Large scale structures in a turbulent boundary layer and their imprint on wall shear stress

NASA Astrophysics Data System (ADS)

Pabon, Rommel; Barnard, Casey; Ukeiley, Lawrence; Sheplak, Mark

2015-11-01

Experiments were performed on a turbulent boundary layer developing on a flat plate model under zero pressure gradient flow. A MEMS differential capacitive shear stress sensor with a 1 mm × 1 mm floating element was used to capture the fluctuating wall shear stress simultaneously with streamwise velocity measurements from a hot-wire anemometer traversed in the wall normal direction. Near the wall, the peak in the cross correlation corresponds to an organized motion inclined 45° from the wall. In the outer region, the peak diminishes in value, but is still significant at a distance greater than half the boundary layer thickness, and corresponds to a structure inclined 14° from the wall. High coherence between the two signals was found for the low-frequency content, reinforcing the belief that large scale structures have a vital impact on wall shear stress. Thus, estimation of the wall shear stress from the low-frequency velocity signal will be performed, and is expected to be statistically significant in the outer boundary layer. Additionally, conditionally averaged mean velocity profiles will be presented to assess the effects of high and low shear stress. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1315138.

Pressure and wall shear stress in blood hammer - Analytical theory.

PubMed

Mei, Chiang C; Jing, Haixiao

2016-10-01

We describe an analytical theory of blood hammer in a long and stiffened artery due to sudden blockage. Based on the model of a viscous fluid in laminar flow, we derive explicit expressions of oscillatory pressure and wall shear stress. To examine the effects on local plaque formation we also allow the blood vessel radius to be slightly nonuniform. Without resorting to discrete computation, the asymptotic method of multiple scales is utilized to deal with the sharp contrast of time scales. The effects of plaque and blocking time on blood pressure and wall shear stress are studied. The theory is validated by comparison with existing water hammer experiments. Copyright © 2016. Published by Elsevier Inc.

Application of mean wall shear stress boundary condition to complex turbulent flows using a wall-modeled large eddy simulation

NASA Astrophysics Data System (ADS)

Cho, Minjeong; Lee, Jungil; Choi, Haecheon

2012-11-01

The mean wall shear stress boundary condition was successfully applied to turbulent channel and boundary flows using large eddy simulation without resolving near-wall region (see Lee, Cho & Choi in this book of abstracts). In the present study, we apply this boundary condition to more complex flows where flow separation and redeveloping flow exist. As a test problem, we consider flow over a backward-facing step at Reh = 22860 based on the step height. Turbulent boundary layer flow at the inlet (Reθ = 1050) is obtained using inflow generation technique by Lund et al. (1998) but with wall shear stress boundary condition. First, we prescribe the mean wall shear stress distribution obtained from DNS (Kim, 2011, Ph.D. Thesis, Stanford U.) as the boundary condition of present simulation. Here we give no-slip boundary condition at flow-reversal region. The present results are in good agreements with the flow statistics by DNS. Currently, a dynamic approach of obtaining mean wall shear stress based on the log-law is being applied to the flow having flow separation and its results will be shown in the presentation. Supported by the WCU and NRF programs.

Thinner regions of intracranial aneurysm wall correlate with regions of higher wall shear stress: a 7.0 tesla MRI

PubMed Central

Blankena, Roos; Kleinloog, Rachel; Verweij, Bon H.; van Ooij, Pim; ten Haken, Bennie; Luijten, Peter R.; Rinkel, Gabriel J.E.; Zwanenburg, Jaco J.M.

2016-01-01

Purpose To develop a method for semi-quantitative wall thickness assessment on in vivo 7.0 tesla (7T) MRI images of intracranial aneurysms for studying the relation between apparent aneurysm wall thickness and wall shear stress. Materials and Methods Wall thickness was analyzed in 11 unruptured aneurysms in 9 patients, who underwent 7T MRI with a TSE based vessel wall sequence (0.8 mm isotropic resolution). A custom analysis program determined the in vivo aneurysm wall intensities, which were normalized to signal of nearby brain tissue and were used as measure for apparent wall thickness (AWT). Spatial wall thickness variation was determined as the interquartile range in AWT (the middle 50% of the AWT range). Wall shear stress was determined using phase contrast MRI (0.5 mm isotropic resolution). We performed visual and statistical comparisons (Pearson’s correlation) to study the relation between wall thickness and wall shear stress. Results 3D colored AWT maps of the aneurysms showed spatial AWT variation, which ranged from 0.07 to 0.53, with a mean variation of 0.22 (a variation of 1.0 roughly means a wall thickness variation of one voxel (0.8mm)). In all aneurysms, AWT was inversely related to WSS (mean correlation coefficient −0.35, P<0.05). Conclusions A method was developed to measure the wall thickness semi-quantitatively, using 7T MRI. An inverse correlation between wall shear stress and AWT was determined. In future studies, this non-invasive method can be used to assess spatial wall thickness variation in relation to pathophysiologic processes such as aneurysm growth and –rupture. PMID:26892986

Numerical design and optimization of hydraulic resistance and wall shear stress inside pressure-driven microfluidic networks.

PubMed

Damiri, Hazem Salim; Bardaweel, Hamzeh Khalid

2015-11-07

Microfluidic networks represent the milestone of microfluidic devices. Recent advancements in microfluidic technologies mandate complex designs where both hydraulic resistance and pressure drop across the microfluidic network are minimized, while wall shear stress is precisely mapped throughout the network. In this work, a combination of theoretical and modeling techniques is used to construct a microfluidic network that operates under minimum hydraulic resistance and minimum pressure drop while constraining wall shear stress throughout the network. The results show that in order to minimize the hydraulic resistance and pressure drop throughout the network while maintaining constant wall shear stress throughout the network, geometric and shape conditions related to the compactness and aspect ratio of the parent and daughter branches must be followed. Also, results suggest that while a "local" minimum hydraulic resistance can be achieved for a geometry with an arbitrary aspect ratio, a "global" minimum hydraulic resistance occurs only when the aspect ratio of that geometry is set to unity. Thus, it is concluded that square and equilateral triangular cross-sectional area microfluidic networks have the least resistance compared to all rectangular and isosceles triangular cross-sectional microfluidic networks, respectively. Precise control over wall shear stress through the bifurcations of the microfluidic network is demonstrated in this work. Three multi-generation microfluidic network designs are considered. In these three designs, wall shear stress in the microfluidic network is successfully kept constant, increased in the daughter-branch direction, or decreased in the daughter-branch direction, respectively. For the multi-generation microfluidic network with constant wall shear stress, the design guidelines presented in this work result in identical profiles of wall shear stresses not only within a single generation but also through all the generations of the

Correlation between vortices and wall shear stress in a curved artery model under pulsatile flow conditions

NASA Astrophysics Data System (ADS)

Cox, Christopher; Plesniak, Michael W.

2017-11-01

One of the most physiologically relevant factors within the cardiovascular system is the wall shear stress. The wall shear stress affects endothelial cells via mechanotransduction and atherosclerotic regions are strongly correlated with curvature and branching in the human vasculature, where the shear stress is both oscillatory and multidirectional. Also, the combined effect of curvature and pulsatility in cardiovascular flows produces unsteady vortices. In this work, our goal is to assess the correlation between multiple vortex pairs and wall shear stress. To accomplish this, we use an in-house high-order flux reconstruction Navier-Stokes solver to simulate pulsatile flow of a Newtonian blood-analog fluid through a rigid 180° curved artery model. We use a physiologically relevant flow rate and generate results using both fully developed and uniform entrance conditions, the latter motivated by the fact that flow upstream to a curved artery may not be fully developed. Under these two inflow conditions, we characterize the evolution of various vortex pairs and their subsequent effect on several wall shear stress metrics. Supported by GW Center for Biomimetics and Bioinspired Engineering.

Simultaneous wall-shear-stress and wide-field PIV measurements in a turbulent boundary layer

NASA Astrophysics Data System (ADS)

Gomit, Guillaume; Fourrie, Gregoire; de Kat, Roeland; Ganapathisubramani, Bharathram

2015-11-01

Simultaneous particle image velocimetry (PIV) and hot-film shear stress sensor measurements were performed to study the large-scale structures associated with shear stress events in a flat plate turbulent boundary layer at a high Reynolds number (Reτ ~ 4000). The PIV measurement was performed in a streamwise-wall normal plane using an array of six high resolution cameras (4 ×16MP and 2 ×29MP). The resulting field of view covers 8 δ (where δ is the boundary layer thickness) in the streamwise direction and captures the entire boundary layer in the wall-normal direction. The spatial resolution of the measurement is approximately is approximately 70 wall units (1.8 mm) and sampled each 35 wall units (0.9 mm). In association with the PIV setup, a spanwise array of 10 skin-friction sensors (spanning one δ) was used to capture the footprint of the large-scale structures. This combination of measurements allowed the analysis of the three-dimensional conditional structures in the boundary layer. Particularly, from conditional averages, the 3D organisation of the wall normal and streamwise velocity components (u and v) and the Reynolds shear stress (-u'v') related to a low and high shear stress events can be extracted. European Research Council Grant No-277472-WBT.

A multi-component parallel-plate flow chamber system for studying the effect of exercise-induced wall shear stress on endothelial cells.

PubMed

Wang, Yan-Xia; Xiang, Cheng; Liu, Bo; Zhu, Yong; Luan, Yong; Liu, Shu-Tian; Qin, Kai-Rong

2016-12-28

In vivo studies have demonstrated that reasonable exercise training can improve endothelial function. To confirm the key role of wall shear stress induced by exercise on endothelial cells, and to understand how wall shear stress affects the structure and the function of endothelial cells, it is crucial to design and fabricate an in vitro multi-component parallel-plate flow chamber system which can closely replicate exercise-induced wall shear stress waveforms in artery. The in vivo wall shear stress waveforms from the common carotid artery of a healthy volunteer in resting and immediately after 30 min acute aerobic cycling exercise were first calculated by measuring the inner diameter and the center-line blood flow velocity with a color Doppler ultrasound. According to the above in vivo wall shear stress waveforms, we designed and fabricated a parallel-plate flow chamber system with appropriate components based on a lumped parameter hemodynamics model. To validate the feasibility of this system, human umbilical vein endothelial cells (HUVECs) line were cultured within the parallel-plate flow chamber under abovementioned two types of wall shear stress waveforms and the intracellular actin microfilaments and nitric oxide (NO) production level were evaluated using fluorescence microscope. Our results show that the trends of resting and exercise-induced wall shear stress waveforms, especially the maximal, minimal and mean wall shear stress as well as oscillatory shear index, generated by the parallel-plate flow chamber system are similar to those acquired from the common carotid artery. In addition, the cellular experiments demonstrate that the actin microfilaments and the production of NO within cells exposed to the two different wall shear stress waveforms exhibit different dynamic behaviors; there are larger numbers of actin microfilaments and higher level NO in cells exposed in exercise-induced wall shear stress condition than resting wall shear stress condition

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.

Role of Wall Shear Stress in Cryptosporidium parvum Oocyst Attachment to Environmental Biofilms.

PubMed

Luo, Xia; Jedlicka, Sabrina S; Jellison, Kristen L

2017-12-15

This study investigated Cryptosporidium parvum oocyst deposition onto biofilms as a function of shear stress under laminar or turbulent flow. Annular rotating bioreactors were used to grow stabilized stream biofilms at shear stresses ranging from 0.038 to 0.46 Pa. These steady-state biofilms were then used to assess the impact of hydrodynamic conditions on C. parvum oocyst attachment. C. parvum deposition onto biofilms followed a pseudo-second-order model under both laminar (after a lag phase) and turbulent flows. The total number of oocysts attached to the biofilm at steady state decreased as the hydrodynamic wall shear stress increased. The oocyst deposition rate constant increased with shear stress but decreased at high shear, suggesting that increasing wall shear stress results in faster attachment of Cryptosporidium due to higher mass transport until the shear forces exceed a critical limit that prevents oocyst attachment. These data show that oocyst attachment in the short and long term are impacted differently by shear: higher shear (to a certain limit) may be associated with faster initial oocyst attachment, but lower shear is associated with greater numbers of oocysts attached at equilibrium. IMPORTANCE This research provides experimental evidence to demonstrate that shear stress plays a critical role in protozoan-pathogen transport and deposition in environmental waters. The data presented in this work expand scientific understanding of Cryptosporidium attachment and fate, which will further influence the development of timely and accurate sampling strategies, as well as advanced water treatment technologies, to target protozoan pathogens in surface waters that serve as municipal drinking water sources. Copyright © 2017 American Society for Microbiology.

Pulsatile flows and wall-shear stresses in models simulating normal and stenosed aortic arches

NASA Astrophysics Data System (ADS)

Huang, Rong Fung; Yang, Ten-Fang; Lan, Y.-K.

2010-03-01

Pulsatile aqueous glycerol solution flows in the models simulating normal and stenosed human aortic arches are measured by means of particle image velocimetry. Three transparent models were used: normal, 25% stenosed, and 50% stenosed aortic arches. The Womersley parameter, Dean number, and time-averaged Reynolds number are 17.31, 725, and 1,081, respectively. The Reynolds numbers based on the peak velocities of the normal, 25% stenosed, and 50% stenosed aortic arches are 2,484, 3,456, and 3,931, respectively. The study presents the temporal/spatial evolution processes of the flow pattern, velocity distribution, and wall-shear stress during the systolic and diastolic phases. It is found that the flow pattern evolving in the central plane of normal and stenosed aortic arches exhibits (1) a separation bubble around the inner arch, (2) a recirculation vortex around the outer arch wall upstream of the junction of the brachiocephalic artery, (3) an accelerated main stream around the outer arch wall near the junctions of the left carotid and the left subclavian arteries, and (4) the vortices around the entrances of the three main branches. The study identifies and discusses the reasons for the flow physics’ contribution to the formation of these features. The oscillating wall-shear stress distributions are closely related to the featured flow structures. On the outer wall of normal and slightly stenosed aortas, large wall-shear stresses appear in the regions upstream of the junction of the brachiocephalic artery as well as the corner near the junctions of the left carotid artery and the left subclavian artery. On the inner wall, the largest wall-shear stress appears in the region where the boundary layer separates.

Pulsatile extracorporeal circulation during on-pump cardiac surgery enhances aortic wall shear stress.

PubMed

Assmann, Alexander; Benim, Ali Cemal; Gül, Fethi; Lux, Philipp; Akhyari, Payam; Boeken, Udo; Joos, Franz; Feindt, Peter; Lichtenberg, Artur

2012-01-03

Controversy on superiority of pulsatile versus non-pulsatile extracorporeal circulation in cardiac surgery still continues. Stroke as one of the major adverse events during cardiopulmonary bypass is, in the majority of cases, caused by mobilization of aortic arteriosclerotic plaques that is inducible by pathologically elevated wall shear stress values. The present study employs computational fluid dynamics to evaluate the aortic blood flow and wall shear stress profiles under the influence of antegrade or retrograde perfusion with pulsatile versus non-pulsatile extracorporeal circulation. While, compared to physiological flow, a non-pulsatile perfusion resulted in generally decreased blood velocities and only moderately increased shear forces (48 Pa versus 20 Pa antegradely and 127 Pa versus 30 Pa retrogradely), a pulsatile perfusion extensively enhanced the occurrence of turbulences, maximum blood flow speed and maximum wall shear stress (1020 Pa versus 20 Pa antegradely and 1178 Pa versus 30 Pa retrogradely). Under these circumstances arteriosclerotic embolism has to be considered. Further simulations and experimental work are necessary to elucidate the impact of our findings on the scientific discourse of pulsatile versus non-pulsatile extracorporeal circulation. Copyright © 2011 Elsevier Ltd. All rights reserved.

Wall shear measurement in sand-water mixture flows

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yucel, O.; Grad, W.H.

1975-07-01

The wall shear stress was measured in clear-water and sand-water mixture flows with the use of a flush-mounting hot-film shear-sensor. Data were obtained with 2 shear-sensors and 2 different sands (d50 = 0.45 mm and d50 = 0.88 mm) with solids concentrations of up to Cmax = 1.6% by vol, and for flow Reynolds number of 10/sup 5/ < RD < 6 x 10/sup 5/. The measured sensor wall shear stresses were compared with the true wall shear stresses obtained with the energy head loss measurements conducted in a pipeline system. The results of the tests in the clear-water flowsmore » confirmed the relationship between the sensor power output, Ps, and the wall shear stress, tauo, given by tauo1/3 = APs + B, in which A and B are calibration coefficients. The tests with the low-concentration sand-water mixtures in a vertical pipe indicated that for the present range of experiments, sensor power outputs with the mixtures exceeded those for clear-water by an average of 5%. It is shown that the shear sensors are delicate but accurate instruments that can be used for the measurement of the wall shear stress. (13 refs.)« less

Spatio-temporal characteristics of large scale motions in a turbulent boundary layer from direct wall shear stress measurement

NASA Astrophysics Data System (ADS)

Pabon, Rommel; Barnard, Casey; Ukeiley, Lawrence; Sheplak, Mark

2016-11-01

Particle image velocimetry (PIV) and fluctuating wall shear stress experiments were performed on a flat plate turbulent boundary layer (TBL) under zero pressure gradient conditions. The fluctuating wall shear stress was measured using a microelectromechanical 1mm × 1mm floating element capacitive shear stress sensor (CSSS) developed at the University of Florida. The experiments elucidated the imprint of the organized motions in a TBL on the wall shear stress through its direct measurement. Spatial autocorrelation of the streamwise velocity from the PIV snapshots revealed large scale motions that scale on the order of boundary layer thickness. However, the captured inclination angle was lower than that determined using the classic method by means of wall shear stress and hot-wire anemometry (HWA) temporal cross-correlations and a frozen field hypothesis using a convection velocity. The current study suggests the large size of these motions begins to degrade the applicability of the frozen field hypothesis for the time resolved HWA experiments. The simultaneous PIV and CSSS measurements are also used for spatial reconstruction of the velocity field during conditionally sampled intense wall shear stress events. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1315138.

Wall shear stress fixed points in blood flow

NASA Astrophysics Data System (ADS)

Arzani, Amirhossein; Shadden, Shawn

2017-11-01

Patient-specific computational fluid dynamics produces large datasets, and wall shear stress (WSS) is one of the most important parameters due to its close connection with the biological processes at the wall. While some studies have investigated WSS vectorial features, the WSS fixed points have not received much attention. In this talk, we will discuss the importance of WSS fixed points from three viewpoints. First, we will review how WSS fixed points relate to the flow physics away from the wall. Second, we will discuss how certain types of WSS fixed points lead to high biochemical surface concentration in cardiovascular mass transport problems. Finally, we will introduce a new measure to track the exposure of endothelial cells to WSS fixed points.

Measurements of wall shear stress in a planar turbulent Couette flow with porous walls

NASA Astrophysics Data System (ADS)

Beuther, Paul

2013-11-01

Measurements of drag on a moving web in a multi-span festoon show a stronger than expected dependency on the porosity of the web. The experiments suggest a wall shear stress 3-4 times larger than non-porous webs or historical Couette flow data for solid walls. Previous DNS studies by Jimenez et al. (JFM Vol 442) of boundary layers with passive porous surfaces predict a much smaller increase in wall shear stress for a porous wall of only 40%. Other DNS studies by Quadrio et al. (JFM Vol 576) of porous walls with periodic transpiration do show a large increase in drag under certain periodic conditions of modest amplitude. Although those results are aligned in magnitude with this study, the exact reason for the observed high drag for porous webs in this present study is not understood because there was no external disturbance applied to the web. It can be hypothesized that natural flutter of the web results in a similar mechanism shown in the periodic DNS study, but when the natural flutter was reduced by increasing web tension, there was only a small decrease of the drag. A key difference in this study is that because of the multiple parallel spans in a festoon, any transpiration in one layer must act in the opposite manner on the adjacent span.

Steel Shear Walls, Behavior, Modeling and Design

NASA Astrophysics Data System (ADS)

Astaneh-Asl, Abolhassan

2008-07-01

In recent years steel shear walls have become one of the more efficient lateral load resisting systems in tall buildings. The basic steel shear wall system consists of a steel plate welded to boundary steel columns and boundary steel beams. In some cases the boundary columns have been concrete-filled steel tubes. Seismic behavior of steel shear wall systems during actual earthquakes and based on laboratory cyclic tests indicates that the systems are quite ductile and can be designed in an economical way to have sufficient stiffness, strength, ductility and energy dissipation capacity to resist seismic effects of strong earthquakes. This paper, after summarizing the past research, presents the results of two tests of an innovative steel shear wall system where the boundary elements are concrete-filled tubes. Then, a review of currently available analytical models of steel shear walls is provided with a discussion of capabilities and limitations of each model. We have observed that the tension only "strip model", forming the basis of the current AISC seismic design provisions for steel shear walls, is not capable of predicting the behavior of steel shear walls with length-to-thickness ratio less than about 600 which is the range most common in buildings. The main reasons for such shortcomings of the AISC seismic design provisions for steel shear walls is that it ignores the compression field in the shear walls, which can be significant in typical shear walls. The AISC method also is not capable of incorporating stresses in the shear wall due to overturning moments. A more rational seismic design procedure for design of shear walls proposed in 2000 by the author is summarized in the paper. The design method, based on procedures used for design of steel plate girders, takes into account both tension and compression stress fields and is applicable to all values of length-to-thickness ratios of steel shear walls. The method is also capable of including the effect of

Orbitally shaken shallow fluid layers. II. An improved wall shear stress model

NASA Astrophysics Data System (ADS)

Alpresa, Paola; Sherwin, Spencer; Weinberg, Peter; van Reeuwijk, Maarten

2018-03-01

A new model for the analytical prediction of wall shear stress distributions at the base of orbitally shaken shallow fluid layers is developed. This model is a generalisation of the classical extended Stokes solution and will be referred to as the potential theory-Stokes model. The model is validated using a large set of numerical simulations covering a wide range of flow regimes representative of those used in laboratory experiments. It is demonstrated that the model is in much better agreement with the simulation data than the classical Stokes solution, improving the prediction in 63% of the studied cases. The central assumption of the model—which is to link the wall shear stress with the surface velocity—is shown to hold remarkably well over all regimes covered.

Quantification of wall shear stress in large blood vessels using Lagrangian interpolation functions with cine phase-contrast magnetic resonance imaging.

PubMed

Cheng, Christopher P; Parker, David; Taylor, Charles A

2002-09-01

Arterial wall shear stress is hypothesized to be an important factor in the localization of atherosclerosis. Current methods to compute wall shear stress from magnetic resonance imaging (MRI) data do not account for flow profiles characteristic of pulsatile flow in noncircular vessel lumens. We describe a method to quantify wall shear stress in large blood vessels by differentiating velocity interpolation functions defined using cine phase-contrast MRI data on a band of elements in the neighborhood of the vessel wall. Validation was performed with software phantoms and an in vitro flow phantom. At an image resolution corresponding to in vivo imaging data of the human abdominal aorta, time-averaged, spatially averaged wall shear stress for steady and pulsatile flow were determined to be within 16% and 23% of the analytic solution, respectively. These errors were reduced to 5% and 8% with doubling in image resolution. For the pulsatile software phantom, the oscillation in shear stress was predicted to within 5%. The mean absolute error of circumferentially resolved shear stress for the nonaxisymmetric phantom decreased from 28% to 15% with a doubling in image resolution. The irregularly shaped phantom and in vitro investigation demonstrated convergence of the calculated values with increased image resolution. We quantified the shear stress at the supraceliac and infrarenal regions of a human abdominal aorta to be 3.4 and 2.3 dyn/cm2, respectively.

The impact of wall shear stress and pressure drop on the stability of the atherosclerotic plaque.

PubMed

Li, Zhi-Yong; Taviani, Valentina; Gillard, Jonathan H

2008-01-01

Rupture of vulnerable atheromatous plaque in the carotid and coronary arteries often leads to stroke and heart attack respectively. The mechanism of blood flow and plaque rupture in stenotic arteries is still not fully understood. A three dimensional rigid wall model was solved under steady state conditions and unsteady conditions by assuming a time-varying inlet velocity profile to investigate the relative importance of axial forces and pressure drops in arteries with asymmetric stenosis. Flow-structure interactions were investigated for the same geometry and the results were compared with those retrieved with the corresponding 2D cross-section structural models. The Navier-Stokes equations were used as the governing equations for the fluid. The tube wall was assumed hyperelastic, homogeneous, isotropic and incompressible. The analysis showed that the three dimensional behavior of velocity, pressure and wall shear stress is in general very different from that predicted by cross-section models. Pressure drop across the stenosis was found to be much higher than shear stress. Therefore, pressure may be the more important mechanical trigger for plaque rupture other than shear stress, although shear stress is closely related to plaque formation and progression.

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

Sustained expression of MCP-1 by low wall shear stress loading concomitant with turbulent flow on endothelial cells of intracranial aneurysm.

PubMed

Aoki, Tomohiro; Yamamoto, Kimiko; Fukuda, Miyuki; Shimogonya, Yuji; Fukuda, Shunichi; Narumiya, Shuh

2016-05-09

Enlargement of a pre-existing intracranial aneurysm is a well-established risk factor of rupture. Excessive low wall shear stress concomitant with turbulent flow in the dome of an aneurysm may contribute to progression and rupture. However, how stress conditions regulate enlargement of a pre-existing aneurysm remains to be elucidated. Wall shear stress was calculated with 3D-computational fluid dynamics simulation using three cases of unruptured intracranial aneurysm. The resulting value, 0.017 Pa at the dome, was much lower than that in the parent artery. We loaded wall shear stress corresponding to the value and also turbulent flow to the primary culture of endothelial cells. We then obtained gene expression profiles by RNA sequence analysis. RNA sequence analysis detected hundreds of differentially expressed genes among groups. Gene ontology and pathway analysis identified signaling related with cell division/proliferation as overrepresented in the low wall shear stress-loaded group, which was further augmented by the addition of turbulent flow. Moreover, expression of some chemoattractants for inflammatory cells, including MCP-1, was upregulated under low wall shear stress with concomitant turbulent flow. We further examined the temporal sequence of expressions of factors identified in an in vitro study using a rat model. No proliferative cells were detected, but MCP-1 expression was induced and sustained in the endothelial cell layer. Low wall shear stress concomitant with turbulent flow contributes to sustained expression of MCP-1 in endothelial cells and presumably plays a role in facilitating macrophage infiltration and exacerbating inflammation, which leads to enlargement or rupture.

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

Wall shear stress characterization of a 3D bluff-body separated flow

NASA Astrophysics Data System (ADS)

Fourrié, Grégoire; Keirsbulck, Laurent; Labraga, Larbi

2013-10-01

Efficient flow control strategies aimed at reducing the aerodynamic drag of road vehicles require a detailed knowledge of the reference flow. In this work, the flow around the rear slanted window of a generic car model was experimentally studied through wall shear stress measurements using an electrochemical method. The mean and fluctuating wall shear stress within the wall impact regions of the recirculation bubble and the main longitudinal vortex structures which develop above the rear window are presented. Correlations allow a more detailed characterization of the recirculation phenomenon within the separation bubble. In the model symmetry plane the recirculation structure compares well with simpler 2D configurations; specific lengths, flapping motion and shedding of large-scale vortices are observed, these similarities diminish when leaving the middle plane due to the strong three-dimensionality of the flow. A specific attention is paid to the convection processes occurring within the recirculation: a downstream convection velocity is observed, in accordance with 2D recirculations from the literature, and an upstream convection is highlighted along the entire bubble length which has not been underlined in some previous canonical configurations.

Oscillatory motion based measurement method and sensor for measuring wall shear stress due to fluid flow

DOEpatents

Armstrong, William D [Laramie, WY; Naughton, Jonathan [Laramie, WY; Lindberg, William R [Laramie, WY

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.

Development of a wall-shear-stress sensor and measurements in mini-channels with partial blockages

NASA Astrophysics Data System (ADS)

Afara, Samer; Medvescek, James; Mydlarski, Laurent; Baliga, Bantwal R.; MacDonald, Mark

2014-05-01

The design, construction, operation and validation of a wall-shear-stress sensor, and measurements obtained using this sensor in air flows downstream of partial blockages in a mini-channel are presented. The sensor consisted of a hot wire mounted over a small rectangular slot and operated using a constant-temperature anemometer. It was used to investigate flows similar to those within the mini-channels inside notebook computers. The overall goal of the present work was to develop a sensor suitable for measurements of the wall-shear stress in such flows, which can be used to validate corresponding numerical simulations, as the latter are known to be often surprisingly inaccurate. To this end, measurements of the wall-shear stress, and the corresponding statistical moments and power spectral densities, were obtained at different distances downstream of the partial blockage, with blockage ratios of 39.7, 59.2, and 76.3 %. The Reynolds number (based on average velocity and hydraulic diameter) ranged from 100 to 900. The results confirmed the presence of unsteadiness, separation, reattachment, and laminar-turbulent transition in the ostensibly laminar flow of air in mini-channels with partial blockages. The present results demonstrate why accurate numerical predictions of cooling air flows in laptop and notebook computers remain a challenging task.

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.

Understanding the fluid mechanics behind transverse wall shear stress.

PubMed

Mohamied, Yumnah; Sherwin, Spencer J; Weinberg, Peter D

2017-01-04

The patchy distribution of atherosclerosis within arteries is widely attributed to local variation in haemodynamic wall shear stress (WSS). A recently-introduced metric, the transverse wall shear stress (transWSS), which is the average over the cardiac cycle of WSS components perpendicular to the temporal mean WSS vector, correlates particularly well with the pattern of lesions around aortic branch ostia. Here we use numerical methods to investigate the nature of the arterial flows captured by transWSS and the sensitivity of transWSS to inflow waveform and aortic geometry. TransWSS developed chiefly in the acceleration, peak systolic and deceleration phases of the cardiac cycle; the reverse flow phase was too short, and WSS in diastole was too low, for these periods to have a significant influence. Most of the spatial variation in transWSS arose from variation in the angle by which instantaneous WSS vectors deviated from the mean WSS vector rather than from variation in the magnitude of the vectors. The pattern of transWSS was insensitive to inflow waveform; only unphysiologically high Womersley numbers produced substantial changes. However, transWSS was sensitive to changes in geometry. The curvature of the arch and proximal descending aorta were responsible for the principal features, the non-planar nature of the aorta produced asymmetries in the location and position of streaks of high transWSS, and taper determined the persistence of the streaks down the aorta. These results reflect the importance of the fluctuating strength of Dean vortices in generating transWSS. Copyright © 2016 Elsevier Ltd. All rights reserved.

Impact of Wall Shear Stress and Pressure Variation on the Stability of Atherosclerotic Plaque

NASA Astrophysics Data System (ADS)

Taviani, V.; Li, Z. Y.; Sutcliffe, M.; Gillard, J.

Rupture of vulnerable atheromatous plaque in the carotid and coronary arteries often leads to stroke and heart attack respectively. The mechanism of blood flow and plaque rupture in stenotic arteries is still not fully understood. A three dimensional rigid wall model was solved under steady and unsteady conditions assuming a time-varying inlet velocity profile to investigate the relative importance of axial forces and pressure drops in arteries with asymmetric stenosis. Flow-structure interactions were investigated for the same geometry and the results were compared with those retrieved with the corresponding one dimensional models. The Navier-Stokes equations were used as the governing equations for the fluid. The tube wall was assumed linearly elastic, homogeneous isotropic. The analysis showed that wall shear stress is small (less than 3.5%) with respect to pressure drop throughout the cycle even for severe stenosis. On the contrary, the three dimensional behavior of velocity, pressure and wall shear stress is in general very different from that predicted by one dimensional models. This suggests that the primary source of mistakes in one dimensional studies comes from neglecting the three dimensional geometry of the plaque. Neglecting axial forces only involves minor errors.

Analysis of wall shear stress around a competitive swimmer using 3D Navier-Stokes equations in CFD.

PubMed

Popa, C V; Zaidi, H; Arfaoui, A; Polidori, G; Taiar, R; Fohanno, S

2011-01-01

This paper deals with the flow dynamics around a competitive swimmer during underwater glide phases occurring at the start and at every turn. The influence of the head position, namely lifted up, aligned and lowered, on the wall shear stress and the static pressure distributions is analyzed. The problem is considered as 3D and in steady hydrodynamic state. Three velocities (1.4 m/s, 2.2 m/s and 3.1 m/s) that correspond to inter-regional, national and international swimming levels are studied. The flow around the swimmer is assumed turbulent. The Reynolds-averaged Navier-Stokes (RANS) equations are solved with the standard k-ω turbulent model by using the CFD (computational fluid dynamics) numerical method based on a volume control approach. Numerical simulations are carried out with the ANSYS FLUENT® CFD code. The results show that the wall shear stress increases with the velocity and consequently the drag force opposing the movement of the swimmer increases as well. Also, high wall shear stresses are observed in the areas where the body shape, globally rigid in form, presents complex surface geometries such as the head, shoulders, buttocks, heel and chest.

Evaluation of Aortic Blood Flow and Wall Shear Stress in Aortic Stenosis and Its Association With Left Ventricular Remodeling.

PubMed

von Knobelsdorff-Brenkenhoff, Florian; Karunaharamoorthy, Achudhan; Trauzeddel, Ralf Felix; Barker, Alex J; Blaszczyk, Edyta; Markl, Michael; Schulz-Menger, Jeanette

2016-03-01

Aortic stenosis (AS) leads to variable stress for the left ventricle (LV) and consequently a broad range of LV remodeling. The aim of this study was to describe blood flow patterns in the ascending aorta of patients with AS and determine their association with remodeling. Thirty-seven patients with AS (14 mild, 8 moderate, 15 severe; age, 63±13 years) and 37 healthy controls (age, 60±10 years) underwent 4-dimensional-flow magnetic resonance imaging. Helical and vortical flow formations and flow eccentricity were assessed in the ascending aorta. Normalized flow displacement from the vessel center and peak systolic wall shear stress in the ascending aorta were quantified. LV remodeling was assessed based on LV mass index and the ratio of LV mass:end-diastolic volume (relative wall mass). Marked helical and vortical flow formation and eccentricity were more prevalent in patients with AS than in healthy subjects, and patients with AS exhibited an asymmetrical and elevated distribution of peak systolic wall shear stress. In AS, aortic orifice area was strongly negatively associated with vortical flow formation (P=0.0274), eccentricity (P=0.0070), and flow displacement (P=0.0021). Bicuspid aortic valve was associated with more intense helical (P=0.0098) and vortical flow formation (P=0.0536), higher flow displacement (P=0.11), and higher peak systolic wall shear stress (P=0.0926). LV mass index and relative wall mass were significantly associated with aortic orifice area (P=0.0611, P=0.0058) and flow displacement (P=0.0058, P=0.0283). In this pilot study, AS leads to abnormal blood flow pattern and peak systolic wall shear stress in the ascending aorta. In addition to aortic orifice area, normalized flow displacement was significantly associated with LV remodeling. © 2016 American Heart Association, Inc.

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

Shear stress enhances microcin B17 production in a rotating wall bioreactor, but ethanol stress does not

NASA Technical Reports Server (NTRS)

Gao, Q.; Fang, A.; Pierson, D. L.; Mishra, S. K.; Demain, A. L.

2001-01-01

Stress, including that caused by ethanol, has been shown to induce or promote secondary metabolism in a number of microbial systems. Rotating-wall bioreactors provide a low stress and simulated microgravity environment which, however, supports only poor production of microcin B17 by Escherichia coli ZK650, as compared to production in agitated flasks. We wondered whether the poor production is due to the low level of stress and whether increasing stress in the bioreactors would raise the amount of microcin B17 formed. We found that applying shear stress by addition of a single Teflon bead to a rotating wall bioreactor improved microcin B17 production. By contrast, addition of various concentrations of ethanol to such bioreactors (or to shaken flasks) failed to increase microcin B17 production. Ethanol stress merely decreased production and, at higher concentrations, inhibited growth. Interestingly, cells growing in the bioreactor were much more resistant to the growth-inhibitory and production-inhibitory effects of ethanol than cells growing in shaken flasks.

Rôle of contrast media viscosity in altering vessel wall shear stress and relation to the risk of contrast extravasations.

PubMed

Sakellariou, Sophia; Li, Wenguang; Paul, Manosh C; Roditi, Giles

2016-12-01

Iodinated contrast media (CM) are the most commonly used injectables in radiology today. A range of different media are commercially available, combining various physical and chemical characteristics (ionic state, osmolality, viscosity) and thus exhibiting distinct in vivo behaviour and safety profiles. In this paper, numerical simulations of blood flow with contrast media were conducted to investigate the effects of contrast viscosity on generated vessel wall shear stress and vessel wall pressure to elucidate any possible relation to extravasations. Five different types of contrast for Iodine fluxes ranging at 1.5-2.2gI/s were modelled through 18G and 20G cannulae placed in an ideal vein at two different orientation angles. Results demonstrate that the least viscous contrast media generate the least maximum wall shear stress as well as the lowest total pressure for the same flow rate. This supports the empirical clinical observations and hypothesis that more viscous contrast media are responsible for a higher percentage of contrast extravasations. In addition, results support the clinical hypothesis that a catheter tip directed obliquely to the vein wall always produces the highest maximum wall shear stress and total pressure due to impingement of the contrast jet on the vessel wall. Copyright © 2016 IPEM. Published by Elsevier Ltd. All rights reserved.

Wall shear stress fixed points in cardiovascular fluid mechanics.

PubMed

Arzani, Amirhossein; Shadden, Shawn C

2018-05-17

Complex blood flow in large arteries creates rich wall shear stress (WSS) vectorial features. WSS acts as a link between blood flow dynamics and the biology of various cardiovascular diseases. WSS has been of great interest in a wide range of studies and has been the most popular measure to correlate blood flow to cardiovascular disease. Recent studies have emphasized different vectorial features of WSS. However, fixed points in the WSS vector field have not received much attention. A WSS fixed point is a point on the vessel wall where the WSS vector vanishes. In this article, WSS fixed points are classified and the aspects by which they could influence cardiovascular disease are reviewed. First, the connection between WSS fixed points and the flow topology away from the vessel wall is discussed. Second, the potential role of time-averaged WSS fixed points in biochemical mass transport is demonstrated using the recent concept of Lagrangian WSS structures. Finally, simple measures are proposed to quantify the exposure of the endothelial cells to WSS fixed points. Examples from various arterial flow applications are demonstrated. Copyright © 2018 Elsevier Ltd. All rights reserved.

Polymer-surfactant complex formation and its effect on turbulent wall shear stress.

PubMed

Suksamranchit, Siriluck; Sirivat, Anuvat; Jamieson, Alexander M

2006-02-01

Turbulent drag reduction in Couette flow was investigated in terms of a decrease in wall shear stress for aqueous solutions of a nonionic polymer, poly(ethylene oxide) (PEO), a cationic surfactant, hexadecyltrimethylammonium chloride (HTAC), and their mixtures. Consistent with literature data, drag reduction was observed for PEO solutions above a critical molecular weight, 0.91 x 10(5) < Mc < 3.04 x 10(5) g/mol. Maximum drag reduction occurred at an optimum concentration, c(PEO)*, which scales inversely with molecular weight, and the % maximum drag reduction increases with molecular weight. For aqueous HTAC solutions, wall shear stress decreased with increasing HTAC concentration and leveled off at an optimum concentration, c(HTAC)*, comparable to the critical micelle concentration. For HTAC/PEO mixtures, the critical PEO molecular weight for drag reduction decreases, interpreted as due to an increase in hydrodynamic volume because of binding of HTAC micelles to PEO. Consistent with this interpretation, at fixed PEO concentration, maximum drag reduction was observed at an optimum HTAC concentration, c(HTAC/PEO)*, comparable to the maximum binding concentration, MBC. Also, with HTAC concentration fixed at the MBC, the optimum PEO concentration for drag reduction, c(PEO/HTAC)*, decreases relative to that, c(PEO)*, in the absence of HTAC.

Direct measurements of wall shear stress by buried wire gages in a shock-wave boundary-layer interaction region

NASA Technical Reports Server (NTRS)

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

1977-01-01

Detailed measurements of wall shear stress (skin friction) were made with specially developed buried wire gages in the interaction regions of a Mach 2.9 turbulent boundary layer with externally generated shocks. Separation and reattachment points inferred by these measurements support the findings of earlier experiments which used a surface oil flow technique and pitot profile measurements. The measurements further indicate that the boundary layer tends to attain significantly higher skin-friction values downstream of the interaction region as compared to upstream. Comparisons between measured wall shear stress and published results of some theoretical calculation schemes show that the general, but not detailed, behavior is predicted well by such schemes.

Interaction between a normal shock wave and a turbulent boundary layer at high transonic speeds. Part 2: Wall shear stress

NASA Technical Reports Server (NTRS)

Liou, M. S.; Adamson, T. C., Jr.

1979-01-01

An analysis is presented of the flow in the two inner layers, the Reynolds stress sublayer and the wall layer. Included is the calculation of the shear stress at the wall in the interaction region. The limit processes considered are those used for an inviscid flow.

Numerical study of wall shear stress-based descriptors in the human left coronary artery.

