanical properties of these suspensions are well captured within

  15. Dynamic fluid loss in hydraulic fracturing under realistic shear conditions in high-permeability rocks

    SciTech Connect

    Navarrete, R.C.; Cawiezel, K.E.; Constien, V.G.

    1996-08-01

    A study of the dynamic fluid loss of hydraulic fracturing fluids under realistic shear conditions is presented. During a hydraulic fracturing treatment, a polymeric solution is pumped under pressure down the well to create and propagate a fracture. Part of the fluid leaks into the rock formation, leaving a skin layer of polymer or polymer filter cake, at the rock surface or in the pore space. This study focuses on the effects of shear rate and permeability on dynamic fluid-loss behavior of crosslinked and linear fracturing gels. Previous studies of dynamic fluid loss have mainly been with low-permeability cores and constant shear rates. Here, the effect of shear history and fluid-loss additive on the dynamic leakoff of high-permeability cores is examined.

  16. Full-field predictions of ice dynamic recrystallisation under simple shear conditions

    NASA Astrophysics Data System (ADS)

    Llorens, Maria-Gema; Griera, Albert; Bons, Paul D.; Lebensohn, Ricardo A.; Evans, Lynn A.; Jansen, Daniela; Weikusat, Ilka

    2016-09-01

    Understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large-scale ice dynamics, and thus our ability to predict future changes of ice sheets. Polar ice behaves anisotropically during flow, which can lead to strain localisation. In order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets, we present a series of numerical simulations of ice polycrystals deformed under simple-shear conditions. The models explicitly simulate the evolution of microstructures using a full-field approach, based on the coupling of a viscoplastic deformation code (VPFFT) with dynamic recrystallisation codes. The simulations provide new insights into the distribution of stress, strain rate and lattice orientation fields with progressive strain, up to a shear strain of three. Our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure (grain size and shape), while the development of lattice preferred orientations (LPO) appears to be less affected. Activation of non-basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour. Simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration. Therefore, the finite-strain history is non-directly reflected by the final microstructure. Masked strain localisation can be recognised in ice cores, such as the EDML, from the presence of stepped boundaries, microshear and grains with zig-zag geometries.

  17. Molecular dynamics at constant Cauchy stress

    NASA Astrophysics Data System (ADS)

    Miller, Ronald E.; Tadmor, Ellad B.; Gibson, Joshua S.; Bernstein, Noam; Pavia, Fabio

    2016-05-01

    The Parrinello-Rahman algorithm for imposing a general state of stress in periodic molecular dynamics simulations is widely used in the literature and has been implemented in many readily available molecular dynamics codes. However, what is often overlooked is that this algorithm controls the second Piola-Kirchhoff stress as opposed to the true (Cauchy) stress. This can lead to misinterpretation of simulation results because (1) the true stress that is imposed during the simulation depends on the deformation of the periodic cell, (2) the true stress is potentially very different from the imposed second Piola-Kirchhoff stress, and (3) the true stress can vary significantly during the simulation even if the imposed second Piola-Kirchhoff is constant. We propose a simple modification to the algorithm that allows the true Cauchy stress to be controlled directly. We then demonstrate the efficacy of the new algorithm with the example of martensitic phase transformations under applied stress.

  18. Molecular dynamics at constant Cauchy stress.

    PubMed

    Miller, Ronald E; Tadmor, Ellad B; Gibson, Joshua S; Bernstein, Noam; Pavia, Fabio

    2016-05-14

    The Parrinello-Rahman algorithm for imposing a general state of stress in periodic molecular dynamics simulations is widely used in the literature and has been implemented in many readily available molecular dynamics codes. However, what is often overlooked is that this algorithm controls the second Piola-Kirchhoff stress as opposed to the true (Cauchy) stress. This can lead to misinterpretation of simulation results because (1) the true stress that is imposed during the simulation depends on the deformation of the periodic cell, (2) the true stress is potentially very different from the imposed second Piola-Kirchhoff stress, and (3) the true stress can vary significantly during the simulation even if the imposed second Piola-Kirchhoff is constant. We propose a simple modification to the algorithm that allows the true Cauchy stress to be controlled directly. We then demonstrate the efficacy of the new algorithm with the example of martensitic phase transformations under applied stress. PMID:27179471

  19. Flow Shear Stress and Atherosclerosis: A Matter of Site Specificity

    PubMed Central

    Nigro, Patrizia; Abe, Jun-ichi

    2011-01-01

    Abstract It is well accepted that atherosclerosis occurs in a site-specific manner especially at branch points where disturbed blood flow (d-flow) predisposes to the development of plaques. Investigations both in vivo and in vitro have shown that d-flow is pro-atherogenic by promoting oxidative and inflammatory states in the artery wall. In contrast, steady laminar blood flow (s-flow) is atheroprotective by inhibition of oxidative stress and inflammation in the vessel wall. The mechanism for inflammation in endothelial cells (ECs) exposed to d-flow has been well studied and includes redox-dependent activation of apoptosis signal-regulating kinase 1 (ASK1) and Jun NH2-terminal kinase (JNK) that ultimately lead to the expression of adhesive molecules. In contrast, s-flow leads to the activation of the mitogen extracellular-signal-regulated kinase kinase 5/extracellular signal-regulated kinase-5 (MEK5/ERK5) pathway that prevents pro-inflammatory signaling. Important transcriptional events that reflect the pro-oxidant and pro-inflammatory condition of ECs in d-flow include the activation of activator protein 1 (AP-1) and nuclear factor kappaB (NFκB), whereas in s-flow, activation of Krüppel-like factor 2 (KLF2) and nuclear factor erythroid 2-like 2 (Nrf2) are dominant. Recent studies have shown that protein kinase c zeta (PKCζ) is highly activated under d-flow conditions and may represent a molecular switch for EC signaling and gene expression. The targeted modulation of proteins activated in a site-specific manner holds the promise for a new approach to limit atherosclerosis. Antioxid. Redox Signal. 15, 1405–1414. PMID:21050140