PubMed

Pinto, S I S; Campos, J B L M

2016-10-01

The present work is about the application of wall shear stress descriptors - time averaged wall shear stress (TAWSS), oscillating shear index (OSI) and relative residence time (RRT) - to the study of blood flow in the left coronary artery (LCA). These descriptors aid the prediction of disturbed flow conditions in the vessels and play a significant role in the detection of potential zones of atherosclerosis development. Hemodynamic descriptors data were obtained, numerically, through ANSYS® software, for the LCA of a patient-specific geometry and for a 3D idealized model. Comparing both cases, the results are coherent, in terms of location and magnitude. Low TAWSS, high OSI and high RRT values are observed in the bifurcation - potential zone of atherosclerosis appearance. The dissimilarities observed in the TAWSS values, considering blood as a Newtonian or non-Newtonian fluid, releases the importance of the correct blood rheologic caracterization. Moreover, for a higher Reynolds number, the TAWSS values decrease in the bifurcation and along the LAD branch, increasing the probability of plaques deposition. Furthermore, for a stenotic LCA model, very low TAWSS and high RRT values in front and behind the stenosis are observed, indicating the probable extension, in the flow direction, of the lesion.

Interaction between a normal shock wave and a turbulent boundary layer at high transonic speeds. II - Wall shear stress

NASA Technical Reports Server (NTRS)

Liou, M. S.; Adamson, T. C., Jr.

1980-01-01

Asymptotic methods are used to calculate the shear stress at the wall for the interaction between a normal shock wave and a turbulent boundary layer on a flat plate. A mixing length model is used for the eddy viscosity. The shock wave is taken to be strong enough that the sonic line is deep in the boundary layer and the upstream influence is thus very small. It is shown that unlike the result found for laminar flow an asymptotic criterion for separation is not found; however, conditions for incipient separation are computed numerically using the derived solution for the shear stress at the wall. Results are compared with available experimental measurements.

Structure of high and low shear-stress events in a turbulent boundary layer

NASA Astrophysics Data System (ADS)

Gomit, G.; de Kat, R.; Ganapathisubramani, B.

2018-01-01

Simultaneous particle image velocimetry (PIV) and wall-shear-stress sensor measurements were performed to study structures associated with shear-stress events in a flat plate turbulent boundary layer at a Reynolds number Reτ≈4000 . The PIV field of view covers 8 δ (where δ is the boundary layer thickness) along the streamwise direction and captures the entire boundary layer in the wall-normal direction. Simultaneously, wall-shear-stress measurements that capture the large-scale fluctuations were taken using a spanwise array of hot-film skin-friction sensors (spanning 2 δ ). Based on this combination of measurements, the organization of the conditional wall-normal and streamwise velocity fluctuations (u and v ) and of the Reynolds shear stress (-u v ) can be extracted. Conditional averages of the velocity field are computed by dividing the histogram of the large-scale wall-shear-stress fluctuations into four quartiles, each containing 25% of the occurrences. The conditional events corresponding to the extreme quartiles of the histogram (positive and negative) predominantly contribute to a change of velocity profile associated with the large structures and in the modulation of the small scales. A detailed examination of the Reynolds shear-stress contribution related to each of the four quartiles shows that the flow above a low wall-shear-stress event carries a larger amount of Reynolds shear stress than the other quartiles. The contribution of the small and large scales to this observation is discussed based on a scale decomposition of the velocity field.

PIV Measurement of Wall Shear Stress and Flow Structures within an Intracranial Aneurysm Model

NASA Astrophysics Data System (ADS)

Chow, Ricky; Sparrow, Eph; Campbell, Gary; Divani, Afshin; Sheng, Jian

2012-11-01

The formation and rupture of an intracranial aneurysm (IA) is a debilitating and often lethal event. Geometric features of the aneurysm bulb and upstream artery, such as bulb size, bulb shape, and curvature of the artery, are two groups of factors that define the flow and stresses within an IA. Abnormal flow stresses are related to rupture. This presentation discusses the development of a quasi-3D PIV technique and its application in various glass models at Re = 275 and 550 to experimentally assess at a preliminary level the impact of geometry and flow rate. Some conclusions are to be drawn linking geometry of the flow domain to rupture risk. The extracted results also serve as the baseline case and as a precursor to a companion presentation by the authors discussing the impact of flow diverters, a new class of medical devices. The PIV experiments were performed in a fully index-matched flow facility, allowing for unobstructed observations over complex geometry. A reconstruction and analysis method was devised to obtain 3D mean wall stress distributions and flow fields. The quasi 3D measurements were reconstructed from orthogonal planes encompassing the entire glass model, spaced 0.4mm apart. Wall shear stresses were evaluated from the near-wall flow viscous stresses.

Measurement of wall shear stress in chick embryonic heart using optical coherence tomography

NASA Astrophysics Data System (ADS)

Ma, Zhenhe; Dou, Shidan; Zhao, Yuqian; Wang, Yi; Suo, Yanyan; Wang, Fengwen

2015-03-01

The cardiac development is a complicated process affected by genetic and environmental factors. Wall shear stress (WSS) is one of the components which have been proved to influence the morphogenesis during early stages of cardiac development. To study the mechanism, WSS measurement is a step with significant importance. WSS is caused by blood flow imposed on the inner surface of the heart wall and it can be determined by calculating velocity gradients of blood flow in a direction perpendicular to the wall. However, the WSS of the early stage embryonic heart is difficult to measure since the embryonic heart is tiny and beating fast. Optical coherence tomography (OCT) is a non-invasive imaging modality with high spatial and temporal resolution, which is uniquely suitable for the study of early stage embryonic heart development. In this paper, we introduce a method to measure the WSS of early stage chick embryonic heart based on high speed spectral domain optical coherence tomography (SDOCT). 4D (x,y,z,t) scan was performed on the outflow tract (OFT) of HH18 (~3 days of incubation) chick embryonic heart. After phase synchronization, OFT boundary segmentation, and OFT center line calculation, Doppler angle of the blood flow in the OFT can be achieved (This method has been described in previous publications). Combining with the Doppler OCT results, we calculate absolute blood flow velocity distribution in the OFT. The boundary of the OFT was segmented at each cross-sectional structural image, then geometrical center of the OFT can be calculated. Thus, the gradients of blood flow in radial direction can be calculated. This velocity gradient near the wall is termed wall shear rate and the WSS value is proportional to the wall shear rate. Based on this method, the WSS at different heart beating phase are compare. The result demonstrates that OCT is capable of early stage chicken embryonic heart WSS study.

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.

Production of functional proteins: balance of shear stress and gravity

NASA Technical Reports Server (NTRS)

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

Production of functional proteins: balance of shear stress and gravity

NASA Technical Reports Server (NTRS)

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

Production of functional proteins: balance of shear stress and gravity

NASA Technical Reports Server (NTRS)

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

Wall shear stress distributions on stented patent ductus arteriosus

NASA Astrophysics Data System (ADS)

Kori, Mohamad Ikhwan; Jamalruhanordin, Fara Lyana; Taib, Ishkrizat; Mohammed, Akmal Nizam; Abdullah, Mohammad Kamil; Ariffin, Ahmad Mubarak Tajul; Osman, Kahar

2017-04-01

A formation of thrombosis due to hemodynamic conditions after the implantation of stent in patent ductus arteriosus (PDA) will derived the development of re-stenosis. The phenomenon of thrombosis formation is significantly related to the distribution of wall shear stress (WSS) on the arterial wall. Thus, the aims of this study is to investigate the distribution of WSS on the arterial wall after the insertion of stent. Three dimensional model of patent ductus arteriosus inserted with different types of commercial stent are modelled. Computational modelling is used to calculate the distributions of WSS on the arterial stented PDA. The hemodynamic parameters such as high WSS and WSSlow are considered in this study. The result shows that the stented PDA with Type III stent has better hemodynamic performance as compared to others stent. This model has the lowest distributions of WSSlow and also the WSS value more than 20 dyne/cm2. From the observed, the stented PDA with stent Type II showed the highest distributions area of WSS more than 20 dyne/cm2. This situation revealed that the high possibility of atherosclerosis to be developed. However, the highest distribution of WSSlow for stented PDA with stent Type II indicated that high possibility of thrombosis to be formed. In conclusion, the stented PDA model calculated with the lowest distributions of WSSlow and WSS value more than 20dyne/cm2 are considered to be performed well in stent hemodynamic performance as compared to other stents.

Wall shear stress effects of different endodontic irrigation techniques and systems.

PubMed

Goode, Narisa; Khan, Sara; Eid, Ashraf A; Niu, Li-na; Gosier, Johnny; Susin, Lisiane F; Pashley, David H; Tay, Franklin R

2013-07-01

This study examined débridement efficacy as a result of wall shear stresses created by different irrigant delivery/agitation techniques in an inaccessible recess of a curved root canal model. A reusable, curved canal cavity containing a simulated canal fin was milled into mirrored titanium blocks. Calcium hydroxide (Ca(OH)2) paste was used as debris and loaded into the canal fin. The titanium blocks were bolted together to provide a fluid-tight seal. Sodium hypochlorite was delivered at a previously-determined flow rate of 1 mL/min that produced either negligible or no irrigant extrusion pressure into the periapex for all the techniques examined. Nine irrigation delivery/agitation techniques were examined: NaviTip passive irrigation control, Max-i-Probe(®) side-vented needle passive irrigation, manual dynamic agitation (MDA) using non-fitting and well-fitting gutta-percha points, EndoActivator™ sonic agitation with medium and large points, VPro™ EndoSafe™ irrigation system, VPro™ StreamClean™ continuous ultrasonic irrigation and EndoVac apical negative pressure irrigation. Débridement efficacies were analysed with Kruskal-Wallis ANOVA and Dunn's multiple comparisons tests (α=0.05). EndoVac was the only technique that removed more than 99% calcium hydroxide debris from the canal fin at the predefined flow rate. This group was significantly different (p<0.05) from the other groups that exhibited incomplete Ca(OH)2 removal. The ability of the EndoVac system to significantly clean more debris from a mechanically inaccessible recess of the model curved root canal may be caused by robust bubble formation during irrigant delivery, creating higher wall shear stresses by a two-phase air-liquid flow phenomenon that is well known in other industrial débridement systems. Copyright © 2013 Elsevier Ltd. All rights reserved.

Flow Instability and Wall Shear Stress Ocillation in Intracranial Aneurysms

NASA Astrophysics Data System (ADS)

Baek, Hyoungsu; Jayamaran, Mahesh; Richardson, Peter; Karniadakis, George

2009-11-01

We investigate the flow dynamics and oscillatory behavior of wall shear stress (WSS) vectors in intracranial aneurysms using high-order spectral/hp simulations. We analyze four patient- specific internal carotid arteries laden with aneurysms of different characteristics : a wide-necked saccular aneurysm, a hemisphere-shaped aneurysm, a narrower-necked saccular aneurysm, and a case with two adjacent saccular aneurysms. Simulations show that the pulsatile flow in aneurysms may be subject to a hydrodynamic instability during the decelerating systolic phase resulting in a high-frequency oscillation in the range of 30-50 Hz. When the aneurysmal flow becomes unstable, both the magnitude and the directions of WSS vectors fluctuate. In particular, the WSS vectors around the flow impingement region exhibit significant spatial and temporal changes in direction as well as in magnitude.

Shear stress along the conduit wall as a plausible source of tilt at Soufrière Hills volcano, Montserrat

NASA Astrophysics Data System (ADS)

Green, D. N.; Neuberg, J.; Cayol, V.

2006-05-01

Surface deformations recorded in close proximity to the active lava dome at Soufrière Hills volcano, Montserrat, can be used to infer stresses within the uppermost 1000 m of the conduit system. Most deformation source models consider only isotropic pressurisation of the conduit. We show that tilt recorded during rapid magma extrusion in 1997 could have also been generated by shear stresses sustained along the conduit wall; these stresses are a consequence of pressure gradients that develop along the conduit. Numerical modelling, incorporating realistic topography, can reproduce both the morphology and half the amplitude of the measured deformation field using a realistic shear stress amplitude, equivalent to a pressure gradient of 3.5 × 104 Pa m-1 along a 1000 m long conduit with a 15 m radius. This shear stress model has advantages over the isotropic pressure models because it does not require either physically unattainable overpressures or source radii larger than 200 m to explain the same deformation.

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.

Bicuspid aortic valves are associated with increased wall and turbulence shear stress levels compared to trileaflet aortic valves.

PubMed

Saikrishnan, Neelakantan; Mirabella, Lucia; Yoganathan, Ajit P

2015-06-01

Congenital bicuspid aortic valves (BAVs) are associated with accelerated disease progression, such as leaflet calcification and ascending aorta dilatation. Although common underlying genetic factors have been implicated in accelerated disease in BAV patients, several studies have suggested that altered hemodynamics also play a role in this disease process. The present study compares turbulence and wall shear stress (WSS) measurements between various BAV and trileaflet aortic valve (TAV) models to provide information for mechanobiological models of BAV disease. BAV and TAV models were constructed from excised porcine aortic valves to simulate parametric variations in BAV stenosis, hemodynamics and geometry. Particle image velocimetry experiments were conducted at physiological pressure conditions to characterize velocity fields in the ascending aorta. The velocity fields were post-processed to calculate turbulence, viscous and wall shear stresses in the ascending aorta. Stenosed BAV models showed the presence of eccentric systolic jets, causing increased WSS. Lower cardiac output resulted in a narrower jet, lower turbulence and lower viscous shear stress (VSS). The specific severe stenosis BAV model studied here showed reduced WSS due to reduction in non-fused leaflet mobility. Dilation of the aorta did not affect any turbulence or VSS, but reduced the WSS. In comparison with BAVs, TAVs have similar VSS values, but much smaller WSS and turbulence levels. These increased turbulence  and WSS levels in BAVs may play a key role in amplifying the biological responses of the ascending aorta wall and valvular leaflets, and support the hemodynamic underpinnings of BAV disease processes.

Wall shear stress estimation in the aorta: Impact of wall motion, spatiotemporal resolution, and phase noise.

PubMed

Zimmermann, Judith; Demedts, Daniel; Mirzaee, Hanieh; Ewert, Peter; Stern, Heiko; Meierhofer, Christian; Menze, Bjoern; Hennemuth, Anja

2018-04-01

Wall shear stress (WSS) presents an important parameter for assessing blood flow characteristics and evaluating flow-mediated lesions in the aorta. To investigate the robustness of WSS and oscillatory shear index (OSI) estimation based on 4D flow MRI against vessel wall motion, spatiotemporal resolution, and velocity encoding (VENC). Simulated and prospective. Synthetic 4D flow MRI data of the aorta, simulated using the Lattice-Boltzmann method; in vivo 4D flow MRI data of the aorta from healthy volunteers (n = 11) and patients with congenital heart defects (n = 17). 1.5T; 4D flow MRI with PEAK-GRAPPA acceleration and prospective electrocardiogram triggering. Predicated upon 3D cubic B-splines interpolation of the image velocity field, WSS was estimated in mid-systole, early-diastole, and late-diastole and OSI was derived. We assessed the impact of spatiotemporal resolution and phase noise, and compared results based on tracked-using deformable registration-and static vessel wall location. Bland-Altman analysis to assess WSS/OSI differences; Hausdorff distance (HD) to assess wall motion; and Pearson's correlation coefficient (PCC) to assess correlation of HD with WSS. Synthetic data results show systematic over-/underestimation of WSS when different spatial resolution (mean ± 1.96 SD up to -0.24 ± 0.40 N/m 2 and 0.5 ± 1.38 N/m 2 for 8-fold and 27-fold voxel size, respectively) and VENC-depending phase noise (mean ± 1.96 SD up to 0.31 ± 0.12 N/m 2 and 0.94 ± 0.28 N/m 2 for 2-fold and 4-fold VENC increase, respectively) are given. Neglecting wall motion when defining the vessel wall perturbs WSS estimates to a considerable extent (1.96 SD up to 1.21 N/m 2 ) without systematic over-/underestimation (Bland-Altman mean range -0.06 to 0.05). In addition to sufficient spatial resolution and velocity to noise ratio, accurate tracking of the vessel wall is essential for reliable image-based WSS estimation and should not be

Log-layer mismatch and modeling of the fluctuating wall stress in wall-modeled large-eddy simulations

NASA Astrophysics Data System (ADS)

Yang, Xiang I. A.; Park, George Ilhwan; Moin, Parviz

2017-10-01

Log-layer mismatch refers to a chronic problem found in wall-modeled large-eddy simulation (WMLES) or detached-eddy simulation, where the modeled wall-shear stress deviates from the true one by approximately 15 % . Many efforts have been made to resolve this mismatch. The often-used fixes, which are generally ad hoc, include modifying subgrid-scale stress models, adding a stochastic forcing, and moving the LES-wall-model matching location away from the wall. An analysis motivated by the integral wall-model formalism suggests that log-layer mismatch is resolved by the built-in physics-based temporal filtering. In this work we investigate in detail the effects of local filtering on log-layer mismatch. We show that both local temporal filtering and local wall-parallel filtering resolve log-layer mismatch without moving the LES-wall-model matching location away from the wall. Additionally, we look into the momentum balance in the near-wall region to provide an alternative explanation of how LLM occurs, which does not necessarily rely on the numerical-error argument. While filtering resolves log-layer mismatch, the quality of the wall-shear stress fluctuations predicted by WMLES does not improve with our remedy. The wall-shear stress fluctuations are highly underpredicted due to the implied use of LES filtering. However, good agreement can be found when the WMLES data are compared to the direct numerical simulation data filtered at the corresponding WMLES resolutions.

Development of a MEMS dual-axis differential capacitance floating element shear stress sensor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barnard, Casey; Griffin, Benjamin

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 rangingmore » 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.« less

Wall shear stress as measured in vivo: consequences for the design of the arterial system.

PubMed

Reneman, Robert S; Hoeks, Arnold P G

2008-05-01

Based upon theory, wall shear stress (WSS), an important determinant of endothelial function and gene expression, has been assumed to be constant along the arterial tree and the same in a particular artery across species. In vivo measurements of WSS, however, have shown that these assumptions are far from valid. In this survey we will discuss the assessment of WSS in the arterial system in vivo and present the results obtained in large arteries and arterioles. In vivo WSS can be estimated from wall shear rate, as derived from non-invasively recorded velocity profiles, and whole blood viscosity in large arteries and plasma viscosity in arterioles, avoiding theoretical assumptions. In large arteries velocity profiles can be recorded by means of a specially designed ultrasound system and in arterioles via optical techniques using fluorescent flow velocity tracers. It is shown that in humans mean WSS is substantially higher in the carotid artery (1.1-1.3 Pa) than in the brachial (0.4-0.5 Pa) and femoral (0.3-0.5 Pa) arteries. Also in animals mean WSS varies substantially along the arterial tree. Mean WSS in arterioles varies between about 1.0 and 5.0 Pa in the various studies and is dependent on the site of measurement in these vessels. Across species mean WSS in a particular artery decreases linearly with body mass, e.g., in the infra-renal aorta from 8.8 Pa in mice to 0.5 Pa in humans. The observation that mean WSS is far from constant along the arterial tree implies that Murray's cube law on flow-diameter relations cannot be applied to the whole arterial system. Because blood flow velocity is not constant along the arterial tree either, a square law also does not hold. The exponent in the power law likely varies along the arterial system, probably from 2 in large arteries near the heart to 3 in arterioles. The in vivo findings also imply that in in vitro studies no average shear stress value can be taken to study effects on endothelial cells derived from different

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.

The effect of shear wall location in resisting earthquake

NASA Astrophysics Data System (ADS)

Tarigan, J.; Manggala, J.; Sitorus, T.

2018-02-01

Shear wall is one of lateral resisting structure which is used commonly. Shear wall gives high stiffness to the structure so as the structure will be stable. Applying shear wall can effectively reduce the displacement and story-drift of the structure. This will reduce the destruction comes from lateral loads such as an earthquake. Earlier studies showed that shear wall gives different performance based on its position in structures. In this paper, seismic analysis has been performed using response spectrum method for different Model of structures; they are the open frame, the shear wall at core symmetrically, the shear wall at periphery symmetrically, and the shear wall at periphery asymmetrically. The results are observed by comparing the displacement and story-drift. Based on the analysis, the placement of shear wall at the core of structure symmetrically gives the best performance to reduce the displacement and story-drift. It can reduce the displacement up to 61.16% (X-dir) and 70.60% (Y-dir). The placement of shear wall at periphery symmetrically will reduce the displacement up to 53.85% (X-dir) and 47.87% (Y-dir) while the placement of shear wall at periphery asymmetrically reducing the displacement up to 59.42% (X-dir) and 66.99% (Y-dir).

Impact of stent mis-sizing and mis-positioning on coronary fluid wall shear and intramural stress

PubMed Central

Chen, Henry Y.; Koo, Bon-Kwon; Bhatt, Deepak L.

2013-01-01

Stent deployments with geographical miss (GM) are associated with increased risk of target-vessel revascularization and periprocedural myocardial infarction. The aim of the current study was to investigate the underlying biomechanical mechanisms for adverse events with GM. The hypothesis is that stent deployment with GM [longitudinal GM, or LGM (i.e., stent not centered on the lesion); or radial GM, RGM (i.e., stent oversizing)] results in unfavorable fluid wall shear stress (WSS), WSS gradient (WSSG), oscillatory shear index (OSI), and intramural circumferential wall stress (CWS). Three-dimensional computational models of stents and plaque were created using a computer-assisted design package. The models were then solved with validated finite element and computational fluid dynamic packages. The dynamic process of large deformation stent deployment was modeled to expand the stent to the desired vessel size. Stent deployed with GM resulted in a 45% increase in vessel CWS compared with stents that were centered and fully covered the lesion. A 20% oversized stent resulted in 72% higher CWS than a correct sized stent. The linkages between the struts had much higher stress than the main struts (i.e., 180 MPa vs. 80 MPa). Additionally, LGM and RGM reduced endothelial WSS and increased WSSG and OSI. The simulations suggest that both LGM and RGM adversely reduce WSS but increase WSSG, OSI, and CWS. These findings highlight the potential mechanical mechanism of the higher adverse events and underscore the importance of stent positioning and sizing for improved clinical outcome. PMID:23722708

Pulsatility Index as a Diagnostic Parameter of Reciprocating Wall Shear Stress Parameters in Physiological Pulsating Waveforms

PubMed Central

Avrahami, Idit; Kersh, Dikla

2016-01-01

Arterial wall shear stress (WSS) parameters are widely used for prediction of the initiation and development of atherosclerosis and arterial pathologies. Traditional clinical evaluation of arterial condition relies on correlations of WSS parameters with average flow rate (Q) and heart rate (HR) measurements. We show that for pulsating flow waveforms in a straight tube with flow reversals that lead to significant reciprocating WSS, the measurements of HR and Q are not sufficient for prediction of WSS parameters. Therefore, we suggest adding a third quantity—known as the pulsatility index (PI)—which is defined as the peak-to-peak flow rate amplitude normalized by Q. We examine several pulsating flow waveforms with and without flow reversals using a simulation of a Womersley model in a straight rigid tube and validate the simulations through experimental study using particle image velocimetry (PIV). The results indicate that clinically relevant WSS parameters such as the percentage of negative WSS (P[%]), oscillating shear index (OSI) and the ratio of minimum to maximum shear stress rates (min/max), are better predicted when the PI is used in conjunction with HR and Q. Therefore, we propose to use PI as an additional and essential diagnostic quantity for improved predictability of the reciprocating WSS. PMID:27893801

Electrochemical wall shear rate microscopy of collapsing bubbles

NASA Astrophysics Data System (ADS)

Reuter, Fabian; Mettin, Robert

2018-06-01

An electrochemical high-speed wall shear raster microscope is presented. It involves chronoamperometric measurements on a microelectrode that is flush-mounted in a submerged test specimen. Wall shear rates are derived from the measured microelectrode signal by numerically solving a convection-diffusion equation with an optimization approach. This way, the unsteady wall shear rates from the collapse of a laser pulse seeded cavitation bubble close to a substrate are measured. By planar scanning, they are resolved in high spatial resolution. The wall shear rates are related to the bubble dynamics via synchronized high-speed imaging of the bubble shape.

Debonding Stress Concentrations in a Pressurized Lobed Sandwich-Walled Generic Cryogenic Tank

NASA Technical Reports Server (NTRS)

Ko, William L.

2004-01-01

A finite-element stress analysis has been conducted on a lobed composite sandwich tank subjected to internal pressure and cryogenic cooling. The lobed geometry consists of two obtuse circular walls joined together with a common flat wall. Under internal pressure and cryogenic cooling, this type of lobed tank wall will experience open-mode (a process in which the honeycomb is stretched in the depth direction) and shear stress concentrations at the junctures where curved wall changes into flat wall (known as a curve-flat juncture). Open-mode and shear stress concentrations occur in the honeycomb core at the curve-flat junctures and could cause debonding failure. The levels of contributions from internal pressure and temperature loading to the open-mode and shear debonding failure are compared. The lobed fuel tank with honeycomb sandwich walls has been found to be a structurally unsound geometry because of very low debonding failure strengths. The debonding failure problem could be eliminated if the honeycomb core at the curve-flat juncture is replaced with a solid core.

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

Shear stress and flow dynamics of the femoral vein among obese patients who qualify for bariatric surgery.

PubMed

Wiewiora, Maciej; Piecuch, Jerzy; Glűck, Marek; Slowinska-Lozynska, Ludmila; Sosada, Krystyn

2013-01-01

The aim of this study was to evaluate the effects of obesity on wall shear stress and its relationship to erythrocyte aggregation. We studied 35 morbidly obese patients who were qualified for bariatric surgery. The control group consisted of 20 non-obese people. Blood rheological measurements were performed using the Laser-assisted Optical Rotational Cell Analyzer (Mechatronics, the Netherlands) and a cone-plate viscometer (Brookfield DV-II). The venous flow dynamics were assessed using a duplex ultrasound. The shear rate was estimated from the measured blood flow velocity and the diameter of the femoral vein. Venous wall shear stress was calculated from the whole blood viscosity and the shear rate. The shear rate (P < 0.005) and the venous wall shear stress (P < 0.05) were significantly lower in obese patients compared with the controls. The aggregation index (P < 0.001), syllectogram amplitude - AMP (P < 0.05) and Tslow (P < 0.001) were significantly higher in the obese patients; the aggregation half-time (P < 0.001) and Tfast (P < 0.001) were decreased compared with the control group. Multivariate regression analyses found waist circumference (β -0.31, P < 0.05), thigh circumference (β 0.33, P < 0.05) and Tslow (β -0.47, P < 0.005) to be variables that independently influenced the shear rate. Nevertheless, the AMP (β 0.34, P < 0.05) and Tslow (β -0.47, P < 0.01) were independent predictors that influenced the wall shear stress. This study indicates that there is a relationship between wall shear stress in the femoral vein and the rheological impairment of the RBC among obese patients, but further studies are necessary to confirm this suggestion.

Inverse Interscale Transport of the Reynolds Shear Stress in Plane Couette Turbulence

NASA Astrophysics Data System (ADS)

Kawata, Takuya; Alfredsson, P. Henrik

2018-06-01

Interscale interaction between small-scale structures near the wall and large-scale structures away from the wall plays an increasingly important role with increasing Reynolds number in wall-bounded turbulence. While the top-down influence from the large- to small-scale structures is well known, it has been unclear whether the small scales near the wall also affect the large scales away from the wall. In this Letter we show that the small-scale near-wall structures indeed play a role to maintain the large-scale structures away from the wall, by showing that the Reynolds shear stress is transferred from small to large scales throughout the channel. This is in contrast to the turbulent kinetic energy transport which is from large to small scales. Such an "inverse" interscale transport of the Reynolds shear stress eventually supports the turbulent energy production at large scales.

Shear Tests and Calculation of Shear Resistance with the PC Program RFEM from Thin Partition Walls of Brick in Old Buildings

NASA Astrophysics Data System (ADS)

Korjenic, Sinan; Nowak, Bernhard; Löffler, Philipp; Vašková, Anna

2015-11-01

This paper is about the shear capacity of partition walls in old buildings based on shear tests which were carried out under real conditions in an existing building. There were experiments conducted on different floors and in each case, the maximum recordable horizontal force and the horizontal displacement of the respective mortar were measured. At the same time material studies and material investigations were carried out in the laboratory. The material parameters were used for the calculation of the precise shear capacity of each joint. In the shear tests, the maximum displacement of a mortar joint was determined at a maximum of two to four millimetres. Furthermore, no direct linear relationship between the theoretical load (wall above it) and the shear stress occurred could be detected in the analysis of the experiment, as it was previously assumed.

Shear flow of dense granular materials near smooth walls. I. Shear localization and constitutive laws in the boundary region.

PubMed

Shojaaee, Zahra; Roux, Jean-Noël; Chevoir, François; Wolf, Dietrich E

2012-07-01

We report on a numerical study of the shear flow of a simple two-dimensional model of a granular material under controlled normal stress between two parallel smooth frictional walls moving with opposite velocities ± V. Discrete simulations, which are carried out with the contact dynamics method in dense assemblies of disks, reveal that, unlike rough walls made of strands of particles, smooth ones can lead to shear strain localization in the boundary layer. Specifically, we observe, for decreasing V, first a fluidlike regime (A), in which the whole granular layer is sheared, with a homogeneous strain rate except near the walls, then (B) a symmetric velocity profile with a solid block in the middle and strain localized near the walls, and finally (C) a state with broken symmetry in which the shear rate is confined to one boundary layer, while the bulk of the material moves together with the opposite wall. Both transitions are independent of system size and occur for specific values of V. Transient times are discussed. We show that the first transition, between regimes A and B, can be deduced from constitutive laws identified for the bulk material and the boundary layer, while the second one could be associated with an instability in the behavior of the boundary layer. The boundary zone constitutive law, however, is observed to depend on the state of the bulk material nearby.

Flow and wall shear stress characterization after endovascular aneurysm repair and endovascular aneurysm sealing in an infrarenal aneurysm model.

PubMed

Boersen, Johannes T; Groot Jebbink, Erik; Versluis, Michel; Slump, Cornelis H; Ku, David N; de Vries, Jean-Paul P M; Reijnen, Michel M P J

2017-12-01

Endovascular aneurysm repair (EVAR) with a modular endograft has become the preferred treatment for abdominal aortic aneurysms. A novel concept is endovascular aneurysm sealing (EVAS), consisting of dual endoframes surrounded by polymer-filled endobags. This dual-lumen configuration is different from a bifurcation with a tapered trajectory of the flow lumen into the two limbs and may induce unfavorable flow conditions. These include low and oscillatory wall shear stress (WSS), linked to atherosclerosis, and high shear rates that may result in thrombosis. An in vitro study was performed to assess the impact of EVAR and EVAS on flow patterns and WSS. Four abdominal aortic aneurysm phantoms were constructed, including three stented models, to study the influence of the flow divider on flow (Endurant [Medtronic, Minneapolis, Minn], AFX [Endologix, Irvine, Calif], and Nellix [Endologix]). Experimental models were tested under physiologic resting conditions, and flow was visualized with laser particle imaging velocimetry, quantified by shear rate, WSS, and oscillatory shear index (OSI) in the suprarenal aorta, renal artery (RA), and common iliac artery. WSS and OSI were comparable for all models in the suprarenal aorta. The RA flow profile in the EVAR models was comparable to the control, but a region of lower WSS was observed on the caudal wall compared with the control. The EVAS model showed a stronger jet flow with a higher shear rate in some regions compared with the other models. Small regions of low WSS and high OSI were found near the distal end of all stents in the common iliac artery compared with the control. Maximum shear rates in each region of interest were well below the pathologic threshold for acute thrombosis. The different stent designs do not influence suprarenal flow. Lower WSS is observed in the caudal wall of the RA after EVAR and a higher shear rate after EVAS. All stented models have a small region of low WSS and high OSI near the distal outflow

Vascular wall shear stress in zebrafish model of early atherosclerosis

NASA Astrophysics Data System (ADS)

Choi, Woorak; Seo, Eunseok; Yeom, Eunseop; Lee, Sang Joon

2016-11-01

Although atherosclerosis is a multifactorial disease, the role of hemodynamic force has strong influence on the outbreak of the disease. Low and oscillating wall shear stress (WSS) is associated with the incidence of atherosclerosis. Many researchers have investigated relationships between WSS and the occurrence of atherosclerosis using in vitro and in vivo models. However, these models possess technological limitations in mimicking real biophysiological conditions and monitoring the temporal progression of atherosclerosis. In this study, a hypercholesterolaemic zebrafish model was established as a novel model to resolve these technical limitations. WSS in blood vessels of 15 days post-fertilisation zebrafish was measured using a micro PIV technique, and the spatial distribution of lipids inside blood vessels was quantitatively visualized using a confocal microscopy. As a result, lipids are mainly deposited in the regions of low WSS. The oscillating WSS is not induced by blood flows in the zebrafish disease model. The present hypercholesterolaemic zebrafish model would be useful for understanding the effect of WSS on the early stage of atherosclerosis. This work was supported by the National Research Foundation of Korea (NRF) under a Grant funded by the Korean government (MSIP) (No. 2008-0061991).

Parametric Study of Wall Shear Stress in Idealized Avian Airways

NASA Astrophysics Data System (ADS)

Farnsworth, Michael S.; Riede, Tobias; Thomson, Scott L.

2017-11-01

Because wall shear stress (WSS) affects cell response, WSS patterns in avian respiratory airways may be related to the origin of the syrinx and corresponding voice-producing tissue structures (e.g., membranes or vocal folds) in birds. To explore possible linkages between WSS patterns and the locations of avian voice-producing structures, a computational model of flow through an idealized portion of the avian respiratory airway, including trachea and primary bronchi sections, has been developed. The flow is governed by the Navier-Stokes equations, with velocity boundary conditions derived from pressure-flow data in an adult zebra finch during quiet respiration. Geometric parameters such as tracheal/bronchial diameter and length, as well as bronchial branching angle, are parametrically varied based on data for different avian species. Simulation results predict elevated WSS in the vicinity of the tracheobronchial juncture, the location at which voice-producing tissues are found in avian species. In this presentation, the model will be described and spatial distributions of WSS during inspiration and expiration will be presented and compared for different geometric configurations and respiration rates and waveforms. Funding for this project from the Gordon and Betty Moore Foundation (Grant 4498) is gratefully acknowledged.

An Experimental Investigation of the Risk of Triggering Geological Disasters by Injection under Shear Stress

PubMed Central

Liu, Yixin; Xu, Jiang; Peng, Shoujian

2016-01-01

Fluid injection has been applied in many fields, such as hazardous waste deep well injection, forced circulation in geothermal fields, hydraulic fracturing, and CO2 geological storage. However, current research mainly focuses on geological data statistics and the dominating effects of pore pressure. There are only a few laboratory-conditioned studies on the role of drilling boreholes and the effect of injection pressure on the borehole wall. Through experimental phenomenology, this study examines the risk of triggering geological disasters by fluid injection under shear stress. We developed a new direct shear test apparatus, coupled Hydro-Mechanical (HM), to investigate mechanical property variations when an intact rock experienced step drilling borehole, fluid injection, and fluid pressure acting on the borehole and fracture wall. We tested the peak shear stress of sandstone under different experimental conditions, which showed that drilling borehole, water injection, and increased pore pressure led to the decrease in peak shear stress. Furthermore, as pore pressure increased, peak shear stress dispersion increased due to crack propagation irregularity. Because the peak shear stress changed during the fluid injection steps, we suggest that the risk of triggering geological disaster with injection under shear stress, pore, borehole, and fluid pressure should be considered. PMID:27929142

An Experimental Investigation of the Risk of Triggering Geological Disasters by Injection under Shear Stress.

PubMed

Liu, Yixin; Xu, Jiang; Peng, Shoujian

2016-12-08

Fluid injection has been applied in many fields, such as hazardous waste deep well injection, forced circulation in geothermal fields, hydraulic fracturing, and CO 2 geological storage. However, current research mainly focuses on geological data statistics and the dominating effects of pore pressure. There are only a few laboratory-conditioned studies on the role of drilling boreholes and the effect of injection pressure on the borehole wall. Through experimental phenomenology, this study examines the risk of triggering geological disasters by fluid injection under shear stress. We developed a new direct shear test apparatus, coupled Hydro-Mechanical (HM), to investigate mechanical property variations when an intact rock experienced step drilling borehole, fluid injection, and fluid pressure acting on the borehole and fracture wall. We tested the peak shear stress of sandstone under different experimental conditions, which showed that drilling borehole, water injection, and increased pore pressure led to the decrease in peak shear stress. Furthermore, as pore pressure increased, peak shear stress dispersion increased due to crack propagation irregularity. Because the peak shear stress changed during the fluid injection steps, we suggest that the risk of triggering geological disaster with injection under shear stress, pore, borehole, and fluid pressure should be considered.

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. Copyright © 2015 Elsevier Ltd. All rights reserved.

Manufacture of high aspect ratio micro-pillar wall shear stress sensor arrays

NASA Astrophysics Data System (ADS)

Gnanamanickam, Ebenezer P.; Sullivan, John P.

2012-12-01

In the field of experimental fluid mechanics the measurement of unsteady, distributed wall shear stress has proved historically challenging. Recently, sensors based on an array of flexible micro-pillars have shown promise in carrying out such measurements. Similar sensors find use in other applications such as cellular mechanics. This work presents a manufacturing technique that can manufacture micro-pillar arrays of high aspect ratio. An electric discharge machine (EDM) is used to manufacture a micro-drilling tool. This micro-drilling tool is used to form holes in a wax sheet which acts as the mold for the micro-pillar array. Silicone rubber is cast in these molds to yield a micro-pillar array. Using this technique, micro-pillar arrays with a maximum aspect ratio of about 10 have been manufactured. Manufacturing issues encountered, steps to alleviate them and the potential of the process to manufacture similar micro-pillar arrays in a time-efficient manner are also discussed.

An experimental investigation for external RC shear wall applications

NASA Astrophysics Data System (ADS)

Kaltakci, M. Y.; Ozturk, M.; Arslan, M. H.

2010-09-01

The strength and rigidity of most reinforced concrete (RC) buildings in Turkey, which are frequently hit by destructive earthquakes, is not at a sufficient level. Therefore, the result of earthquakes is a significant loss of life and property. The strengthening method most commonly preferred for these type of RC buildings is the application of RC infilled walls (shear walls) in the frame openings of the building. However, since the whole building has to be emptied and additional heavy costs arise during this type of strengthening, users prefer not to strengthen their buildings despite the heavy risk they are exposed to. Therefore, it is necessary to develop easier-to-apply and more effective methods for the rapid strengthening of housing and the heavily-used public buildings which cannot be emptied during the strengthening process (such as hospitals and schools). This study empirically analyses the different methods of a new system which can meet this need. In this new system, named "external shear wall application", RC shear walls are applied on the external surface of the building, along the frame plane rather than in the building. To this end, 7 test samples in 1/2 and 1/3 geometrical scale were designed to analyse the efficiency of the strengthening technique where the shear wall leans on the frame from outside of the building (external shear wall application) and of the strengthening technique where a specific space is left between the frame and the external shear wall by using a coupling beam to connect elements (application of external shear wall with coupling beam). Test results showed that the maximum lateral load capacity, initial rigidity and energy dissipation behaviours of the samples strengthened with external shear wall were much better than those of the bare frames.

Significance of Shear Wall in Multi-Storey Structure With Seismic Analysis

NASA Astrophysics Data System (ADS)

Bongilwar, Rajat; Harne, V. R.; Chopade, Aditya

2018-03-01

In past decades, shear walls are one of the most appropriate and important structural component in multi-storied building. Therefore, it would be very interesting to study the structural response and their systems in multi-storied structure. Shear walls contribute the stiffness and strength during earthquakes which are often neglected during design of structure and construction. This study shows the effect of shear walls which significantly affect the vulnerability of structures. In order to test this hypothesis, G+8 storey building was considered with and without shear walls and analyzed for various parameters like base shear, storey drift ratio, lateral displacement, bending moment and shear force. Significance of shear wall has been studied with the help of two models. First model is without shear wall i.e. bare frame and other another model is with shear wall considering opening also in it. For modeling and analysis of both the models, FEM based software ETABS 2016 were used. The analysis of all models was done using Equivalent static method. The comparison of results has been done based on same parameters like base shear, storey drift ratio, lateral displacement, bending moment and shear force.

Polymer stress tensor in turbulent shear flows.

PubMed

L'vov, Victor S; Pomyalov, Anna; Procaccia, Itamar; Tiberkevich, Vasil

2005-01-01

The interaction of polymers with turbulent shear flows is examined. We focus on the structure of the elastic stress tensor, which is proportional to the polymer conformation tensor. We examine this object in turbulent flows of increasing complexity. First is isotropic turbulence, then anisotropic (but homogenous) shear turbulence, and finally wall bounded turbulence. The main result of this paper is that for all these flows the polymer stress tensor attains a universal structure in the limit of large Deborah number De > 1. We present analytic results for the suppression of the coil-stretch transition at large Deborah numbers. Above the transition the turbulent velocity fluctuations are strongly correlated with the polymer's elongation: there appear high-quality "hydroelastic" waves in which turbulent kinetic energy turns into polymer potential energy and vice versa. These waves determine the trace of the elastic stress tensor but practically do not modify its universal structure. We demonstrate that the influence of the polymers on the balance of energy and momentum can be accurately described by an effective polymer viscosity that is proportional to the cross-stream component of the elastic stress tensor. This component is smaller than the streamwise component by a factor proportional to De2. Finally we tie our results to wall bounded turbulence and clarify some puzzling facts observed in the problem of drag reduction by polymers.

Development of a MEMS shear stress sensor for use in wind tunnel applications

NASA Astrophysics Data System (ADS)

Barnard, Casey; Meloy, Jessica; Sheplak, Mark; Interdisciplinary Microsystems Group Team

2013-11-01

The measurement of mean and fluctuating wall shear-stress in laminar, transitional, and turbulent boundary layers and channel flows has applications both in industry and the scientific community. Currently there is no method for time resolved, direct measurement of wall shear stress at the spatial and temporal scales of turbulent flow structures inside model testing facilities. To address this need, a silicon micromachined differential capacitance shear stress sensor system has been developed. Mean measurements are enabled by custom synchronous modulation/demodulation circuitry, which allows for measurement of both magnitude and phase of incident wall shear stress. Sizes of the largest device features are on the order of relevant viscous length scales, to minimize flow disturbance and provide a hydraulically smooth sensing surface. Static calibration is performed in a flow cell setup, and an acoustic plane wave tube is used for dynamic response data. Normalized sensitivity of 1.34 mV/V/Pa has been observed over a bandwidth of 4.8 kHz, with a minimum detectable signal of 6.5 mPa. Initial results show qualitative agreement with contemporary measurement techniques. The design, fabrication, support electronics, characterization, and preliminary experimental performance of this sensor will be presented. The support of NASA SFW-NRA NNX11AI30A, AFOSR grant #FA 9550-12-1-0469, and Sandia Campus Executive Fellowship are gratefully acknowledged.

A wall shear stress sensor using a pair of sidewall doped cantilevers

NASA Astrophysics Data System (ADS)

Nguyen, Thanh-Vinh; Kazama, Ryohei; Takahashi, Hidetoshi; Takahata, Tomoyuki; Matsumoto, Kiyoshi; Shimoyama, Isao

2017-07-01

In this paper, we report on a micro-electro mechanical system (MEMS)-based piezoresistive sensor for measuring shear stress induced by an airflow. The advantages of the proposed sensor include a simple sensing method and a high resonance frequency due to the small size of the sensing elements. Our sensor consists of a pair of 3 µm thick cantilevers with piezoresistors formed on the sidewall of their hinges to detect lateral deformation in the cantilevers induced by an airflow. Each cantilever has a 200 µm  ×  400 µm plate supported by two 150 µm long, 4 µm wide beams. The piezoresistors on the two cantilevers are designed to deform in opposite manners when a shear stress is applied and in the same manner when a pressure is applied. Therefore, the applied shear stress can be detected from the difference in the responses of the two cantilevers without becoming conflated with pressure. In this paper, the design, fabrication and evaluation of the proposed sensor are reported and compared to numerical simulation results. From the experimental results, the resolution of the sensor and its first resonance frequency are 1.3 Pa and 3.9 kHz, respectively. Moreover, we show that the effect of temperature on the readout of the sensor can be eliminated using a temperature-compensating piezoresistor fabricated on the same sensor chip. Finally, using the fabricated sensor, the measurement of the shear stress induced by an airflow with velocity between  -10 and 10 m s-1 is demonstrated.

Anisotropic shear stress patterns predict the orientation of convergent tissue movements in the embryonic heart

PubMed Central

2017-01-01

Myocardial contractility and blood flow provide essential mechanical cues for the morphogenesis of the heart. In general, endothelial cells change their migratory behavior in response to shear stress patterns, according to flow directionality. Here, we assessed the impact of shear stress patterns and flow directionality on the behavior of endocardial cells, the specialized endothelial cells of the heart. At the early stages of zebrafish heart valve formation, we show that endocardial cells are converging to the valve-forming area and that this behavior depends upon mechanical forces. Quantitative live imaging and mathematical modeling allow us to correlate this tissue convergence with the underlying flow forces. We predict that tissue convergence is associated with the direction of the mean wall shear stress and of the gradient of harmonic phase-averaged shear stresses, which surprisingly do not match the overall direction of the flow. This contrasts with the usual role of flow directionality in vascular development and suggests that the full spatial and temporal complexity of the wall shear stress should be taken into account when studying endothelial cell responses to flow in vivo. PMID:29183943

Analysis and seismic tests of composite shear walls with CFST columns and steel plate deep beams

NASA Astrophysics Data System (ADS)

Dong, Hongying; Cao, Wanlin; Wu, Haipeng; Zhang, Jianwei; Xu, Fangfang

2013-12-01

A composite shear wall concept based on concrete filled steel tube (CFST) columns and steel plate (SP) deep beams is proposed and examined in this study. The new wall is composed of three different energy dissipation elements: CFST columns; SP deep beams; and reinforced concrete (RC) strips. The RC strips are intended to allow the core structural elements — the CFST columns and SP deep beams — to work as a single structure to consume energy. Six specimens of different configurations were tested under cyclic loading. The resulting data are analyzed herein. In addition, numerical simulations of the stress and damage processes for each specimen were carried out, and simulations were completed for a range of location and span-height ratio variations for the SP beams. The simulations show good agreement with the test results. The core structure exhibits a ductile yielding mechanism characteristic of strong column-weak beam structures, hysteretic curves are plump and the composite shear wall exhibits several seismic defense lines. The deformation of the shear wall specimens with encased CFST column and SP deep beam design appears to be closer to that of entire shear walls. Establishing optimal design parameters for the configuration of SP deep beams is pivotal to the best seismic behavior of the wall. The new composite shear wall is therefore suitable for use in the seismic design of building structures.

Spatial variations in shear stress in a 3-D bifurcation model at low Reynolds numbers.

PubMed

Rouhanizadeh, Mahsa; Lin, Tiantian C; Arcas, Diego; Hwang, Juliana; Hsiai, Tzung K

2005-10-01

Real-time wall shear stress is difficult to monitor precisely because it varies in space and time. Microelectromechanical systems sensor provides high spatial resolution to resolve variations in shear stress in a 3-D bifurcation model for small-scaled hemodynamics. At low Reynolds numbers from 1.34 to 6.7 skin friction coefficients (C(f)) varied circumferentially by a factor of two or more within the bifurcation. At a Reynolds number of 6.7, the C(f) value at the lateral wall of the bifurcation along the 270 degree plane was 7.1, corresponding to a shear stress value of 0.0061 dyn/cm(2). Along the 180 degree plane, C(f) was 13 or 0.0079 dyn/cm(2), and at the medial wall along the 90 degree plane, C(f) was 10.3 or 0.0091 dyn/cm(2). The experimental skin friction coefficients correlated with values derived from the Navier-Stokes solutions.

Relation between wall shear stress and carotid artery wall thickening MRI versus CFD.

PubMed

Cibis, Merih; Potters, Wouter V; Selwaness, Mariana; Gijsen, Frank J; Franco, Oscar H; Arias Lorza, Andres M; de Bruijne, Marleen; Hofman, Albert; van der Lugt, Aad; Nederveen, Aart J; Wentzel, Jolanda J

2016-03-21

Wall shear stress (WSS), a parameter associated with endothelial function, is calculated by computational fluid dynamics (CFD) or phase-contrast (PC) MRI measurements. Although CFD is common in WSS (WSSCFD) calculations, PC-MRI-based WSS (WSSMRI) is more favorable in population studies; since it is straightforward and less time consuming. However, it is not clear if WSSMRI and WSSCFD show similar associations with vascular pathology. Our aim was to test the associations between wall thickness (WT) of the carotid arteries and WSSMRI and WSSCFD. The subjects (n=14) with an asymptomatic carotid plaque who underwent MRI scans two times within 4 years of time were selected from the Rotterdam Study. We compared WSSCFD and WSSMRI at baseline and follow-up. Baseline WSSMRI and WSSCFD values were divided into 3 categories representing low, medium and high WSS tertiles. WT of each tertile was compared by a one-way ANOVA test. The WSSMRI and WSSCFD were 0.50±0.13Pa and 0.73±0.25Pa at baseline. Although WSSMRI was underestimated, a significant regression was found between WSSMRI and WSSCFD (r(2)=0.71). No significant difference was found between baseline and follow-up WSS by CFD and MRI-based calculations. The WT at baseline was 1.36±0.16mm and did not change over time. The WT was 1.55±0.21mm in low, 1.33±0.20mm in medium and 1.21±0.21mm in the high WSSMRI tertiles. Similarly, the WT was 1.49±0.21mm in low, 1.33±0.20mm in medium and 1.26±0.21mm in high WSSCFD tertiles. We found that WSSMRI and WSSCFD were inversely related with WT. WSSMRI and WSSCFD patterns were similar although MRI-based calculations underestimated WSS. Copyright © 2016 Elsevier Ltd. All rights reserved.

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).

Hydraulic properties of 3D rough-walled fractures during shearing: An experimental study

NASA Astrophysics Data System (ADS)

Yin, Qian; Ma, Guowei; Jing, Hongwen; Wang, Huidong; Su, Haijian; Wang, Yingchao; Liu, Richeng

2017-12-01

This study experimentally analyzed the influence of shear processes on nonlinear flow behavior through 3D rough-walled rock fractures. A high-precision apparatus was developed to perform stress-dependent fluid flow tests of fractured rocks. Then, water flow tests on rough-walled fractures with different mechanical displacements were conducted. At each shear level, the hydraulic pressure ranged from 0 to 0.6 MPa, and the normal load varied from 7 to 35 kN. The results show that (i) the relationship between the volumetric flow rate and hydraulic gradient of rough-walled fractures can be well fit using Forchheimer's law. Notably, both the linear and nonlinear coefficients in Forchheimer's law decrease during shearing; (ii) a sixth-order polynomial function is used to evaluate the transmissivity based on the Reynolds number of fractures during shearing. The transmissivity exhibits a decreasing trend as the Reynolds number increases and an increasing trend as the shear displacement increases; (iii) the critical hydraulic gradient, critical Reynolds number and equivalent hydraulic aperture of the rock fractures all increase as the shear displacement increases. When the shear displacement varies from 0 to 15 mm, the critical hydraulic gradient ranges from 0.3 to 2.2 for a normal load of 7 kN and increases to 1.8-8.6 for a normal load of 35 kN; and (iv) the Forchheimer law results are evaluated by plotting the normalized transmissivity of the fractures during shearing against the Reynolds number. An increase in the normal load shifts the fitted curves downward. Additionally, the Forchheimer coefficient β decreases with the shear displacement but increases with the applied normal load.

Shear-layer structures in near-wall turbulence

NASA Technical Reports Server (NTRS)

Johansson, A. V.; Alfredsson, P. H.; Kim, J.

1987-01-01

The structure of internal shear layer observed in the near-wall region of turbulent flows is investigated by analyzing flow fields obtained from numerical simulations of channel and boundary-layer flows. It is found that the shear layer is an important contributor to the turbulence production. The conditionally averaged production at the center of the structure was almost twice as large as the long-time mean value. The shear-layer structure is also found to retain its coherence over streamwise distances on the order of a thousand viscous length units, and propagates with a constant velocity of about 10.6 u sub rho throughout the near wall region.

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.

Wall shear stress in portal vein of cirrhotic patients with portal hypertension.

PubMed

Wei, Wei; Pu, Yan-Song; Wang, Xin-Kai; Jiang, An; Zhou, Rui; Li, Yu; Zhang, Qiu-Juan; Wei, Ya-Juan; Chen, Bin; Li, Zong-Fang

2017-05-14

To investigate wall shear stress (WSS) magnitude and distribution in cirrhotic patients with portal hypertension using computational fluid dynamics. Idealized portal vein (PV) system models were reconstructed with different angles of the PV-splenic vein (SV) and superior mesenteric vein (SMV)-SV. Patient-specific models were created according to enhanced computed tomography images. WSS was simulated by using a finite-element analyzer, regarding the blood as a Newtonian fluid and the vessel as a rigid wall. Analysis was carried out to compare the WSS in the portal hypertension group with that in healthy controls. For the idealized models, WSS in the portal hypertension group (0-10 dyn/cm 2 ) was significantly lower than that in the healthy controls (10-20 dyn/cm 2 ), and low WSS area (0-1 dyn/cm 2 ) only occurred in the left wall of the PV in the portal hypertension group. Different angles of PV-SV and SMV-SV had different effects on the magnitude and distribution of WSS, and low WSS area often occurred in smaller PV-SV angle and larger SMV-SV angle. In the patient-specific models, WSS in the cirrhotic patients with portal hypertension (10.13 ± 1.34 dyn/cm 2 ) was also significantly lower than that in the healthy controls ( P < 0.05). Low WSS area often occurred in the junction area of SV and SMV into the PV, in the area of the division of PV into left and right PV, and in the outer wall of the curving SV in the control group. In the cirrhotic patients with portal hypertension, the low WSS area extended to wider levels and the magnitude of WSS reached lower levels, thereby being more prone to disturbed flow occurrence. Cirrhotic patients with portal hypertension show dramatic hemodynamic changes with lower WSS and greater potential for disturbed flow, representing a possible causative factor of PV thrombosis.

Wall shear stress in portal vein of cirrhotic patients with portal hypertension

PubMed Central

Wei, Wei; Pu, Yan-Song; Wang, Xin-Kai; Jiang, An; Zhou, Rui; Li, Yu; Zhang, Qiu-Juan; Wei, Ya-Juan; Chen, Bin; Li, Zong-Fang

2017-01-01

AIM To investigate wall shear stress (WSS) magnitude and distribution in cirrhotic patients with portal hypertension using computational fluid dynamics. METHODS Idealized portal vein (PV) system models were reconstructed with different angles of the PV-splenic vein (SV) and superior mesenteric vein (SMV)-SV. Patient-specific models were created according to enhanced computed tomography images. WSS was simulated by using a finite-element analyzer, regarding the blood as a Newtonian fluid and the vessel as a rigid wall. Analysis was carried out to compare the WSS in the portal hypertension group with that in healthy controls. RESULTS For the idealized models, WSS in the portal hypertension group (0-10 dyn/cm2) was significantly lower than that in the healthy controls (10-20 dyn/cm2), and low WSS area (0-1 dyn/cm2) only occurred in the left wall of the PV in the portal hypertension group. Different angles of PV-SV and SMV-SV had different effects on the magnitude and distribution of WSS, and low WSS area often occurred in smaller PV-SV angle and larger SMV-SV angle. In the patient-specific models, WSS in the cirrhotic patients with portal hypertension (10.13 ± 1.34 dyn/cm2) was also significantly lower than that in the healthy controls (P < 0.05). Low WSS area often occurred in the junction area of SV and SMV into the PV, in the area of the division of PV into left and right PV, and in the outer wall of the curving SV in the control group. In the cirrhotic patients with portal hypertension, the low WSS area extended to wider levels and the magnitude of WSS reached lower levels, thereby being more prone to disturbed flow occurrence. CONCLUSION Cirrhotic patients with portal hypertension show dramatic hemodynamic changes with lower WSS and greater potential for disturbed flow, representing a possible causative factor of PV thrombosis. PMID:28566887

Impact of Age and Aerobic Exercise Training on Conduit Artery Wall Thickness: Role of the Shear Pattern.

PubMed

Tanahashi, Koichiro; Kosaki, Keisei; Sawano, Yuriko; Yoshikawa, Toru; Tagawa, Kaname; Kumagai, Hiroshi; Akazawa, Nobuhiko; Maeda, Seiji

2017-01-01

Hemodynamic shear stress is the frictional force of blood on the arterial wall. The shear pattern in the conduit artery affects the endothelium and may participate in the development and progression of atherosclerosis. We investigated the role of the shear pattern in age- and aerobic exercise-induced changes in conduit artery wall thickness via cross-sectional and interventional studies. In a cross-sectional study, we found that brachial shear rate patterns and brachial artery intima-media thickness (IMT) correlated with age. Additionally, brachial artery shear rate patterns were associated with brachial artery IMT in 102 middle-aged and older individuals. In an interventional study, 39 middle-aged and older subjects were divided into 2 groups: control and exercise. The exercise group completed 12 weeks of aerobic exercise training. Aerobic exercise training significantly increased the antegrade shear rate and decreased the retrograde shear rate and brachial artery IMT. Moreover, changes in the brachial artery antegrade shear rate and the retrograde shear rate correlated with the change in brachial artery IMT. The results of the present study indicate that changes in brachial artery shear rate patterns may contribute to age- and aerobic exercise training-induced changes in brachial artery wall thickness. © 2017 S. Karger AG, Basel.

Turbulence significantly increases pressure and fluid shear stress in an aortic aneurysm model under resting and exercise flow conditions.

PubMed

Khanafer, Khalil M; Bull, Joseph L; Upchurch, Gilbert R; Berguer, Ramon

2007-01-01

The numerical models of abdominal aortic aneurysm (AAA) in use do not take into account the non-Newtonian behavior of blood and the development of local turbulence. This study examines the influence of pulsatile, turbulent, non-Newtonian flow on fluid shear stresses and pressure changes under rest and exercise conditions. We numerically analyzed pulsatile turbulent flow, using simulated physiological rest and exercise waveforms, in axisymmetric-rigid aortic aneurysm models (AAMs). Discretization of governing equations was achieved using a finite element scheme. Maximum turbulence-induced shear stress was found at the distal end of an AAM. In large AAMs (dilated to undilated diameter ratio = 3.33) at peak systolic flow velocity, fluid shear stress during exercise is 70.4% higher than at rest. Our study provides a numerical, noninvasive method for obtaining detailed data on the forces generated by pulsatile turbulent flow in AAAs that are difficult to study in humans and in physical models. Our data suggest that increased flow turbulence results in increased shear stress in aneurysms. While pressure readings are fairly uniform along the length of an aneurysm, the kinetic energy generated by turbulence impacting on the wall of the distal half of the aneurysm increases fluid and wall shear stress at this site. If the increased fluid shear stress results in further dilation and hence further turbulence, wall stress may be a mechanism for aneurysmal growth and eventual rupture.

Shear localization and effective wall friction in a wall bounded granular flow

NASA Astrophysics Data System (ADS)

Artoni, Riccardo; Richard, Patrick

2017-06-01

In this work, granular flow rheology is investigated by means of discrete numerical simulations of a torsional, cylindrical shear cell. Firstly, we focus on azimuthal velocity profiles and study the effect of (i) the confining pressure, (ii) the particle-wall friction coefficient, (iii) the rotating velocity of the bottom wall and (iv) the cell diameter. For small cell diameters, azimuthal velocity profiles are nearly auto-similar, i.e. they are almost linear with the radial coordinate. Different strain localization regimes are observed : shear can be localized at the bottom, at the top of the shear cell, or it can be even quite distributed. This behavior originates from the competition between dissipation at the sidewalls and dissipation in the bulk of the system. Then we study the effective friction at the cylindrical wall, and point out the strong link between wall friction, slip and fluctuations of forces and velocities. Even if the system is globally below the sliding threshold, force fluctuations trigger slip events, leading to a nonzero wall slip velocity and an effective wall friction coefficient different from the particle-wall one. A scaling law was found linking slip velocity, granular temperature in the main flow direction and effective friction. Our results suggest that fluctuations are an important ingredient for theories aiming to capture the interface rheology of granular materials.

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

In vivo wall shear measurements within the developing zebrafish heart.

PubMed

Jamison, R Aidan; Samarage, Chaminda R; Bryson-Richardson, Robert J; Fouras, Andreas

2013-01-01

Physical forces can influence the embryonic development of many tissues. Within the cardiovascular system shear forces resulting from blood flow are known to be one of the regulatory signals that shape the developing heart. A key challenge in investigating the role of shear forces in cardiac development is the ability to obtain shear force measurements in vivo. Utilising the zebrafish model system we have developed a methodology that allows the shear force within the developing embryonic heart to be determined. Accurate wall shear measurement requires two essential pieces of information; high-resolution velocity measurements near the heart wall and the location and orientation of the heart wall itself. We have applied high-speed brightfield imaging to capture time-lapse series of blood flow within the beating heart between 3 and 6 days post-fertilization. Cardiac-phase filtering is applied to these time-lapse images to remove the heart wall and other slow moving structures leaving only the red blood cell movement. Using particle image velocimetry to calculate the velocity of red blood cells in different regions within the heart, and using the signal-to-noise ratio of the cardiac-phase filtered images to determine the boundary of blood flow, and therefore the position of the heart wall, we have been able to generate the necessary information to measure wall shear in vivo. We describe the methodology required to measure shear in vivo and the application of this technique to the developing zebrafish heart. We identify a reduction in shear at the ventricular-bulbar valve between 3 and 6 days post-fertilization and demonstrate that the shear environment of the ventricle during systole is constantly developing towards a more uniform level.

Shear-induced desorption of isolated polymer molecules from a planar wall

NASA Astrophysics Data System (ADS)

Dutta, Sarit; Dorfman, Kevin; Kumar, Satish

2014-03-01

Shear-induced desorption of isolated polymer molecules is studied using Brownian dynamics simulations. The polymer molecules are modeled as freely jointed bead-spring chains interacting with a planar wall via a short-range potential. The simulations include both intrachain and chain-wall hydrodynamic interactions. Shear flow is found to cause chain flattening, resulting at low shear rates in an increased fraction of chain segments bound to the wall. However, above a critical shear rate the chains desorb completely. The desorption process is nucleated by random protrusions in the shear gradient direction which evolve under the combined effect of drag, hydrodynamic interaction, and vorticity-induced rotation, and subsequently lead to recapture. Above the critical shear rate, these protrusions grow in length until the entire chain is peeled off the wall. For free-draining chains, the protrusions are not sustained and no desorption is observed even at shear rates much higher than the critical value. These simulations can help in interpreting experiments on shear-induced desorption of polymer films and brushes.

Direct detection and measurement of wall shear stress using a filamentous bio-nanoparticle

PubMed Central

Lobo, Daniela P.; Wemyss, Alan M.; Smith, David J.; Straube, Anne; Betteridge, Kai B.; Salmon, Andrew H. J.; Foster, Rebecca R.; Elhegni, Hesham E.; Satchell, Simon C.; Little, Haydn A.; Pacheco-Gómez, Raúl; Simmons, Mark J.; Hicks, Matthew R.; Bates, David O.; Dafforn, Timothy R.; Arkill, Kenton P.

2016-01-01

The wall shear stress (WSS) that a moving fluid exerts on a surface affects many processes including those relating to vascular function. WSS plays an important role in normal physiology (e.g. angiogenesis) and affects the microvasculature’s primary function of molecular transport. Points of fluctuating WSS show abnormalities in a number of diseases; however, there is no established technique for measuring WSS directly in physiological systems. All current methods rely on estimates obtained from measured velocity gradients in bulk flow data. In this work, we report a nanosensor that can directly measure WSS in microfluidic chambers with sub-micron spatial resolution by using a specific type of virus, the bacteriophage M13, which has been fluorescently labeled and anchored to a surface. It is demonstrated that the nanosensor can be calibrated and adapted for biological tissue, revealing WSS in micro-domains of cells that cannot be calculated accurately from bulk flow measurements. This method lends itself to a platform applicable to many applications in biology and microfluidics. PMID:27570611

Development of buried wire gages for measurement of wall shear stress in Blastane experiments

NASA Technical Reports Server (NTRS)

Murthy, S. V.; Steinle, F. W.

1986-01-01

Buried Wire Gages operated from a Constant Temperature Anemometer System are among the special types of instrumentation to be used in the Boundary Layer Apparatus for Subsonic and Transonic flow Affected by Noise Environment (BLASTANE). These Gages are of a new type and need to be adapted for specific applications. Methods were developed to fabricate Gage inserts and mount those in the BLASTANE Instrumentation Plugs. A large number of Gages were prepared and operated from a Constant Temperature Anemometer System to derive some of the calibration constants for application to fluid-flow wall shear-stress measurements. The final stage of the calibration was defined, but could not be accomplished because of non-availability of a suitable flow simulating apparatus. This report provides a description of the Buried Wire Gage technique, an explanation of the method evolved for making proper Gages and the calibration constants, namely Temperature Coefficient of Resistance and Conduction Loss Factor.

Wall shear stress promotes intimal hyperplasia through the paracrine H2O2-mediated NOX-AKT-SVV axis.

PubMed

Zhang, Haolong; Yang, Zhipeng; Wang, Jing; Wang, Xuehu; Zhao, Yu; Zhu, Fangyu

2018-05-27

Oscillatory wall shear stress (WSS)-linked oxidative stress promotes intimal hyperplasia (IH) development, but the underlying mechanisms are not completely understood. We used an in vivo rabbit carotid arterial stenosis model representing different levels of WSS and found that WSS was increased at 1 month with 50% stenosis and was accompanied by VSMCs proliferation and interstitial collagen accumulation. Increased WSS promoted the expression of NOX, AKT, and survivin (SVV) and the proliferation/migration of VSMCs and reduced apoptosis. Our in vitro study suggested that H 2 O 2 promoted proliferation and migration while suppressing apoptosis in cultured human umbilical vascular endothelial cells. We demonstrated that the elevation of WSS promotes VSMC proliferation and migration through the H 2 O 2 -mediated NOX-AKT-SVV axis, thereby accelerating IH development. Copyright © 2017. Published by Elsevier Inc.

Research Status on Bonding Behavior of Prefabricated Concrete Shear Wall

NASA Astrophysics Data System (ADS)

Wang, Donghui; Liu, Xudong; Wang, Sheng; Li, Shanshan

2018-03-01

Prefabricated shear wall structure adapts to the development and requirements of China’s residential industrialization. The key to the prefabricated concrete shear wall structure is the connection between the prefabricated members, where the reliability of the connection of the concrete joint is related to the overall performance and seismic effect of the structure. In this paper, the microstructures of the joint surface and shear properties are analysed, and the formula for calculating the shear strength of the joint is obtained.

Measurement of surface shear stress vector beneath high-speed jet flow using liquid crystal coating

NASA Astrophysics Data System (ADS)

Wang, Cheng-Peng; Zhao, Ji-Song; Jiao, Yun; Cheng, Ke-Ming

2018-05-01

The shear-sensitive liquid crystal coating (SSLCC) technique is investigated in the high-speed jet flow of a micro-wind-tunnel. An approach to measure surface shear stress vector distribution using the SSLCC technique is established, where six synchronous cameras are used to record the coating color at different circumferential view angles. Spatial wall shear stress vector distributions on the test surface are obtained at different velocities. The results are encouraging and demonstrate the great potential of the SSLCC technique in high-speed wind-tunnel measurement.

Modeling of Wall-Bounded Complex Flows and Free Shear Flows

NASA Technical Reports Server (NTRS)

Shih, Tsan-Hsing; Zhu, Jiang; Lumley, John L.

1994-01-01

Various wall-bounded flows with complex geometries and free shear flows have been studied with a newly developed realizable Reynolds stress algebraic equation model. The model development is based on the invariant theory in continuum mechanics. This theory enables us to formulate a general constitutive relation for the Reynolds stresses. Pope was the first to introduce this kind of constitutive relation to turbulence modeling. In our study, realizability is imposed on the truncated constitutive relation to determine the coefficients so that, unlike the standard k-E eddy viscosity model, the present model will not produce negative normal stresses in any situations of rapid distortion. The calculations based on the present model have shown an encouraging success in modeling complex turbulent flows.

Calculations of wall shear stress in harmonically oscillated turbulent pipe flow using a low-Reynolds-number {kappa}-{epsilon} model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ismael, J.O.; Cotton, M.A.

1996-03-01

The low-Reynolds-number {kappa}-{epsilon} turbulence model of Launder and Sharma is applied to the calculation of wall shear stress in spatially fully-developed turbulent pipe flow oscillated at small amplitudes. It is believed that the present study represents the first systematic evaluation of the turbulence closure under consideration over a wide range of frequency. Model results are well correlated in terms of the parameter {omega}{sup +} = {omega}{nu}/{bar U}{sub {tau}}{sup 2} at high frequencies, whereas at low frequencies there is an additional Reynolds number dependence. Comparison is made with the experimental data of Finnicum and Hanratty.

High wall shear stress and spatial gradients in vascular pathology: a review.

PubMed

Dolan, Jennifer M; Kolega, John; Meng, Hui

2013-07-01

Cardiovascular pathologies such as intracranial aneurysms (IAs) and atherosclerosis preferentially localize to bifurcations and curvatures where hemodynamics are complex. While extensive knowledge about low wall shear stress (WSS) has been generated in the past, due to its strong relevance to atherogenesis, high WSS (typically >3 Pa) has emerged as a key regulator of vascular biology and pathology as well, receiving renewed interests. As reviewed here, chronic high WSS not only stimulates adaptive outward remodeling, but also contributes to saccular IA formation (at bifurcation apices or outer curves) and atherosclerotic plaque destabilization (in stenosed vessels). Recent advances in understanding IA pathogenesis have shed new light on the role of high WSS in pathological vascular remodeling. In complex geometries, high WSS can couple with significant spatial WSS gradient (WSSG). A combination of high WSS and positive WSSG has been shown to trigger aneurysm initiation. Since endothelial cells (ECs) are sensors of WSS, we have begun to elucidate EC responses to high WSS alone and in combination with WSSG. Understanding such responses will provide insight into not only aneurysm formation, but also plaque destabilization and other vascular pathologies and potentially lead to improved strategies for disease management and novel targets for pharmacological intervention.

Computational solution of the velocity and wall shear stress distribution inside a left carotid artery under pulsatile flow conditions

NASA Astrophysics Data System (ADS)

Arslan, Nurullah; Turmuş, Hakan

2014-08-01

Stroke is still one of the leading causes for death after heart diseases and cancer in all over the world. Strokes happen because an artery that carries blood uphill from the heart to the head is clogged. Most of the time, as with heart attacks, the problem is atherosclerosis, hardening of the arteries, calcified buildup of fatty deposits on the vessel wall. In this study, the fluid dynamic simulations were done in a left carotid bifurcation under the pulsatile flow conditions computationally. Pulsatile flow waveform is given in the paper. In vivo geometry and boundary conditions were obtained from a patient who has stenosis located at external carotid artery (ECA) and internal carotid artery (ICA) of his common carotid artery (CCA). The location of critical flow fields such as low wall shear stress (WSS), stagnation regions and separation regions were detected near the highly stenosed region and at branching region.

The shear-lag effect of thin-walled box girder under vertical earthquake excitation

NASA Astrophysics Data System (ADS)

Zhai, Zhipeng; Li, Yaozhuang; Guo, Wei

2017-03-01

The variation method based on the energy variation principle is proved to be accurate and valid for analyzing the shear lag effect of box girder under static and dynamic load. Meanwhile, dynamic problems gradually become the key factors in engineering practice. Therefore, a method for calculating the shear lag effect in thin-walled box girder under vertical seismic excitation is proposed by applying Hamilton Principle in this paper. The Timoshenko shear deformation is taken into account. And a new definition of shear lag ratio for box girder is given. What's more, some conclusions are drawn by analysis of numerical example. The results show that small amplitude of earthquake ground motion can generate high stress and obvious shear lag, especially in the region of resonance. And the influence of rotary inertia cannot be ignored for analyzing the shear lag effect. With the increase of span to width ratio, shear lag effect becomes smaller and smaller. These research conclusions will be useful for the engineering practice and enrich the theoretical studies of box girders.

Modeling Periodic Adiabatic Shear Bands Evolution in a 304L Stainless Steel Thick-Walled Cylinder

NASA Astrophysics Data System (ADS)

Liu, Mingtao; Hu, Haibo; Fan, Cheng; Tang, Tiegang

2015-06-01

The self-organization of multiple shear bands in a 304L stainless steel thick-walled cylinder (TWC) was numerically studied. The microstructures of material lead to the non-uniform distribution of local yield stress, which plays a key role in the formation of spontaneous shear localization. We introduced a probability factor satisfied Gauss distribution into the macroscopic constitutive relationship to describe the non-uniformity of local yield stress. Using the probability factor, the initiation and propagation of multiple shear bands in TWC were numerically replicated in our 2D FEM simulation. Experimental results in the literature indicate that the machined surface at the internal boundary of a 304L stainless steel cylinder provides a work-hardened layer (about 20 μm) which has significantly different microstructures from base material. The work-hardened layer leads to the phenomenon that most shear bands are in clockwise or counterclockwise direction. In our simulation, periodic oriented perturbations were applied to describe the grain orientation in the work-hardened layer, and the spiral pattern of shear bands was successfully replicated.

Modeling of flow-induced shear stress applied on 3D cellular scaffolds: Implications for vascular tissue engineering.

PubMed

Lesman, Ayelet; Blinder, Yaron; Levenberg, Shulamit

2010-02-15

Novel tissue-culture bioreactors employ flow-induced shear stress as a means of mechanical stimulation of cells. We developed a computational fluid dynamics model of the complex three-dimensional (3D) microstructure of a porous scaffold incubated in a direct perfusion bioreactor. Our model was designed to predict high shear-stress values within the physiological range of those naturally sensed by vascular cells (1-10 dyne/cm(2)), and will thereby provide suitable conditions for vascular tissue-engineering experiments. The model also accounts for cellular growth, which was designed as an added cell layer grown on all scaffold walls. Five model variants were designed, with geometric differences corresponding to cell-layer thicknesses of 0, 50, 75, 100, and 125 microm. Four inlet velocities (0.5, 1, 1.5, and 2 cm/s) were applied to each model. Wall shear-stress distribution and overall pressure drop calculations were then used to characterize the relation between flow rate, shear stress, cell-layer thickness, and pressure drop. The simulations showed that cellular growth within 3D scaffolds exposes cells to elevated shear stress, with considerably increasing average values in correlation to cell growth and inflow velocity. Our results provide in-depth analysis of the microdynamic environment of cells cultured within 3D environments, and thus provide advanced control over tissue development in vitro. 2009 Wiley Periodicals, Inc.

Wall Shear Stress Distribution in a Patient-Specific Cerebral Aneurysm Model using Reduced Order Modeling

NASA Astrophysics Data System (ADS)

Han, Suyue; Chang, Gary Han; Schirmer, Clemens; Modarres-Sadeghi, Yahya

2016-11-01

We construct a reduced-order model (ROM) to study the Wall Shear Stress (WSS) distributions in image-based patient-specific aneurysms models. The magnitude of WSS has been shown to be a critical factor in growth and rupture of human aneurysms. We start the process by running a training case using Computational Fluid Dynamics (CFD) simulation with time-varying flow parameters, such that these parameters cover the range of parameters of interest. The method of snapshot Proper Orthogonal Decomposition (POD) is utilized to construct the reduced-order bases using the training CFD simulation. The resulting ROM enables us to study the flow patterns and the WSS distributions over a range of system parameters computationally very efficiently with a relatively small number of modes. This enables comprehensive analysis of the model system across a range of physiological conditions without the need to re-compute the simulation for small changes in the system parameters.

Effects of biaxial oscillatory shear stress on endothelial cell proliferation and morphology.

PubMed

Chakraborty, Amlan; Chakraborty, Sutirtha; Jala, Venkatakrishna R; Haribabu, Bodduluri; Sharp, M Keith; Berson, R Eric

2012-03-01

Wall shear stress (WSS) on anchored cells affects their responses, including cell proliferation and morphology. In this study, the effects of the directionality of pulsatile WSS on endothelial cell proliferation and morphology were investigated for cells grown in a Petri dish orbiting on a shaker platform. Time and location dependent WSS was determined by computational fluid dynamics (CFD). At low orbital speed (50 rpm), WSS was shown to be uniform (0-1 dyne/cm(2)) across the bottom of the dish, while at higher orbital speed (100 and 150 rpm), WSS remained fairly uniform near the center and fluctuated significantly (0-9 dyne/cm(2)) near the side walls of the dish. Since WSS on the bottom of the dish is two-dimensional, a new directional oscillatory shear index (DOSI) was developed to quantify the directionality of oscillating shear. DOSI approached zero for biaxial oscillatory shear of equal magnitudes near the center and approached one for uniaxial pulsatile shear near the wall, where large tangential WSS dominated a much smaller radial component. Near the center (low DOSI), more, smaller and less elongated cells grew, whereas larger cells with greater elongation were observed in the more uniaxial oscillatory shear (high DOSI) near the periphery of the dish. Further, cells aligned with the direction of the largest component of shear but were randomly oriented in low magnitude biaxial shear. Statistical analyses of the individual and interacting effects of multiple factors (DOSI, shear magnitudes and orbital speeds) showed that DOSI significantly affected all the responses, indicating that directionality is an important determinant of cellular responses. Copyright © 2011 Wiley Periodicals, Inc.

Shear stress-induced mitochondrial biogenesis decreases the release of microparticles from endothelial cells.

PubMed

Kim, Ji-Seok; Kim, Boa; Lee, Hojun; Thakkar, Sunny; Babbitt, Dianne M; Eguchi, Satoru; Brown, Michael D; Park, Joon-Young

2015-08-01

The concept of enhancing structural integrity of mitochondria has emerged as a novel therapeutic option for cardiovascular disease. Flow-induced increase in laminar shear stress is a potent physiological stimulant associated with exercise, which exerts atheroprotective effects in the vasculature. However, the effect of laminar shear stress on mitochondrial remodeling within the vascular endothelium and its related functional consequences remain largely unknown. Using in vitro and in vivo complementary studies, here, we report that aerobic exercise alleviates the release of endothelial microparticles in prehypertensive individuals and that these salutary effects are, in part, mediated by shear stress-induced mitochondrial biogenesis. Circulating levels of total (CD31(+)/CD42a(-)) and activated (CD62E(+)) microparticles released by endothelial cells were significantly decreased (∼40% for both) after a 6-mo supervised aerobic exercise training program in individuals with prehypertension. In cultured human endothelial cells, laminar shear stress reduced the release of endothelial microparticles, which was accompanied by an increase in mitochondrial biogenesis through a sirtuin 1 (SIRT1)-dependent mechanism. Resveratrol, a SIRT1 activator, treatment showed similar effects. SIRT1 knockdown using small-interfering RNA completely abolished the protective effect of shear stress. Disruption of mitochondrial integrity by either antimycin A or peroxisome proliferator-activated receptor-γ coactivator-1α small-interfering RNA significantly increased the number of total, and activated, released endothelial microparticles, and shear stress restored these back to basal levels. Collectively, these data demonstrate a critical role of endothelial mitochondrial integrity in preserving endothelial homeostasis. Moreover, prolonged laminar shear stress, which is systemically elevated during aerobic exercise in the vessel wall, mitigates endothelial dysfunction by promoting

Cyclic Behavior of Low Rise Concrete Shear Walls Containing Recycled Coarse and Fine Aggregates.

PubMed

Qiao, Qiyun; Cao, Wanlin; Qian, Zhiwei; Li, Xiangyu; Zhang, Wenwen; Liu, Wenchao

2017-12-07

In this study, the cyclic behaviors of low rise concrete shear walls using recycled coarse or fine aggregates were investigated. Eight low rise Recycled Aggregates Concrete (RAC) shear wall specimens were designed and tested under a cyclic loading. The following parameters were varied: replacement percentages of recycled coarse or fine aggregates, reinforcement ratio, axial force ratio and X-shaped rebars brace. The failure characteristics, hysteretic behavior, strength and deformation capacity, strain characteristics and stiffness were studied. Test results showed that the using of the Recycled Coarse Aggregates (RCA) and its replacement ratio had almost no influence on the mechanical behavior of the shear wall; however, the using of Recycled Fine Aggregates (RFA) had a certain influence on the ductility of the shear wall. When the reinforcement ratio increased, the strength and ductility also increased. By increasing the axial force ratio, the strength increased but the ductility decreased significantly. The encased brace had a significant effect on enhancing the RAC shear walls. The experimental maximum strengths were evaluated with existing design codes, it was indicated that the strength evaluation of the low rise RAC shear walls can follow the existing design codes of the conventional concrete shear walls.

Seismic Behaviour of Composite Steel Fibre Reinforced Concrete Shear Walls

NASA Astrophysics Data System (ADS)

Boita, Ioana-Emanuela; Dan, Daniel; Stoian, Valeriu

2017-10-01

In this paper is presented an experimental study conducted at the “Politehnica” University of Timisoara, Romania. This study provides results from a comprehensive experimental investigation on the behaviour of composite steel fibre reinforced concrete shear walls (CSFRCW) with partially or totally encased profiles. Two experimental composite steel fibre reinforced concrete walls (CSFRCW) and, as a reference specimen, a typical reinforced concrete shear wall (RCW), (without structural reinforcement), were fabricated and tested under constant vertical load and quasi-static reversed cyclic lateral loads, in displacement control. The tests were performed until failure. The tested specimens were designed as 1:3 scale steel-concrete composite elements, representing a three storeys and one bay element from the base of a lateral resisting system made by shear walls. Configuration/arrangement of steel profiles in cross section were varied within the specimens. The main objective of this research consisted in identifying innovative solutions for composite steel-concrete shear walls with enhanced performance, as steel fibre reinforced concrete which was used in order to replace traditional reinforced concrete. A first conclusion was that replacing traditional reinforcement with steel fibre changes the failure mode of the elements, as from a flexural mode, in case of element RCW, to a shear failure mode for CSFRCW. The maximum lateral force had almost similar values but test results indicated an improvement in cracking response, and a decrease in ductility. The addition of steel fibres in the concrete mixture can lead to an increase of the initial cracking force, and can change the sudden opening of a crack in a more stable process.

Experimental Verification of Same Simple Equilibrium Models of Masonry Shear Walls

NASA Astrophysics Data System (ADS)

Radosław, Jasiński

2017-10-01

This paper contains theoretical fundamentals of strut and tie models, used in unreinforced horizontal shear walls. Depending on support conditions and wall loading, we can distinguish models with discrete bars when point load is applied to the wall (type I model) or with continuous bars (type II model) when load is uniformly distributed at the wall boundary. The main part of this paper compares calculated results with the own tests on horizontal shear walls made of solid brick, silicate elements and autoclaved aerated concrete. The tests were performed in Poland. The model required some modifications due to specific load and static diagram.