  20. A New Model to Calculate Friction Coefficients and Shear Stresses in Thermal Drilling

    SciTech Connect

    Qu, Jun; Blau, Peter Julian

    2008-01-01

    A new analytical model for thermal drilling (also known as friction drilling) has been developed. The model distinguishes itself from recent work of other investigators by improving on two aspects: (1) the new model defines material plastic flow in terms of the yield in shear rather than the yield in compression, and (2) it uses a single, variable friction coefficient instead of assuming two unrelated friction coefficients in fixed values. The time dependence of the shear stress and friction coefficient at the hole walls, which cannot be measured directly in thermal drilling, can be calculated using this model from experimentally-measured values of the instantaneous thrust force and torque. Good matches between the calculated shear strengths and the handbook values for thermally drilling low carbon steel confirm the model's validity.

  1. A finite element method for shear stresses calculation in composite blade models

    NASA Astrophysics Data System (ADS)

    Paluch, B.

    1991-09-01

    A finite-element method is developed for accurately calculating shear stresses in helicopter blade models, induced by torsion and shearing forces. The method can also be used to compute the equivalent torsional stiffness of the section, their transverse shear coefficient, and the position of their center of torsion. A grid generator method which is a part of the calculation program is also described and used to discretize the sections quickly and to condition the grid data reliably. The finite-element method was validated on a few sections composed of isotropic materials and was then applied to a blade model sections made of composite materials. Good agreement was obtained between the calculated and experimental data.

  2. Experimental Study of Loosening of Threaded Fasteners due to Dynamic Shear Loads

    NASA Astrophysics Data System (ADS)

    PAI, N. G.; HESS, D. P.

    2002-06-01

    This paper presents a study on loosening of threaded fasteners subjected to dynamic shear loads. A fundamental analysis of loosening reveals that a fastener can loosen at lower loads than previously expected due to localized slip at the contact surfaces. Four different loosening processes of a screw under different conditions of slip at the head and thread contact regions are identified. Experimental results illustrating these loosening processes are presented. In addition, the minimum dynamic shear force required to initiate loosening is determined experimentally.

  3. Resolved shear stress intensity coefficient and fatigue crack growth in large crystals

    NASA Technical Reports Server (NTRS)

    Chen, QI; Liu, Hao-Wen

    1988-01-01

    Fatigue crack growth in large grain Al alloy was studied. Fatigue crack growth is caused primarily by shear decohesion due to dislocation motion in the crack tip region. The crack paths in the large crystals are very irregular and zigzag. The crack planes are often inclined to the loading axis both in the inplane direction and the thickness direction. The stress intensity factors of such inclined cracks are approximated from the two dimensional finite element calculations. The plastic deformation in a large crystal is highly anisotropic, and dislocation motion in such crystals are driven by the resolved shear stress. The resolved shear stress intensity coefficient in a crack solid, RSSIC, is defined, and the coefficients for the slip systems at a crack tip are evaluated from the calculated stress intensity factors. The orientations of the crack planes are closely related to the slip planes with the high RSSIC values. If a single slip system has a much higher RSSIC than all the others, the crack will follow the slip plane, and the slip plane becomes the crack plane. If two or more slip systems have a high RSSIC, the crack plane is the result of the decohesion processes on these active slip planes.

  4. Estimate Interface Shear Stress of Woven Ceramic Matrix Composites from Hysteresis Loops

    NASA Astrophysics Data System (ADS)

    Li, Longbiao; Song, Yingdong

    2013-12-01

    An approach to estimate the fiber/matrix interface shear stress of woven ceramic matrix composites during fatigue loading has been developed in this paper. Based on the analysis of the microstructure, the woven ceramic matrix composites were divided into four elements of 0o warp yarns, 90o weft yarns, matrix outside of the yarns and the open porosity. When matrix cracking and fiber/matrix interface debonding occur upon first loading to the peak stress, it is assumed that fiber slipping relative to matrix in the interface debonded region of the 0o warp yarns is the mainly reason for the occurrence of the hysteresis loops of woven ceramic matrix composiets during unloading and subsequent reloading. The unloading interface reverse slip length and reloading interface new slip length are determined by the interface slip mechanisms. The hysteresis loops of three different cases have been derived. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulated in terms of the fiber/matrix interface shear stress. By comparing the experimental hysteresis loss energy with the computational values, the fiber/matrix interface shear stress of woven ceramic matrix composites corresponding to different cycles can then be derived. The theoretical results have been compared with experimental data of two different woven ceramic composites.

  5. Modeling bed shear-stress fluctuations in a shallow tidal channel

    NASA Astrophysics Data System (ADS)

    Mathis, R.; Marusic, I.; Cabrit, O.; Jones, N. L.; Ivey, G. N.