Circulatory shear flow alters the viability and proliferation of circulating colon cancer cells

NASA Astrophysics Data System (ADS)

Fan, Rong; Emery, Travis; Zhang, Yongguo; Xia, Yuxuan; Sun, Jun; Wan, Jiandi

2016-06-01

During cancer metastasis, circulating tumor cells constantly experience hemodynamic shear stress in the circulation. Cellular responses to shear stress including cell viability and proliferation thus play critical roles in cancer metastasis. Here, we developed a microfluidic approach to establish a circulatory microenvironment and studied circulating human colon cancer HCT116 cells in response to a variety of magnitude of shear stress and circulating time. Our results showed that cell viability decreased with the increase of circulating time, but increased with the magnitude of wall shear stress. Proliferation of cells survived from circulation could be maintained when physiologically relevant wall shear stresses were applied. High wall shear stress (60.5 dyne/cm2), however, led to decreased cell proliferation at long circulating time (1 h). We further showed that the expression levels of β-catenin and c-myc, proliferation regulators, were significantly enhanced by increasing wall shear stress. The presented study provides a new insight to the roles of circulatory shear stress in cellular responses of circulating tumor cells in a physiologically relevant model, and thus will be of interest for the study of cancer cell mechanosensing and cancer metastasis.

Near-wall similarity in a pressure-driven three-dimensional turbulent boundary layer

NASA Technical Reports Server (NTRS)

Pierce, F. J.; Mcallister, J. E.

1980-01-01

Mean velocity, measured wall pressure and wall shear stress fields were made in a three dimensional pressure-driven turbulent boundary layer created by a cylinder with trailing edge placed normal to a flat plate floor. The direct force wall shear stress measurements were made with floating element direct force sensing shear meter that responded to both the magnitude and direction of the local wall shear stress. The ability of 10 near wall similarity models to describe the near wall velocity field for the measured flow under a wide range of skewing conditions and a variety of pressure gradient and wall shear vector orientations was used.

In vitro shear stress measurements using particle image velocimetry in a family of carotid artery models: effect of stenosis severity, plaque eccentricity, and ulceration.

PubMed

Kefayati, Sarah; Milner, Jaques S; Holdsworth, David W; Poepping, Tamie L

2014-01-01

Atherosclerotic disease, and the subsequent complications of thrombosis and plaque rupture, has been associated with local shear stress. In the diseased carotid artery, local variations in shear stress are induced by various geometrical features of the stenotic plaque. Greater stenosis severity, plaque eccentricity (symmetry) and plaque ulceration have been associated with increased risk of cerebrovascular events based on clinical trial studies. Using particle image velocimetry, the levels and patterns of shear stress (derived from both laminar and turbulent phases) were studied for a family of eight matched-geometry models incorporating independently varied plaque features - i.e. stenosis severity up to 70%, one of two forms of plaque eccentricity, and the presence of plaque ulceration). The level of laminar (ensemble-averaged) shear stress increased with increasing stenosis severity resulting in 2-16 Pa for free shear stress (FSS) and approximately double (4-36 Pa) for wall shear stress (WSS). Independent of stenosis severity, marked differences were found in the distribution and extent of shear stress between the concentric and eccentric plaque formations. The maximum WSS, found at the apex of the stenosis, decayed significantly steeper along the outer wall of an eccentric model compared to the concentric counterpart, with a 70% eccentric stenosis having 249% steeper decay coinciding with the large outer-wall recirculation zone. The presence of ulceration (in a 50% eccentric plaque) resulted in both elevated FSS and WSS levels that were sustained longer (∼20 ms) through the systolic phase compared to the non-ulcerated counterpart model, among other notable differences. Reynolds (turbulent) shear stress, elevated around the point of distal jet detachment, became prominent during the systolic deceleration phase and was widely distributed over the large recirculation zone in the eccentric stenoses.

Seismic Performance of Composite Shear Walls Constructed Using Recycled Aggregate Concrete and Different Expandable Polystyrene Configurations.

PubMed

Liu, Wenchao; Cao, Wanlin; Zhang, Jianwei; Qiao, Qiyun; Ma, Heng

2016-03-02

The seismic performance of recycled aggregate concrete (RAC) composite shear walls with different expandable polystyrene (EPS) configurations was investigated. Six concrete shear walls were designed and tested under cyclic loading to evaluate the effect of fine RAC in designing earthquake-resistant structures. Three of the six specimens were used to construct mid-rise walls with a shear-span ratio of 1.5, and the other three specimens were used to construct low-rise walls with a shear-span ratio of 0.8. The mid-rise and low-rise shear walls consisted of an ordinary recycled concrete shear wall, a composite wall with fine aggregate concrete (FAC) protective layer (EPS modules as the external insulation layer), and a composite wall with sandwiched EPS modules as the insulation layer. Several parameters obtained from the experimental results were compared and analyzed, including the load-bearing capacity, stiffness, ductility, energy dissipation, and failure characteristics of the specimens. The calculation formula of load-bearing capacity was obtained by considering the effect of FAC on composite shear walls as the protective layer. The damage process of the specimen was simulated using the ABAQUS Software, and the results agreed quite well with those obtained from the experiments. The results show that the seismic resistance behavior of the EPS module composite for shear walls performed better than ordinary recycled concrete for shear walls. Shear walls with sandwiched EPS modules had a better seismic performance than those with EPS modules lying outside. Although the FAC protective layer slightly improved the seismic performance of the structure, it undoubtedly slowed down the speed of crack formation and the stiffness degradation of the walls.

Seismic Performance of Composite Shear Walls Constructed Using Recycled Aggregate Concrete and Different Expandable Polystyrene Configurations

PubMed Central

Liu, Wenchao; Cao, Wanlin; Zhang, Jianwei; Qiao, Qiyun; Ma, Heng

2016-01-01

The seismic performance of recycled aggregate concrete (RAC) composite shear walls with different expandable polystyrene (EPS) configurations was investigated. Six concrete shear walls were designed and tested under cyclic loading to evaluate the effect of fine RAC in designing earthquake-resistant structures. Three of the six specimens were used to construct mid-rise walls with a shear-span ratio of 1.5, and the other three specimens were used to construct low-rise walls with a shear-span ratio of 0.8. The mid-rise and low-rise shear walls consisted of an ordinary recycled concrete shear wall, a composite wall with fine aggregate concrete (FAC) protective layer (EPS modules as the external insulation layer), and a composite wall with sandwiched EPS modules as the insulation layer. Several parameters obtained from the experimental results were compared and analyzed, including the load-bearing capacity, stiffness, ductility, energy dissipation, and failure characteristics of the specimens. The calculation formula of load-bearing capacity was obtained by considering the effect of FAC on composite shear walls as the protective layer. The damage process of the specimen was simulated using the ABAQUS Software, and the results agreed quite well with those obtained from the experiments. The results show that the seismic resistance behavior of the EPS module composite for shear walls performed better than ordinary recycled concrete for shear walls. Shear walls with sandwiched EPS modules had a better seismic performance than those with EPS modules lying outside. Although the FAC protective layer slightly improved the seismic performance of the structure, it undoubtedly slowed down the speed of crack formation and the stiffness degradation of the walls. PMID:28773274

Critical bed shear stress and threshold of motion of maerl biogenic gravel

NASA Astrophysics Data System (ADS)

Joshi, Siddhi; Duffy, Garret Patrick; Brown, Colin

2017-07-01

A determination of the critical bed shear stress of maerl is a prerequisite for quantifying its mobility, rate of erosion and deposition in conservation management. The critical bed shear stress for incipient motion has been determined for the first time for samples from biogenic free-living maerl beds in three contrasting environments (open marine, intertidal and beach) in Galway Bay, west of Ireland. The bed shear stress was determined using two methods, Law of the Wall and Turbulent Kinetic Energy, in a rotating annular flume and in a linear flume. The velocity profile of flowing water above a bed of natural maerl grains was measured in four runs of progressively increasing flow velocity until the flow exceeded the critical shear stress of grains on the bed. The critical Shields parameter and the mobility number are estimated and compared with the equivalent curves for natural quartz sand. The critical Shields parameters for the maerl particles from all three environments fall below the Shields curve. Along with a previously reported correlation between maerl grain shape and settling velocity, these results suggest that the highly irregular shapes also allow maerl grains to be mobilised more easily than quartz grains with the same sieve diameter. The intertidal beds with the roughest particles exhibit the greatest critical shear stress because the particle thalli interlock and resist entrainment. In samples with a high percentage of maerl and low percentage of siliciclastic sand, the lower density, lower settling velocity and lower critical bed shear stress of maerl results in its preferential transport over the siliciclastic sediment. At velocities ∼10 cm s-1 higher than the threshold velocity of grain motion, rarely-documented subaqueous maerl dunes formed in the annular flume.

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

Wall Shear Stress Restoration in Dialysis Patient's Venous Stenosis: Elucidation via 3D CFD and Shape Optimization

NASA Astrophysics Data System (ADS)

Mahmoudzadeh Akherat, S. M. Javid; Cassel, Kevin; Hammes, Mary; Boghosian, Michael; Illinois Institute of Technology Team; University of Chicago Team

2016-11-01

Venous stenosis developed after the growth of excessive neointimal hyperplasia (NH) in chronic dialysis treatment is a major cause of mortality in renal failure patients. It has been hypothesized that the low wall shear stress (WSS) triggers an adaptive response in patients' venous system that through the growth of neointimal hyperplastic lesions restores WSS and transmural pressure, which also regulates the blood flow rate back to physiologically acceptable values which is violated by dialysis treatment. A strong coupling of three-dimensional CFD and shape optimization analyses were exploited to elucidate and forecast this adaptive response which correlates very well topographically with patient-specific clinical data. Based on the framework developed, a medical protocol is suggested to predict and prevent dialysis treatment failure in clinical practice. Supported by the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health (R01 DK90769).

A numerical study of multiple adiabatic shear bands evolution in a 304LSS thick-walled cylinder

NASA Astrophysics Data System (ADS)

Liu, Mingtao; Hu, Haibo; Fan, Cheng; Tang, Tiegang

2017-01-01

The self-organization of multiple shear bands in a 304L stainless steel(304LSS) thick-walled cylinder (TWC) was numerically studied. The microstructures of material lead to the non-uniform distribution of the local yield stress, which play a key role in the formation of spontaneous shear localization. We introduced a probability factor satisfied the Gaussian distribution into the macroscopic constitutive relationship to describe the non-uniformity of local yield stress. Using the probability factor, the initiation and propagation of multiple shear bands in TWC were numerically replicated in our 2D FEM simulation. Experimental results in the literature indicated that the machined surface at the internal boundary of a 304L stainless steel cylinder provides a work-hardened layer (about 20˜30μm) which has significantly different microstructures from the base material. The work-hardened layer leads to the phenomenon that most shear bands propagate along a given direction, clockwise or counterclockwise. In our simulation, periodical single direction spiral perturbations were applied to describe the grain orientation in the work-hardened layer, and the single direction spiral pattern of shear bands was successfully replicated.

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.

Shear stress and the endothelial transport barrier.

PubMed

Tarbell, John M

2010-07-15

The shear stress of flowing blood on the surfaces of endothelial cells that provide the barrier to transport of solutes and water between blood and the underlying tissue modulates the permeability to solutes and the hydraulic conductivity. This review begins with a discussion of transport pathways across the endothelium and then considers the experimental evidence from both in vivo and in vitro studies that shows an influence of shear stress on endothelial transport properties after both acute (minutes to hours) and chronic (hours to days) changes in shear stress. Next, the effects of shear stress on individual transport pathways (tight junctions, adherens junctions, vesicles and leaky junctions) are described, and this information is integrated with the transport experiments to suggest mechanisms controlling both acute and chronic responses of transport properties to shear stress. The review ends with a summary of future research challenges.

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.

A Near-Wall Reynolds-Stress Closure Without Wall Normals

NASA Technical Reports Server (NTRS)

Yuan, S. P.; So, R. M. C.

1997-01-01

Turbulent wall-bounded complex flows are commonly encountered in engineering practice and are of considerable interest in a variety of industrial applications. The presence of a wall significantly affects turbulence characteristics. In addition to the wall effects, turbulent wall-bounded flows become more complicated by the presence of additional body forces (e.g. centrifugal force and Coriolis force) and complex geometry. Most near-wall Reynolds stress models are developed from a high-Reynolds-number model which assumes turbulence is homogenous (or quasi-homogenous). Near-wall modifications are proposed to include wall effects in near-wall regions. In this process, wall normals are introduced. Good predictions could be obtained by Reynolds stress models with wall normals. However, ambiguity arises when the models are applied in flows with multiple walls. Many models have been proposed to model turbulent flows. Among them, Reynolds stress models, in which turbulent stresses are obtained by solving the Reynolds stress transport equations, have been proved to be the most successful ones. To apply the Reynolds stress models to wall-bounded flows, near-wall corrections accounting for the wall effects are needed, and the resulting models are called near-wall Reynolds stress models. In most of the existing near-wall models, the near-wall corrections invoke wall normals. These wall-dependent near-wall models are difficult to implement for turbulent flows with complex geometry and may give inaccurate predictions due to the ambiguity of wall normals at corners connecting multiple walls. The objective of this study is to develop a more general and flexible near-wall Reynolds stress model without using any wall-dependent variable for wall-bounded turbulent flows. With the aid of near-wall asymptotic analysis and results of direct numerical simulation, a new near-wall Reynolds stress model (NNWRS) is formulated based on Speziale et al.'s high-Reynolds-stress model with wall

Modeling Force Transfer around Openings in Wood-Frame Shear Walls

Treesearch

Minghao Li; Frank Lam; Borjen Yeh; Tom Skaggs; Doug Rammer; James Wacker

2012-01-01

This paper presented a modeling study on force transfer around openings (FTAO) in wood-frame shear walls detailed for FTAO. To understand the load transfer in the walls, this study used a finite-element model WALL2D, which is able to model individual wall components, including framing members, sheathing panels, oriented panel-frame nailed connections, framing...

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.

Shear stress and the endothelial transport barrier

PubMed Central

Tarbell, John M.

2010-01-01

The shear stress of flowing blood on the surfaces of endothelial cells that provide the barrier to transport of solutes and water between blood and the underlying tissue modulates the permeability to solutes and the hydraulic conductivity. This review begins with a discussion of transport pathways across the endothelium and then considers the experimental evidence from both in vivo and in vitro studies that shows an influence of shear stress on endothelial transport properties after both acute (minutes to hours) and chronic (hours to days) changes in shear stress. Next, the effects of shear stress on individual transport pathways (tight junctions, adherens junctions, vesicles and leaky junctions) are described, and this information is integrated with the transport experiments to suggest mechanisms controlling both acute and chronic responses of transport properties to shear stress. The review ends with a summary of future research challenges. PMID:20543206

Role of endothelium sensitivity to shear stress in noradrenaline-induced constriction of feline femoral arterial bed under constant flow and constant pressure perfusions.

PubMed

Kartamyshev, Sergey P; Balashov, Sergey A; Melkumyants, Arthur M

2007-01-01

The effect of shear stress at the endothelium in the attenuation of the noradrenaline-induced constriction of the femoral vascular bed perfused at a constant blood flow was investigated in 16 anesthetized cats. It is known that the adrenergic vasoconstriction of the femoral vascular bed is considerably greater at a constant pressure perfusion than at a constant blood flow. This difference may depend on the ability of the endothelium to relax smooth muscle in response to an increase in wall shear stress. Since the shear stress is directly related to the blood flow and inversely related to the third power of vessel diameter, vasoconstriction at a constant blood flow increases the wall shear stress that is the stimulus for smooth muscle relaxation opposing constriction. On the other hand, at a constant perfusion pressure, vasoconstriction is accompanied by a decrease in flow rate, which prevents a wall shear stress increase. To reveal the effect of endothelial sensitivity to shear stress, we compared noradrenaline-induced changes in total and proximal arterial resistances during perfusion of the hind limb at a constant blood flow and at a constant pressure in vessels with intact and injured endothelium. We found that in the endothelium-intact bed the same concentration of noradrenaline at a constant flow caused an increase in overall vascular peripheral resistance that was half as large as at a constant perfusion pressure. This difference is mainly confined to the proximal arterial vessels (arteries and large arterioles) whose resistance at a constant flow increased only 0.19 +/- 0.03 times compared to that at a constant pressure. The removal of the endothelium only slightly increased constrictor responses at the perfusion under a constant pressure (noradrenaline-induced increases of both overall and proximal arterial resistance augmented by 12%), while the responses of the proximal vessels at a constant flow became 4.7 +/- 0.4 times greater than in the endothelium

Carotid artery wall shear stress is independently correlated with renal function in the elderly.

PubMed

Guo, Yuqi; Wei, Fang; Wang, Juan; Zhao, Yingxin; Sun, Shangwen; Zhang, Hua; Liu, Zhendong

2018-01-12

Hemodynamic has increasingly been regarded as an important factor of renal function. However, the relationship between carotid artery wall shear stress (WSS) and renal function is not clarified. To investigate the relationship between carotid WSS and renal function, we recruited 761 older subjects aged 60 years and over from community-dwelling in the Shandong area, China. Carotid WSS, endothelial function, and estimated glomerular filtration rate (eGFR) were assessed in all subjects. Subjects were grouped by the interquartile of the carotid artery mean WSS. We found that the eGFRs derived from serum creatinine and/or cystatin C using three CKD-EPI equations were significantly higher and albumin/creatinine ratio was lower in the higher interquartile groups than in the lower interquartile groups ( P <0.05). The mean WSS was independently correlated with eGFRs even after adjustment for confounders. Similar findings were found between carotid artery peak WSS and eGFRs and albumin/creatinine ratio. In addition, we found that endothelial function was strongly related to carotid WSS and renal function after adjustment for confounders. In conclusion, there is an independent correlation of carotid WSS with renal function in the elderly. The local rheologic forces may play an important role in renal function changing. The correlation may be mediated by regulation of endothelial function.

An investigation on near wall transport characteristics in an adiabatic upward gas-liquid two-phase slug flow

NASA Astrophysics Data System (ADS)

Zheng, Donghong; Che, Defu

2007-08-01

The near-wall transport characteristics, inclusive of mass transfer coefficient and wall shear stress, which have a great effect on gas-liquid two-phase flow induced internal corrosion of low alloy pipelines in vertical upward oil and gas mixing transport, have been both mechanistically and experimentally investigated in this paper. Based on the analyses on the hydrodynamic characteristics of an upward slug unit, the mass transfer in the near wall can be divided into four zones, Taylor bubble nose zone, falling liquid film zone, Taylor bubble wake zone and the remaining liquid slug zone; the wall shear stress can be divided into two zones, the positive wall shear stress zone associated with the falling liquid film and the negative wall shear stress zone associated with the liquid slug. Based on the conventional mass transfer and wall shear stress characteristics formulas of single phase liquid full-pipe turbulent flow, corrected normalized mass transfer coefficient formula and wall shear stress formula are proposed. The calculated results are in good agreement with the experimental data. The shear stress and the mass transfer coefficient in the near wall zone are increased with the increase of superficial gas velocity and decreased with the increase of superficial liquid velocity. The mass transfer coefficients in the falling liquid film zone and the wake zone of leading Taylor bubble are lager than those in the Taylor bubble nose zone and the remaining liquid slug zone, and the wall shear stress associated falling liquid film is larger than that associated the liquid slug. The mass transfer coefficient is within 10-3 m/s, and the wall shear stress below 103 Pa. It can be concluded that the alternate wall shear stress due to upward gas-liquid slug flow is considered to be the major cause of the corrosion production film fatigue cracking.

Simulation of NO and O2 transport facilitated by polymerized hemoglobin solutions in an arteriole that takes into account wall shear stress-induced NO production.

PubMed

Zhou, Yipin; Cabrales, Pedro; Palmer, Andre F

2012-03-01

A mathematical model was developed to study nitric oxide (NO) and oxygen (O(2)) transport in an arteriole and surrounding tissues exposed to a mixture of red blood cells (RBCs) and hemoglobin (Hb)-based O(2) carriers (HBOCs). A unique feature of this model is the inclusion of blood vessel wall shear stress-induced production of endothelial-derived NO, which is very sensitive to the viscosity of the RBC and HBOC mixture traversing the blood vessel lumen. Therefore in this study, a series of polymerized bovine Hb (PolyHb) solutions with high viscosity, varying O(2) affinities, NO dioxygenation rate constants and O(2) dissociation rate constants that were previously synthesized and characterized by our group was evaluated via mathematical modeling, in order to investigate the effect of these biophysical properties on the transport of NO and O(2) in an arteriole and its surrounding tissues subjected to anemia with the commercial HBOC Oxyglobin® and cell-free bovine Hb (bHb) serving as appropriate controls. The computer simulation results indicated that transfusion of high viscosity PolyHb solutions promoted blood vessel wall shear stress dependent generation of the vasodilator NO, especially in the blood vessel wall and should transport enough NO inside the smooth muscle layer to activate vasodilation compared to the commercial HBOC Oxyglobin® and cell-free bHb. However, NO scavenging in the arteriole lumen was unavoidable due to the intrinsic high NO dioxygenation rate constant of the HBOCs being studied. This study also observed that all PolyHbs could potentially improve tissue oxygenation under hypoxic conditions, while low O(2) affinity PolyHbs were more effective in oxygenating tissues under normoxic conditions compared with high O(2) affinity PolyHbs. In addition, all ultrahigh molecular weight PolyHbs displayed higher O(2) transfer rates than the commercial HBOC Oxyglobin® and cell-free bHb. Therefore, these results suggest that ultrahigh molecular weight

Augmentative effect of pulsatility on the wall shear stress in tube flow.

PubMed

Nakata, M; Tatsumi, E; Tsukiya, T; Taenaka, Y; Nishimura, T; Nishinaka, T; Takano, H; Masuzawa, T; Ohba, K

1999-08-01

Wall shear stress (WSS) has been considered to play an important role in the physiological and metabolic functions of the vascular endothelial cells. We investigated the effects of the pulse rate and the maximum flow rate on the WSS to clarify the influence of pulsatility. Water was perfused in a 1/2 inch transparent straight cylinder with a nonpulsatile centrifugal pump and a pulsatile pneumatic ventricular assist device (VAD). In nonpulsatile flow (NF), the flow rate was changed 1 to 6 L/min by 1 L/min increments to obtain standard values of WSS at each flow rate. In pulsatile flow (PF), the pulse rate was controlled at 40, 60, and 80 bpm, and the maximum flow rate was varied from 3.3 to 12.0 L/min while the mean flow rate was kept at 3 L/min. The WSS was estimated from the velocity profile at measuring points using the laser illuminated fluorescence method. In NF, the WSS was 12.0 dyne/cm2 at 3 L/min and 33.0 dyne/cm2 at 6 L/min. In PF, the pulse rate change with the same mean, and the maximum flow rate did not affect WSS. On the other hand, the increase in the maximum flow rate at the constant mean flow rate of 3 L/min augmented the mean WSS from 13.1 to 32.9 dyne/cm2. We concluded that the maximum flow rate exerted a substantial augmentative effect on WSS, and the maximum flow rate was a dominant factor of pulsatility in this effect.

Shake-table testing of a self-centering precast reinforced concrete frame with shear walls

NASA Astrophysics Data System (ADS)

Lu, Xilin; Yang, Boya; Zhao, Bin

2018-04-01

The seismic performance of a self-centering precast reinforced concrete (RC) frame with shear walls was investigated in this paper. The lateral force resistance was provided by self-centering precast RC shear walls (SPCW), which utilize a combination of unbonded prestressed post-tensioned (PT) tendons and mild steel reinforcing bars for flexural resistance across base joints. The structures concentrated deformations at the bottom joints and the unbonded PT tendons provided the self-centering restoring force. A 1/3-scale model of a five-story self-centering RC frame with shear walls was designed and tested on a shake-table under a series of bi-directional earthquake excitations with increasing intensity. The acceleration response, roof displacement, inter-story drifts, residual drifts, shear force ratios, hysteresis curves, and local behaviour of the test specimen were analysed and evaluated. The results demonstrated that seismic performance of the test specimen was satisfactory in the plane of the shear wall; however, the structure sustained inter-story drift levels up to 2.45%. Negligible residual drifts were recorded after all applied earthquake excitations. Based on the shake-table test results, it is feasible to apply and popularize a self-centering precast RC frame with shear walls as a structural system in seismic regions.

Measuring Interlayer Shear Stress in Bilayer Graphene

NASA Astrophysics Data System (ADS)

Wang, Guorui; Dai, Zhaohe; Wang, Yanlei; Tan, PingHeng; Liu, Luqi; Xu, Zhiping; Wei, Yueguang; Huang, Rui; Zhang, Zhong

2017-07-01

Monolayer two-dimensional (2D) crystals exhibit a host of intriguing properties, but the most exciting applications may come from stacking them into multilayer structures. Interlayer and interfacial shear interactions could play a crucial role in the performance and reliability of these applications, but little is known about the key parameters controlling shear deformation across the layers and interfaces between 2D materials. Herein, we report the first measurement of the interlayer shear stress of bilayer graphene based on pressurized microscale bubble loading devices. We demonstrate continuous growth of an interlayer shear zone outside the bubble edge and extract an interlayer shear stress of 40 kPa based on a membrane analysis for bilayer graphene bubbles. Meanwhile, a much higher interfacial shear stress of 1.64 MPa was determined for monolayer graphene on a silicon oxide substrate. Our results not only provide insights into the interfacial shear responses of the thinnest structures possible, but also establish an experimental method for characterizing the fundamental interlayer shear properties of the emerging 2D materials for potential applications in multilayer systems.

Microbubbles and Blood Brain Barrier Opening: A Numerical Study on Acoustic Emissions and Wall Stress Predictions

PubMed Central

Goertz, David E.; Hynynen, Kullervo

2015-01-01

Focused ultrasound with microbubbles is an emerging technique for blood brain barrier (BBB) opening. Here, a comprehensive theoretical model of a bubble-fluid-vessel system has been developed which accounts for the bubble’s non-spherical oscillations inside a microvessel, and its resulting acoustic emissions. Numerical simulations of unbound and confined encapsulated bubbles were performed to evaluate the effect of the vessel wall on acoustic emissions and vessel wall stresses. Using a Marmottant shell model, the normalized second harmonic to fundamental emissions first decreased as a function of pressure (>50 kPa) until reaching a minima ("transition point") at which point they increased. The transition point of unbound compared to confined bubble populations occurred at different pressures and was associated with an accompanying increase in shear and circumferential wall stresses. As the wall stresses depend on the bubble to vessel wall distance, the stresses were evaluated for bubbles with their wall at a constant distance to a flat wall. As a result, the wall stresses were bubble size and frequency dependent and the peak stress values induced by bubbles larger than resonance remained constant versus frequency at a constant mechanical index. PMID:25546853

COMPUTATIONAL SIMULATIONS DEMONSTRATE ALTERED WALL SHEAR STRESS IN AORTIC COARCTATION PATIENTS TREATED BY RESECTION WITH END-TO-END ANASTOMOSIS

PubMed Central

LaDisa, John F.; Dholakia, Ronak J.; Figueroa, C. Alberto; Vignon-Clementel, Irene E.; Chan, Frandics P.; Samyn, Margaret M.; Cava, Joseph R.; Taylor, Charles A.; Feinstein, Jeffrey A.

2011-01-01

Background Atherosclerotic plaque in the descending thoracic aorta (dAo) is related to altered wall shear stress (WSS) for normal patients. Resection with end-to-end anastomosis (RWEA) is the gold standard for coarctation of the aorta (CoA) repair, but may lead to altered WSS indices that contribute to morbidity. Methods Computational fluid dynamics (CFD) models were created from imaging and blood pressure data for control subjects and age- and gender-matched CoA patients treated by RWEA (4 male, 2 female, 15±8 years). CFD analysis incorporated downstream vascular resistance and compliance to generate blood flow velocity, time-averaged WSS (TAWSS) and oscillatory shear index (OSI) results. These indices were quantified longitudinally and circumferentially in the dAo, and several visualization methods were used to highlight regions of potential hemodynamic susceptibility. Results The total dAo area exposed to subnormal TAWSS and OSI was similar between groups, but several statistically significant local differences were revealed. Control subjects experienced left-handed rotating patterns of TAWSS and OSI down the dAo. TAWSS was elevated in CoA patients near the site of residual narrowings and OSI was elevated distally, particularly along the left dAo wall. Differences in WSS indices between groups were negligible more than 5 dAo diameters distal to the aortic arch. Conclusions Localized differences in WSS indices within the dAo of CoA patients treated by RWEA suggest that plaque may form in unique locations influenced by the surgical repair. These regions can be visualized in familiar and intuitive ways allowing clinicians to track their contribution to morbidity in longitudinal studies. PMID:21801315

Experimental study on the bed shear stress under breaking waves

NASA Astrophysics Data System (ADS)

Hao, Si-yu; Xia, Yun-feng; Xu, Hua

2017-06-01

The object of present study is to investigate the bed shear stress on a slope under regular breaking waves by a novel instrument named Micro-Electro-Mechanical System (MEMS) flexible hot-film shear stress sensor. The sensors were calibrated before application, and then a wave flume experiment was conducted to study the bed shear stress for the case of regular waves spilling and plunging on a 1:15 smooth PVC slope. The experiment shows that the sensor is feasible for the measurement of the bed shear stress under breaking waves. For regular incident waves, the bed shear stress is mainly periodic in both outside and inside the breaking point. The fluctuations of the bed shear stress increase significantly after waves breaking due to the turbulence and vortexes generated by breaking waves. For plunging breaker, the extreme value of the mean maximum bed shear stress appears after the plunging point, and the more violent the wave breaks, the more dramatic increase of the maximum bed shear stress will occur. For spilling breaker, the increase of the maximum bed shear stress along the slope is gradual compared with the plunging breaker. At last, an empirical equation about the relationship between the maximum bed shear stress and the surf similarity parameter is given, which can be used to estimate the maximum bed shear stress under breaking waves in practice.

Model-based image analysis of a tethered Brownian fibre for shear stress sensing

PubMed Central

2017-01-01

The measurement of fluid dynamic shear stress acting on a biologically relevant surface is a challenging problem, particularly in the complex environment of, for example, the vasculature. While an experimental method for the direct detection of wall shear stress via the imaging of a synthetic biology nanorod has recently been developed, the data interpretation so far has been limited to phenomenological random walk modelling, small-angle approximation, and image analysis techniques which do not take into account the production of an image from a three-dimensional subject. In this report, we develop a mathematical and statistical framework to estimate shear stress from rapid imaging sequences based firstly on stochastic modelling of the dynamics of a tethered Brownian fibre in shear flow, and secondly on a novel model-based image analysis, which reconstructs fibre positions by solving the inverse problem of image formation. This framework is tested on experimental data, providing the first mechanistically rational analysis of the novel assay. What follows further develops the established theory for an untethered particle in a semi-dilute suspension, which is of relevance to, for example, the study of Brownian nanowires without flow, and presents new ideas in the field of multi-disciplinary image analysis. PMID:29212755

Hydrodynamic shear stress and mass transport modulation of endothelial cell metabolism.

PubMed

Nollert, M U; Diamond, S L; McIntire, L V

1991-09-01

Mammalian cells responds to physical forces by altering their growth rate, morphology, metabolism, and genetic expression. We have studied the mechanism by which these cells detect the presence of mechanical stress and convert this force into intracellular signals. As our model systems, we have studied cultured human endothelial cells, which line the blood vessels and forms the interface between the blood and the vessel wall. These cell responds within minutes to the initiation of flow by increasing their arachidonic acid metabolism and increasing the level of the intracellular second messengers inositol trisphosphate and calcium ion concentration. With continued exposure to arterial levels of wall shear stress for up to 24 h, endothelial cells increase the expression of tissue plasminogen activator (tPA) and tPA messenger RNA (mRNA) and decrease the expression of endothelin peptide and endothelin mRNA. Since the initiation of flow also causes enhanced convective mass transfer to the endothelial cell monolayer, we have investigated the role of enhanced convection of adenosine trisphosphate (ATP) to the cell surface in eliciting a cellular response by monitoring cytosolic calcium concentrations on the single cell level and by computing the concentration profile of ATP in a parallel-plate flow geometry. Our result demonstrate that endothelial cells respond in very specific ways to the initiation of flow and that mass transfer and fluid shear stress can both play a role in the modulation of intracellular signal transduction and metabolism.

Role of shear stress in nitric oxide-dependent modulation of renal angiotensin II vasoconstriction.

PubMed

Endlich, K; Muller, C; Barthelmebs, M; Helwig, J J

1999-08-01

1. Renal vasoconstriction in response to angiotensin II (ANGII) is known to be modulated by nitric oxide (NO). Since shear stress stimulates the release of a variety of vasoactive compounds from endothelial cells, we studied the impact of shear stress on the haemodynamic effect of ANGII in isolated perfused kidneys of rats under control conditions and during NO synthase inhibition with L-NAME (100 microM). 2. Kidneys were perfused in the presence of cyclo-oxygenase inhibitor (10 microM indomethacin) with Tyrode's solution of relative viscosity zeta=1 (low viscosity perfusate, LVP) or, in order to augment shear stress, with Tyrode's solution containing 7% Ficoll 70 of relative viscosity zeta=2 (high viscosity perfusate, HVP). 3. Vascular conductance was 3.5+/-0.4 fold larger in HVP as compared with LVP kidneys, associated with an augmentation of overall wall shear stress by 37+/-5%. During NO inhibition, vascular conductance was only 2.5+/-0.2 fold elevated in HVP vs LVP kidneys, demonstrating shear stress-induced vasodilatation by NO and non-NO/non-prostanoid compound(s). 4. ANGII (10 - 100 pM) constricted the vasculature in LVP kidneys, but was without effect in HVP kidneys. During NO inhibition, in contrast, ANGII vasoconstriction was potentiated in HVP as compared with LVP kidneys. 5. The potentiation of ANGII vasoconstriction during NO inhibition has been shown to be mediated by endothelium-derived P450 metabolites and to be sensitive to AT2 receptor blockade in our earlier studies. Accordingly, in HVP kidneys, increasing concentrations of the AT2 receptor antagonist PD123319 (5 and 500 nM) gradually abolished the potentiation of ANGII vasoconstriction during NO inhibition, but did not affect vasoconstriction in response to ANGII in LVP kidneys. 6. Our results demonstrate, that augmentation of shear stress by increasing perfusate viscosity induces vasodilatation in the rat kidney, which is partially mediated by NO. Elevated levels of shear stress attenuate

Role of shear stress in nitric oxide-dependent modulation of renal angiotensin II vasoconstriction

PubMed Central

Endlich, Karlhans; Muller, Catherine; Barthelmebs, Mariette; Helwig, Jean-Jacques

1999-01-01

Renal vasoconstriction in response to angiotensin II (ANGII) is known to be modulated by nitric oxide (NO). Since shear stress stimulates the release of a variety of vasoactive compounds from endothelial cells, we studied the impact of shear stress on the haemodynamic effect of ANGII in isolated perfused kidneys of rats under control conditions and during NO synthase inhibition with L-NAME (100 μM).Kidneys were perfused in the presence of cyclo-oxygenase inhibitor (10 μM indomethacin) with Tyrode's solution of relative viscosity ζ=1 (low viscosity perfusate, LVP) or, in order to augment shear stress, with Tyrode's solution containing 7% Ficoll 70 of relative viscosity ζ=2 (high viscosity perfusate, HVP).Vascular conductance was 3.5±0.4 fold larger in HVP as compared with LVP kidneys, associated with an augmentation of overall wall shear stress by 37±5%. During NO inhibition, vascular conductance was only 2.5±0.2 fold elevated in HVP vs LVP kidneys, demonstrating shear stress-induced vasodilatation by NO and non-NO/non-prostanoid compound(s).ANGII (10–100 pM) constricted the vasculature in LVP kidneys, but was without effect in HVP kidneys. During NO inhibition, in contrast, ANGII vasoconstriction was potentiated in HVP as compared with LVP kidneys.The potentiation of ANGII vasoconstriction during NO inhibition has been shown to be mediated by endothelium-derived P450 metabolites and to be sensitive to AT2 receptor blockade in our earlier studies. Accordingly, in HVP kidneys, increasing concentrations of the AT2 receptor antagonist PD123319 (5 and 500 nM) gradually abolished the potentiation of ANGII vasoconstriction during NO inhibition, but did not affect vasoconstriction in response to ANGII in LVP kidneys.Our results demonstrate, that augmentation of shear stress by increasing perfusate viscosity induces vasodilatation in the rat kidney, which is partially mediated by NO. Elevated levels of shear stress attenuate renal ANGII vasoconstriction

Effects of shear stress on the microalgae Chaetoceros muelleri

PubMed Central

van der Goot, Atze J.; Norsker, Niels-Henrik; Wijffels, René H.

2010-01-01

The effect of shear stress on the viability of Chaetoceros muelleri was studied using a combination of a rheometer and dedicated shearing devices. Different levels of shear stress were applied by varying the shear rates and the medium viscosities. It was possible to quantify the effect of shear stress over a wide range, whilst preserving laminar flow conditions through the use of a thickening agent. The threshold value at which the viability of algae was negatively influenced was between 1 and 1.3 Pa. Beyond the threshold value the viability decreased suddenly to values between 52 and 66%. The effect of shear stress was almost time independent compared to normal microalgae cultivation times. The main shear stress effect was obtained within 1 min, with a secondary effect of up to 8 min. PMID:20191365

Evolution of allowable stresses in shear for lumber

Treesearch

Robert L. Ethington; William L. Galligan; Henry M. Montrey; Alan D. Freas

1979-01-01

This paper surveys research leading to allowable shear stress parallel to grain for lumber. In early flexure tests of lumber, some pieces failed in shear. The estimated shear stress at time of failure was generally lower than shear strength measured on small, clear, straight-grained specimens. This and other engineering observations gave rise to adjustments that...

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.

Effect of shear stress on water and LDL transport through cultured endothelial cell monolayers.

PubMed

Kang, Hongyan; Cancel, Limary M; Tarbell, John M

2014-04-01

Previous animal experiments have shown that the transport of LDL into arterial walls is shear stress dependent. However, little work has probed shear effects on LDL transport in vitro where conditions are well defined and mechanisms are more easily explored. Therefore, we measured shear induced water and LDL fluxes across cultured bovine aortic endothelial (BAEC) monolayers in vitro and developed a three-pore model to describe the transport dynamics. Cell apoptosis was quantified by TdT-mediated dUTP nick end labeling (TUNEL) assay. We also examined the role of nitric oxide (NO) in shear induced water and LDL fluxes by incubating BAEC monolayers with an NO synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA). Our results show that direct exposure of endothelial monolayers to 12 dyn/cm2 shear stress for 3 h elicited a 2.37-fold increase in water flux (Jv), a 3.00-fold increase in LDL permeability (Pe), a 1.32-fold increase in LDL uptake, and a 1.68-fold increase in apoptotic rate. L-NMMA treatment of BAEC monolayers blocked shear induced Jv response, but had no significant effect on shear responses of Pe and cell apoptosis. A long time shear exposure (12 h) of endothelial monolayers reduced Pe and apoptotic rate close to the baseline. These results suggest that an acute change in shear stress from a static baseline state induces increases in water flux that are mediated by an NO dependent mechanism. On the other hand, the permeability of endothelial monolayers to LDL is enhanced by a short term-shear application and reduced nearly to the baseline level by a longer time shear exposure, positively correlated to the leaky junctions forming around apoptotic cells. Copyright © 2014 Elsevier Ireland Ltd. All rights reserved.

Effect of shear stress on water and LDL transport through cultured endothelial cell monolayers

PubMed Central

Kang, Hongyan; Cancel, Limary M.; Tarbell, John M.

2014-01-01

Previous animal experiments have shown that the transport of LDL into arterial walls is shear stress dependent. However, little work has probed shear effects on LDL transport in vitro where conditions are well defined and mechanisms are more easily explored. Therefore, we measured shear induced water and LDL fluxes across cultured bovine aortic endothelial (BAEC) monolayers in vitro and developed a three-pore model to describe the transport dynamics. Cell apoptosis was quantified by TdT-mediated dUTP nick end labeling (TUNEL) assay. We also examined the role of nitric oxide (NO) in shear induced water and LDL fluxes by incubating BAEC monolayers with a NO synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA). Our results show that direct exposure of endothelial monolayers to 12 dyn/cm2 shear stress for 3 hours elicited a 2.37-fold increase in water flux (Jv), a 3.00-fold increase in LDL permeability (Pe), a 1.32-fold increase in LDL uptake, and a 1.68-fold increase in apoptotic rate. L-NMMA treatment of BAEC monolayers blocked shear induced Jv response, but had no significant effect on shear responses of Pe and cell apoptosis. A long time shear exposure (12 h) of endothelial monolayers reduced Pe and apoptotic rate close to the baseline. These results suggest that an acute change in shear stress from a static baseline state induces increases in water flux that are mediated by a NO dependent mechanism. On the other hand, the permeability of endothelial monolayers to LDL is enhanced by a short term-shear application and reduced nearly to the baseline level by a longer time shear exposure, positively correlated to the leaky junctions forming around apoptotic cells. PMID:24583416

Normal stresses in shear thickening granular suspensions.