    2014-05-01

    Recently, Mathis et al. (2013) developed a model for predicting the instantaneous fluctuations of the wall shear-stress in turbulent boundary layers. This model is based on an inner-outer scale interaction mechanism, incorporating superposition, and amplitude-modulation effects, and the only input required for the model is a time series measurement of the streamwise velocity signal taken in the logarithmic region of the flow. The present study applies this new approach for the first time to environmental flows, for which the near-bed information is typically inaccessible. The data used here are acoustic Doppler velocimeter time series measurements from a shallow tidal channel (Suisun Slough in North San Francisco Bay). We first extract segments of data sharing properties with canonical turbulent boundary layers. The wall (bed) shear-stress model is then applied to these selected data. Statistical and spectral analysis demonstrates that the field data predictions are consistent with laboratory and DNS results. The model is also applied to the whole available data set to demonstrate, even for situations far from the canonical boundary layer case, its ability to preserve the overall Reynolds number trend. The predicted instantaneous bed stress is highly skewed and amplitude modulated with the variations in the large-scale streamwise velocity. Finally, the model is compared to conventional methods employed to predict the bed shear-stress. A large disparity is observed, but the present model is the only one able to predict both the correct spectral content and the probability density function.

  6. Nature of stress accommodation in sheared granular material: Insights from 3D numerical modeling

    NASA Astrophysics Data System (ADS)

    Mair, Karen; Hazzard, James F.

    2007-07-01

    Active faults often contain distinct accumulations of granular wear material. During shear, this granular material accommodates stress and strain in a heterogeneous manner that may influence fault stability. We present new work to visualize the nature of contact force distributions during 3D granular shear. Our 3D discrete numerical models consist of granular layers subjected to normal loading and direct shear, where gouge particles are simulated by individual spheres interacting at points of contact according to simple laws. During shear, we observe the transient microscopic processes and resulting macroscopic mechanical behavior that emerge from interactions of thousands of particles. We track particle translations and contact forces to determine the nature of internal stress accommodation with accumulated slip for different initial configurations. We view model outputs using novel 3D visualization techniques. Our results highlight the prevalence of transient directed contact force networks that preferentially transmit enhanced stresses across our granular layers. We demonstrate that particle size distribution (psd) controls the nature of the force networks. Models having a narrow (i.e. relatively uniform) psd exhibit discrete pipe-like force clusters with a dominant and focussed orientation oblique to but in the plane of shear. Wider psd models (e.g. power law size distributions D = 2.6) also show a directed contact force network oblique to shear but enjoy a wider range of orientations and show more out-of-plane linkages perpendicular to shear. Macroscopic friction level, is insensitive to these distinct force network morphologies, however, force network evolution appears to be linked to fluctuations in macroscopic friction. Our results are consistent with predictions, based on recent laboratory observations, that force network morphologies are sensitive to grain characteristics such as particle size distribution of a sheared granular layer. Our numerical

  7. Bed Forms Modulating Temporal Peaks on Near-Bank Shear Stresses, the Wabash River Case

    NASA Astrophysics Data System (ADS)

    Abad, J. D.; Frias, C. E.; Langendoen, E. J.; Best, J.; Rhoads, B. L.; Konsoer, K. M.; Garcia, M. H.

    2013-12-01

    There is a great body of experimental work showing how bed forms modulate bed roughness, flow field structure, and sediment transport rates in straight flumes. Recently, it was observed that migrating bed forms produce temporal and spatial peaks of shear stresses along the outer bank of an experimental meandering channel. These stresses are about 50% larger than the shear stresses exerted by the mean near-bank flow. As fluvial erosion bank erosion rates are typically linearly related to applied shear stress, the migration rate of the bend may be significantly increased. However, this hypothesis has never been tested in the field, where bed forms could be more complex than those found in experimental cases. Herein, only fluvial erosion is considered, while geotechnical processes occurring at the outer bank are not accounted for. Detailed measurements of hydrodynamics (using acoustic Doppler profiler), bed morphology (using multibeam and RTK GPS) and bank morphology (using laser scanner) were conducted at two bends on the Wabash River along the Illinois and Indiana Stateline. The bed morphology exhibited different scales of bed forms, ranging from dunes to ripples. Using Wavelet analysis to discriminate the bed morphology it was possible to separate the ripples and dunes structures resulting in a bed without bed forms, which shows the typical erosion (outer bank)/deposition (inner bank) arrangement in meandering channels. Using a fully three-dimensional Reynolds-Averaged Navier-Stokes (RANS) numerical model, two cases are simulated: [1] bend with bed forms, and [2] bend without bed forms to test the above hypothesis. The results show that the three-dimensional flow field is compares well to that observed for both scenarios. Further, peaks in shear stresses along the outer bank are indeed observed, which are correlated to the location of the bed forms with respect to the bend. Further conclusion and its importance for long-term morphodynamics of meandering channels

  8. Additional Shear Resistance from Fault Roughness and its Role in Determining Stress Levels on Mature and Immature Faults

    NASA Astrophysics Data System (ADS)

    Fang, Z.; Dunham, E. M.

    2011-12-01

    The majority of crustal faults host earthquakes at τ /(σ - p) ˜ 0.6 (τ is shear stress and (σ - p) is the effective normal stress), while mature plate-boundary faults, like the San Andreas Fault (SAF), host earthquakes at τ /(σ - p) ˜ 0.2. A leading explaination for the weakness of the SAF is the existence of dynamic weakening, which, on planar faults, allows self-sustaining rupture at a critical background stress level τ pulse/(σ - p) ˜ 0.25. Provided that dynamic weakening also occurs on less mature faults, which seems likely given the ubiquity of dynamic weakening in high velocity friction experiments, the stress levels on the less mature faults are puzzling. We offer a self-consistent explanation for the relatively high stress levels on immature faults that is compatible with dynamic weakening and low coseismic strength of all faults. Our explanation is that increased geometrical complexity of less mature faults introduces an additional resistance to slip that must be overcome in order for the fault to host ruptures. Lab and field observations suggest that faults are self-similar surfaces with amplitude-to-wavelength ratio α in the range of 10-3 (mature faults) to 10-2 (immature faults). Slip on such faults induces huge stress perturbations near the fault. Projection of these stress perturbations back onto the rough fault surface results in an additional shear resistance to slip, the 'roughness drag' τ drag, that exists even if the fault is frictionless. A perturbation analysis, accurate to second order in α , shows that τ drag = 8π 3 α 2[G/(1-&nu)][Δ u/λ min], in which G is shear modulus, ν is the Poisson's ratio, Δ u is the amount of slip, and λ min is the minimum wavelength of roughness. Estimates indicate that τ drag is negligible on mature faults (α ˜ 10-3) but can become substantial on immature faults (α ˜ 10-2). We expect that the finite strength of the off-fault material ultimately bounds τ drag to a value determined by the

  9. Methicillin resistant Staphylococcus aureus adhesion to human umbilical vein endothelial cells demonstrates wall shear stress dependent behaviour

    PubMed Central

    2011-01-01

    Background Methicillin-resistant Staphylococcus aureus (MRSA) is an increasingly prevalent pathogen capable of causing severe vascular infections. The goal of this work was to investigate the role of shear stress in early adhesion events. Methods Human umbilical vein endothelial cells (HUVEC) were exposed to MRSA for 15-60 minutes and shear stresses of 0-1.2 Pa in a parallel plate flow chamber system. Confocal microscopy stacks were captured and analyzed to assess the number of MRSA. Flow chamber parameters were validated using micro-particle image velocimetry (PIV) and computational fluid dynamics modelling (CFD). Results Under static conditions, MRSA adhered to, and were internalized by, more than 80% of HUVEC at 15 minutes, and almost 100% of the cells at 1 hour. At 30 minutes, there was no change in the percent HUVEC infected between static and low flow (0.24 Pa), but a 15% decrease was seen at 1.2 Pa. The average number of MRSA per HUVEC decreased 22% between static and 0.24 Pa, and 37% between 0.24 Pa and 1.2 Pa. However, when corrected for changes in bacterial concentration near the surface due to flow, bacteria per area was shown to increase at 0.24 Pa compared to static, with a subsequent decline at 1.2 Pa. Conclusions This study demonstrates that MRSA adhesion to endothelial cells is strongly influenced by flow conditions and time, and that MSRA adhere in greater numbers to regions of low shear stress. These areas are common in arterial bifurcations, locations also susceptible to generation of atherosclerosis. PMID:21426581

  10. Profile blunting and flow blockage in a yield-stress fluid: a molecular dynamics study.

    PubMed

    Varnik, F; Raabe, D

    2008-01-01

    The flow of a simple glass forming system (a 80:20 binary Lennard-Jones mixture) through a planar channel is studied via molecular dynamics simulations. The flow is driven by an external body force similar to gravity. Previous studies show that the model exhibits both a static [F. Varnik, J. Chem. Phys. 120, 2788 (2004)] and a dynamic [F. Varnik and O. Henrich, Phys. Rev. B 73, 174209 (2006)] yield stress in the glassy phase. These observations are corroborated by the present work, where we investigate how the presence of a yield stress may affect the system behavior in a Poiseuille-type flow geometry. In particular, we observe a blunted velocity profile across the channel: A relatively wide region in the channel center flows with a constant velocity (zero shear rate) followed by a nonlinear change of the shear rate as the walls are approached. The observed velocity gradients are compared to those obtained from the knowledge of the shear stress across the channel and the flow curves (stress versus shear rate), the latter being determined in our previous simulations of homogeneous shear flow. Furthermore, using the value of the (dynamic) yield stress known from previous simulations, we estimate the threshold body force for a complete arrest of the flow. Indeed, a blockage is observed as the imposed force falls below this threshold value. Small but finite shear rates are observed at stresses above the dynamic but below the static yield stress. We discuss the possible role of the stick-slip-like motion for this observation. PMID:18351859

  11. Brownian dynamics simulations of confined tethered polymers in shear flow: the effect of attractive surfaces.

    PubMed

    Ibáñez-García, Gabriel O; Goldstein, Patricia; Hanna, S

    2013-05-01

    Coarse grain Brownian dynamics simulations of the bead-spring model are used to investigate the effect of attractive surfaces on the stretching of confined tethered polymers under shear flow. The weak and strong adsorbed regimes have been addressed by means of a coarse grain van der Waals potential to simulate polymer substrate interactions. Different stationary cyclic dynamics are observed upon varying shear flow intensity and surface potential strength. Polymer stretching decreases as increasing the attractive potential strength, breaking down the scaling predictions for non-adsorbed polymers. We found that adsorption is enhanced by the shear flow strength in agreement to simulations of adsorbed non-tethered polymers. PMID:23715882

  12. Experiments and simulations of MEMS thermal sensors for wall shear-stress measurements in aerodynamic control applications

    NASA Astrophysics Data System (ADS)

    Lin, Qiao; Jiang, Fukang; Wang, Xuan-Qi; Xu, Yong; Han, Zhigang; Tai, Yu-Chong; Lew, James; Ho, Chih-Ming