PubMed

Pan, Zhongcheng; de Cagny, Henri; Habibi, Mehdi; Bonn, Daniel

2017-05-24

When subjected to shear, granular suspensions exhibit normal stresses perpendicular to the shear plane but the magnitude and sign of the different components of the normal stresses are still under debate. By performing both oscillatory and rotational rheology measurements on shear thickening granular suspensions and systematically varying the particle diameters and the gap sizes between two parallel-plates, we show that a transition from a positive to a negative normal stress can be observed. We find that frictional interactions which determine the shear thickening behavior of suspensions contribute to the positive normal stresses. Increasing the particle diameters or decreasing the gap sizes leads to a growing importance of hydrodynamic interactions, which results in negative normal stresses. We determine a relaxation time for the system, set by both the pore and the gap sizes, that governs the fluid flow through the inter-particle space. Finally, using a two-fluid model we determine the relative contributions from the particle phase and the liquid phase.

Meniscal shear stress for punching.

PubMed

Tuijthof, Gabrielle J M; Meulman, Hubert N; Herder, Just L; van Dijk, C Niek

2009-01-01

Experimental determination of the shear stress for punching meniscal tissue. Meniscectomy (surgical treatment of a lesion of one of the menisci) is the most frequently performed arthroscopic procedure. The performance of a meniscectomy is not optimal with the currently available instruments. To design new instruments, the punching force of meniscal tissue is an important parameter. Quantitative data are unavailable. The meniscal punching process was simulated by pushing a rod through meniscal tissue at constant speed. Three punching rods were tested: a solid rod of Oslash; 3.00 mm, and two hollow tubes (Oslash; 3.00-2.60 mm) with sharpened cutting edges of 0.15 mm and 0.125 mm thick, respectively. Nineteen menisci acquired from 10 human cadaveric knee joints were punched (30 tests). The force and displacement were recorded from which the maximum shear stress was determined (average added with three times the standard deviation). The maximum shear stress for the solid rod was determined at 10.2 N/mm2. This rod required a significantly lower punch force in comparison with the hollow tube having a 0.15 mm cutting edge (plt;0.01). The maximum shear stress for punching can be applied to design instruments, and virtual reality training environments. This type of experiment is suitable to form a database with material properties of human tissue similar to databases for the manufacturing industry.

Experimental Research on Boundary Shear Stress in Typical Meandering Channel

NASA Astrophysics Data System (ADS)

Chen, Kai-hua; Xia, Yun-feng; Zhang, Shi-zhao; Wen, Yun-cheng; Xu, Hua

2018-06-01

A novel instrument named Micro-Electro-Mechanical System (MEMS) flexible hot-film shear stress sensor was used to study the boundary shear stress distribution in the generalized natural meandering open channel, and the mean sidewall shear stress distribution along the meandering channel, and the lateral boundary shear stress distribution in the typical cross-section of the meandering channel was analysed. Based on the measurement of the boundary shear stress, a semi-empirical semi-theoretical computing approach of the boundary shear stress was derived including the effects of the secondary flow, sidewall roughness factor, eddy viscosity and the additional Reynolds stress, and more importantly, for the first time, it combined the effects of the cross-section central angle and the Reynolds number into the expressions. Afterwards, a comparison between the previous research and this study was developed. Following the result, we found that the semi-empirical semi-theoretical boundary shear stress distribution algorithm can predict the boundary shear stress distribution precisely. Finally, a single factor analysis was conducted on the relationship between the average sidewall shear stress on the convex and concave bank and the flow rate, water depth, slope ratio, or the cross-section central angle of the open channel bend. The functional relationship with each of the above factors was established, and then the distance from the location of the extreme sidewall shear stress to the bottom of the open channel was deduced based on the statistical theory.

An experimental study of near wall flow parameters in the blade end-wall corner region

NASA Technical Reports Server (NTRS)

Bhargava, Rakesh K.; Raj, Rishi S.

1989-01-01

The near wall flow parameters in the blade end-wall corner region is investigated. The blade end-wall corner region was simulated by mounting an airfoil section (NACA 65-015 base profile) symmetric blades on both sides of the flat plate with semi-circular leading edge. The initial 7 cm from the leading edge of the flat plate was roughened by gluing No. 4 floor sanding paper to artificially increase the boundary layer thickness on the flat plate. The initial flow conditions of the boundary layer upstream of the corner region are expected to dictate the behavior of flow inside the corner region. Therefore, an experimental investigation was extended to study the combined effect of initial roughness and increased level of free stream turbulence on the development of a 2-D turbulent boundary layer in the absence of the blade. The measurement techniques employed in the present investigation included, the conventional pitot and pitot-static probes, wall taps, the Preston tube, piezoresistive transducer and the normal sensor hot-wire probe. The pitot and pitot-static probes were used to obtain mean velocity profile measurements within the boundary layer. The measurements of mean surface static pressure were obtained with the surface static tube and the conventional wall tap method. The wall shear vector measurements were made with a specially constructed Preston tube. The flush mounted piezoresistive type pressure transducer were employed to measure the wall pressure fluctuation field. The velocity fluctuation measurements, used in obtaining the wall pressure-velocity correlation data, were made with normal single sensor hot-wire probe. At different streamwise stations, in the blade end-wall corner region, the mean values of surface static pressure varied more on the end-wall surface in the corner region were mainly caused by the changes in the curvature of the streamlines. The magnitude of the wall shear stress in the blade end-wall corner region increased significantly

High shear microfluidics and its application in rheological measurement

NASA Astrophysics Data System (ADS)

Kang, Kai; Lee, L. James; Koelling, Kurt W.

2005-02-01

High shear rheology was explored experimentally in microchannels (150×150 μm). Two aqueous polymer solutions, polyethylene oxide (viscoelastic fluid) and hydroxyethyl cellulose (viscous fluid) were tested. Bagley correction was applied to remove the end effect. Wall slip was investigated with Mooney’s analysis. Shear rates as high as 106 s-1 were obtained in the pressure-driven microchannel flow, allowing a smooth extension of the low shear rheological data obtained from the conventional rheometers. At high shear rates, polymer degradation was observed for PEO solutions at a critical microchannel wall shear stress of 4.1×103 Pa. Stresses at the ends of the microchannel also contributed to PEO degradation significantly.

Seismic behavior of outrigger truss-wall shear connections using multiple steel angles

NASA Astrophysics Data System (ADS)

Li, Xian; Wang, Wei; Lü, Henglin; Zhang, Guangchang

2016-06-01

An experimental investigation on the seismic behavior of a type of outrigger truss-reinforced concrete wall shear connection using multiple steel angles is presented. Six large-scale shear connection models, which involved a portion of reinforced concrete wall and a shear tab welded onto a steel endplate with three steel angles, were constructed and tested under combined actions of cyclic axial load and eccentric shear. The effects of embedment lengths of steel angles, wall boundary elements, types of anchor plates, and thicknesses of endplates were investigated. The test results indicate that properly detailed connections exhibit desirable seismic behavior and fail due to the ductile fracture of steel angles. Wall boundary elements provide beneficial confinement to the concrete surrounding steel angles and thus increase the strength and stiffness of connections. Connections using whole anchor plates are prone to suffer concrete pry-out failure while connections with thin endplates have a relatively low strength and fail due to large inelastic deformations of the endplates. The current design equations proposed by Chinese Standard 04G362 and Code GB50011 significantly underestimate the capacities of the connection models. A revised design method to account for the influence of previously mentioned test parameters was developed.

A Microfluidic-Based Multi-Shear Device for Investigating the Effects of Low Fluid-Induced Stresses on Osteoblasts

PubMed Central

Yu, Weiliang; Qu, Hong; Hu, Guoqing; Zhang, Qian; Song, Kui; Guan, Haijie; Liu, Tingjiao; Qin, Jianhua

2014-01-01

Interstitial fluid flow (IFF) within the extracellular matrix (ECM) produces low magnitude shear stresses on cells. Fluid flow-induced stress (FSS) plays an important role during tissue morphogenesis. To investigate the effect of low FSS generated by IFF on cells, we developed a microfluidic-based cell culture device that can generate multiple low shear stresses. By changing the length and width of the flow-in channels, different continuous low level shear stresses could be generated in individual cell culture chambers. Numerical calculations demonstrate uniform shear stress distributions of the major cell culture area of each chamber. This calculation is further confirmed by the wall shear stress curves. The effects of low FSS on MC3T3-E1 proliferation and differentiation were studied using this device. It was found that FSS ranging from 1.5 to 52.6 µPa promoted MC3T3-E1 proliferation and differentiation, but FSS over 412 µPa inhibited the proliferation and differentiation of MC3T3-E1 cells. FSS ranging from 1.5 to 52.6 µPa also increased the expression of Runx2, a key transcription factor regulating osteoblast differentiation. It is suggested that Runx2 might be an important regulator in low FSS-induced MC3T3-E1 differentiation. This device allows for detailed study of the effect of low FSS on the behaviors of cells; thus, it would be a useful tool for analysis of the effects of IFF-induced shear stresses on cells. PMID:24587156

Combined effects of physiologically relevant disturbed wall shear stress and glycated albumin on endothelial cell functions associated with inflammation, thrombosis and cytoskeletal dynamics.

PubMed

Maria, Zahra; Yin, Wei; Rubenstein, David Alan

2014-07-01

Diabetes mellitus is a major risk factor in the development of cardiovascular diseases (CVDs). The presence of advanced glycation end-products (AGEs) promotes CVDs by upregulating endothelial cell (EC) inflammatory and thrombotic responses, in a similar manner as disturbed shear stress. However, the combined effect of disturbed shear stress and AGEs on EC function has yet to be determined. Our goal was to evaluate these effects on EC responses. ECs were incubated with AGEs for 5 days. ECs were then subjected to physiological or pathological shear stress. Cell metabolic activity, surface expression of intercellular adhesion molecule-1, thrombomodulin, connexin-43 and caveolin-1, and cytoskeleton organization were quantified. The results show that irreversibly glycated albumin and pathological shear stress increased EC metabolic activity, and upregulated and downregulated the EC surface expression of intercellular adhesion molecule-1 and thrombomodulin, respectively. Expression of connexin-43, caveolin-1 and cytoskeletal organization was independent of shear stress; however, the presence of irreversibly glycated AGEs markedly increased connexin-43, and decreased caveolin-1 expression and actin cytoskeletal connectivity. Our data suggest that irreversibly glycated albumin and disturbed shear stress could promote CVD pathogenesis by enhancing EC inflammatory and thrombotic responses, and through the deterioration of the cytoskeletal organization.

Study on Shear Performance of Cold-formed Steel Composite Wall with New Type of stud

NASA Astrophysics Data System (ADS)

Wang, Chungang; Yue, Sizhe; Liu, Hong; Zhang, Zhuangnan

2018-03-01

The shear resistance of single oriented-strand board wall and single gypsum board wall can be improved in different degrees by increasing strength of steel. The experimental data of literatures were used, and the test specimens had been simulated and validated by ABAQUS finite element analysis. According to the research, it showed that the compressive bearing capacity of the new stud composite wall was much better than the common stud composite wall, so the establishment and research of all models had been based on the new section stud. The analysis results show that when using new type of stud the shear resistance of the single oriented-strand board wall can be improved efficiently by increasing strength of steel, but the shear resistance of the single gypsum wall can be increased little.

Wrinkling of Stretched Films: Shear Stress

NASA Technical Reports Server (NTRS)

Zak, M. A.

1982-01-01

Report presents theoretical investigation on nonlinear shearing characteristics of wrinkling films under applied shear stress. Report helps explain force/deflection characteristic of in-planeboom and solar-array blanket structural combinations.

The Fluid Mechanics of a Wavy-Wall Bioreactor

NASA Astrophysics Data System (ADS)

Sucosky, Philippe; Bilgen, Bahar; Aleem, Alexander; Neitzel, Paul; Barabino, Gilda

2004-11-01

Bioreactors are devices used for the production of mammalian tissue in vitro. Although mixing has been shown to stimulate the growth of cartilage constructs, high shear-stress levels can damage the cells. In order to enhance mixing while minimizing shear, a wavy-wall bioreactor (WWB) featuring a sinusoidal internal profile has been designed. The turbulent hydrodynamic environment produced in this device is investigated experimentally using particle-image velocimetry. A model bioreactor made of acrylic and filled with an index-matching solution of zinc iodide is used to compensate for the refraction of light at the walls. The flow observed in different planes is shown to be periodic, spatially dependent, and dominated by mean-shear rather than Reynolds stresses in the vicinity of constructs. Finally, a comparison between the mean-shear stresses obtained in the WWB and in a standard spinner flask reveals similar stress levels near the construct walls.

An Automatic Braking System That Stabilizes Leukocyte Rolling by an Increase in Selectin Bond Number with Shear

PubMed Central

Chen, Shuqi; Springer, Timothy A.

1999-01-01

Wall shear stress in postcapillary venules varies widely within and between tissues and in response to inflammation and exercise. However, the speed at which leukocytes roll in vivo has been shown to be almost constant within a wide range of wall shear stress, i.e., force on the cell. Similarly, rolling velocities on purified selectins and their ligands in vitro tend to plateau. This may be important to enable rolling leukocytes to be exposed uniformly to activating stimuli on endothelium, independent of local hemodynamic conditions. Wall shear stress increases the rate of dissociation of individual selectin–ligand tether bonds exponentially (1, 4) thereby destabilizing rolling. We find that this is compensated by a shear-dependent increase in the number of bonds per rolling step. We also find an increase in the number of microvillous tethers to the substrate. This explains (a) the lack of firm adhesion through selectins at low shear stress or high ligand density, and (b) the stability of rolling on selectins to wide variation in wall shear stress and ligand density, in contrast to rolling on antibodies (14). Furthermore, our data successfully predict the threshold wall shear stress below which rolling does not occur. This is a special case of the more general regulation by shear of the number of bonds, in which the number of bonds falls below one. PMID:9885254

An automatic braking system that stabilizes leukocyte rolling by an increase in selectin bond number with shear.

PubMed

Chen, S; Springer, T A

1999-01-11

Wall shear stress in postcapillary venules varies widely within and between tissues and in response to inflammation and exercise. However, the speed at which leukocytes roll in vivo has been shown to be almost constant within a wide range of wall shear stress, i.e., force on the cell. Similarly, rolling velocities on purified selectins and their ligands in vitro tend to plateau. This may be important to enable rolling leukocytes to be exposed uniformly to activating stimuli on endothelium, independent of local hemodynamic conditions. Wall shear stress increases the rate of dissociation of individual selectin-ligand tether bonds exponentially (, ) thereby destabilizing rolling. We find that this is compensated by a shear-dependent increase in the number of bonds per rolling step. We also find an increase in the number of microvillous tethers to the substrate. This explains (a) the lack of firm adhesion through selectins at low shear stress or high ligand density, and (b) the stability of rolling on selectins to wide variation in wall shear stress and ligand density, in contrast to rolling on antibodies (). Furthermore, our data successfully predict the threshold wall shear stress below which rolling does not occur. This is a special case of the more general regulation by shear of the number of bonds, in which the number of bonds falls below one.

Direct Assessment of Wall Shear Stress by Signal Intensity Gradient from Time-of-Flight Magnetic Resonance Angiography

PubMed Central

Lee, Sang Hyuk; Ryu, Han Uk; Park, Se-Hyoung; Chung, Gyung-Ho; Cho, Young I.

2017-01-01

The aim of the study was to calculate the arterial wall signal intensity gradient (SIG) from time-of-flight MR angiography (TOF-MRA) and represent arterial wall shear stress. We developed a new algorithm that uses signal intensity (SI) of a TOF-MRA to directly calculate the signal intensity gradient (SIG). The results from our phantom study showed that the TOF-MRA SIG could be used to distinguish the magnitude of blood flow rate as high (mean SIG ± SD, 2.2 ± 0.4 SI/mm for 12.5 ± 2.3 L/min) and low (0.9 ± 0.3 SI/mm for 8.5 ± 2.6 L/min) in vessels (p < 0.001). Additionally, we found that the TOF-MRA SIG values were highly correlated with various flow rates (β = 0.96, p < 0.001). Remarkably, the correlation coefficient between the WSS obtained from the computational fluid dynamics (CFD) analysis and the TOF-MRA SIG was greater than 0.8 in each section at the carotid artery (p < 0.001 for all β values). This new technique using TOF-MRA could enable the rapid calculation of the TOF-MRA SIG and thereby the WSS. Thus, the TOF-MRA SIG can provide clinicians with an accurate and efficient screening method for making rapid decisions on the risk of vascular disease for a patient in clinical practice. PMID:28900625

Direct Assessment of Wall Shear Stress by Signal Intensity Gradient from Time-of-Flight Magnetic Resonance Angiography.

PubMed

Han, Kap-Soo; Lee, Sang Hyuk; Ryu, Han Uk; Park, Se-Hyoung; Chung, Gyung-Ho; Cho, Young I; Jeong, Seul-Ki

2017-01-01

The aim of the study was to calculate the arterial wall signal intensity gradient (SIG) from time-of-flight MR angiography (TOF-MRA) and represent arterial wall shear stress. We developed a new algorithm that uses signal intensity (SI) of a TOF-MRA to directly calculate the signal intensity gradient (SIG). The results from our phantom study showed that the TOF-MRA SIG could be used to distinguish the magnitude of blood flow rate as high (mean SIG ± SD, 2.2 ± 0.4 SI/mm for 12.5 ± 2.3 L/min) and low (0.9 ± 0.3 SI/mm for 8.5 ± 2.6 L/min) in vessels ( p < 0.001). Additionally, we found that the TOF-MRA SIG values were highly correlated with various flow rates ( β = 0.96, p < 0.001). Remarkably, the correlation coefficient between the WSS obtained from the computational fluid dynamics (CFD) analysis and the TOF-MRA SIG was greater than 0.8 in each section at the carotid artery ( p < 0.001 for all β values). This new technique using TOF-MRA could enable the rapid calculation of the TOF-MRA SIG and thereby the WSS. Thus, the TOF-MRA SIG can provide clinicians with an accurate and efficient screening method for making rapid decisions on the risk of vascular disease for a patient in clinical practice.

Localized stress fluctuations drive shear thickening in dense suspensions

NASA Astrophysics Data System (ADS)

Rathee, Vikram; Blair, Daniel L.; Urbach, Jeffrey S.

2017-08-01

Dense particulate suspensions exhibit a dramatic increase in average viscosity above a critical, material-dependent shear stress. This thickening changes from continuous to discontinuous as the concentration is increased. Using direct measurements of spatially resolved surface stresses in the continuous thickening regime, we report the existence of clearly defined dynamic localized regions of substantially increased stress that appear intermittently at stresses above the critical stress. With increasing applied stress, these regions occupy an increasing fraction of the system, and the increase accounts quantitatively for the observed shear thickening. The regions represent high-viscosity fluid phases, with a size determined by the distance between the shearing surfaces and a viscosity that is nearly independent of shear rate but that increases rapidly with concentration. Thus, we find that continuous shear thickening arises from increasingly frequent localized discontinuous transitions between distinct fluid phases with widely differing viscosities.

Finite element analyses for seismic shear wall international standard problem

DOE Office of Scientific and Technical Information (OSTI.GOV)

Park, Y.J.; Hofmayer, C.H.

Two identical reinforced concrete (RC) shear walls, which consist of web, flanges and massive top and bottom slabs, were tested up to ultimate failure under earthquake motions at the Nuclear Power Engineering Corporation`s (NUPEC) Tadotsu Engineering Laboratory, Japan. NUPEC provided the dynamic test results to the OECD (Organization for Economic Cooperation and Development), Nuclear Energy Agency (NEA) for use as an International Standard Problem (ISP). The shear walls were intended to be part of a typical reactor building. One of the major objectives of the Seismic Shear Wall ISP (SSWISP) was to evaluate various seismic analysis methods for concrete structuresmore » used for design and seismic margin assessment. It also offered a unique opportunity to assess the state-of-the-art in nonlinear dynamic analysis of reinforced concrete shear wall structures under severe earthquake loadings. As a participant of the SSWISP workshops, Brookhaven National Laboratory (BNL) performed finite element analyses under the sponsorship of the U.S. Nuclear Regulatory Commission (USNRC). Three types of analysis were performed, i.e., monotonic static (push-over), cyclic static and dynamic analyses. Additional monotonic static analyses were performed by two consultants, F. Vecchio of the University of Toronto (UT) and F. Filippou of the University of California at Berkeley (UCB). The analysis results by BNL and the consultants were presented during the second workshop in Yokohama, Japan in 1996. A total of 55 analyses were presented during the workshop by 30 participants from 11 different countries. The major findings on the presented analysis methods, as well as engineering insights regarding the applicability and reliability of the FEM codes are described in detail in this report. 16 refs., 60 figs., 16 tabs.« less

Calculation Method of Lateral Strengths and Ductility Factors of Constructions with Shear Walls of Different Ductility

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yamaguchi, Nobuyoshi; Nakao, Masato; Murakami, Masahide

2008-07-08

For seismic design, ductility-related force modification factors are named R factor in Uniform Building Code of U.S, q factor in Euro Code 8 and Ds (inverse of R) factor in Japanese Building Code. These ductility-related force modification factors for each type of shear elements are appeared in those codes. Some constructions use various types of shear walls that have different ductility, especially for their retrofit or re-strengthening. In these cases, engineers puzzle the decision of force modification factors of the constructions. Solving this problem, new method to calculate lateral strengths of stories for simple shear wall systems is proposed andmore » named 'Stiffness--Potential Energy Addition Method' in this paper. This method uses two design lateral strengths for each type of shear walls in damage limit state and safety limit state. Two lateral strengths of stories in both limit states are calculated from these two design lateral strengths for each type of shear walls in both limit states. Calculated strengths have the same quality as values obtained by strength addition method using many steps of load-deformation data of shear walls. The new method to calculate ductility factors is also proposed in this paper. This method is based on the new method to calculate lateral strengths of stories. This method can solve the problem to obtain ductility factors of stories with shear walls of different ductility.« less

Determination of seismic performance factors for CLT shear wall systems

Treesearch

M. Omar Amini; John W. van de Lindt; Douglas Rammer; Shiling Pei; Philip Line; Marjan Popovski

2016-01-01

This paper presents selected results of connector testing and wall testing which were part of a Forest Products Lab-funded project undertaken at Colorado State University in an effort to determine seismic performance factors for cross laminated timber (CLT) shear walls in the United States. Archetype development, which is required as part of the process, is also...

Response of hot element flush wall gauges in oscillating laminar flow

NASA Technical Reports Server (NTRS)

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

1986-01-01

The time dependent response characteristics of flush-mounted hot element gauges used as instruments to measure wall shear stress in unsteady periodic air flows were investigated. The study was initiated because anomalous results were obtained from the gauges in oscillating turbulent flows for the phase relation of the wall shear stress variation, indicating possible gauge response problems. Flat plate laminar oscillating turbulent flows characterized by a mean free stream velocity with a superposed sinusoidal variation were performed. Laminar rather than turbulent flows were studied, because a numerical solution for the phase angle between the free stream velocity and the wall shear stress variation that is known to be correct can be obtained. The focus is on comparing the phase angle indicated by the hot element gauges with corresponding numerical prediction for the phase angle, since agreement would indicate that the hot element gauges faithfully follow the true wall shear stress variation.

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.

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.

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.

Effects of wall curvature on turbulence statistics

NASA Technical Reports Server (NTRS)

Moser, R. D.; Moin, P.

1985-01-01

A three-dimensional, time-dependent, direct numerical simulation of low-Reynolds number turbulent flow in a mildly curved channel was performed, and the results examined to determine the mechanism by which curvature affects wall-bounded turbulent shear flows. A spectral numerical method with about one-million modes was employed, and no explicit subgrid scale model was used. The effects of curvature on this flow were determined by comparing the concave and convex sides of the channel. The observed effects are consistent with experimental observations for mild curvature. The most significant difference in the turbulence statistics between the concave and convex sides is in the Reynolds shear stress. This is accompanied by significant differences in the terms of the Reynolds shear stress balance equations. In addition, it was found that stationary Taylor-Goertler vortices were present and that they had a significant effect on the flow by contributing to the mean Reynolds shear stress, and by enhancing the difference between the wall shear stresses.

Fluid shear stress and tumor metastasis

PubMed Central

Huang, Qiong; Hu, Xingbin; He, Wanming; Zhao, Yang; Hao, Shihui; Wu, Qijing; Li, Shaowei; Zhang, Shuyi; Shi, Min

2018-01-01

The tumor microenvironment (TME) is a key factor regulating tumor cell invasion and metastasis. The effects of biochemical factors such as stromal cells, immune cells, and cytokines have been previously investigated. Owing to restrictions by the natural barrier between physical and biochemical disciplines, the role of physical factors in tumorigenesis is unclear. However, with the emergence of interdisciplinary mechanobiology and continuous advancements therein in the past 30 years, studies on the effect of physical properties such as hardness or shear stress on tumorigenesis and tumor progression are constantly renewing our understanding of mechanotransduction mechanisms. Shear stress, induced by liquid flow, is known to actively participate in proliferation, apoptosis, invasion, and metastasis of tumor cells. The present review discusses the progress and achievements in studies on tumor fluid microenvironment in recent years, especially fluid shear stress, on tumor metastasis, and presents directions for future study.

Computational fluid dynamics comparisons of wall shear stress in patient-specific coronary artery bifurcation using coronary angiography and optical coherence tomography

NASA Astrophysics Data System (ADS)

Poon, Eric; Thondapu, Vikas; Chin, Cheng; Scheerlinck, Cedric; Zahtila, Tony; Mamon, Chris; Nguyen, Wilson; Ooi, Andrew; Barlis, Peter

2016-11-01

Blood flow dynamics directly influence biology of the arterial wall, and are closely linked with the development of coronary artery disease. Computational fluid dynamics (CFD) solvers may be employed to analyze the hemodynamic environment in patient-specific reconstructions of coronary arteries. Although coronary X-ray angiography (CA) is the most common medical imaging modality for 3D arterial reconstruction, models reconstructed from CA assume a circular or elliptical cross-sectional area. This limitation can be overcome with a reconstruction technique fusing CA with intravascular optical coherence tomography (OCT). OCT scans the interior of an artery using near-infrared light, achieving a 10-micron resolution and providing unprecedented detail of vessel geometry. We compared 3D coronary artery bifurcation models generated using CA alone versus OCT-angiography fusion. The model reconstructed from CA alone is unable to identify the detailed geometrical variations of diseased arteries, and also under-estimates the cross-sectional vessel area compared to OCT-angiography fusion. CFD was performed in both models under pulsatile flow in order to identify and compare regions of low wall shear stress, a hemodynamic parameter directly linked with progression of atherosclerosis. Supported by ARC LP150100233 and VLSCI VR0210.

Shear-coupled grain-boundary migration dependence on normal strain/stress

NASA Astrophysics Data System (ADS)

Combe, N.; Mompiou, F.; Legros, M.

2017-08-01

In specific conditions, grain-boundary (GB) migration occurs in polycrystalline materials as an alternative vector of plasticity compared to the usual dislocation activity. The shear-coupled GB migration, the expected most efficient GB based mechanism, couples the GB motion to an applied shear stress. Stresses on GB in polycrystalline materials seldom have, however, a unique pure shear component. This work investigates the influence of a normal strain on the shear coupled migration of a Σ 13 (320 )[001 ] GB in a copper bicrystal using atomistic simulations. We show that the yield shear stress inducing the GB migration strongly depends on the applied normal stress. Beyond, the application of a normal stress on this GB qualitatively modifies the GB migration: while the Σ 13 (320 )[001 ] GB shear couples following the 〈110 〉 migration mode without normal stress, we report the observation of the 〈010 〉 mode under a sufficiently high tensile normal stress. Using the nudge elastic band method, we uncover the atomistic mechanism of this 〈010 〉 migration mode and energetically characterize it.

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.

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.

A Near-Wall Reynolds-Stress Closure without Wall Normals

NASA Technical Reports Server (NTRS)

Yuan, S. P.; So, R. M. C.

1997-01-01

With the aid of near-wall asymptotic analysis and results of direct numerical simulation, a new near-wall Reynolds stress model (NNWRS) is formulated based on the SSG high-Reynolds-stress model with wall-independent near-wall corrections. Only one damping function is used for flows with a wide range of Reynolds numbers to ensure that the near-wall modifications diminish away from the walls. The model is able to reproduce complicated flow phenomena induced by complex geometry, such as flow recirculation, reattachment and boundary-layer redevelopment in backward-facing step flow and secondary flow in three-dimensional square duct flow. In simple flows, including fully developed channel/pipe flow, Couette flow and boundary-layer flow, the wall effects are dominant, and the NNWRS model predicts less degree of turbulent anisotropy in the near-wall region compared with a wall-dependent near-wall Reynolds Stress model (NWRS) developed by So and colleagues. The comparison of the predictions given by the two models rectifies the misconception that the overshooting of skin friction coefficient in backward-facing step flow prevalent in those near-wall, models with wall normal is caused by he use of wall normal.

Shear Stress in Magnetorheological FInishing for Glasses

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miao, C.; Shafrir, S.N.; Lambropoulos, J.C.

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 themore » 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.« less

Human endothelial cell responses to cardiovascular inspired pulsatile shear stress

NASA Astrophysics Data System (ADS)

Watson, Matthew; Baugh, Lauren; Black, Lauren, III; Kemmerling, Erica

2016-11-01

It is well established that hemodynamic shear stress regulates blood vessel structure and the development of vascular pathology. This process can be studied via in vitro models of endothelial cell responses to pulsatile shear stress. In this study, a macro-scale cone and plate viscometer was designed to mimic various shear stress waveforms found in the body and apply these stresses to human endothelial cells. The device was actuated by a PID-controlled DC gear-motor. Cells were exposed to 24 hours of pulsatile shear and then imaged and stained to track their morphology and secretions. These measurements were compared with control groups of cells exposed to constant shear and no shear. The results showed that flow pulsatility influenced levels of secreted proteins such as VE-cadherin and neuroregulin IHC. Cell morphology was also influenced by flow pulsatility; in general cells exposed to pulsatile shear stress developed a higher aspect ratio than cells exposed to no flow but a lower aspect ratio than cells exposed to steady flow.

Left Ventricular Trabeculations Decrease the Wall Shear Stress and Increase the Intra-Ventricular Pressure Drop in CFD Simulations

PubMed Central

Sacco, Federica; Paun, Bruno; Lehmkuhl, Oriol; Iles, Tinen L.; Iaizzo, Paul A.; Houzeaux, Guillaume; Vázquez, Mariano; Butakoff, Constantine; Aguado-Sierra, Jazmin

2018-01-01

The aim of the present study is to characterize the hemodynamics of left ventricular (LV) geometries to examine the impact of trabeculae and papillary muscles (PMs) on blood flow using high performance computing (HPC). Five pairs of detailed and smoothed LV endocardium models were reconstructed from high-resolution magnetic resonance images (MRI) of ex-vivo human hearts. The detailed model of one LV pair is characterized only by the PMs and few big trabeculae, to represent state of art level of endocardial detail. The other four detailed models obtained include instead endocardial structures measuring ≥1 mm2 in cross-sectional area. The geometrical characterizations were done using computational fluid dynamics (CFD) simulations with rigid walls and both constant and transient flow inputs on the detailed and smoothed models for comparison. These simulations do not represent a clinical or physiological scenario, but a characterization of the interaction of endocardial structures with blood flow. Steady flow simulations were employed to quantify the pressure drop between the inlet and the outlet of the LVs and the wall shear stress (WSS). Coherent structures were analyzed using the Q-criterion for both constant and transient flow inputs. Our results show that trabeculae and PMs increase the intra-ventricular pressure drop, reduce the WSS and disrupt the dominant single vortex, usually present in the smoothed-endocardium models, generating secondary small vortices. Given that obtaining high resolution anatomical detail is challenging in-vivo, we propose that the effect of trabeculations can be incorporated into smoothed ventricular geometries by adding a porous layer along the LV endocardial wall. Results show that a porous layer of a thickness of 1.2·10−2 m with a porosity of 20 kg/m2 on the smoothed-endocardium ventricle models approximates the pressure drops, vorticities and WSS observed in the detailed models. PMID:29760665

A potential model for drug screening by simulating the effect of shear stress in vivo on endothelium.

PubMed

Xu, Yingqian; Wang, Bochu; Deng, Jia; Liu, Zerong; Zhu, Liancai

2013-01-01

The purpose of this paper was to research the potential of a dynamic cell model in drug screening by studying the influence of microvascular wall shear stress on the drug absorption of endothelial cells compared to that in the static state. The cells were grown and seeded on gelatin-coated glass slides and were pretreated with extracts of Salviae miltiorrhizae (200 μg/ml) for 1 h. Then oxidative stress damage was produced by H2O2 (300 μmol/l) for 0.5 h under the 1.5 dyn/cm2 shear stress incorporated in a parallel plate flow chamber. Morphological analysis was conducted with an inverted microscope and image analysis software, and high performance liquid chromatography-mass spectrometry was used for the detection of active compounds. We compared the drug absorption in the dynamic group with that in the static group. In the dynamic model, five compounds and two new metabolite peaks were detected. However, in the static model, four compounds were absorbed by cells, and one metabolite peak was found. This study indicated that there were some effects on the absorption and metabolism of drugs under the microvascular shear stress compared to that under stasis. We infer that shear stress in the microcirculation situation in vivo played a role in causing the differences between drug screening in vitro and in vivo.

SIP Shear Walls: Cyclic Performance of High-Aspect-Ratio Segments and Perforated Walls

Treesearch

Vladimir Kochkin; Douglas R. Rammer; Kevin Kauffman; Thomas Wiliamson; Robert J. Ross

2015-01-01

Increasing stringency of energy codes and the growing market demand for more energy efficient buildings gives structural insulated panel (SIP) construction an opportunity to increase its use in commercial and residential buildings. However, shear wall aspect ratio limitations and lack of knowledge on how to design SIPs with window and door openings are barriers to the...

Evolution and dynamics of shear-layer structures in near-wall turbulence

NASA Technical Reports Server (NTRS)

Johansson, Arne V.; Alfredsson, P. H.; Kim, John

1991-01-01

Near-wall flow structures in turbulent shear flows are analyzed, with particular emphasis on the study of their space-time evolution and connection to turbulence production. The results are obtained from investigation of a database generated from direct numerical simulation of turbulent channel flow at a Reynolds number of 180 based on half-channel width and friction velocity. New light is shed on problems associated with conditional sampling techniques, together with methods to improve these techniques, for use both in physical and numerical experiments. The results clearly indicate that earlier conceptual models of the processes associated with near-wall turbulence production, based on flow visualization and probe measurements need to be modified. For instance, the development of asymmetry in the spanwise direction seems to be an important element in the evolution of near-wall structures in general, and for shear layers in particular. The inhibition of spanwise motion of the near-wall streaky pattern may be the primary reason for the ability of small longitudinal riblets to reduce turbulent skin friction below the value for a flat surface.

Evaluation of Aortic Blood Flow and Wall Shear Stress in Aortic Stenosis and Its Association With Left Ventricular Remodeling

PubMed Central

von Knobelsdorff-Brenkenhoff, Florian; Karunaharamoorthy, Achudhan; Trauzeddel, Ralf Felix; Barker, Alex J; Blaszczyk, Edyta; Markl, Michael; Schulz-Menger, Jeanette

2016-01-01

Background Aortic stenosis (AS) leads to variable stress for the left ventricle (LV) and consequently a broad range of LV remodeling. Study aim was to describe blood flow patterns in the ascending aorta of AS patients and determine their association with remodeling. Methods and Results Thirty-seven patients with AS (14 mild, 8 moderate, 15 severe; age 63±13 years) and 37 healthy controls (age 60±10 years) underwent 4D-flow MRI. Helical and vortical flow formations and flow eccentricity were assessed in the ascending aorta. Normalized flow displacement from the vessel center and peak systolic wall shear stress (WSSpeak) in the ascending aorta were quantified. LV remodeling was assessed based on LV mass index (LVMI-I) and the ratio of LV mass to enddiastolic volume (relative wall mass; RWM). Marked helical and vortical flow formation and eccentricity were more prevalent in patients with AS than in healthy subjects, and AS patients exhibited an asymmetric and elevated distribution of WSSpeak. In AS, aortic orifice area was strongly negatively associated with vortical flow formation (p=0.0274), eccentricity (p=0.0070) and flow displacement (p=0.0021). Bicuspid aortic valve was associated with more intense helical (p=0.0098) and vortical flow formation (p=0.0536), higher flow displacement (p=0.11) and higher WSSpeak (p=0.0926). LVM-I and RWM were significantly associated with aortic orifice area (p=0.0611, p=0.0058) and flow displacement (p=0.0058, p=0.0283). Conclusions In this pilot study, AS leads to abnormal blood flow pattern and WSSpeak in the ascending aorta. In addition to aortic orifice area, normalized flow displacement was significantly associated with LV remodeling. PMID:26917824

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.

Predicting equilibrium states with Reynolds stress closures in channel flow and homogeneous shear flow

NASA Technical Reports Server (NTRS)

Abid, R.; Speziale, C. G.

1993-01-01

Turbulent channel flow and homogeneous shear flow have served as basic building block flows for the testing and calibration of Reynolds stress models. A direct theoretical connection is made between homogeneous shear flow in equilibrium and the log-layer of fully-developed turbulent channel flow. It is shown that if a second-order closure model is calibrated to yield good equilibrium values for homogeneous shear flow it will also yield good results for the log-layer of channel flow provided that the Rotta coefficient is not too far removed from one. Most of the commonly used second-order closure models introduce an ad hoc wall reflection term in order to mask deficient predictions for the log-layer of channel flow that arise either from an inaccurate calibration of homogeneous shear flow or from the use of a Rotta coefficient that is too large. Illustrative model calculations are presented to demonstrate this point which has important implications for turbulence modeling.

Predicting equilibrium states with Reynolds stress closures in channel flow and homogeneous shear flow

NASA Technical Reports Server (NTRS)

Abid, R.; Speziale, C. G.

1992-01-01

Turbulent channel flow and homogeneous shear flow have served as basic building block flows for the testing and calibration of Reynolds stress models. A direct theoretical connection is made between homogeneous shear flow in equilibrium and the log-layer of fully-developed turbulent channel flow. It is shown that if a second-order closure model is calibrated to yield good equilibrium values for homogeneous shear flow it will also yield good results for the log-layer of channel flow provided that the Rotta coefficient is not too far removed from one. Most of the commonly used second-order closure models introduce an ad hoc wall reflection term in order to mask deficient predictions for the log-layer of channel flow that arise either from an inaccurate calibration of homogeneous shear flow or from the use of a Rotta coefficient that is too large. Illustrative model calculations are presented to demonstrate this point which has important implications for turbulence modeling.

Hydrostatic pressure and shear stress affect endothelin-1 and nitric oxide release by endothelial cells in bioreactors.

PubMed

Vozzi, Federico; Bianchi, Francesca; Ahluwalia, Arti; Domenici, Claudio

2014-01-01

Abundant experimental evidence demonstrates that endothelial cells are sensitive to flow; however, the effect of fluid pressure or pressure gradients that are used to drive viscous flow is not well understood. There are two principal physical forces exerted on the blood vessel wall by the passage of intra-luminal blood: pressure and shear. To analyze the effects of pressure and shear independently, these two stresses were applied to cultured cells in two different types of bioreactors: a pressure-controlled bioreactor and a laminar flow bioreactor, in which controlled levels of pressure or shear stress, respectively, can be generated. Using these bioreactor systems, endothelin-1 (ET-1) and nitric oxide (NO) release from human umbilical vein endothelial cells were measured under various shear stress and pressure conditions. Compared to the controls, a decrease of ET-1 production by the cells cultured in both bioreactors was observed, whereas NO synthesis was up-regulated in cells under shear stress, but was not modulated by hydrostatic pressure. These results show that the two hemodynamic forces acting on blood vessels affect endothelial cell function in different ways, and that both should be considered when planning in vitro experiments in the presence of flow. Understanding the individual and synergic effects of the two forces could provide important insights into physiological and pathological processes involved in vascular remodeling and adaptation. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Failure mechanism of shear-wall dominant multi-story buildings

USGS Publications Warehouse

Yuksel, S.B.; Kalkan, E.

2008-01-01

The recent trend in the building industry of Turkey as well as in many European countries is towards utilizing the tunnel form (shear-wall dominant) construction system for development of multi-story residential units. The tunnel form buildings diverge from other conventional reinforced concrete (RC) buildings due to the lack of beams and columns in their structural integrity. The vertical load-carrying members of these buildings are the structural-walls only, and the floor system is a flat plate. Besides the constructive advantages, tunnel form buildings provide superior seismic performance compared to conventional RC frame and dual systems as observed during the recent devastating earthquakes in Turkey (1999 Mw 7.4 Kocaeli, Mw 7.2 Duzce, and 2004 Mw 6.5 Bingol). With its proven earthquake performance, the tunnel form system is becoming the primary construction technique in many seismically active regions. In this study, a series of nonlinear analyses were conducted using finite element (FE) models to augment our understanding on their failure mechanism under lateral forces. In order to represent the nonlinear behavior adequately, The FE models were verified with the results of experimental studies performed on three dimensional (3D) scaled tunnel form building specimens. The results of this study indicate that the structural walls of tunnel form buildings may exhibit brittle flexural failure under lateral loading, if they are not properly reinforced. The global tension/compression couple triggers this failure mechanism by creating pure axial tension in the outermost shear-walls.

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.

Effect of shear stress on the migration of hepatic stellate cells.