    2004-12-01

    MEMS thermal shear-stress sensors exploit heat-transfer effects to measure the shear stress exerted by an air flow on its solid boundary, and have promising applications in aerodynamic control. Classical theory for conventional, macroscale thermal shear-stress sensors states that the rate of heat removed by the flow from the sensor is proportional to the 1/3-power of the shear stress. However, we have observed that this theory is inconsistent with experimental data from MEMS sensors. This paper seeks to develop an understanding of MEMS thermal shear-stress sensors through a study including both experimental and theoretical investigations. We first obtain experimental data that confirm the inadequacy of the classical theory by wind-tunnel testing of prototype MEMS shear-stress sensors with different dimensions and materials. A theoretical analysis is performed to identify that this inadequacy is due to the lack of a thin thermal boundary layer in the fluid flow at the sensor surface, and then a two-dimensional MEMS shear-stress sensor theory is presented. This theory incorporates important heat-transfer effects that are ignored by the classical theory, and consistently explains the experimental data obtained from prototype MEMS sensors. Moreover, the prototype MEMS sensors are studied with three-dimensional simulations, yielding results that quantitatively agree with experimental data. This work demonstrates that classical assumptions made for conventional thermal devices should be carefully examined for miniature MEMS devices.

  13. Endothelial cell activation by hemodynamic shear stress derived from arteriovenous fistula for hemodialysis access.

    PubMed

    Franzoni, Marco; Cattaneo, Irene; Longaretti, Lorena; Figliuzzi, Marina; Ene-Iordache, Bogdan; Remuzzi, Andrea

    2016-01-01

    Intimal hyperplasia (IH) is the first cause of failure of an arteriovenous fistula (AVF). The aim of the present study was to investigate the effects on endothelial cells (ECs) of shear stress waveforms derived from AVF areas prone to develop IH. We used a cone-and-plate device to obtain real-time control of shear stress acting on EC cultures. We exposed human umbilical vein ECs for 48 h to different shear stimulations calculated in a side-to-end AVF model. Pulsatile unidirectional flow, representative of low-risk stenosis areas, induced alignment of ECs and actin fiber orientation with flow. Shear stress patterns of reciprocating flow, derived from high-risk stenosis areas, did not affect EC shape or cytoskeleton organization, which remained similar to static cultures. We also evaluated flow-induced EC expression of genes known to be involved in cytoskeletal remodeling and expression of cell adhesion molecules. Unidirectional flow induced a significant increase in Kruppel-like factor 2 mRNA expression, whereas it significantly reduced phospholipase D1, α4-integrin, and Ras p21 protein activator 1 mRNA expression. Reciprocating flow did not increase Kruppel-like factor 2 mRNA expression compared with static controls but significantly increased mRNA expression of phospholipase D1, α4-integrin, and Ras p21 protein activator 1. Reciprocating flow selectively increased monocyte chemoattractant protein-1 and IL-8 production. Furthermore, culture medium conditioned by ECs exposed to reciprocating flows selectively increased smooth muscle cell proliferation compared with unidirectional flow. Our results indicate that protective vascular effects induced in ECs by unidirectional pulsatile flow are not induced by reciprocating shear forces, suggesting a mechanism by which oscillating flow conditions may induce the development of IH in AVF and vascular access dysfunction. PMID:26497959

  14. Frequency- and stress-dependent changes in shear-wave velocity dispersion in water-saturated, unconsolidated sand

    NASA Astrophysics Data System (ADS)

    Ghose, R.

    2009-04-01

    Anelastic processes in the earth causes dissipation of seismic energy. Because of the fundamental laws of causality, the dissipation effects demand a frequency-dependent change of elastic moduli, and therefore, dispersion in elastic wave velocities. Assessing the dispersion of seismic shear waves in the unconsolidated subsoil is important for at least 3 reasons: 1) shear-wave velocity (Vs) is a key parameter in all dynamic loading problems; the frequency of the observed shear waves in field, downhole and laboratory measurements varies widely (20 Hz - 10 kHz), and consequently an uncertainty resulting from an unknown or poorly known estimate of dispersion may translate into erroneous evaluation and potential risks, 2) generally Vs-dispersion is considered negligible for the frequency range of practical interest; it is important to check this assumption and modify the site evaluation results, if necessary, and 3) the underlying soil-physics of any observed dispersion can be useful in estimating an unknown soil physical parameter. In the present research, we have concentrated on Vs dispersion in saturated sand in laboratory, under varying vertical and horizontal stress levels that are realistic in the context of shallow subsoil investigations. We explored theoretical models to obtain insight from our experimental findings. Laboratory experiments involving array seismic measurements and accurate stress control present clear evidence of dispersive shear-wave velocity in saturated sand in the frequency range 2-16 kHz. The change of Vs as a function of frequency is clearly nonlinear. For low frequencies, as observed in the field data, our result indicates significant dispersion and, therefore, nonlinear variation of attenuation. This has important implication on site evaluation using Vs. Significantly, the data allows us to distinguish a frequency-dependence of the velocity dispersion. The relative importance of fluid motion relative to the skeleton frame (Biot theory

  15. Interfibrillar shear stress is the loading mechanism of collagen fibrils in tendon

    PubMed Central

    Szczesny, Spencer E.; Elliott, Dawn M.

    2014-01-01

    Despite the critical role tendons play in transmitting loads throughout the musculoskeletal system, little is known about the microstructural mechanisms underlying their mechanical function. Of particular interest is whether collagen fibrils in tendon fascicles bear load independently or if load is transferred between fibrils through interfibrillar shear forces. We conducted multiscale experimental testing and developed a microstructural shear lag model to explicitly test whether interfibrillar shear load transfer is indeed the fibrillar loading mechanism in tendon. Experimental correlations between fascicle macroscale mechanics and microscale interfibrillar sliding suggest that fibrils are discontinuous and share load. Moreover, for the first time, we demonstrate that a shear lag model can replicate the fascicle macroscale mechanics as well as predict the microscale fibrillar deformations. Since interfibrillar shear stress is the fundamental loading mechanism assumed in the model, this result provides strong evidence that load is transferred between fibrils in tendon and possibly other aligned collagenous tissues. Conclusively establishing this fibrillar loading mechanism and identifying the involved structural components should help develop repair strategies for tissue degeneration and guide the design of tissue engineered replacements. PMID:24530560

  16. Characterizing wave- and current- induced bottom shear stress: U.S. middle Atlantic continental shelf

    USGS Publications Warehouse

    Dalyander, P. Soupy; Butman, Bradford; Sherwood, Christopher R.; Signell, Richard P.; Wilkin, John L.

    2013-01-01

    Waves and currents create bottom shear stress, a force at the seabed that influences sediment texture distribution, micro-topography, habitat, and anthropogenic use. This paper presents a methodology for assessing the magnitude, variability, and driving mechanisms of bottom stress and resultant sediment mobility on regional scales using numerical model output. The analysis was applied to the Middle Atlantic Bight (MAB), off the U.S. East Coast, and identified a tidally-dominated shallow region with relatively high stress southeast of Massachusetts over Nantucket Shoals, where sediment mobility thresholds are exceeded over 50% of the time; a coastal band extending offshore to about 30 m water depth dominated by waves, where mobility occurs more than 20% of the time; and a quiescent low stress region southeast of Long Island, approximately coincident with an area of fine-grained sediments called the “Mud Patch”. The regional high in stress and mobility over Nantucket Shoals supports the hypothesis that fine grain sediment winnowed away in this region maintains the Mud Patch to the southwest. The analysis identified waves as the driving mechanism for stress throughout most of the MAB, excluding Nantucket Shoals and sheltered coastal bays where tides dominate; however, the relative dominance of low-frequency events varied regionally, and increased southward toward Cape Hatteras. The correlation between wave stress and local wind stress was lowest in the central MAB, indicating a relatively high contribution of swell to bottom stress in this area, rather than locally generated waves. Accurate prediction of the wave energy spectrum was critical to produce good estimates of bottom shear stress, which was sensitive to energy in the long period waves.

  17. Endothelial dysfunction following prolonged sitting is mediated by a reduction in shear stress.

    PubMed

    Restaino, Robert M; Walsh, Lauren K; Morishima, Takuma; Vranish, Jennifer R; Martinez-Lemus, Luis A; Fadel, Paul J; Padilla, Jaume

    2016-03-01

    We and others have recently reported that prolonged sitting impairs endothelial function in the leg vasculature; however, the mechanism(s) remain unknown. Herein, we tested the hypothesis that a sustained reduction in flow-induced shear stress is the underlying mechanism by which sitting induces leg endothelial dysfunction. Specifically, we examined whether preventing the reduction in shear stress during sitting would abolish the detrimental effects of sitting on popliteal artery endothelial function. In 10 young healthy men, bilateral measurements of popliteal artery flow-mediated dilation were performed before and after a 3-h sitting period during which one foot was submerged in 42°C water (i.e., heated) to increase blood flow and thus shear stress, whereas the contralateral leg remained dry and served as internal control (i.e., nonheated). During sitting, popliteal artery mean shear rate was reduced in the nonheated leg (pre-sit, 42.9 ± 4.5 s(-1); and 3-h sit, 23.6 ± 3.3 s(-1); P < 0.05) but not in the heated leg (pre-sit, 38.9 ± 3.4 s(-1); and 3-h sit, 63.9 ± 16.9 s(-1); P > 0.05). Popliteal artery flow-mediated dilation was impaired after 3 h of sitting in the nonheated leg (pre-sit, 7.1 ± 1.4% vs. post-sit, 2.8 ± 0.9%; P < 0.05) but not in the heated leg (pre-sit: 7.3 ± 1.5% vs. post-sit, 10.9 ± 1.8%; P > 0.05). Collectively, these data suggest that preventing the reduction of flow-induced shear stress during prolonged sitting with local heating abolishes the impairment in popliteal artery endothelial function. Thus these findings are consistent with the hypothesis that sitting-induced leg endothelial dysfunction is mediated by a reduction in shear stress. PMID:26747508

  18. Role of the cytoskeleton in flow (shear stress)-induced dilation and remodeling in resistance arteries

    PubMed Central

    Loufrani, Laurent; Henrion, Daniel

    2008-01-01

    Cytoskeletal proteins determine cell shape and integrity and membrane-bound structures connected to extracellular components allow tissue integrity. These structural elements have an active role in the interaction of blood vessels with their environment. Shear stress due to blood flow is the most important force stimulating the endothelium. The role of cytoskeletal proteins in endothelial responses to flow has been studied in resistance arteries using pharmacological tools and transgenic models. Shear stress activates extracellular “flow sensing” elements associated with a thick glycocalyx communicating the signal to membrane-bound complexes (integrins and/or dystrophin-dystroglycans) and to eNOS through a pathway involving the intermediate filament vimentin, the microtubule network and actin. When blood flow increases chronically the endothelium triggers diameter enlargement and medial hypertrophy. This is facilitated by the genetic absence of the intermediate filaments, vimentin and desmin suggesting that these elements oppose the process. PMID:18246377

  19. Local mass transport coefficients and local wall shear stresses at flow disturbances

    SciTech Connect

    Schmitt, G.; Gudde, T.