PubMed

Sera, Toshihiro; Sumii, Tateki; Fujita, Ryosuke; Kudo, Susumu

2018-01-01

When the liver is damaged, hepatic stellate cells (HSCs) can change into an activated, highly migratory state. The migration of HSCs may be affected by shear stress due not only to sinusoidal flow but also by the flow in the space of Disse because this space is filled with blood plasma. In this study, we evaluated the effects of shear stress on HSC migration in a scratch-wound assay with a parallel flow chamber. At regions upstream of the wound area, the migration was inhibited by 0.6 Pa and promoted by 2.0 Pa shear stress, compared to the static condition. The platelet-derived growth factor (PDGF)-BB receptor, PDGFR-β, was expressed in all conditions and the differences were not significant. PDGF increased HSC migration, except at 0.6 Pa shear stress, which was still inhibited. These results indicate that another molecular factor, such as PDGFR-α, may act to inhibit the migration under low shear stress. At regions downstream of the wound area, the migration was smaller under shear stress than under the static condition, although the expression of PDGFR-β was significantly higher. In particular, the migration direction was opposite to the wound area under high shear stress; therefore, migration might be influenced by the intercellular environment. Our results indicate that HSC migration was influenced by shear stress intensity and the intercellular environment.

Shear-stress fluctuations and relaxation in polymer glasses

NASA Astrophysics Data System (ADS)

Kriuchevskyi, I.; Wittmer, J. P.; Meyer, H.; Benzerara, O.; Baschnagel, J.

2018-01-01

We investigate by means of molecular dynamics simulation a coarse-grained polymer glass model focusing on (quasistatic and dynamical) shear-stress fluctuations as a function of temperature T and sampling time Δ t . The linear response is characterized using (ensemble-averaged) expectation values of the contributions (time averaged for each shear plane) to the stress-fluctuation relation μsf for the shear modulus and the shear-stress relaxation modulus G (t ) . Using 100 independent configurations, we pay attention to the respective standard deviations. While the ensemble-averaged modulus μsf(T ) decreases continuously with increasing T for all Δ t sampled, its standard deviation δ μsf(T ) is nonmonotonic with a striking peak at the glass transition. The question of whether the shear modulus is continuous or has a jump singularity at the glass transition is thus ill posed. Confirming the effective time-translational invariance of our systems, the Δ t dependence of μsf and related quantities can be understood using a weighted integral over G (t ) .

Self-sustaining processes at all scales in wall-bounded turbulent shear flows

PubMed Central

Hwang, Yongyun

2017-01-01

We collect and discuss the results of our recent studies which show evidence of the existence of a whole family of self-sustaining motions in wall-bounded turbulent shear flows with scales ranging from those of buffer-layer streaks to those of large-scale and very-large-scale motions in the outer layer. The statistical and dynamical features of this family of self-sustaining motions, which are associated with streaks and quasi-streamwise vortices, are consistent with those of Townsend’s attached eddies. Motions at each relevant scale are able to sustain themselves in the absence of forcing from larger- or smaller-scale motions by extracting energy from the mean flow via a coherent lift-up effect. The coherent self-sustaining process is embedded in a set of invariant solutions of the filtered Navier–Stokes equations which take into full account the Reynolds stresses associated with the residual smaller-scale motions. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’. PMID:28167581

Self-sustaining processes at all scales in wall-bounded turbulent shear flows.

PubMed

Cossu, Carlo; Hwang, Yongyun

2017-03-13

We collect and discuss the results of our recent studies which show evidence of the existence of a whole family of self-sustaining motions in wall-bounded turbulent shear flows with scales ranging from those of buffer-layer streaks to those of large-scale and very-large-scale motions in the outer layer. The statistical and dynamical features of this family of self-sustaining motions, which are associated with streaks and quasi-streamwise vortices, are consistent with those of Townsend's attached eddies. Motions at each relevant scale are able to sustain themselves in the absence of forcing from larger- or smaller-scale motions by extracting energy from the mean flow via a coherent lift-up effect. The coherent self-sustaining process is embedded in a set of invariant solutions of the filtered Navier-Stokes equations which take into full account the Reynolds stresses associated with the residual smaller-scale motions.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'. © 2017 The Author(s).

Mass Transport and Shear Stress in the Carotid Artery Bifurcation

NASA Astrophysics Data System (ADS)

Gorder, Riley; Aliseda, Alberto

2010-11-01

The carotid artery bifurcation (CAB) is one of the leading sites for atherosclerosis, a major cause of death and disability in the developed world. The specific processes by which the complex flow found at the bifurcation and carotid sinus promotes plaque formation and growth are not fully understood. Shear stress, mass transport, and flow residence times are considered key factors. Although the governing equations closely link shear stress and mass transfer, the pulsatile, transitional, and detached flow found at the CAB can lead to differences between regions of WSS and mass transfer statistics. In this study, CAB geometries are reconstructed from patient specific 3D ultrasound medical imaging. Using ANSYS FLUENT, the fluid flow and scalar transport was solved using realistic flow conditions and various mass transfer boundary conditions. The spatial and temporal resolution was validated against the analytical solution of the Graetz-Nusselt problem with constant wall flux to ensure the scalar transport is resolved for a Peclet number up to 100,000. High residence time regions are investigated by determining the number of cardiac cycles required to flush out the carotid sinus. The correlations between regions of low WSS, high OSI, and scalar concentration are computed and interpreted in the context of atherosclerotic plaque origin and progression.

Behaviour of Masonry Walls under Horizontal Shear in Mining Areas

NASA Astrophysics Data System (ADS)

Kadela, Marta; Bartoszek, Marek; Fedorowicz, Jan

2017-12-01

The paper discusses behaviour of masonry walls constructed with small-sized elements under the effects of mining activity. It presents some mechanisms of damage occurring in such structures, its forms in real life and the behaviour of large fragments of masonry walls subjected to specific loads in FEM computational models. It offers a constitutive material model, which enables numerical analyses and monitoring of the behaviour of numerical models as regards elastic-plastic performance of the material, with consideration of its degradation. Results from the numerical analyses are discussed for isolated fragments of the wall subjected to horizontal shear, with consideration of degradation, impact of imposed vertical load as well as the effect of weakening of the wall, which was achieved by introducing openings in it, on the performance and deformation of the wall.

Impact of competitive flow on wall shear stress in coronary surgery: computational fluid dynamics of a LIMA-LAD model.

PubMed

Nordgaard, Håvard; Swillens, Abigail; Nordhaug, Dag; Kirkeby-Garstad, Idar; Van Loo, Denis; Vitale, Nicola; Segers, Patrick; Haaverstad, Rune; Lovstakken, Lasse

2010-12-01

Competitive flow from native coronary vessels is considered a major factor in the failure of coronary bypass grafts. However, the pathophysiological effects are not fully understood. Low and oscillatory wall shear stress (WSS) is known to induce endothelial dysfunction and vascular disease, like atherosclerosis and intimal hyperplasia. The aim was to investigate the impact of competitive flow on WSS in mammary artery bypass grafts. Using computational fluid dynamics, WSS was calculated in a left internal mammary artery (LIMA) graft to the left anterior descending artery in a three-dimensional in vivo porcine coronary artery bypass graft model. The following conditions were investigated: high competitive flow (non-significant coronary lesion), partial competitive flow (significant coronary lesion), and no competitive flow (totally occluded coronary vessel). Time-averaged WSS of LIMA at high, partial, and no competitive flow were 0.3-0.6, 0.6-3.0, and 0.9-3.0 Pa, respectively. Further, oscillatory WSS quantified as the oscillatory shear index (OSI) ranged from (maximum OSI = 0.5 equals zero net WSS) 0.15 to 0.35, <0.05, and <0.05, respectively. Thus, high competitive flow resulted in substantial oscillatory and low WSS. Moderate competitive flow resulted in WSS and OSI similar to the no competitive flow condition. Graft flow is highly dependent on the degree of competitive flow. High competitive flow was found to produce unfavourable WSS consistent with endothelial dysfunction and subsequent graft narrowing and failure. Partial competitive flow, however, may be better tolerated as it was found to be similar to the ideal condition of no competitive flow.

The Effect of Spatial and Temporal Resolution of Cine Phase Contrast MRI on Wall Shear Stress and Oscillatory Shear Index Assessment

PubMed Central

Gijsen, Frank J.; Marquering, Henk; van Ooij, Pim; vanBavel, Ed; Wentzel, Jolanda J.; Nederveen, Aart J.

2016-01-01

Introduction Wall shear stress (WSS) and oscillatory shear index (OSI) are associated with atherosclerotic disease. Both parameters are derived from blood velocities, which can be measured with phase-contrast MRI (PC-MRI). Limitations in spatiotemporal resolution of PC-MRI are known to affect these measurements. Our aim was to investigate the effect of spatiotemporal resolution using a carotid artery phantom. Methods A carotid artery phantom was connected to a flow set-up supplying pulsatile flow. MRI measurement planes were placed at the common carotid artery (CCA) and internal carotid artery (ICA). Two-dimensional PC-MRI measurements were performed with thirty different spatiotemporal resolution settings. The MRI flow measurement was validated with ultrasound probe measurements. Mean flow, peak flow, flow waveform, WSS and OSI were compared for these spatiotemporal resolutions using regression analysis. The slopes of the regression lines were reported in %/mm and %/100ms. The distribution of low and high WSS and OSI was compared between different spatiotemporal resolutions. Results The mean PC-MRI CCA flow (2.5±0.2mL/s) agreed with the ultrasound probe measurements (2.7±0.02mL/s). Mean flow (mL/s) depended only on spatial resolution (CCA:-13%/mm, ICA:-49%/mm). Peak flow (mL/s) depended on both spatial (CCA:-13%/mm, ICA:-17%/mm) and temporal resolution (CCA:-19%/100ms, ICA:-24%/100ms). Mean WSS (Pa) was in inverse relationship only with spatial resolution (CCA:-19%/mm, ICA:-33%/mm). OSI was dependent on spatial resolution for CCA (-26%/mm) and temporal resolution for ICA (-16%/100ms). The regions of low and high WSS and OSI matched for most of the spatiotemporal resolutions (CCA:30/30, ICA:28/30 cases for WSS; CCA:23/30, ICA:29/30 cases for OSI). Conclusion We show that both mean flow and mean WSS are independent of temporal resolution. Peak flow and OSI are dependent on both spatial and temporal resolution. However, the magnitude of mean and peak flow, WSS and

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.

Reynolds number invariance of the structure inclination angle in wall turbulence.

PubMed

Marusic, Ivan; Heuer, Weston D C

2007-09-14

Cross correlations of the fluctuating wall-shear stress and the streamwise velocity in the logarithmic region of turbulent boundary layers are reported over 3 orders of magnitude change in Reynolds number. These results are obtained using hot-film and hot-wire anemometry in a wind tunnel facility, and sonic anemometers and a purpose-built wall-shear stress sensor in the near-neutral atmospheric surface layer on the salt flats of Utah's western desert. The direct measurement of fluctuating wall-shear stress in the atmospheric surface layer has not been available before. Structure inclination angles are inferred from the cross correlation results and are found to be invariant over the large range of Reynolds number. The findings justify the prior use of low Reynolds number experiments for obtaining structure angles for near-wall models in the large-eddy simulation of atmospheric surface layer flows.

Measurement of the temperature-dependent threshold shear-stress of red blood cell aggregation.

PubMed

Lim, Hyun-Jung; Nam, Jeong-Hun; Lee, Yong-Jin; Shin, Sehyun

2009-09-01

Red blood cell (RBC) aggregation is becoming an important hemorheological parameter, which typically exhibits temperature dependence. Quite recently, a critical shear-stress was proposed as a new dimensional index to represent the aggregative and disaggregative behaviors of RBCs. The present study investigated the effect of the temperature on the critical shear-stress that is required to keep RBC aggregates dispersed. The critical shear-stress was measured at various temperatures (4, 10, 20, 30, and 37 degrees C) through the use of a transient microfluidic aggregometry. The critical shear-stress significantly increased as the blood temperature lowered, which accorded with the increase in the low-shear blood viscosity with the lowering of the temperature. Furthermore, the critical shear-stress also showed good agreement with the threshold shear-stress, as measured in a rotational Couette flow. These findings assist in rheologically validating the critical shear-stress, as defined in the microfluidic aggregometry.

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.

Heat transfer, velocity-temperature correlation, and turbulent shear stress from Navier-Stokes computations of shock wave/turbulent boundary layer interaction flows

NASA Technical Reports Server (NTRS)

Wang, C. R.; Hingst, W. R.; Porro, A. R.

1991-01-01

The properties of 2-D shock wave/turbulent boundary layer interaction flows were calculated by using a compressible turbulent Navier-Stokes numerical computational code. Interaction flows caused by oblique shock wave impingement on the turbulent boundary layer flow were considered. The oblique shock waves were induced with shock generators at angles of attack less than 10 degs in supersonic flows. The surface temperatures were kept at near-adiabatic (ratio of wall static temperature to free stream total temperature) and cold wall (ratio of wall static temperature to free stream total temperature) conditions. The computational results were studied for the surface heat transfer, velocity temperature correlation, and turbulent shear stress in the interaction flow fields. Comparisons of the computational results with existing measurements indicated that (1) the surface heat transfer rates and surface pressures could be correlated with Holden's relationship, (2) the mean flow streamwise velocity components and static temperatures could be correlated with Crocco's relationship if flow separation did not occur, and (3) the Baldwin-Lomax turbulence model should be modified for turbulent shear stress computations in the interaction flows.

Seismic performance of RC shear wall structure with novel shape memory alloy dampers in coupling beams

NASA Astrophysics Data System (ADS)

Mao, Chenxi; Dong, Jinzhi; Li, Hui; Ou, Jinping

2012-04-01

Shear wall system is widely adopted in high rise buildings because of its high lateral stiffness in resisting earthquakes. According to the concept of ductility seismic design, coupling beams in shear wall structure are required to yield prior to the damage of wall limb. However, damage in coupling beams results in repair cost post earthquake and even in some cases it is difficult to repair the coupling beams if the damage is severe. In order to solve this problem, a novel passive SMA damper was proposed in this study. The coupling beams connecting wall limbs are split in the middle, and the dampers are installed between the ends of the two cantilevers. Then the relative flexural deformation of the wall limbs is transferred to the ends of coupling beams and then to the SMA dampers. After earthquakes the deformation of the dampers can recover automatically because of the pseudoelasticity of austenite SMA material. In order to verify the validity of the proposed dampers, seismic responses of a 12-story coupled shear wall with such passive SMA dampers in coupling beams was investigated. The additional stiffness and yielding deformation of the dampers and their ratios to the lateral stiffness and yielding displacements of the wall limbs are key design parameters and were addressed. Analytical results indicate that the displacement responses of the shear wall structure with such dampers are reduced remarkably. The deformation of the structure is concentrated in the dampers and the damage of coupling beams is reduced.

Behavior of Steel-Sheathed Shear Walls Subjected to Seismic and Fire Loads.

PubMed

Hoehler, Matthew S; Smith, Christopher M; Hutchinson, Tara C; Wang, Xiang; Meacham, Brian J; Kamath, Praveen

2017-07-01

A series of tests was conducted on six 2.7 m × 3.7 m shear wall specimens consisting of cold-formed steel framing sheathed on one side with sheet steel adhered to gypsum board and on the opposite side with plain gypsum board. The specimens were subjected to various sequences of simulated seismic shear deformation and fire exposure to study the influence of multi-hazard interactions on the lateral load resistance of the walls. The test program was designed to complement a parallel effort at the University of California, San Diego to investigate a six-story building subjected to earthquakes and fires. The test results reported here indicate that the fire exposure caused a shift in the failure mode of the walls from local buckling of the sheet steel in cases without fire exposure, to global buckling of the sheet steel with an accompanying 35 % reduction in lateral load capacity after the wall had been exposed to fire. This behavior appears to be predictable, which is encouraging from the standpoint of residual lateral load capacity under these severe multi-hazard actions.

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.

Research Status on Reinforcement Connection Form of Precast Concrete Shear Wall Structure

NASA Astrophysics Data System (ADS)

Zhang, Zhuangnan; Zhang, Yan

2018-03-01

With the rapid development of Chinese economy and the speeding up the process of urbanization, housing industrialization has been paid more and more attention. And the fabricated structure has been widely used in China. The key of precast concrete shear wall structure is the connection of precast components. The reinforcement connection can directly affect the entirety performance and seismic behavior of the structure. Different reinforcement connections have a great impact on the overall behavior of the structure. By studying the characteristics of the reinforcement connection forms used in the vertical connection and horizontal connection of precast concrete shear wall, it can provide reference for the research and development of the reinforcement connection forms in the future.

Experimental, numerical, and analytical studies on the seismic response of steel-plate concrete (SC) composite shear walls

NASA Astrophysics Data System (ADS)

Epackachi, Siamak

The seismic performance of rectangular steel-plate concrete (SC) composite shear walls is assessed for application to buildings and mission-critical infrastructure. The SC walls considered in this study were composed of two steel faceplates and infill concrete. The steel faceplates were connected together and to the infill concrete using tie rods and headed studs, respectively. The research focused on the in-plane behavior of flexure- and flexure-shear-critical SC walls. An experimental program was executed in the NEES laboratory at the University at Buffalo and was followed by numerical and analytical studies. In the experimental program, four large-size specimens were tested under displacement-controlled cyclic loading. The design variables considered in the testing program included wall thickness, reinforcement ratio, and slenderness ratio. The aspect ratio (height-to-length) of the four walls was 1.0. Each SC wall was installed on top of a re-usable foundation block. A bolted baseplate to RC foundation connection was used for all four walls. The walls were identified to be flexure- and flexure-shear critical. The progression of damage in the four walls was identical, namely, cracking and crushing of the infill concrete at the toes of the walls, outward buckling and yielding of the steel faceplates near the base of the wall, and tearing of the faceplates at their junctions with the baseplate. A robust finite element model was developed in LS-DYNA for nonlinear cyclic analysis of the flexure- and flexure-shear-critical SC walls. The DYNA model was validated using the results of the cyclic tests of the four SC walls. The validated and benchmarked models were then used to conduct a parametric study, which investigated the effects of wall aspect ratio, reinforcement ratio, wall thickness, and uniaxial concrete compressive strength on the in-plane response of SC walls. Simplified analytical models, suitable for preliminary analysis and design of SC walls, were

Human brain microvascular endothelial cells resist elongation due to shear stress.

PubMed

Reinitz, Adam; DeStefano, Jackson; Ye, Mao; Wong, Andrew D; Searson, Peter C

2015-05-01

Endothelial cells in straight sections of vessels are known to elongate and align in the direction of flow. This phenotype has been replicated in confluent monolayers of bovine aortic endothelial cells and human umbilical vein endothelial cells (HUVECs) in cell culture under physiological shear stress. Here we report on the morphological response of human brain microvascular endothelial cells (HBMECs) in confluent monolayers in response to shear stress. Using a microfluidic platform we image confluent monolayers of HBMECs and HUVECs under shear stresses up to 16 dyne cm(-2). From live-cell imaging we quantitatively analyze the cell morphology and cell speed as a function of time. We show that HBMECs do not undergo a classical transition from cobblestone to spindle-like morphology in response to shear stress. We further show that under shear stress, actin fibers are randomly oriented in the cells indicating that there is no cytoskeletal remodeling. These results suggest that HBMECs are programmed to resist elongation and alignment under shear stress, a phenotype that may be associated with the unique properties of the blood-brain barrier. Copyright © 2015 Elsevier Inc. All rights reserved.

Effect of friction on shear jamming

NASA Astrophysics Data System (ADS)

Wang, Dong; Ren, Jie; Dijksman, Joshua; Behringer, Robert

2014-03-01

Shear Jamming of granular materials was first found for systems of frictional disks, with a static friction coefficients μs ~= 0 . 6 . Jamming by shear is obtained by starting from a zero-stress state with a packing fraction ϕS <= ϕ <=ϕJ between ϕJ (isotropic jamming) and a lowest ϕS for shear jamming. This phenomenon is associated with strong anisotropy in stress and the contact network in the form of ``force chains,'' which are stabilized and/or enhanced by the presence of friction. We address experimentally how reducing friction affects shear jamming by using either teflon disks of teflon wrapped photoelastic particles. The teflon disks were placed in a wall driven 2D shear apparatus, in which we can probe shear stresses mechanically. Teflon-wrapped disks were placed in a bottom driven 2D shear apparatus (Ren et al., PRL 2013). Both apparatuses provide uniform simple shear. In all low- μ experiments, the shear jamming occurred, as observed through stress increases on the packing. However, the low- μ differences observed for ϕJ -ϕS were smaller than for higher friction particles. Ongoing work is studying systems using hydrogel disks, which have a lower friction coefficient than teflon. We acknowledge support from NSF Grant No. DMR12-06351, ARO Grant No. W911NF-1-11-0110, and NASA Grant No. NNX10AU01G.

Analysis of a dusty wall jet

NASA Technical Reports Server (NTRS)

Lim, Hock-Bin; Roberts, Leonard

1991-01-01

An analysis is given for the entrainment of dust into a turbulent radial wall jet. Equations are solved based on incompressible flow of a radial wall jet into which dust is entrained from the wall and transported by turbulent diffusion and convection throughout the flow. It is shown that the resulting concentration of dust particles in the flow depends on the difference between the applied shear stress at the surface and the maximum level of shear stress that the surface can withstand (varies as rho(sub d)a(sub g)D) i.e., the pressure due to the weight of a single layer of dust. The analysis is expected to have application to the downflow that results from helicopter and VTOL aircraft.

Link between deviations from Murray's Law and occurrence of low wall shear stress regions in the left coronary artery.

PubMed

Doutel, E; Pinto, S I S; Campos, J B L M; Miranda, J M

2016-08-07

Murray developed two laws for the geometry of bifurcations in the circulatory system. Based on the principle of energy minimization, Murray found restrictions for the relation between the diameters and also between the angles of the branches. It is known that bifurcations are prone to the development of atherosclerosis, in regions associated to low wall shear stresses (WSS) and high oscillatory shear index (OSI). These indicators (size of low WSS regions, size of high OSI regions and size of high helicity regions) were evaluated in this work. All of them were normalized by the size of the outflow branches. The relation between Murray's laws and the size of low WSS regions was analysed in detail. It was found that the main factor leading to large regions of low WSS is the so called expansion ratio, a relation between the cross section areas of the outflow branches and the cross section area of the main branch. Large regions of low WSS appear for high expansion ratios. Furthermore, the size of low WSS regions is independent of the ratio between the diameters of the outflow branches. Since the expansion ratio in bifurcations following Murray's law is kept in a small range (1 and 1.25), all of them have regions of low WSS with similar size. However, the expansion ratio is not small enough to completely prevent regions with low WSS values and, therefore, Murray's law does not lead to atherosclerosis minimization. A study on the effect of the angulation of the bifurcation suggests that the Murray's law for the angles does not minimize the size of low WSS regions. Copyright © 2016 Elsevier Ltd. All rights reserved.

Literature Review of Shear Performance of Light-weight Steel Framing Wall Panels

NASA Astrophysics Data System (ADS)

Zhang, Zhuangnan; Liu, Shen; Liu, Hong

2018-03-01

In this paper, a comprehensive review of light-weight steel framing wall panels was carried out. The structure and force characteristics of light-weight steel framing wall panels were introduced. The testing and theoretical research results on the shear behaviour of light-weight steel framing wall panels were summarized in the domestic and foreign. And combined with the existing standards in China, the author's views and ideas are put forward to the problems in the research field of this kind of structural system.

Shear stress-induced calcium transients in endothelial cells from human umbilical cord veins.

PubMed Central

Schwarz, G; Callewaert, G; Droogmans, G; Nilius, B

1992-01-01

1. Changes of the free cytosolic Ca2+ concentration induced by shear stress were measured in Fura-2 acetoxymethyl ester-loaded endothelial cells from human umbilical cord veins. 2. We were able to induce Ca2+ transients in almost every cell by blowing a stream of physiological solution onto a single endothelial cell thereby inducing shear stress between 0 and 50 dyn cm-2. The Ca2+ response could be graded by varying the shear stress, and reached a half-maximal value at a shear stress of 30 dyn cm-2. 3. The shear stress responses critically depended on the extracellular Ca2+ concentration and were absent in a Ca(2+)-free solution. Repetitive application of short pulses of shear stress induced cumulative effects because of the slow decay of the shear stress Ca2+ responses (time constants 82.3 +/- 17.8 s from twenty-five cells). Application of a depolarizing high potassium solution to reduce the driving force for Ca2+ entry decreased the Ca2+ transients in some of the cells. 4. Application of shear stress in the presence of other divalent cations, such as nickel, cobalt or barium, always produced substantial changes in the ratio of the 390/360 nm fluorescence signal, indicating influx of these cations and subsequent quenching of the Fura-2 fluorescence. 5. Shear stress responses in the presence of 10 mM Ca2+ were completely blocked by application of 1 mM La3+. 6. Incubation of the cells with the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) did not alter the shear stress response, but completely blocked histamine-induced Ca2+ transients. 7. Small submaximal shear stress potentiated the Ca2+ transients induced by histamine. 8. We conclude that shear stress-dependent Ca2+ signals are induced by an influx of calcium that is not modulated via protein kinase C and not activated by membrane depolarization. The influx pathway is also permeable to divalent cations such as Ni2+, Co2+ and Ba2+, but is blocked by La3+. PMID:1338792

Estimating boundary shear stress along vegetated streambanks with turbulent kinetic energy

NASA Astrophysics Data System (ADS)

Hopkinson, L. C.; Wynn, T.

2010-12-01

Boundary shear stress (BSS) is critical to correctly predict streambank erosion rates and stable channel design and has been estimated using turbulent kinetic energy (TKE). Typically TKE is used in ocean and fluvial environments to determine bed shear stress where the proportionality coefficient (C1) ranges from 0.19 to 0.21. TKE has only recently been used to estimate boundary shear stress along sloping streambanks. This study examined the relationship between boundary shear stress and turbulent kinetic energy along vegetated streambanks for three vegetation treatments: bare, tree, and shrub. A flume study was conducted, modeling a second order prototype stream (Tom’s Creek in Blacksburg, VA) with individual reaches dominated by two vegetation types (trees and shrubs). Boundary shear stress was measured using a flush-mount hot-film anemometer, and three-dimensional velocity was measured using an acoustic Doppler velocimeter 0.5 cm from the boundary. The relationship between TKE and BSS (BSS=C1TKE) was examined by calculating a no-intercept regression model. The calculated C1 ranged from 0.11 to 0.53 for all vegetation types (MRes=0.018-0.086). No-intercept regression models with TKE and the Reynolds stresses (τuv, τuw, and τvw) were also examined as Reynolds stresses have been used to calculate C1. There was better agreement with the reported C1 range for the TKE and Reynolds stress relationship (C1=0.17-0.21 and MRes<0.0072 for the τvw relationship) than with the measured values of shear stress, likely due to the dominance of turbulence generation. While these results are consistent with previously reported values, the relationship should be further explored with measured values of shear stress to determine the trends along hydraulically rough boundaries.

Low shear stress gravel-bed river

USGS Publications Warehouse

Milhous, Robert T.

1997-01-01

A low stress gravel bed river is a river where the cross-sectional average dimensionless shear stress (??*) rarely exceeds 0.047. That is the case for the Gunnison River below Delta in Western Colorado. The cross-sectional average ??* in the Gunnison River has not exceeded 0.047, except at one cross section during one year, in the 87 years of record. A ??* of 0.047 is the critical ??* in the bed-load equation considered to be most applicable to gravel/cobble bed rivers (the Meyer-Peter, Mueller equation). According to this equation, there has been no bed-material movement in the Gunnison River since 1920; in fact there has been bed-material movement and this movement is biologically important. Bed-material is moved when the ??* is 0.016 or larger. Streamflows that cause a ??* of at least 0.016 maintain the aquatic habitat in a low shear stress river.

Pressure measurements in a rapidly sheared turbulent wall layer

NASA Astrophysics Data System (ADS)

Diwan, Sourabh; Morrison, Jonathan

2014-11-01

The aim of the present work is to improve understanding of the role of pressure fluctuations in the generation of coherent structures in wall-bounded turbulent flows, with particular regard to the rapid and slow source terms. The work is in part motivated by the recent numerical simulations of Sharma et al. (Phy. Fluids, 23, 2011), which showed the importance of pressure fluctuations (and their spatial gradients) in the dynamics of large-scale turbulent motions. Our experimental design consists of first generating a shearless boundary layer in a wind tunnel by passing a grid-generated turbulent flow over a moving floor whose speed is matched to the freestream velocity, and then shearing it rapidly by passing it over a stationary floor further downstream. Close to the leading edge of the stationary floor, the resulting flow is expected to satisfy the approximations of the Rapid Distortion Theory and therefore would be an ideal candidate for studying linear processes in wall turbulence. We carry out pressure measurements on the wall as well as within the flow - the former using surface mounted pressure transducers and the latter using a static pressure probe similar in design to that used by Tsuji et al. (J. Fluid. Mech. 585, 2007). We also present a comparison between the rapidly sheared flow and a more conventional boundary layer subjected to a turbulent free stream. We acknowledge the financial support from EPSRC (Grant No. EP/I037938).

Distance-from-the-wall scaling of turbulent motions in wall-bounded flows

NASA Astrophysics Data System (ADS)

Baidya, R.; Philip, J.; Hutchins, N.; Monty, J. P.; Marusic, I.

2017-02-01

An assessment of self-similarity in the inertial sublayer is presented by considering the wall-normal velocity, in addition to the streamwise velocity component. The novelty of the current work lies in the inclusion of the second velocity component, made possible by carefully conducted subminiature ×-probe experiments to minimise the errors in measuring the wall-normal velocity. We show that not all turbulent stress quantities approach the self-similar asymptotic state at an equal rate as the Reynolds number is increased, with the Reynolds shear stress approaching faster than the streamwise normal stress. These trends are explained by the contributions from attached eddies. Furthermore, the Reynolds shear stress cospectra, through its scaling with the distance from the wall, are used to assess the wall-normal limits where self-similarity applies within the wall-bounded flow. The results are found to be consistent with the recent prediction from the work of Wei et al. ["Properties of the mean momentum balance in turbulent boundary layer, pipe and channel flows," J. Fluid Mech. 522, 303-327 (2005)], Klewicki ["Reynolds number dependence, scaling, and dynamics of turbulent boundary layers," J. Fluids Eng. 132, 094001 (2010)], and others that the self-similar region starts and ends at z+˜O (√{δ+}) and O (δ+) , respectively. Below the self-similar region, empirical evidence suggests that eddies responsible for turbulent stresses begin to exhibit distance-from-the-wall scaling at a fixed z+ location; however, they are distorted by viscous forces, which remain a leading order contribution in the mean momentum balance in the region z+≲O (√{δ+}) , and thus result in a departure from self-similarity.

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.

Shear load transfer in high and low stress tendons.

PubMed

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

2015-05-01

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 microstructures (helical versus linear) may redistribute loads differently. 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% to 60% of the tendon width, to create various magnitudes of shear. Differences in fascicular orientation were quantified using polarized light microscopy. 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. Copyright © 2015 Elsevier Ltd. All rights reserved.

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

Rac1 mediates laminar shear stress-induced vascular endothelial cell migration

PubMed Central

Huang, Xianliang; Shen, Yang; Zhang, Yi; Wei, Lin; Lai, Yi; Wu, Jiang; Liu, Xiaojing; Liu, Xiaoheng

2013-01-01

The migration of endothelial cells (ECs) plays an important role in vascular remodeling and regeneration. ECs are constantly subjected to shear stress resulting from blood flow and are able to convert mechanical stimuli into intracellular signals that affect cellular behaviors and functions. The aim of this study is to elucidate the effects of Rac1, which is the member of small G protein family, on EC migration under different laminar shear stress (5.56, 10.02, and 15.27 dyn/cm2). The cell migration distance under laminar shear stress increased significantly than that under the static culture condition. Especially, under relative high shear stress (15.27 dyn/cm2) there was a higher difference at 8 h (P < 0.01) and 2 h (P < 0.05) compared with static controls. RT-PCR results further showed increasing mRNA expression of Rac1 in ECs exposed to laminar shear stress than that exposed to static culture. Using plasmids encoding the wild-type (WT), an activated mutant (Q61L), and a dominant-negative mutant (T17N), plasmids encoding Rac1 were transfected into EA.hy 926 cells. The average net migration distance of Rac1Q61L group increased significantly, while Rac1T17N group decreased significantly in comparison with the static controls. These results indicated that Rac1 mediated shear stress-induced EC migration. Our findings conduce to elucidate the molecular mechanisms of EC migration induced by shear stress, which is expected to understand the pathophysiological basis of wound healing in health and diseases. PMID:24430179

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.

Difference in hemodynamic and wall stress of ascending thoracic aortic aneurysms with bicuspid and tricuspid aortic valve

PubMed Central

Pasta, Salvatore; Rinaudo, Antonino; Luca, Angelo; Pilato, Michele; Scardulla, Cesare; Gleason, Thomas G.; Vorp, David A.

2014-01-01

The aortic dissection (AoD) of an ascending thoracic aortic aneurysm (ATAA) initiates when the hemodynamic loads exerted on the aneurysmal wall overcome the adhesive forces holding the elastic layers together. Parallel coupled, two-way fluid–structure interaction (FSI) analyses were performed on patient-specific ATAAs obtained from patients with either bicuspid aortic valve (BAV) or tricuspid aortic valve (TAV) to evaluate hemodynamic predictors and wall stresses imparting aneurysm enlargement and AoD. Results showed a left-handed circumferential flow with slower-moving helical pattern in the aneurysm's center for BAV ATAAs whereas a slight deviation of the blood flow toward the anterolateral region of the ascending aorta was observed for TAV ATAAs. Blood pressure and wall shear stress were found key hemodynamic predictors of aneurysm dilatation, and their dissimilarities are likely associated to the morphological anatomy of the aortic valve. We also observed discontinues, wall stresses on aneurysmal aorta, which was modeled as a composite with two elastic layers (i.e., inhomogeneity of vessel structural organization). This stress distribution was caused by differences on elastic material properties of aortic layers. Wall stress distribution suggests AoD just above sinotubular junction. Moreover, abnormal flow and lower elastic material properties that are likely intrinsic in BAV individuals render the aneurysm susceptible to the initiation of AoD. PMID:23664314

A modification of Murray's law for shear-thinning rheology.

PubMed

McGah, Patrick M; Capobianchi, Massimo

2015-05-01

This study reformulates Murray's well-known principle of minimum work as applied to the cardiovascular system to include the effects of the shear-thinning rheology of blood. The viscous behavior is described using the extended modified power law (EMPL), which is a time-independent, but shear-thinning rheological constitutive equation. The resulting minimization problem is solved numerically for typical parameter ranges. The non-Newtonian analysis still predicts the classical cubic diameter dependence of the volume flow rate and the cubic branching law. The current analysis also predicts a constant wall shear stress throughout the vascular tree, albeit with a numerical value about 15-25% higher than the Newtonian analysis. Thus, experimentally observed deviations from the cubic branching law or the predicted constant wall shear stress in the vasculature cannot likely be attributed to blood's shear-thinning behavior. Further differences between the predictions of the non-Newtonian and the Newtonian analyses are highlighted, and the limitations of the Newtonian analysis are discussed. Finally, the range and limits of applicability of the current results as applied to the human arterial tree are also discussed.

Methodology for calculating shear stress in a meandering channel

Treesearch

Kyung-Seop Sin

2010-01-01

Shear stress in meandering channels is the key parameter to predict bank erosion and bend migration. A representative study reach of the Rio Grande River in central New Mexico has been modeled in the Hydraulics Laboratory at CSU. To determine the shear stress distribution in a meandering channel, the large scale (1:12) physical modeling study was conducted in the...

Modelling Force Transfer Around Openings of Full-Scale Shear Walls

Treesearch

Tom Skaggs; Borjen Yeh; Frank Lam; Minghao Li; Doug Rammer; James Wacker

2011-01-01

Wood structural panel (WSP) sheathed shear walls and diaphragms are the primary lateralload-resisting elements in wood-frame construction. The historical performance of lightframe structures in North America has been very good due, in part, to model building codes that are designed to preserve life safety. These model building codes have spawned continual improvement...

Bayesian decision and mixture models for AE monitoring of steel-concrete composite shear walls

NASA Astrophysics Data System (ADS)

Farhidzadeh, Alireza; Epackachi, Siamak; Salamone, Salvatore; Whittaker, Andrew S.

2015-11-01

This paper presents an approach based on an acoustic emission technique for the health monitoring of steel-concrete (SC) composite shear walls. SC composite walls consist of plain (unreinforced) concrete sandwiched between steel faceplates. Although the use of SC system construction has been studied extensively for nearly 20 years, little-to-no attention has been devoted to the development of structural health monitoring techniques for the inspection of damage of the concrete behind the steel plates. In this work an unsupervised pattern recognition algorithm based on probability theory is proposed to assess the soundness of the concrete infill, and eventually provide a diagnosis of the SC wall’s health. The approach is validated through an experimental study on a large-scale SC shear wall subjected to a displacement controlled reversed cyclic loading.

Shear stress increases nitric oxide production in thick ascending limbs

PubMed Central

Cabral, Pablo D.; Hong, Nancy J.

2010-01-01

We showed that luminal flow stimulates nitric oxide (NO) production in thick ascending limbs. Ion delivery, stretch, pressure, and shear stress all increase when flow is enhanced. We hypothesized that shear stress stimulates NO in thick ascending limbs, whereas stretch, pressure, and ion delivery do not. We measured NO in isolated, perfused rat thick ascending limbs using the NO-sensitive dye DAF FM-DA. NO production rose from 21 ± 7 to 58 ± 12 AU/min (P < 0.02; n = 7) when we increased luminal flow from 0 to 20 nl/min, but dropped to 16 ± 8 AU/min (P < 0.02; n = 7) 10 min after flow was stopped. Flow did not increase NO in tubules from mice lacking NO synthase 3 (NOS 3). Flow stimulated NO production by the same extent in tubules perfused with ion-free solution and physiological saline (20 ± 7 vs. 24 ± 6 AU/min; n = 7). Increasing stretch while reducing shear stress and pressure lowered NO generation from 42 ± 9 to 17 ± 6 AU/min (P < 0.03; n = 6). In the absence of shear stress, increasing pressure and stretch had no effect on NO production (2 ± 8 vs. 8 ± 8 AU/min; n = 6). Similar results were obtained in the presence of tempol (100 μmol/l), a O2− scavenger. Primary cultures of thick ascending limb cells subjected to shear stresses of 0.02 and 0.55 dyne/cm2 produced NO at rates of 55 ± 10 and 315 ± 93 AU/s, respectively (P < 0.002; n = 7). Pretreatment with the NOS inhibitor l-NAME (5 mmol/l) blocked the shear stress-induced increase in NO production. We concluded that shear stress rather than pressure, stretch, or ion delivery mediates flow-induced stimulation of NO by NOS 3 in thick ascending limbs. PMID:20719980

Computational modeling for prediction of the shear stress of three-dimensional isotropic and aligned fiber networks.

PubMed

Park, Seungman

2017-09-01

Interstitial flow (IF) is a creeping flow through the interstitial space of the extracellular matrix (ECM). IF plays a key role in diverse biological functions, such as tissue homeostasis, cell function and behavior. Currently, most studies that have characterized IF have focused on the permeability of ECM or shear stress distribution on the cells, but less is known about the prediction of shear stress on the individual fibers or fiber networks despite its significance in the alignment of matrix fibers and cells observed in fibrotic or wound tissues. In this study, I developed a computational model to predict shear stress for different structured fibrous networks. To generate isotropic models, a random growth algorithm and a second-order orientation tensor were employed. Then, a three-dimensional (3D) solid model was created using computer-aided design (CAD) software for the aligned models (i.e., parallel, perpendicular and cubic models). Subsequently, a tetrahedral unstructured mesh was generated and flow solutions were calculated by solving equations for mass and momentum conservation for all models. Through the flow solutions, I estimated permeability using Darcy's law. Average shear stress (ASS) on the fibers was calculated by averaging the wall shear stress of the fibers. By using nonlinear surface fitting of permeability, viscosity, velocity, porosity and ASS, I devised new computational models. Overall, the developed models showed that higher porosity induced higher permeability, as previous empirical and theoretical models have shown. For comparison of the permeability, the present computational models were matched well with previous models, which justify our computational approach. ASS tended to increase linearly with respect to inlet velocity and dynamic viscosity, whereas permeability was almost the same. Finally, the developed model nicely predicted the ASS values that had been directly estimated from computational fluid dynamics (CFD). The present

Experimental and numerical investigations of higher mode effects on seismic inelastic response of reinforced concrete shear walls

NASA Astrophysics Data System (ADS)

Ghorbanirenani, Iman

This thesis presents two experimental programs together with companion numerical studies that were carried out on reinforced concrete shear walls: static tests and dynamic (shake table) tests. The first series of experiments were monotonic and cyclic quasi-static testing on ductile reinforced concrete shear wall specimens designed and detailed according to the seismic provisions of NBCC 2005 and CSA-A23.3-04 standard. The tests were carried out on full-scale and 1:2.37 reduced scale wall specimens to evaluate the seismic design provisions and similitude law and determine the appropriate scaling factor that could be applied for further studies such as dynamic tests. The second series of experiments were shake table tests conducted on two identical 1:2.33 scaled, 8-storey moderately ductile reinforced concrete shear wall specimens to investigate the effects of higher modes on the inelastic response of slender walls under high frequency ground motions expected in Eastern North America. The walls were designed and detailed according to the seismic provisions of NBCC 2005 and CSA-A23.3-04 standard. The objectives were to validate and understand the inelastic response and interaction of shear, flexure and axial loads in plastic hinge zones of the walls considering the higher mode effects and to investigate the formation of second hinge in upper part of the wall due to higher mode responses. Second mode response significantly affected the response of the walls. This caused inelastic flexural response to develop at the 6th level with approximately the same rotation ductility compared to that observed at the base. Dynamic amplification of the base shear forces was also observed in both walls. Numerical modeling of these two shake table tests was performed to evaluate the test results and validate current modeling approaches. Nonlinear time history analyses were carried out by the reinforced concrete fibre element (OpenSees program) and finite element (VecTor2 program

Full-scale shear wall tests for force transfer around openings

Treesearch

Tom Skaggs; Borjen Yeh; Frank Lam; Douglas Rammer; James Wacker

2010-01-01

Wood structural panel sheathed shear walls and diaphragms are the primary lateral-load resisting elements in wood-frame construction. The historical performance of light-frame structures in North America are very good due, in part, to model building codes that are designed to preserve life safety, as well as the inherent redundancy of wood-frame construction using wood...