    1995-10-01

    Electrochemical measurements were performed with micro and ultramicro electrode arrays to evaluate local mass transfer rates with high lateral resolution in order to explain extreme corrosion rates during flow induced localized corrosion at leading edges of small flow disturbances. It was found that the mass transport coefficient close to the leading edge of a rectangular cavity in the wall of a rectangular flow channel is higher by a factor of 4--7 than at the plain channel wall. A parabolic correlation was found between wall shear stress and mass transfer rate at the plain channel wall. Assuming the validity of this correlation also in the high turbulent areas at leading edges of cavities enhancement factors in the order of 200 were assessed for the wall shear stress at the cavity compared to the plain channel wall.

  20. Dynamic stresses, Coulomb failure, and remote triggering

    USGS Publications Warehouse

    Hill, D.P.

    2008-01-01

    Dynamic stresses associated with crustal surface waves with 15-30-sec periods and peak amplitudes 5 km). The latter is consistent with the observation that extensional or transtensional tectonic regimes are more susceptible to remote triggering by Rayleigh-wave dynamic stresses than compressional or transpressional regimes. Locally elevated pore pressures may have a role in the observed prevalence of dynamic triggering in extensional regimes and geothermal/volcanic systems.

  1. Notch1 inhibition reduces low shear stress-induced plaque formation.

    PubMed

    Qin, Wei-Dong; Zhang, Fan; Qin, Xiao-Jun; Wang, Juan; Meng, Xiao; Wang, Hao; Guo, Hai-Peng; Wu, Qun-Zheng; Wu, Da-Wei; Zhang, Ming-Xiang

    2016-03-01

    Low shear stress (LSS) contributes to the pathogenesis of inflammatory diseases, such as atherosclerosis. Notch1 is a type I transmembrane receptor that critically determines the growth, differentiation, and survival of various cell types, but its role and mechanism in LSS-induced inflammatory response remains undetermined. Apolipoprotein E-deficient (ApoE(-/-)) mice were fed with high fat diet and administered intraperitoneally with DAPT (a γ-secretase inhibitor). Perivascular shear stress modifiers were placed around the right carotid arteries to induce LSS. The left carotid arteries with undisturbed shear stress (USS) were used as the control. LSS increased Delta-like 1 (DLL-1) protein expression and the expression of Notch1 and NICD, while DAPT administration reduced NICD expression. Compared with the LSS group, DAPT reduced LSS-induced plaque formation and intercellular adhesion molecule 1 (ICAM-1) expression. Human umbilical vein endothelial cells (HUVECs) were exposure to undisturbed shear stress (USS, 1Pa) or LSS (0.4Pa). Notch1 was inhibited by siRNA or DAPT. RT-PCR and western blotting analysis showed that LSS upregulated the expression of Notch1 in a time-dependent manner. Caveolin-1 (CAV1) inhibition by siRNA could reduce Notch1 and NICD expression. Compared with USS, LSS increased inflammatory response, including IL-1β and IL-6 secretion, ICAM-1 and inducible nitric oxide synthase (iNOS) expression, and THP-1 cells adhesion. Notch1 inhibition by siRNA or DAPT could reduce these inflammatory responses by reduction of NF-κB phosphorylation, upregulation of IkBα expression, and inhibition of nuclear translocation of NF-κB, while Notch1 activation by DLL-4 had an adverse effect. The Notch signaling system is therefore a potential target for modulating LSS-induced inflammation response during atherosclerosis. PMID:26783939

  2. Investigations of the wall-shearing stress in turbulent boundary layers

    NASA Technical Reports Server (NTRS)

    Ludwieg, Hubert; Tillmann, W

    1950-01-01

    Because of the unsatisfactory state of knowledge concerning the surface shearing stress of boundary layers with pressure gradients, the problem is re-examined. It is found that for general turbulent boundary layers in wall proximity, that is, in the laminar sublayer, in the transition zone and in the part of the completely turbulent zone near the wall, the same universal law applies as for the plate flow.

  3. Two-Axis Direct Fluid Shear Stress Sensor for Aerodynamic Applications

    NASA Technical Reports Server (NTRS)

    Bajikar, Sateesh S.; Scott, Michael A.; Adcock, Edward E.

    2011-01-01

    This miniature or micro-sized semiconductor sensor design provides direct, nonintrusive measurement of skin friction or wall shear stress in fluid flow situations in a two-axis configuration. The sensor is fabricated by microelectromechanical system (MEMS) technology, enabling small size and multiple, low-cost reproductions. The sensors may be fabricated by bonding a sensing element wafer to a fluid-coupling element wafer. Using this layered machine structure provides a truly three-dimensional device.