Periodic Viscous Shear Heating Instability in Fine-Grained Shear Zones: Mechanism for Intermediate Depth Earthquakes

NASA Astrophysics Data System (ADS)

Coon, E.; Kelemen, P.; Hirth, G.; Spiegelman, M.

2005-12-01

initial T of 600 to 850 C, this produced periodic viscous shear heating events with periods of 100's to 1000's of years. Strain rates during these events approach 1 per second as temperatures reach 1400. Cooling between events returns the shear zone almost to its initial temperature, though ultimately shear zone temperature between events exceeds 850 C resulting in stable viscous creep. Analysis shows that our system of equations jumps from one steady state to another, depending on a non-dimensional number relating the rate of shear heating to the rate of diffusive cooling. This year, Kelemen and Hirth show that the rate of stress drop during shear heating events is greater than the rate of elastic stress relaxation, so that shear heating events are a runaway instability. Rather than capping the temperature at 1400 C, we parameterize melt fraction as a function of T, and shear viscosity as a function of melt fraction. A problem with our 1D model is that predicted displacements are too large (1 to 20 m) during shear heating events, essentially because there is no resistance at shear zone ends. To address this, Coon and Spiegelman have embarked on a 3D model, incorporating a pre-existing fine-grained, tabular shear zone of finite extent, with a visco-elastic rheology for both shear zone and wall rocks. Preliminary 1D models using this approach show that the more complicated rheology yields the same result as the simpler model. We will present preliminary results, and determine the Maxwell time for this problem, since low strain rates could produce viscous relaxation in both shear zone and wall rocks with negligible shear heating.

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.

Linear and weakly nonlinear aspects of free shear layer instability, roll-up, subharmonic interaction and wall influence

NASA Technical Reports Server (NTRS)

Cain, A. B.; Thompson, M. W.

1986-01-01

The growth of the momentum thickness and the modal disturbance energies are examined to study the nature and onset of nonlinearity in a temporally growing free shear layer. A shooting technique is used to find solutions to the linearized eigenvalue problem, and pseudospectral weakly nonlinear simulations of this flow are obtained for comparison. The roll-up of a fundamental disturbance follows linear theory predictions even with a 20 percent disturbance amplitude. A weak nonlinear interaction of the disturbance creates a finite-amplitude mean shear stress which dominates the growth of the layer momentum thickness, and the disturbance growth rate changes until the fundamental disturbance dominates. The fundamental then becomes an energy source for the harmonic, resulting in an increase in the growth rate of the subharmonic over the linear prediction even when the fundamental has no energy to give. Also considered are phase relations and the wall influence.

Navier-Stokes Computations With One-Equation Turbulence Model for Flows Along Concave Wall Surfaces

NASA Technical Reports Server (NTRS)

Wang, Chi R.

2005-01-01

This report presents the use of a time-marching three-dimensional compressible Navier-Stokes equation numerical solver with a one-equation turbulence model to simulate the flow fields developed along concave wall surfaces without and with a downstream extension flat wall surface. The 3-D Navier- Stokes numerical solver came from the NASA Glenn-HT code. The one-equation turbulence model was derived from the Spalart and Allmaras model. The computational approach was first calibrated with the computations of the velocity and Reynolds shear stress profiles of a steady flat plate boundary layer flow. The computational approach was then used to simulate developing boundary layer flows along concave wall surfaces without and with a downstream extension wall. The author investigated the computational results of surface friction factors, near surface velocity components, near wall temperatures, and a turbulent shear stress component in terms of turbulence modeling, computational mesh configurations, inlet turbulence level, and time iteration step. The computational results were compared with existing measurements of skin friction factors, velocity components, and shear stresses of the developing boundary layer flows. With a fine computational mesh and a one-equation model, the computational approach could predict accurately the skin friction factors, near surface velocity and temperature, and shear stress within the flows. The computed velocity components and shear stresses also showed the vortices effect on the velocity variations over a concave wall. The computed eddy viscosities at the near wall locations were also compared with the results from a two equation turbulence modeling technique. The inlet turbulence length scale was found to have little effect on the eddy viscosities at locations near the concave wall surface. The eddy viscosities, from the one-equation and two-equation modeling, were comparable at most stream-wise stations. The present one

Diffractive Optic Fluid Shear Stress Sensor

NASA Technical Reports Server (NTRS)

Wilson, D.; Scalf, J.; Forouhar, S.; Muller, R.; Taugwalder, F.; Gharib, M.; Fourguette, D.; Modarress, D.

2000-01-01

Light scattering off particles flowing through a two-slit interference pattern can be used to measure the shear stress of the fluid. We have designed and fabricated a miniature diffractive optic sensor based on this principle.

Monocyte recruitment to endothelial cells in response to oscillatory shear stress

PubMed Central

Hsiai, Tzung K.; Cho, Sung K.; Wong, Pak K.; Ing, Mike; Salazar, Adler; Sevanian, Alex; Navab, Mohamad; Demer, Linda L.; Ho, Chih-Ming

2014-01-01

Leukocyte recruitment to endothelial cells is a critical event in inflammatory responses. The spatial, temporal gradients of shear stress, topology, and outcome of cellular interactions that underlie these responses have so far been inferred from static imaging of tissue sections or studies of statically cultured cells. In this report, we developed micro-electromechanical systems (MEMS) sensors, comparable to a single endothelial cell (EC) in size, to link real-time shear stress with monocyte/EC binding kinetics in a complex flow environment, simulating the moving and unsteady separation point at the arterial bifurcation with high spatial and temporal resolution. In response to oscillatory shear stress (τ) at ± 2.6 dyn/cm2 at a time-averaged shear stress (τave) = 0 and 0.5 Hz, individual monocytes displayed unique to-and-fro trajectories undergoing rolling, binding, and dissociation with other monocyte, followed by solid adhesion on EC. Our study quantified individual monocyte/EC binding kinetics in terms of displacement and velocity profiles. Oscillatory flow induces up-regulation of adhesion molecules and cytokines to mediate monocyte/EC interactions over a dynamic range of shear stress ± 2.6 dyn/cm2 (P= 0.50, n= 10).—Hsiai, T. K., Cho, S. K., Wong, P. K., Ing, M., Salazar, A., Sevanian, A., Navab, M., Demer, L. L., Ho, C.-M. Monocyte recruitment to endothelial cells in response to oscillatory shear stress. FASEB J. 17, 1648–1657 (2003) PMID:12958171

Self-sustaining processes at all scales in wall-bounded turbulent shear flows

NASA Astrophysics Data System (ADS)

Cossu, Carlo; Hwang, Yongyun

2017-03-01

We collect and discuss the results of our recent studies which show evidence of the existence of a whole family of self-sustaining motions in wall-bounded turbulent shear flows with scales ranging from those of buffer-layer streaks to those of large-scale and very-large-scale motions in the outer layer. The statistical and dynamical features of this family of self-sustaining motions, which are associated with streaks and quasi-streamwise vortices, are consistent with those of Townsend's attached eddies. Motions at each relevant scale are able to sustain themselves in the absence of forcing from larger- or smaller-scale motions by extracting energy from the mean flow via a coherent lift-up effect. The coherent self-sustaining process is embedded in a set of invariant solutions of the filtered Navier-Stokes equations which take into full account the Reynolds stresses associated with the residual smaller-scale motions.

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.

Interaction between a normal shock wave and a turbulent boundary layer at high transonic speeds. Part 1: Pressure distribution. Part 2: Wall shear stress. Part 3: Simplified formulas for the prediction of surface pressures and skin friction

NASA Technical Reports Server (NTRS)

Adamson, T. C., Jr.; Liou, M. S.; Messiter, A. F.

1980-01-01

An asymptotic description is derived for the interaction between a shock wave and a turbulent boundary layer in transonic flow, for a particular limiting case. The dimensionless difference between the external flow velocity and critical sound speed is taken to be much smaller than one, but large in comparison with the dimensionless friction velocity. The basic results are derived for a flat plate, and corrections for longitudinal wall curvature and for flow in a circular pipe are also shown. Solutions are given for the wall pressure distribution and the shape of the shock wave. Solutions for the wall shear stress are obtained, and a criterion for incipient separation is derived. Simplified solutions for both the wall pressure and skin friction distributions in the interaction region are given. These results are presented in a form suitable for use in computer programs.

Repetitive Supra-Physiological Shear Stress Impairs Red Blood Cell Deformability and Induces Hemolysis.

PubMed

Horobin, Jarod T; Sabapathy, Surendran; Simmonds, Michael J

2017-11-01

The supra-physiological shear stress that blood is exposed to while traversing mechanical circulatory assist devices affects the physical properties of red blood cells (RBCs), impairs RBC deformability, and may induce hemolysis. Previous studies exploring RBC damage following exposure to supra-physiological shear stress have employed durations exceeding clinical instrumentation, thus we explored changes in RBC deformability following exposure to shear stress below the reported "hemolytic threshold" using shear exposure durations per minute (i.e., duty-cycles) reflective of that employed by circulatory assist devices. Blood collected from 20 male donors, aged 18-38 years, was suspended in a viscous medium and exposed to an intermittent shear stress protocol of 1 s at 100 Pa, every 60 s for 60 duty-cycles. During the remaining 59 s/min, the cells were left at stasis until the subsequent duty-cycle commenced. At discrete time points (15/30/45/60 duty-cycles), an ektacytometer measured RBC deformability immediately after shear exposure at 100 Pa. Plasma-free hemoglobin, a measurement of hemolysis, was quantified via spectrophotometry. Supra-physiological shear stress impaired RBC properties, as indicated by: (1) decreased maximal elongation of RBCs at infinite shear stress following 15 duty-cycles (P <0.05); (2) increased real-time RBC deformability during application of the supra-physiological shear stress protocol (100 Pa) following exposure to 1 duty-cycle (F (1.891, 32.15) = 12.21, P = 0.0001); and (3) increased plasma-free hemoglobin following 60 duty-cycles (P < 0.01). The present study indicates that exposure of RBCs to short-term, repeated supra-physiological shear stress, impairs RBC deformability, with the extent of impairment exacerbated with each duty-cycle, and ultimately precipitates hemolysis. © 2017 International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc.

Exercise-induced heat stress disrupts the shear-dilatory relationship.

PubMed

Ives, Stephen J; Lefferts, Wesley K; Wharton, Margret; Fehling, Patricia C; Smith, Denise L

2016-12-01

What is the central question of this study? Although heat stress is known to increase cardiovascular strain, no study, to date, had explored the potential impact of exercise-induced heat stress on vascular function. What is the main finding and its importance? We found that acute exercise tended to reduce flow-mediated dilatation (FMD), owing in part to reduced reactive hyperaemia/shear stimulus; thus, when FMD is normalized to shear no postexercise deficit exists. Exercise-induced heat stress increased reactive hyperaemia, shear rate, coupled with a sustained FMD postexercise, suggests that exercise-induced heat stress increases the amount of shear stimulus to elicit a similar response, indicating reduced vascular responsiveness, or reserve, which might increase cardiovascular susceptibility. Heat stress increases cardiovascular strain and is of particular concern in occupations, such as firefighting, in which individuals are required to perform strenuous work while wearing personal protective equipment. Sudden cardiac events are associated with strenuous activity and are the leading cause of duty-related death among firefighters, accounting for ∼50% of duty-related fatalities per year. Understanding the acute effects of exercise-induced heat stress (EIHS) on vascular endothelial function may provide insight into the mechanisms precipitating acute coronary events in firefighters. The purpose of this study, therefore, was to determine the effects of EIHS on vascular endothelial function. Using a balanced crossover design, 12 healthy men performed 100 min of moderate-intensity, intermittent exercise with and without EIHS (personal protective equipment or cooling vest, respectively). Measurements of flow-mediated dilatation (FMD), reactive hyperaemia and shear rate area under the curve (SR AUC ) were performed pre- and postexercise. During EIHS, core temperature was significantly higher (38 ± 0.1 versus 37 ± 0.1°C). Postexercise FMD tended to be suppressed

Fluid shear stress activates YAP1 to promote cancer cell motility

NASA Astrophysics Data System (ADS)

Lee, Hyun Jung; Diaz, Miguel F.; Price, Katherine M.; Ozuna, Joyce A.; Zhang, Songlin; Sevick-Muraca, Eva M.; Hagan, John P.; Wenzel, Pamela L.

2017-01-01

Mechanical stress is pervasive in egress routes of malignancy, yet the intrinsic effects of force on tumour cells remain poorly understood. Here, we demonstrate that frictional force characteristic of flow in the lymphatics stimulates YAP1 to drive cancer cell migration; whereas intensities of fluid wall shear stress (WSS) typical of venous or arterial flow inhibit taxis. YAP1, but not TAZ, is strictly required for WSS-enhanced cell movement, as blockade of YAP1, TEAD1-4 or the YAP1-TEAD interaction reduces cellular velocity to levels observed without flow. Silencing of TEAD phenocopies loss of YAP1, implicating transcriptional transactivation function in mediating force-enhanced cell migration. WSS dictates expression of a network of YAP1 effectors with executive roles in invasion, chemotaxis and adhesion downstream of the ROCK-LIMK-cofilin signalling axis. Altogether, these data implicate YAP1 as a fluid mechanosensor that functions to regulate genes that promote metastasis.

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.

Cell Wall Metabolism in Response to Abiotic Stress

PubMed Central

Gall, Hyacinthe Le; Philippe, Florian; Domon, Jean-Marc; Gillet, Françoise; Pelloux, Jérôme; Rayon, Catherine

2015-01-01

This review focuses on the responses of the plant cell wall to several abiotic stresses including drought, flooding, heat, cold, salt, heavy metals, light, and air pollutants. The effects of stress on cell wall metabolism are discussed at the physiological (morphogenic), transcriptomic, proteomic and biochemical levels. The analysis of a large set of data shows that the plant response is highly complex. The overall effects of most abiotic stress are often dependent on the plant species, the genotype, the age of the plant, the timing of the stress application, and the intensity of this stress. This shows the difficulty of identifying a common pattern of stress response in cell wall architecture that could enable adaptation and/or resistance to abiotic stress. However, in most cases, two main mechanisms can be highlighted: (i) an increased level in xyloglucan endotransglucosylase/hydrolase (XTH) and expansin proteins, associated with an increase in the degree of rhamnogalacturonan I branching that maintains cell wall plasticity and (ii) an increased cell wall thickening by reinforcement of the secondary wall with hemicellulose and lignin deposition. Taken together, these results show the need to undertake large-scale analyses, using multidisciplinary approaches, to unravel the consequences of stress on the cell wall. This will help identify the key components that could be targeted to improve biomass production under stress conditions. PMID:27135320

A High-Temperature MEMS Surface Fence for Wall-Shear-Stress Measurement in Scramjet Flow

PubMed Central

Ma, Binghe; Deng, Jinjun; Yuan, Weizheng; Zhou, Zitong; Zhang, Han

2017-01-01

A new variant of MEMS surface fence is proposed for shear-stress estimation under high-speed, high-temperature flow conditions. Investigation of high-temperature resistance including heat-resistant mechanism and process, in conjunction with high-temperature packaging design, enable the sensor to be used in environment up to 400 °C. The packaged sensor is calibrated over a range of ~65 Pa and then used to examine the development of the transient flow of the scramjet ignition process (Mach 2 airflow, stagnation pressure, and a temperature of 0.8 MPa and 950 K, respectively). The results show that the sensor is able to detect the transient flow conditions of the scramjet ignition process including shock impact, flow correction, steady state, and hydrogen off. PMID:29065498

A High-Temperature MEMS Surface Fence for Wall-Shear-Stress Measurement in Scramjet Flow.

PubMed

Ma, Chengyu; Ma, Binghe; Deng, Jinjun; Yuan, Weizheng; Zhou, Zitong; Zhang, Han

2017-10-22

A new variant of MEMS surface fence is proposed for shear-stress estimation under high-speed, high-temperature flow conditions. Investigation of high-temperature resistance including heat-resistant mechanism and process, in conjunction with high-temperature packaging design, enable the sensor to be used in environment up to 400 °C. The packaged sensor is calibrated over a range of ~65 Pa and then used to examine the development of the transient flow of the scramjet ignition process (Mach 2 airflow, stagnation pressure, and a temperature of 0.8 MPa and 950 K, respectively). The results show that the sensor is able to detect the transient flow conditions of the scramjet ignition process including shock impact, flow correction, steady state, and hydrogen off.

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.

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...

Prediction of plantar shear stress distribution by artificial intelligence methods.

PubMed

Yavuz, Metin; Ocak, Hasan; Hetherington, Vincent J; Davis, Brian L

2009-09-01

Shear forces under the human foot are thought to be responsible for various foot pathologies such as diabetic plantar ulcers and athletic blisters. Frictional shear forces might also play a role in the metatarsalgia observed among hallux valgus (HaV) and rheumatoid arthritis (RA) patients. Due to the absence of commercial devices capable of measuring shear stress distribution, a number of linear models were developed. All of these have met with limited success. This study used nonlinear methods, specifically neural network and fuzzy logic schemes, to predict the distribution of plantar shear forces based on vertical loading parameters. In total, 73 subjects were recruited; 17 had diabetic neuropathy, 14 had HaV, 9 had RA, 11 had frequent foot blisters, and 22 were healthy. A feed-forward neural network (NN) and adaptive neurofuzzy inference system (NFIS) were built. These systems were then applied to a custom-built platform, which collected plantar pressure and shear stress data as subjects walked over the device. The inputs to both models were peak pressure, peak pressure-time integral, and time to peak pressure, and the output was peak resultant shear. Root-mean-square error (RMSE) values were calculated to test the models' accuracy. RMSE/actual shear ratio varied between 0.27 and 0.40 for NN predictions. Similarly, NFIS estimations resulted in a 0.28-0.37 ratio for local peak values in all subject groups. On the other hand, error percentages for global peak shear values were found to be in the range 11.4-44.1. These results indicate that there is no direct relationship between pressure and shear magnitudes. Future research should aim to decrease error levels by introducing shear stress dependent variables into the models.

Research Advances on Fabricated Shear Wall System

NASA Astrophysics Data System (ADS)

Liu, Xudong; Wang, Donghui; Wang, Sheng; Zhai, Yu

2018-03-01

With the rapid development of the construction industry, building energy consumption has been increasing, has become a problem that can not be ignored. It is imperative to develop energy-saving buildings. A new type of prefabricated shear wall is assembled and partially assembled by prefabricated parts, and some concrete is spliced together. The new structure has good integrity, seismic resistance and excellent energy saving and environmental protection performance. It reduces building energy consumption to a great extent. Therefore, the design method, manufacturing process, site assembly process and key technical problems of the system are discussed. For the construction industry gradually entered the energy conservation, environmental protection, safety and durability of sustainable development laid the foundation.

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. Copyright © 2013 Elsevier B.V. All rights reserved.

Laser reflection method for determination of shear stress in low density transitional flows

NASA Astrophysics Data System (ADS)

Sathian, Sarith P.; Kurian, Job

2006-03-01

The details of laser reflection method (LRM) for the determination of shear stress in low density transitional flows are presented. The method is employed to determine the shear stress due to impingement of a low density supersonic free jet issuing out from a convergent divergent nozzle on a flat plate. The plate is smeared with a thin oil film and kept parallel to the nozzle axis. For a thin oil film moving under the action of aerodynamic boundary layer, the shear stress at the air-oil interface is equal to the shear stress between the surface and air. A direct and dynamic measurement of the oil film slope generated by the shear force is done using a position sensing detector (PSD). The thinning rate of the oil film is directly measured which is the major advantage of the LRM. From the oil film slope history, calculation of the shear stress is done using a three-point formula. The range of Knudsen numbers investigated is from 0.028 to 0.516. Pressure ratio across the nozzle varied from 3,500 to 8,500 giving highly under expanded free jets. The measured values of shear, in the overlapping region of experimental parameters, show fair agreement with those obtained by force balance method and laser interferometric method.

Deformation and Stress Response of Carbon Nanotubes/UHMWPE Composites under Extensional-Shear Coupling Flow

NASA Astrophysics Data System (ADS)

Wang, Junxia; Cao, Changlin; Yu, Dingshan; Chen, Xudong

2018-02-01

In this paper, the effect of varying extensional-shear couple loading on deformation and stress response of Carbon Nanotubes/ ultra-high molecular weight polyethylene (CNTs/UHMWPE) composites was investigated using finite element numerical simulation, with expect to improve the manufacturing process of UHMWPE-based composites with reduced stress and lower distortion. When applying pure extensional loading and pure X-Y shear loading, it was found that the risk of a structural breakage greatly rises. For identifying the coupling between extensional and shear loading, distinct generations of force loading were defined by adjusting the magnitude of extensional loading and X-Y shear loading. It was shown that with the decrement of X-Y shear loading the deformation decreases obviously where the maximal Mises stress in Z-direction at 0.45 m distance is in the range from 24 to 10 MPa and the maximal shear stress at 0.61 m distance is within the range from 0.9 to 0.3 MPa. In addition, all the stresses determined were clearly below the yield strength of CNTs/UHMWPE composites under extensional-shear couple loading.

Uncertainty quantification of wall shear stress in intracranial aneurysms using a data-driven statistical model of systemic blood flow variability.

PubMed

Sarrami-Foroushani, Ali; Lassila, Toni; Gooya, Ali; Geers, Arjan J; Frangi, Alejandro F

2016-12-08

Adverse wall shear stress (WSS) patterns are known to play a key role in the localisation, formation, and progression of intracranial aneurysms (IAs). Complex region-specific and time-varying aneurysmal WSS patterns depend both on vascular morphology as well as on variable systemic flow conditions. Computational fluid dynamics (CFD) has been proposed for characterising WSS patterns in IAs; however, CFD simulations often rely on deterministic boundary conditions that are not representative of the actual variations in blood flow. We develop a data-driven statistical model of internal carotid artery (ICA) flow, which is used to generate a virtual population of waveforms used as inlet boundary conditions in CFD simulations. This allows the statistics of the resulting aneurysmal WSS distributions to be computed. It is observed that ICA waveform variations have limited influence on the time-averaged WSS (TAWSS) on the IA surface. In contrast, in regions where the flow is locally highly multidirectional, WSS directionality and harmonic content are strongly affected by the ICA flow waveform. As a consequence, we argue that the effect of blood flow variability should be explicitly considered in CFD-based IA rupture assessment to prevent confounding the conclusions. Copyright © 2016 Elsevier Ltd. All rights reserved.

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

Fluid shear stress sensitizes cancer cells to receptor-mediated apoptosis via trimeric death receptors

NASA Astrophysics Data System (ADS)

Mitchell, Michael J.; King, Michael R.

2013-01-01

Cancer metastasis, the process of cancer cell migration from a primary to distal location, typically leads to a poor patient prognosis. Hematogenous metastasis is initiated by intravasation of circulating tumor cells (CTCs) into the bloodstream, which are then believed to adhere to the luminal surface of the endothelium and extravasate into distal locations. Apoptotic agents such as tumor necrosis factor apoptosis-inducing ligand (TRAIL), whether in soluble ligand form or expressed on the surface of natural killer cells, have shown promise in treating CTCs to reduce the probability of metastasis. The role of hemodynamic shear forces in altering the cancer cell response to apoptotic agents has not been previously investigated. Here, we report that human colon cancer COLO 205 and prostate cancer PC-3 cells exposed to a uniform fluid shear stress in a cone-and-plate viscometer become sensitized to TRAIL-induced apoptosis. Shear-induced sensitization directly correlates with the application of fluid shear stress, and TRAIL-induced apoptosis increases in a fluid shear stress force- and time-dependent manner. In contrast, TRAIL-induced necrosis is not affected by the application fluid shear stress. Interestingly, fluid shear stress does not sensitize cancer cells to apoptosis when treated with doxorubicin, which also induces apoptosis in cancer cells. Caspase inhibition experiments reveal that shear stress-induced sensitization to TRAIL occurs via caspase-dependent apoptosis. These results suggest that physiological fluid shear forces can modulate receptor-mediated apoptosis of cancer cells in the presence of apoptotic agents.

Evaluation of bed load transport subject to high shear stress fluctuations

NASA Astrophysics Data System (ADS)

Cheng, Nian-Sheng; Tang, Hongwu; Zhu, Lijun

2004-05-01

Many formulas available in the literature for computing sediment transport rates are often expressed in terms of time mean variables such as time mean bed shear stress or flow velocity, while effects of turbulence intensity, e.g., bed shear stress fluctuation, on sediment transport were seldom considered. This may be due to the fact that turbulence fluctuation is relatively limited in laboratory open-channel flows, which are often used for conducting sediment transport experiments. However, turbulence intensity could be markedly enhanced in practice. This note presents an analytical method to compute bed load transport by including effects of fluctuations in the bed shear stress. The analytical results obtained show that the transport rate enhanced by turbulence can be expressed as a simple function of the relative fluctuation of the bed shear stress. The results are also verified using data that were collected recently from specifically designed laboratory experiments. The present analysis is applicable largely for the condition of a flat bed that is comprised of uniform sand particles subject to unidirectional flows.

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.

Formulating the shear stress distribution in circular open channels based on the Renyi entropy

NASA Astrophysics Data System (ADS)

Khozani, Zohreh Sheikh; Bonakdari, Hossein

2018-01-01

The principle of maximum entropy is employed to derive the shear stress distribution by maximizing the Renyi entropy subject to some constraints and by assuming that dimensionless shear stress is a random variable. A Renyi entropy-based equation can be used to model the shear stress distribution along the entire wetted perimeter of circular channels and circular channels with flat beds and deposited sediments. A wide range of experimental results for 12 hydraulic conditions with different Froude numbers (0.375 to 1.71) and flow depths (20.3 to 201.5 mm) were used to validate the derived shear stress distribution. For circular channels, model performance enhanced with increasing flow depth (mean relative error (RE) of 0.0414) and only deteriorated slightly at the greatest flow depth (RE of 0.0573). For circular channels with flat beds, the Renyi entropy model predicted the shear stress distribution well at lower sediment depth. The Renyi entropy model results were also compared with Shannon entropy model results. Both models performed well for circular channels, but for circular channels with flat beds the Renyi entropy model displayed superior performance in estimating the shear stress distribution. The Renyi entropy model was highly precise and predicted the shear stress distribution in a circular channel with RE of 0.0480 and in a circular channel with a flat bed with RE of 0.0488.

Importance of Tensile Strength on the Shear Behavior of Discontinuities

NASA Astrophysics Data System (ADS)

Ghazvinian, A. H.; Azinfar, M. J.; Geranmayeh Vaneghi, R.

2012-05-01

In this study, the shear behavior of discontinuities possessing two different rock wall types with distinct separate compressive strengths was investigated. The designed profiles consisted of regular artificial joints molded by five types of plaster mortars, each representing a distinct uniaxial compressive strength. The compressive strengths of plaster specimens ranged from 5.9 to 19.5 MPa. These specimens were molded considering a regular triangular asperity profile and were designed so as to achieve joint walls with different strength material combinations. The results showed that the shear behavior of discontinuities possessing different joint wall compressive strengths (DDJCS) tested under constant normal load (CNL) conditions is the same as those possessing identical joint wall strengths, but the shear strength of DDJCS is governed by minor joint wall compressive strength. In addition, it was measured that the predicted values obtained by Barton's empirical criterion are greater than the experimental results. The finding indicates that there is a correlation between the joint roughness coefficient (JRC), normal stress, and mechanical strength. It was observed that the mode of failure of asperities is either pure tensile, pure shear, or a combination of both. Therefore, Barton's strength criterion, which considers the compressive strength of joint walls, was modified by substituting the compressive strength with the tensile strength. The validity of the modified criterion was examined by the comparison of the predicted shear values with the laboratory shear test results reported by Grasselli (Ph.D. thesis n.2404, Civil Engineering Department, EPFL, Lausanne, Switzerland, 2001). These comparisons infer that the modified criterion can predict the shear strength of joints more precisely.

Intermittent hydrostatic pressure inhibits shear stress-induced nitric oxide release in human osteoarthritic chondrocytes in vitro.

PubMed

Lee, Mel S; Trindade, Michael C D; Ikenoue, Takashi; Schurman, David J; Goodman, Stuart B; Smith, R Lane

2003-02-01

To test the effects of intermittent hydrostatic pressure (IHP) on nitric oxide (NO) release induced by shear stress and matrix macromolecule gene expression in human osteoarthritic chondrocytes in vitro. Chondrocytes isolated from cartilage samples from 9 patients with osteoarthritis were cultured and exposed to either shear stress or an NO donor. Nitrite concentration was measured using the Griess reaction. Matrix macromolecule mRNA signal levels were determined using reverse-transcriptase polymerase chain reaction and quantified by imaging analysis software. Exposure to shear stress upregulated NO release in a dose and time-dependent manner. Application of IHP inhibited shear stress induced NO release but did not alter NO release from chondrocytes not exposed to shear stress. Shear stress induced NO or addition of an NO donor (sodium nitroprusside) was associated with decreased mRNA signal levels for the cartilage matrix proteins, aggrecan, and type II collagen. Intermittent hydrostatic pressure blocked the inhibitory effects of sodium nitroprusside but did not alter the inhibitory effects of shear stress on cartilage macromolecule gene expression. Our data show that shear stress and IHP differentially alter chondrocyte metabolism and suggest that a balance of effects between different loading forces preserve cartilage extracellular matrix in vivo.

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.

Brownian dynamics of wall tethered polymers in shear flow

NASA Astrophysics Data System (ADS)

Lin, Tiras Y.; Saadat, Amir; Kushwaha, Amit; Shaqfeh, Eric S. G.

2017-11-01

The dynamics of a wall tethered polymer in shear flow is studied using Brownian dynamics. Simulations are performed with bead-spring chains, and the effect of hydrodynamic interactions (HI) is incorporated through Blake's tensor with a finite size bead correction. We characterize the configuration of the polymer as a function of the Weissenberg number by investigating the regions the polymer explores in both the flow-gradient and flow-vorticity planes. The fractional extension in the flow direction, the width in the vorticity direction, and the thickness in the gradient direction are reported as well, and these quantities are found to compare favorably with the experimental data of the literature. The cyclic motion of the polymer is demonstrated through analysis of the mean velocity field of the end bead. We characterize the collision process of each bead with the wall as a Poisson process and extract an average wall collision rate, which in general varies along the backbone of the chain. The inclusion of HI with the wall for a tethered polymer is found to reduce the average wall collision rate. We anticipate that results from this work will be directly applicable to, e.g., the design of polymer brushes or the use of DNA for making nanowires in molecular electronics. T.Y.L. is supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program.

Shear Stress-Normal Stress (Pressure) Ratio Decides Forming Callus in Patients with Diabetic Neuropathy

PubMed Central

Noguchi, Hiroshi; Takehara, Kimie; Ohashi, Yumiko; Suzuki, Ryo; Yamauchi, Toshimasa; Kadowaki, Takashi; Sanada, Hiromi

2016-01-01

Aim. Callus is a risk factor, leading to severe diabetic foot ulcer; thus, prevention of callus formation is important. However, normal stress (pressure) and shear stress associated with callus have not been clarified. Additionally, as new valuables, a shear stress-normal stress (pressure) ratio (SPR) was examined. The purpose was to clarify the external force associated with callus formation in patients with diabetic neuropathy. Methods. The external force of the 1st, 2nd, and 5th metatarsal head (MTH) as callus predilection regions was measured. The SPR was calculated by dividing shear stress by normal stress (pressure), concretely, peak values (SPR-p) and time integral values (SPR-i). The optimal cut-off point was determined. Results. Callus formation region of the 1st and 2nd MTH had high SPR-i rather than noncallus formation region. The cut-off value of the 1st MTH was 0.60 and the 2nd MTH was 0.50. For the 5th MTH, variables pertaining to the external forces could not be determined to be indicators of callus formation because of low accuracy. Conclusions. The callus formation cut-off values of the 1st and 2nd MTH were clarified. In the future, it will be necessary to confirm the effect of using appropriate footwear and gait training on lowering SPR-i. PMID:28050567

Animal models of surgically manipulated flow velocities to study shear stress-induced atherosclerosis.

PubMed

Winkel, Leah C; Hoogendoorn, Ayla; Xing, Ruoyu; Wentzel, Jolanda J; Van der Heiden, Kim

2015-07-01

Atherosclerosis is a chronic inflammatory disease of the arterial tree that develops at predisposed sites, coinciding with locations that are exposed to low or oscillating shear stress. Manipulating flow velocity, and concomitantly shear stress, has proven adequate to promote endothelial activation and subsequent plaque formation in animals. In this article, we will give an overview of the animal models that have been designed to study the causal relationship between shear stress and atherosclerosis by surgically manipulating blood flow velocity profiles. These surgically manipulated models include arteriovenous fistulas, vascular grafts, arterial ligation, and perivascular devices. We review these models of manipulated blood flow velocity from an engineering and biological perspective, focusing on the shear stress profiles they induce and the vascular pathology that is observed. Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.

Quantification of shear stress in a meandering native topographic channel using a physical hydraulic model

Treesearch

Michael E. Ursic

2011-01-01

Current guidelines for predicting increases in shear stress in open-channel bends were developed from investigations that were primarily prismatic in cross section. This study provides possible increases in shear stress relative to approach flow conditions resulting from planimetric and topographic geometric features. Boundary shear stress estimates were determined by...

Analysis of temporal shear stress gradients during the onset phase of flow over a backward-facing step.

PubMed

Haidekker, M A; White, C R; Frangos, J A

2001-10-01

Endothelial cells in blood vessels are exposed to bloodflow and thus fluid shear stress. In arterial bifurcations and stenoses, disturbed flow causes zones of recirculation and stagnation, which are associated with both spatial and temporal gradients of shear stress. Such gradients have been linked to the generation of atherosclerotic plaques. For in-vitro studies of endothelial cell responses, the sudden-expansion flow chamber has been widely used and described. A two-dimensional numerical simulation of the onset phase of flow through the chamber was performed. The wall shear stress action on the bottom plate was computed as a function of time and distance from the sudden expansion. The results showed that depending on the time for the flow to be established, significant temporal gradients occurred close to the second stagnation point of flow. Slowly ramping the flow over 15 s instead of 200 ms reduces the temporal gradients by a factor of 300, while spatial gradients are reduced by 23 percent. Thus, the effects of spatial and temporal gradients can be observed separately. In experiments on endothelial cells, disturbed flow stimulated cell proliferation only when flow onset was sudden. The spatial patterns of proliferation rate match the exposure to temporal gradients. This study provides information on the dynamics of spatial and temporal gradients to which the cells are exposed in a sudden-expansion flow chamber and relates them to changes in the onset phase of flow.

An analysis of the characteristics of rough bed turbulent shear stresses in an open channel

NASA Astrophysics Data System (ADS)

Keshavarzy, A.; Ball, J. E.

1997-06-01

Entrainment of sediment particles from channel beds into the channel flow is influenced by the characteristics of the flow turbulence which produces stochastic shear stress fluctuations at the bed. Recent studies of the structure of turbulent flow has recognized the importance of bursting processes as important mechanisms for the transfer of momentum into the laminar boundary layer. Of these processes, the sweep event has been recognized as the most important bursting event for entrainment of sediment particles as it imposes forces in the direction of the flow resulting in movement of particles by rolling, sliding and occasionally saltating. Similarly, the ejection event has been recognized as important for sediment transport since these events maintain the sediment particles in suspension. In this study, the characteristics of bursting processes and, in particular, the sweep event were investigated in a flume with a rough bed. The instantaneous velocity fluctuations of the flow were measured in two-dimensions using a small electromagnetic velocity meter and the turbulent shear stresses were determined from these velocity fluctuations. It was found that the shear stress applied to the sediment particles on the bed resulting from sweep events depends on the magnitude of the turbulent shear stress and its probability distribution. A statistical analysis of the experimental data was undertaken and it was found necessary to apply a Box-Cox transformation to transform the data into a normally distributed sample. This enabled determination of the mean shear stress, angle of action and standard error of estimate for sweep and ejection events. These instantaneous shear stresses were found to be greater than the mean flow shear stress and for the sweep event to be approximately 40 percent greater near the channel bed. Results from this analysis suggest that the critical shear stress determined from Shield's diagram is not sufficient to predict the initiation of motion due

Avalanches, and evolution of stress and fabric for a cyclically sheared granular material

NASA Astrophysics Data System (ADS)

Wang, Dengming; Bares, Jonathan; Wang, Dong; Behringer, Bob

2015-03-01

Granular materials yield for large enough shear stress, leading to avalanches. We seek to understand the relation between macroscopic avalanches and the the microscopic granular structure. We present an experimental study of a 2D granular material subjected to cyclic pure shear, which we visualized by a photo-elastic technique. We start from a stress-free sample of frictional particles in the shear-jamming regime (ϕS <= ϕ <=ϕJ). We apply multiple cycles of pure shear: shear in one direction, followed by a reversal to the original boundary configuration. The strain is made in small quasi-static steps: after each small step, we obtain polarized and unpolarized images yielding particle-scale forces and locations. Statistical measures of the avalanches are in reasonable agreement with recent mean-field avalanche models by Dahmen et al. (Nature Physics 7, 554 (2011)) The system structure evolves slowly to reduce the stress at the extrema of strain, similar to the relaxation observed by Ren et al. (Phys. Rev. Lett. 110, 018302 (2013)) in a simple shear experiment. To understand how this relaxation occurs, we track the stress and fabric tensors and measures of the strain field over many cycles of shear. Supported by NASA Grant NNX10AU01G, and NSF Grants DMR1206351 and DMS1248071.

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.

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

2007-08-02

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.

Direct Shear Tests of Sandstone Under Constant Normal Tensile Stress Condition Using a Simple Auxiliary Device

NASA Astrophysics Data System (ADS)

Cen, Duofeng; Huang, Da

2017-06-01

Tension-shear failure is a typical failure mode in the rock masses in unloading zones induced by excavation or river incision, etc., such as in excavation-disturbed zone of deep underground caverns and superficial rocks of high steep slopes. However, almost all the current shear failure criteria for rock are usually derived on the basis of compression-shear failure. This paper proposes a simple device for use with a servo-controlled compression-shear testing machine to conduct the tension-shear tests of cuboid rock specimens, to test the direct shear behavior of sandstone under different constant normal tensile stress conditions ( σ = -1, -1.5, -2, -2.5 and -3 MPa) as well as the uniaxial tension behavior. Generally, the fracture surface roughness decreases and the proportion of comminution areas in fracture surface increases as the change of stress state from tension to tension-shear and to compression-shear. Stepped fracture is a primary fracture pattern in the tension-shear tests. The shear stiffness, shear deformation and normal deformation (except the normal deformation for σ = -1 MPa) decrease during shearing, while the total normal deformation containing the pre-shearing portion increases as the normal tensile stress level (| σ|) goes up. Shear strength is more sensitive to the normal tensile stress than to the normal compressive stress, and the power function failure criterion (or Mohr envelope form of Hoek-Brown criterion) is examined to be the optimal criterion for the tested sandstone in the full region of tested normal stress in this study.

Cell Wall Remodeling Enzymes Modulate Fungal Cell Wall Elasticity and Osmotic Stress Resistance.

PubMed

Ene, Iuliana V; Walker, Louise A; Schiavone, Marion; Lee, Keunsook K; Martin-Yken, Hélène; Dague, Etienne; Gow, Neil A R; Munro, Carol A; Brown, Alistair J P

2015-07-28

The fungal cell wall confers cell morphology and protection against environmental insults. For fungal pathogens, the cell wall is a key immunological modulator and an ideal therapeutic target. Yeast cell walls possess an inner matrix of interlinked β-glucan and chitin that is thought to provide tensile strength and rigidity. Yeast cells remodel their walls over time in response to environmental change, a process controlled by evolutionarily conserved stress (Hog1) and cell integrity (Mkc1, Cek1) signaling pathways. These mitogen-activated protein kinase (MAPK) pathways modulate cell wall gene expression, leading to the construction of a new, modified cell wall. We show that the cell wall is not rigid but elastic, displaying rapid structural realignments that impact survival following osmotic shock. Lactate-grown Candida albicans cells are more resistant to hyperosmotic shock than glucose-grown cells. We show that this elevated resistance is not dependent on Hog1 or Mkc1 signaling and that most cell death occurs within 10 min of osmotic shock. Sudden decreases in cell volume drive rapid increases in cell wall thickness. The elevated stress resistance of lactate-grown cells correlates with reduced cell wall elasticity, reflected in slower changes in cell volume following hyperosmotic shock. The cell wall elasticity of lactate-grown cells is increased by a triple mutation that inactivates the Crh family of cell wall cross-linking enzymes, leading to increased sensitivity to hyperosmotic shock. Overexpressing Crh family members in glucose-grown cells reduces cell wall elasticity, providing partial protection against hyperosmotic shock. These changes correlate with structural realignment of the cell wall and with the ability of cells to withstand osmotic shock. The C. albicans cell wall is the first line of defense against external insults, the site of immune recognition by the host, and an attractive target for antifungal therapy. Its tensile strength is conferred by

The practicality of defensive ice walls: How would the great ice wall in Game of Thrones hold up?