  4. Periodontal Treatment Elevates Carotid Wall Shear Stress in the Medium Term

    PubMed Central

    Carallo, Claudio; Franceschi, Maria Serena De; Tripolino, Cesare; Iovane, Claudio; Catalano, Serena; Giudice, Amerigo; Crispino, Antonio; Figliuzzi, Michele; Irace, Concetta; Fortunato, Leonzio; Gnasso, Agostino

    2015-01-01

    Abstract Periodontal disease is associated with endothelial dysfunction of the brachial artery and hemodynamic alterations of the common carotid artery. Periodontal therapy improves endothelial function. It is not known if it is able also to improve the hemodynamics of the carotid artery. The aim of the current study was to evaluate the efficacy of 2 different periodontal treatments on carotid hemodynamics: scaling and root planing (SRP) alone or together with low-level laser therapy (LLLT). Forty patients were recruited and randomly treated with SRP (n = 20) or SRP + LLLT (n = 20). Periodontal indices (plaque, gingival, and probing depth indices) were measured before and 5 months after treatment. Blood viscosity, common carotid wall shear stress, circumferential wall tension, and Peterson elastic modulus were evaluated before, soon after and 5 months after treatment. It was found that the periodontal indices improved in both groups, but significantly more so for SRP + LLLT than for SRP (decrease in gingival index 69.3% versus 45.4%, respectively, P = 0.04). In the SRP + LLLT group, after a transient reduction by 5% immediately after therapy, shear stress increased by 11% after 5 months. In SRP only group, however, shear stress variations were less marked. No significant changes were found for the other hemodynamic parameters in either of the groups. Periodontal disease treatment by SRP + LLLT can therefore be said to improve common carotid wall shear stress. This suggests a possible mechanism by which the treatment of periodontal disease has beneficial effects on the cardiovascular system. PMID:26496285

  5. Understanding the structural properties of clusters in sheared aggregating systems using Brownian dynamics simulation

    NASA Astrophysics Data System (ADS)

    Markutsya, Sergiy; Fox, Rodney; Vigil, Dennis; Subramaniam, Shankar

    2009-11-01

    Nanoparticle synthesis in turbulent reactors subjects anoparticle aggregates to a homogeneous, time-varying shear flow. The shear flow results in anisotropic clusters and it is of interest to characterize the structural properties of these clusters and their effects on initiation and acceleration of aggregation, the restructuring of clusters, and their breakage. The anisotropic structure of a sheared cluster is characterized by the ratio of the major to minor axis length of the approximating ellipsoid oriented along the cluster moment of inertia tensor's principal axes. Brownian dynamics simulations show that shear flow dramatically changes the structure of aggregates by initiating the formation of more compact structures at smaller length scales perpendicular to the shear direction, and anisotropic, cigar--like structures along the shear direction. More compact clusters correspond to higher local volumetric potential energy density. Therefore, we classify the compactness and anisotropy of sheared clusters on a map of local volumetric potential energy density versus ratio of the principal values of the cluster's moment of inertia tensor. The effect of shear on breakage of clusters is characterized by the radius of gyration Rg^cr of the largest stable aggregate for a given value of the imposed steady shear rate (P'eclet number).

  6. Steady and transient fluid shear stress stimulate NO release in osteoblasts through distinct biochemical pathways

    NASA Technical Reports Server (NTRS)

    McAllister, T. N.; Frangos, J. A.

    1999-01-01

    Fluid flow has been shown to be a potent stimulus in osteoblasts and osteocytes and may therefore play an important role in load-induced bone remodeling. The objective of this study was to investigate the characteristics of flow-activated pathways. Previously we reported that fluid flow stimulates rapid and continuous release of nitric oxide (NO) in primary rat calvarial osteoblasts. Here we demonstrate that flow-induced NO release is mediated by shear stress and that this response is distinctly biphasic. Transients in shear stress associated with the onset of flow stimulated a burst in NO production (8.2 nmol/mg of protein/h), while steady flow stimulated sustained NO production (2.2 nmol/mg of protein/h). Both G-protein inhibition and calcium chelation abolished the burst phase but had no effect on sustained production. Activation of G-proteins stimulated dose-dependent NO release in static cultures of both calvarial osteoblasts and UMR-106 osteoblast-like cells. Pertussis toxin had no effect on NO release. Calcium ionophore stimulated low levels of NO production within 15 minutes but had no effect on sustained production. Taken together, these data suggest that fluid shear stress stimulates NO release by two distinct pathways: a G-protein and calcium-dependent phase sensitive to flow transients, and a G-protein and calcium-independent pathway stimulated by sustained flow.

  7. Mass Transport and Shear Stress as Mediators of Flow Effects on Atherosclerotic Plaque Origin and Growth

    NASA Astrophysics Data System (ADS)

    Gorder, Riley; Aliseda, Alberto

    2009-11-01

    The carotid artery bifurcation (CAB) is one of the leading site for atherosclerosis, a major cause of mortality and morbidity in the developed world. The specific mechanisms by which perturbed flow at the bifurcation and in the carotid bulge promotes plaque formation and growth are not fully understood. Shear stress, mass transport, and flow residence times are considered dominant factors. Shear stress causes restructuring of endothelial cells at the arterial wall which changes the wall's permeability. Long residence times are associated with enhanced mass transport through increased diffusion of lipids and white blood cells into the arterial wall. Although momentum and mass transfer are traditionally coupled by correlations similar to Reynolds Analogy, the complex flow patterns present in this region due to the pulsatile, transitional, detached flow associated with the complex geometry makes the validity of commonly accepted assumptions uncertain. We create solid models of the CAB from MRI or ultrasound medical images, build flow phantoms on clear polyester resin and use an IOR matching, blood mimicking, working fluid. Using PIV and dye injection techniques the shear stress and scalar transport are experimentally investigated. Our goal is to establish a quantitative relationship between momentum and mass transfer under a wide range of physiologically normal and pathological conditions.

  8. The dependence of particle permittivity on the shear stress of electrorheological fluids

    NASA Astrophysics Data System (ADS)

    Lan, Yucheng; Men, Sho