NASA Astrophysics Data System (ADS)

Truffer, M.

2017-12-01

The Game of Thrones great ice wall is a colossal feature stretching several hundred miles and over 200 m high. Its purpose is to defend the realm from the wildlings. It is generally pictured as a near vertical wall. An ice wall of these proportions poses interesting challenges, mainly because ice acts as a non-linear shear-thinning fluid. A 200 m high vertical wall would create a large effective stress near its base of almost 1.8 MPa. Typical stresses responsible for ice flow in glaciers and ice sheets are more than a magnitude lower (0.1 MPa). Extrapolating a commonly used flow law for temperate ice to such high stresses would lead to strain rates at the bottom of the wall in excess of 1/day, meaning the wall would rapidly collapse and spread laterally under its own weight. To keep the wall stable, it would help to cool it significantly, as the flow of ice is also very temperature dependent. Cooling to a chilly -40 C would reduce strain rates by two orders of magnitude, but this still leads to significant slumping of the wall within just a few weeks. A time-dependent similarity solution for simplified ice flow equations that describe the evolving shape of the ice wall was provided by Halfar (1981), and demonstrates the rapid decay of the wall. A simple estimate can be derived by assuming that ice is a perfectly plastic fluid, able to maintain a basal shear stress of about 0.1 MPa. A stable ice wall would then spread laterally to about 4 km width. The resulting slope would only be steep at the very margin and the ice wall would loose much of its defensive capabilities. I conclude that the ice wall as proposed would not be a practicable defense under typical Earth conditions, and special magical powers would be necessary to maintain its shape, even for just a few days.

Inflectional instabilities in the wall region of bounded turbulent shear flows

NASA Technical Reports Server (NTRS)

Swearingen, Jerry D.; Blackwelder, Ron F.; Spalart, Philippe R.

1987-01-01

The primary thrust of this research was to identify one or more mechanisms responsible for strong turbulence production events in the wall region of bounded turbulent shear flows. Based upon previous work in a transitional boundary layer, it seemed highly probable that the production events were preceded by an inflectional velocity profile which formed on the interface between the low-speed streak and the surrounding fluid. In bounded transitional flows, this unstable profile developed velocity fluctuations in the streamwise direction and in the direction perpendicular to the sheared surface. The rapid growth of these instabilities leads to a breakdown and production of turbulence. Since bounded turbulent flows have many of the same characteristics, they may also experience a similar type of breakdown and turbulence production mechanism.

Blanking Method with Aid of Scrap to Reduce Tensile Residual Stress on Sheared Edge

NASA Astrophysics Data System (ADS)

Yasutomi, T.; Yonemura, S.; Yoshida, T.; Mizumura, M.; Hiwatashi, S.

2017-09-01

A simple shearing method to reduce tensile residual stress on a sheared edge is highly desired in the automotive industry because this type of stress deteriorates the fatigue property of automotive parts. In this study, the effect of a coining method with a shearing scrap material on a sheared edge was investigated. The scrap part of a sheared plate has a fracture surface shape similar to that of the product part since these parts are generated by separation of a single plate with crack propagation. Therefore, it is possible to impose plastic strain over the entire fracture surface by using the scrap part as a coining tool. Effectiveness of this method was investigated for high-tensile-strength steel. Using this method, the tensile residual stress on the sheared surface was significantly reduced and work hardening was slightly increased. The effects of shearing clearance and coining stroke were also investigated. Tensile residual stress decreased as the coining stroke increased; however, it saturated at a certain stroke. The stroke at which tensile residual stress saturated was relatively small at a large clearance. In particular, the amount of plastic deformation on fracture surface increased when coining stroke became large. These tendencies could be explained by the conditions of contact, which were investigated using finite element analysis.

Shear Stress Partitioning in Large Patches of Roughness in the Atmospheric Inertial Sublayer

NASA Technical Reports Server (NTRS)

Gillies, John A.; Nickling, William G.; King, James

2007-01-01

Drag partition measurements were made in the atmospheric inertial sublayer for six roughness configurations made up of solid elements in staggered arrays of different roughness densities. The roughness was in the form of a patch within a large open area and in the shape of an equilateral triangle with 60 m long sides. Measurements were obtained of the total shear stress (tau) acting on the surfaces, the surface shear stress on the ground between the elements (tau(sub S)) and the drag force on the elements for each roughness array. The measurements indicated that tau(sub S) quickly reduced near the leading edge of the roughness compared with tau, and a tau(sub S) minimum occurs at a normalized distance (x/h, where h is element height) of approx. -42 (downwind of the roughness leading edge is negative), then recovers to a relatively stable value. The location of the minimum appears to scale with element height and not roughness density. The force on the elements decreases exponentially with normalized downwind distance and this rate of change scales with the roughness density, with the rate of change increasing as roughness density increases. Average tau(sub S): tau values for the six roughness surfaces scale predictably as a function of roughness density and in accordance with a shear stress partitioning model. The shear stress partitioning model performed very well in predicting the amount of surface shear stress, given knowledge of the stated input parameters for these patches of roughness. As the shear stress partitioning relationship within the roughness appears to come into equilibrium faster for smaller roughness element sizes it would also appear the shear stress partitioning model can be applied with confidence for smaller patches of smaller roughness elements than those used in this experiment.

Flow-induced adhesion of shear-activated polymers to a substrate

NASA Astrophysics Data System (ADS)

Hoore, Masoud; Rack, Kathrin; Fedosov, Dmitry A.; Gompper, Gerhard

2018-02-01

Adhesion of polymers and proteins to substrates plays a crucial role in many technological applications and biological processes. A prominent example is the von Willebrand factor (VWF) protein, which is essential in blood clotting as it mediates adhesion of blood platelets to the site of injury at high shear rates. VWF is activated by flow and is able to bind efficiently to damaged vessel walls even under extreme flow-stress conditions; however, its adhesion is reversible when the flow strength is significantly reduced or the flow is ceased. Motivated by the properties and behavior of VWF in flow, we investigate adhesion of shear-activated polymers to a planar wall in flow and whether the adhesion is reversible under flow stasis. The main ingredients of the polymer model are cohesive inter-monomer interactions, a catch bond with the adhesive surface, and the shear activation/deactivation of polymer adhesion correlated with its stretching in flow. The cohesive interactions within the polymer maintain a globular conformation under low shear stresses and allow polymer stretching if a critical shear rate is exceeded, which is directly associated with its activation for adhesion. Our results show that polymer adhesion at high shear rates is significantly stabilized by catch bonds, while at the same time they also permit polymer dissociation from a surface at low or no flow stresses. In addition, the activation/deactivation mechanism for adhesion plays a crucial role in the reversibility of its adhesion. These observations help us better understand the adhesive behavior of VWF in flow and interpret its adhesion malfunctioning in VWF-related diseases.

Unsteady behavior of a reattaching shear layer

NASA Technical Reports Server (NTRS)

Driver, D. M.; Seegmiller, H. L.; Marvin, J.

1983-01-01

A detailed investigation of the unsteadiness in a reattaching, turbulent shear layer is reported. Laser-Doppler velocimeter measurements were conditionally sampled on the basis of instantaneous flow direction near reattachment. Conditions of abnormally short reattachment and abnormally long reattachment were considered. Ensemble-averaging of measurements made during these conditions was used to obtain mean velocities and Rreynolds stresses. In the mean flow, conditional streamlines show a global change in flow pattern which correlates with wall-flow direction. This motion can loosely be described as a 'flapping' of the shear layer. Tuft probes show that the flow direction reversals occur quite randomly and are shortlived. Streses shown also vary with the change in flow pattern. Yet, the global'flapping' motion does not appear to contribute significantly to the stress in the flow. A second type of unsteady motion was identified. Spectral analysis of both wall static pressure and streamwise velocity shows that most of the energy in the flow resides in frequencies that are significantly lower than that of the turbulence. The dominant frequency is at a Strouhal number equal to 0.2, which is the characteristic frequency of roll-up and pairing of vortical structure seen in free shear layers. It is conjectured that the 'flapping' is a disorder of the roll-up and pairing process occurring in the shear layer.

Influence of polymer charge on the shear yield stress of silica aggregated with adsorbed cationic polymers.

PubMed

Zhou, Ying; Yu, Hai; Wanless, Erica J; Jameson, Graeme J; Franks, George V

2009-08-15

Flocs were produced by adding three cationic polymers (10% charge density, 3.0x10(5) g/mol molecular weight; 40% charge density, 1.1x10(5) g/mol molecular weight; and 100% charge density, 1.2x10(5) g/mol molecular weight) to 90 nm diameter silica particles. The shear yield stresses of the consolidated sediment beds from settled and centrifuged flocs were determined via the vane technique. The polymer charge density plays an important role in influencing the shear yield stresses of sediment beds. The shear yield stresses of sediment beds from flocs induced by the 10% charged polymer were observed to increase with an increase in polymer dose, initial solid concentration and background electrolyte concentration at all volume fractions. In comparison, polymer dose has a marginal effect on the shear yield stresses of sediment beds from flocs induced by the 40% and 100% charged polymers. The shear yield stresses of sediments from flocs induced by the 40% charged polymer are independent of salt concentration whereas the addition of salt decreases the shear yield stresses of sediments from flocs induced by the 100% charged polymer. When flocculated at the optimum dose for each polymer (12 mg/g silica for the 10% charged polymer at 0.03 M NaCl, 12 mg/g for 40% and 2 mg/g for 100%), shear yield stress increases as polymer charge increases. The effects observed are related to the flocculation mechanism (bridging, patch attraction or charge neutralisation) and the magnitude of the adhesive force. Comparison of shear and compressive yield stresses show that the network is only slightly weaker in shear than in compression. This is different than many other systems (mainly salt and pH coagulation) which have shear yield stress much less than compressive yield stress. The existing models relating the power law exponent of the volume fraction dependence of the shear yield stress to the network fractal structure are not satisfactory to predict all the experimental behaviour.

Molecular characteristics of stress overshoot for polymer melts under start-up shear flow.

PubMed

Jeong, Sohdam; Kim, Jun Mo; Baig, Chunggi

2017-12-21

Stress overshoot is one of the most important nonlinear rheological phenomena exhibited by polymeric liquids undergoing start-up shear at sufficient flow strengths. Despite considerable previous research, the fundamental molecular characteristics underlying stress overshoot remain unknown. Here, we analyze the intrinsic molecular mechanisms behind the overshoot phenomenon using atomistic nonequilibrium molecular dynamics simulations of entangled linear polyethylene melts under shear flow. Through a detailed analysis of the transient rotational chain dynamics, we identify an intermolecular collision angular regime in the vicinity of the chain orientation angle θ ≈ 20° with respect to the flow direction. The shear stress overshoot occurs via strong intermolecular collisions between chains in the collision regime at θ = 15°-25°, corresponding to a peak strain of 2-4, which is an experimentally well-known value. The normal stress overshoot appears at approximately θ = 10°, at a corresponding peak strain roughly equivalent to twice that for the shear stress. We provide plausible answers to several basic questions regarding the stress overshoot, which may further help understand other nonlinear phenomena of polymeric systems.

Seismic performance evaluation of RC frame-shear wall structures using nonlinear analysis methods

NASA Astrophysics Data System (ADS)

Shi, Jialiang; Wang, Qiuwei

To further understand the seismic performance of reinforced concrete (RC) frame-shear wall structures, a 1/8 model structure is scaled from a main factory structure with seven stories and seven bays. The model with four-stories and two-bays was pseudo-dynamically tested under six earthquake actions whose peak ground accelerations (PGA) vary from 50gal to 400gal. The damage process and failure patterns were investigated. Furthermore, nonlinear dynamic analysis (NDA) and capacity spectrum method (CSM) were adopted to evaluate the seismic behavior of the model structure. The top displacement curve, story drift curve and distribution of hinges were obtained and discussed. It is shown that the model structure had the characteristics of beam-hinge failure mechanism. The two methods can be used to evaluate the seismic behavior of RC frame-shear wall structures well. What’s more, the NDA can be somewhat replaced by CSM for the seismic performance evaluation of RC structures.

The Relationship Between Surface Curvature and Abdominal Aortic Aneurysm Wall Stress.

PubMed

de Galarreta, Sergio Ruiz; Cazón, Aitor; Antón, Raúl; Finol, Ender A

2017-08-01

The maximum diameter (MD) criterion is the most important factor when predicting risk of rupture of abdominal aortic aneurysms (AAAs). An elevated wall stress has also been linked to a high risk of aneurysm rupture, yet is an uncommon clinical practice to compute AAA wall stress. The purpose of this study is to assess whether other characteristics of the AAA geometry are statistically correlated with wall stress. Using in-house segmentation and meshing algorithms, 30 patient-specific AAA models were generated for finite element analysis (FEA). These models were subsequently used to estimate wall stress and maximum diameter and to evaluate the spatial distributions of wall thickness, cross-sectional diameter, mean curvature, and Gaussian curvature. Data analysis consisted of statistical correlations of the aforementioned geometry metrics with wall stress for the 30 AAA inner and outer wall surfaces. In addition, a linear regression analysis was performed with all the AAA wall surfaces to quantify the relationship of the geometric indices with wall stress. These analyses indicated that while all the geometry metrics have statistically significant correlations with wall stress, the local mean curvature (LMC) exhibits the highest average Pearson's correlation coefficient for both inner and outer wall surfaces. The linear regression analysis revealed coefficients of determination for the outer and inner wall surfaces of 0.712 and 0.516, respectively, with LMC having the largest effect on the linear regression equation with wall stress. This work underscores the importance of evaluating AAA mean wall curvature as a potential surrogate for wall stress.

Improving the Efficiency of Abdominal Aortic Aneurysm Wall Stress Computations

PubMed Central

Zelaya, Jaime E.; Goenezen, Sevan; Dargon, Phong T.; Azarbal, Amir-Farzin; Rugonyi, Sandra

2014-01-01

An abdominal aortic aneurysm is a pathological dilation of the abdominal aorta, which carries a high mortality rate if ruptured. The most commonly used surrogate marker of rupture risk is the maximal transverse diameter of the aneurysm. More recent studies suggest that wall stress from models of patient-specific aneurysm geometries extracted, for instance, from computed tomography images may be a more accurate predictor of rupture risk and an important factor in AAA size progression. However, quantification of wall stress is typically computationally intensive and time-consuming, mainly due to the nonlinear mechanical behavior of the abdominal aortic aneurysm walls. These difficulties have limited the potential of computational models in clinical practice. To facilitate computation of wall stresses, we propose to use a linear approach that ensures equilibrium of wall stresses in the aneurysms. This proposed linear model approach is easy to implement and eliminates the burden of nonlinear computations. To assess the accuracy of our proposed approach to compute wall stresses, results from idealized and patient-specific model simulations were compared to those obtained using conventional approaches and to those of a hypothetical, reference abdominal aortic aneurysm model. For the reference model, wall mechanical properties and the initial unloaded and unstressed configuration were assumed to be known, and the resulting wall stresses were used as reference for comparison. Our proposed linear approach accurately approximates wall stresses for varying model geometries and wall material properties. Our findings suggest that the proposed linear approach could be used as an effective, efficient, easy-to-use clinical tool to estimate patient-specific wall stresses. PMID:25007052

Resolved shear stress intensity coefficient and fatigue crack growth in large crystals

NASA Technical Reports Server (NTRS)

Chen, Q.; Liu, H. W.

1988-01-01

Fatigue crack growth tests were carried out on large-grain Al 7029 aluminum alloy and the finite element method was used to calculate the stress field near the tip of a zigzag crack. The resolved shear stresses on all 12 slip systems were computed, and the resolved shear stress intensity coefficient (RSSIC) was defined. The RSSIC was used to analyze the irregular crack path and was correlated with the rate of single-slip-plane shear crack growth. Fatigue crack growth was found to be caused primarily by shear decohesion at a crack tip. When the RSSIC on a single-slip system was much larger than all the others, the crack followed a single-slip plane. When the RSSICs on two conjugate slip systems were comparable, a crack grew in a zigzag manner on these planes and the macrocrack-plane bisected the two active slip planes. The maximum RSSIC on the most active slip system is proposed as a parameter to correlate with the shear fatigue crack growth rate in large crystals.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aisha, M.D.; Nor-Ashikin, M.N.K.; DDH, Universiti Teknologi MARA, ShahAlam 40450, Selangor

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 250more » 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.« less

Low-dimensional representation of near-wall dynamics in shear flows, with implications to wall-models.

PubMed

Schmid, P J; Sayadi, T

2017-03-13

The dynamics of coherent structures near the wall of a turbulent boundary layer is investigated with the aim of a low-dimensional representation of its essential features. Based on a triple decomposition into mean, coherent and incoherent motion and a dynamic mode decomposition to recover statistical information about the incoherent part of the flow field, a driven linear system coupling first- and second-order moments of the coherent structures is derived and analysed. The transfer function for this system, evaluated for a wall-parallel plane, confirms a strong bias towards streamwise elongated structures, and is proposed as an 'impedance' boundary condition which replaces the bulk of the transport between the coherent velocity field and the coherent Reynolds stresses, thus acting as a wall model for large-eddy simulations (LES). It is interesting to note that the boundary condition is non-local in space and time. The extracted model is capable of reproducing the principal Reynolds stress components for the pretransitional, transitional and fully turbulent boundary layer.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'. © 2017 The Author(s).

Aortic dissection simulation models for clinical support: fluid-structure interaction vs. rigid wall models.

PubMed

Alimohammadi, Mona; Sherwood, Joseph M; Karimpour, Morad; Agu, Obiekezie; Balabani, Stavroula; Díaz-Zuccarini, Vanessa

2015-04-15

The management and prognosis of aortic dissection (AD) is often challenging and the use of personalised computational models is being explored as a tool to improve clinical outcome. Including vessel wall motion in such simulations can provide more realistic and potentially accurate results, but requires significant additional computational resources, as well as expertise. With clinical translation as the final aim, trade-offs between complexity, speed and accuracy are inevitable. The present study explores whether modelling wall motion is worth the additional expense in the case of AD, by carrying out fluid-structure interaction (FSI) simulations based on a sample patient case. Patient-specific anatomical details were extracted from computed tomography images to provide the fluid domain, from which the vessel wall was extrapolated. Two-way fluid-structure interaction simulations were performed, with coupled Windkessel boundary conditions and hyperelastic wall properties. The blood was modelled using the Carreau-Yasuda viscosity model and turbulence was accounted for via a shear stress transport model. A simulation without wall motion (rigid wall) was carried out for comparison purposes. The displacement of the vessel wall was comparable to reports from imaging studies in terms of intimal flap motion and contraction of the true lumen. Analysis of the haemodynamics around the proximal and distal false lumen in the FSI model showed complex flow structures caused by the expansion and contraction of the vessel wall. These flow patterns led to significantly different predictions of wall shear stress, particularly its oscillatory component, which were not captured by the rigid wall model. Through comparison with imaging data, the results of the present study indicate that the fluid-structure interaction methodology employed herein is appropriate for simulations of aortic dissection. Regions of high wall shear stress were not significantly altered by the wall motion

KRE5 Suppression Induces Cell Wall Stress and Alternative ER Stress Response Required for Maintaining Cell Wall Integrity in Candida glabrata

PubMed Central

Sasaki, Masato; Ito, Fumie; Aoyama, Toshio; Sato-Okamoto, Michiyo; Takahashi-Nakaguchi, Azusa; Chibana, Hiroji; Shibata, Nobuyuki

2016-01-01

The maintenance of cell wall integrity in fungi is required for normal cell growth, division, hyphae formation, and antifungal tolerance. We observed that endoplasmic reticulum stress regulated cell wall integrity in Candida glabrata, which possesses uniquely evolved mechanisms for unfolded protein response mechanisms. Tetracycline-mediated suppression of KRE5, which encodes a predicted UDP-glucose:glycoprotein glucosyltransferase localized in the endoplasmic reticulum, significantly increased cell wall chitin content and decreased cell wall β-1,6-glucan content. KRE5 repression induced endoplasmic reticulum stress-related gene expression and MAP kinase pathway activation, including Slt2p and Hog1p phosphorylation, through the cell wall integrity signaling pathway. Moreover, the calcineurin pathway negatively regulated cell wall integrity, but not the reduction of β-1,6-glucan content. These results indicate that KRE5 is required for maintaining both endoplasmic reticulum homeostasis and cell wall integrity, and that the calcineurin pathway acts as a regulator of chitin-glucan balance in the cell wall and as an alternative mediator of endoplasmic reticulum stress in C. glabrata. PMID:27548283

A Numerical Experiment on the Role of Surface Shear Stress in the Generation of Sound

NASA Technical Reports Server (NTRS)

Shariff, Karim; Wang, Meng; Merriam, Marshal (Technical Monitor)

1996-01-01

The sound generated due to a localized flow over an infinite flat surface is considered. It is known that the unsteady surface pressure, while appearing in a formal solution to the Lighthill equation, does not constitute a source of sound but rather represents the effect of image quadrupoles. The question of whether a similar surface shear stress term constitutes a true source of dipole sound is less settled. Some have boldly assumed it is a true source while others have argued that, like the surface pressure, it depends on the sound field (via an acoustic boundary layer) and is therefore not a true source. A numerical experiment based on the viscous, compressible Navier-Stokes equations was undertaken to investigate the issue. A small region of a wall was oscillated tangentially. The directly computed sound field was found to to agree with an acoustic analogy based calculation which regards the surface shear as an acoustically compact dipole source of sound.

Direct measurements of local bed shear stress in the presence of pressure gradients

NASA Astrophysics Data System (ADS)

Pujara, Nimish; Liu, Philip L.-F.

2014-07-01

This paper describes the development of a shear plate sensor capable of directly measuring the local mean bed shear stress in small-scale and large-scale laboratory flumes. The sensor is capable of measuring bed shear stress in the range 200 Pa with an accuracy up to 1 %. Its size, 43 mm in the flow direction, is designed to be small enough to give spatially local measurements, and its bandwidth, 75 Hz, is high enough to resolve time-varying forcing. Typically, shear plate sensors are restricted to use in zero pressure gradient flows because secondary forces on the edge of the shear plate caused by pressure gradients can introduce large errors. However, by analysis of the pressure distribution at the edges of the shear plate in mild pressure gradients, we introduce a new methodology for correcting for the pressure gradient force. The developed sensor includes pressure tappings to measure the pressure gradient in the flow, and the methodology for correction is applied to obtain accurate measurements of bed shear stress under solitary waves in a small-scale wave flume. The sensor is also validated by measurements in a turbulent flat plate boundary layer in open channel flow.

Doppler optical coherence tomography imaging of local fluid flow and shear stress within microporous scaffolds

NASA Astrophysics Data System (ADS)

Jia, Yali; Bagnaninchi, Pierre O.; Yang, Ying; Haj, Alicia El; Hinds, Monica T.; Kirkpatrick, Sean J.; Wang, Ruikang K.

2009-05-01

Establishing a relationship between perfusion rate and fluid shear stress in a 3D cell culture environment is an ongoing and challenging task faced by tissue engineers. We explore Doppler optical coherence tomography (DOCT) as a potential imaging tool for in situ monitoring of local fluid flow profiles inside porous chitosan scaffolds. From the measured fluid flow profiles, the fluid shear stresses are evaluated. We examine the localized fluid flow and shear stress within low- and high-porosity chitosan scaffolds, which are subjected to a constant input flow rate of 0.5 ml.min-1. The DOCT results show that the behavior of the fluid flow and shear stress in micropores is strongly dependent on the micropore interconnectivity, porosity, and size of pores within the scaffold. For low-porosity and high-porosity chitosan scaffolds examined, the measured local fluid flow and shear stress varied from micropore to micropore, with a mean shear stress of 0.49+/-0.3 dyn.cm-2 and 0.38+/-0.2 dyn.cm-2, respectively. In addition, we show that the scaffold's porosity and interconnectivity can be quantified by combining analyses of the 3D structural and flow images obtained from DOCT.

Diagnostics of boundary layer transition by shear stress sensitive liquid crystals

NASA Astrophysics Data System (ADS)

Shapoval, E. S.

2016-10-01

Previous research indicates that the problem of boundary layer transition visualization on metal models in wind tunnels (WT) which is a fundamental question in experimental aerodynamics is not solved yet. In TsAGI together with Khristianovich Institute of Theoretical and Applied Mechanics (ITAM) a method of shear stress sensitive liquid crystals (LC) which allows flow visualization was proposed. This method allows testing several flow conditions in one wind tunnel run and does not need covering the investigated model with any special heat-insulating coating which spoils the model geometry. This coating is easily applied on the model surface by spray or even by brush. Its' thickness is about 40 micrometers and it does not spoil the surface quality. At first the coating obtains some definite color. Under shear stress the LC coating changes color and this change is proportional to shear stress. The whole process can be visually observed and during the tests it is recorded by camera. The findings of the research showed that it is possible to visualize boundary layer transition, flow separation, shock waves and the flow image on the whole. It is possible to predict that the proposed method of shear stress sensitive liquid crystals is a promise for future research.

Torsional bridge setup for the characterization of integrated circuits and microsensors under mechanical shear stress.

PubMed

Herrmann, M; Gieschke, P; Ruther, P; Paul, O

2011-12-01

We present a torsional bridge setup for the electro-mechanical characterization of devices integrated in the surface of silicon beams under mechanical in-plane shear stress. It is based on the application of a torsional moment to the longitudinal axis of the silicon beams, which results in a homogeneous in-plane shear stress in the beam surface. The safely applicable shear stresses span the range of ±50 MPa. Thanks to a specially designed clamping mechanism, the unintended normal stress typically stays below 2.5% of the applied shear stress. An analytical model is presented to compute the induced shear stress. Numerical computations verify the analytical results and show that the homogeneity of the shear stress is very high on the beam surface in the region of interest. Measurements with piezoresistive microsensors fabricated using a complementary metal-oxide-semiconductor process show an excellent agreement with both the computational results and comparative measurements performed on a four-point bending bridge. The electrical connection to the silicon beam is performed with standard bond wires. This ensures that minimal forces are applied to the beam by the electrical interconnection to the external instrumentation and that devices with arbitrary bond pad layout can be inserted into the setup.

Strength Tests on Thin-walled Duralumin Cylinders in Torsion

NASA Technical Reports Server (NTRS)

Lundquist, Eugene E

1932-01-01

This report is the first of a series presenting the results of strength tests on thin-walled cylinders and truncated cones of circular and elliptical section; it comprises the results obtained to date from torsion (pure shear) tests on 65 thin-walled duralumin cylinders of circular section with ends clamped to rigid bulkheads. The effect of variations in the length/radius and radius/thickness ratios on the type of failure is indicated, and a semi-empirical equation for the shearing stress at maximum load is given.

Estimation of the supplementary axial wall stress generated at peak flow by an arterial stenosis

NASA Astrophysics Data System (ADS)

Doriot, Pierre-André

2003-01-01

Mechanical stresses in arterial walls are known to be implicated in the development of atherosclerosis. While shear stress and circumferential stress have received a lot of attention, axial stress has not. Yet, stenoses can be intuitively expected to produce a supplementary axial stress during flow systole in the region immediately proximal to the constriction cone. In this paper, a model for the estimation of this effect is presented, and ten numerical examples are computed. These examples show that the cyclic increase in axial stress can be quite considerable in severe stenoses (typically 120% or more of the normal stress value). This result is in best agreement with the known mechanical or morphological risk factors of stenosis progression and restenosis (hypertension, elevated pulse pressure, degree of stenosis, stenosis geometry, residual stenosis, etc). The supplementary axial stress generated by a stenosis might create the damages in the endothelium and in the elastic membranes which potentiate the action of the other risk factors (hyperlipidaemia, diabetes, etc). It could thus be an important cause of stenosis progression and of restenosis.

Origins of the anomalous stress behavior in charged colloidal suspensions under shear.

PubMed

Kumar, Amit; Higdon, Jonathan J L

2010-11-01

Numerical simulations are conducted to determine microstructure and rheology of sheared suspensions of charged colloidal particles at a volume fraction of ϕ=0.33. Over broad ranges of repulsive force strength F0 and Péclet number Pe, dynamic simulations show coexistence of ordered and disordered stable states with the state dependent on the initial condition. In contrast to the common view, at low shear rates, the disordered phase exhibits a lower viscosity (μ(r)) than the ordered phase, while this behavior is reversed at higher shear rates. Analysis shows the stress reversal is associated with different shear induced microstructural distortions in the ordered and disordered systems. Viscosity vs shear rate data over a wide range of F0 and Pe collapses well upon rescaling with the long-time self-diffusivity. Shear thinning viscosity in the ordered phase scaled as μ(r)∼Pe(-0.81) at low shear rates. The microstructural dynamics revealed in these studies explains the anomalous behavior and hysteresis loops in stress data reported in the literature.

Direct and Large Eddy Simulation of non-equilibrium wall-bounded turbulent flows

NASA Astrophysics Data System (ADS)

Park, Hee-Jun

2005-11-01

The performance of several existing SGS models in non-equilibrium wall-bounded turbulent flows is investigated through comparisons of LES and DNS. The test problem is a shear-driven three-dimensional turbulent channel flow at base Reτ˜210 established by impulsive motion of one of the channel walls in the spanwise direction with a spanwise velocity equal to 3/4 of the bulk mean velocity in the channel. The DNS and LES are performed using pseudo-spectral methods with resolutions of 128x128x129 and 32x64x65, respectively. The SGS models tested include the nonlinear Interactions Approximation model (NIA) [Haliloglu and Akhavan (2004)], the Dynamic Smagorinsky model (DSM) [Germano et al. (1991)], and the Dynamic Mixed Model (DMM) [Zang et al. (1993)]. The results show that NIA gives the best overall agreement with DNS. Both DMM and DSM over-predict the decay of the mean streamwise wall shear stress on the moving wall, while NIA gives results in close agreements with DNS. Similarly, NIA gives the best agreement with DNS in the prediction of the mean velocity, the higher-order turbulence statistics, and the lag angle between the mean shear and the turbulent shear stress. These results suggest that non-equilibrium wall-bounded turbulent flows can be accurately computed by LES with NIA as the SGS model.

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.

Effects of Combined Shear and Thermal Forces on Destruction of Microbacterium lacticum

PubMed Central

Bulut, S.; Waites, W. M.; Mitchell, J. R.

1999-01-01

A twin-screw extruder and a rotational rheometer were used to generate shear forces in concentrated gelatin inoculated with a heat-resistant isolate of a vegetative bacterial species, Microbacterium lacticum. Shear forces in the extruder were mainly controlled by varying the water feed rate. The water content of the extrudates changed between 19 and 45% (wet weight basis). Higher shear forces generated at low water contents and the calculated die wall shear stress correlated strongly with bacterial destruction. No surviving microorganisms could be detected at the highest wall shear stress of 409 kPa, giving log reduction of 5.3 (minimum detection level, 2 × 104 CFU/sample). The mean residence time of the microorganism in the extruder was 49 to 58 s, and the maximum temperature measured in the end of the die was 73°C. The D75°C of the microorganism in gelatin at 65% water content was 20 min. It is concluded that the physical forces generated in the reverse screw element and the extruder die rather than heat played a major part in cell destruction. In a rotational rheometer, after shearing of a mix of microorganisms with gelatin at 65% (wt/wt) moisture content for 4 min at a shear stress of 2.8 kPa and a temperature of 75°C, the number of surviving microorganisms in the sheared sample was 5.2 × 106 CFU/g of sample compared with 1.4 × 108 CFU/g of sample in the nonsheared control. The relative effectiveness of physical forces in the killing of bacteria and destruction of starch granules is discussed. PMID:10508076

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.

Localized modelling and feedback control of linear instabilities in 2-D wall bounded shear flows

NASA Astrophysics Data System (ADS)

Tol, Henry; Kotsonis, Marios; de Visser, Coen

2016-11-01

A new approach is presented for control of instabilities in 2-D wall bounded shear flows described by the linearized Navier-Stokes equations (LNSE). The control design accounts both for spatially localized actuators/sensors and the dominant perturbation dynamics in an optimal control framework. An inflow disturbance model is proposed for streamwise instabilities that drive laminar-turbulent transition. The perturbation modes that contribute to the transition process can be selected and are included in the control design. A reduced order model is derived from the LNSE that captures the input-output behavior and the dominant perturbation dynamics. This model is used to design an optimal controller for suppressing the instability growth. A 2-D channel flow and a 2-D boundary layer flow over a flat plate are considered as application cases. Disturbances are generated upstream of the control domain and the resulting flow perturbations are estimated/controlled using wall shear measurements and localized unsteady blowing and suction at the wall. It will be shown that the controller is able to cancel the perturbations and is robust to unmodelled disturbances.

Shear Stress Enhances Chemokine Secretion from Chlamydia pneumoniae-infected Monocytes.

PubMed

Evani, Shankar J; Dallo, Shatha F; Murthy, Ashlesh K; Ramasubramanian, Anand K

2013-09-01

Chlamydia pneumoniae is a common respiratory pathogen that is considered a highly likely risk factor for atherosclerosis. C. pneumoniae is disseminated from the lung into systemic circulation via infected monocytes and lodges at the atherosclerotic sites. During transit, C. pneumoniae -infected monocytes in circulation are subjected to shear stress due to blood flow. The effect of mechanical stimuli on infected monocytes is largely understudied in the context of C. pneumoniae infection and inflammation. We hypothesized that fluid shear stress alters the inflammatory response of C. pneumoniae -infected monocytes and contributes to immune cell recruitment to the site of tissue damage. Using an in vitro model of blood flow, we determined that a physiological shear stress of 7.5 dyn/cm 2 for 1 h on C. pneumoniae -infected monocytes enhances the production of several chemokines, which in turn is correlated with the recruitment of significantly large number of monocytes. Taken together, these results suggest synergistic interaction between mechanical and chemical factors in C. pneumoniae infection and associated inflammation.

High-speed high-stress ring shear tests on granular sods and clayey soils

Treesearch

Hiroshi Fukuoka; Kyoji Sassa

1991-01-01

The purposes of this study is to obtain exact knowledge of the influences on friction angle during shear by shearing speeds. Ring shear tests on sandy and clayey materials have been carried out with a newly developed High-speed High-Stress Ring Shear Apparatus to examine if there are some changes in the frictional behaviors of these materials at high shearing speeds of...

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.

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.

Effect of intraplaque angiogenesis to atherosclerotic rupture-prone plaque induced by high shear stress in rabbit model

PubMed Central

Qiu, Juhui; Lei, Daoxi; Hu, Jianjun; Yin, Tieying; Zhang, Kang; Yu, Donghong

2017-01-01

Abstract Atherosclerotic prone-rupture plaque is mainly localized in the region of the entrance to the stenosis with high shear stress and the reasons are largely unknown. Our hypothesis is that such a distribution of cells in atherosclerotic plaque may depend on the angiogenesis. Silastic collars induced regions of high shear stress (20.68 ± 5.27 dynes/cm2) in the upstream flow and low shear stress (12.25 ± 1.28 dynes/cm2) in the downstream flow in carotid arteries. Compared with the low shear stress region, plaques in the high shear stress region showed more intraplaque haemorrhaging, less collagen and higher apoptotic rates of vascular smooth muscle cells; endothelial cells (ECs) in the high shear stress region were characterized with integrity and high endothelial nitric oxide synthase (eNOS) expression (1570.3 ± 345.5% vs 172.9 ± 49.9%). The number of intraplaque microvessels is very high in the high shear stress region (15 ± 1.8 n/mm2 vs 3.5 ± 0.4 n/mm2), and the microvessels in the plaque show ECs were abnormal, with membrane blebs, intracytoplasmic vacuoles and leukocyte infiltration. Our current study reveals that the integrity of the endothelium and the vulnerability of atherosclerotic plaques are simultaneously localized in high shear stress regions, and we provide evidence for the first time that microvessels in the intraplaque maybe responsible for rupture-prone plaque formation in the high shear stress region. PMID:28798867

A Methodology to Detect Abnormal Relative Wall Shear Stress on the Full Surface of the Thoracic Aorta Using 4D Flow MRI

PubMed Central

van Ooij, Pim; Potters, Wouter V.; Nederveen, Aart J.; Allen, Bradley D.; Collins, Jeremy; Carr, James; Malaisrie, S. Chris; Markl, Michael; Barker, Alex J.

2014-01-01

Purpose To compute cohort-averaged wall shear stress (WSS) maps in the thoracic aorta of patients with aortic dilatation or valvular stenosis and to detect abnormal regional WSS. Methods Systolic WSS vectors, estimated from 4D flow MRI data, were calculated along the thoracic aorta lumen in 10 controls, 10 patients with dilated aortas and 10 patients with aortic valve stenosis. 3D segmentations of each aorta were co-registered by group and used to create a cohort-specific aortic geometry. The WSS vectors of each subject were interpolated onto the corresponding cohort-specific geometry to create cohort-averaged WSS maps. A Wilcoxon rank sum test was used to generate aortic P-value maps (P<0.05) representing regional relative WSS differences between groups. Results Cohort-averaged systolic WSS maps and P-value maps were successfully created for all cohorts and comparisons. The dilation cohort showed significantly lower WSS on 7% of the ascending aorta surface, whereas the stenosis cohort showed significantly higher WSS aorta on 34% the ascending aorta surface. Conclusions The findings of this study demonstrated the feasibility of generating cohort-averaged WSS maps for the visualization and identification of regionally altered WSS in the presence of disease, as compared to healthy controls. PMID:24753241

Determination of Reynolds Shear Stress Level for Hemolysis.

PubMed

Jhun, Choon-Sik; Stauffer, Megan A; Reibson, John D; Yeager, Eric E; Newswanger, Raymond K; Taylor, Joshua O; Manning, Keefe B; Weiss, William J; Rosenberg, Gerson

Reynolds shear stress (RSS) has served as a metric for the effect of turbulence on hemolysis. Forstrom (1969) and Sallam and Hwang (1984) determined the RSS threshold for hemolysis to be 50,000 and 4,000 dyne/cm, respectively, using a turbulent jet. Despite the order of magnitude discrepancy, the threshold by Sallam and Hwang has been frequently cited for hemolytic potential in blood pumps. We recreated a Sallam apparatus (SA) to resolve this discrepancy and provide additional data to be used in developing a more accurate hemolysis model. Hemolysis was measured over a large range of Reynolds numbers (Re) (Re = 1,000-80,000). Washed bovine red blood cells (RBCs) were injected into the free jet of phosphate buffered saline, and hemolysis was quantified using a percent hemolysis, Hp = h (100 - hematocrit [HCT])/Hb, where h (mg/dl) is free hemoglobin and Hb (mg/dl) is total hemoglobin. Reynolds shear stress was calculated using two-dimensional laser Doppler velocimetry. Reynolds shear stress of ≥30,000 dyne/cm corresponding to Re of ≥60,000 appeared to cause hemolysis (p < 0.05). This RSS is an order of magnitude greater than the RSS threshold that Sallam and Hwang suggested, and it is similar to Forstrom's RSS threshold. This study resolved a long-standing uncertainty regarding the critical values of RSS for hemolysis and may provide a foundation for a more accurate hemolysis model.

Numerical analysis of wall shear stress in ascending aorta before tearing in type A aortic dissection.

PubMed

Chi, Qingzhuo; He, Ying; Luan, Yong; Qin, Kairong; Mu, Lizhong

2017-10-01

Although the incidence of many cardiovascular diseases has declined as medical treatments have improved, the prevalence of aortic dissection (AD) has increased. Compared to type B dissections, type A dissections are more severe, and most patients with type A dissections require surgical treatment. The objective of this study was to investigate the relationships between the wall shear stress (WSS) on the aortic endothelium and the frequent tearing positions using computational fluid dynamics. Five type A dissection cases and two normal aortas were included in the study. First, the structures of the aortas before the type A dissection were reconstructed on the basis of the original imaging data. Analyses of flow in the reconstructed premorbid structures reveals that the rupture positions in three of the five cases corresponded to the area of maximum elevated WSS. Moreover, the WSS at the junction of the aortic arch and descending aorta was found to be elevated, which is considered to be related to the locally disturbed helical flow. Meanwhile, the highest WSS in the patients with premorbid AD was found to be almost double that of the control group. Due to the noticeable morphological differences between the AD cases and the control group, the WSSs in the premorbid structures without vasodilation in the ascending part were estimated. The computational results revealed that the WSS was lower in the aorta without vasodilation, but the pressure drop in this situation was higher than that with vasodilation in the ascending aorta. Significant differences were seen between the AD cases and the control group in the angles of the side branches of the aortic arch and its bending degree. Dilation of the ascending aorta and alterations in the branching angles may be the key determinants of a high WSS that leads to type A dissection. Greater tortuosity of the aortic arch leads to stronger helical flow through the distal aortic arch, which may be related to tears in this region

Bonded joint and method. [for reducing peak shear stress in adhesive bonds

NASA Technical Reports Server (NTRS)

Sainsbury-Carter, J. B. (Inventor)

1974-01-01

An improved joint is described for reducing the peak shear stress in adhesive bonds when adhesives are used to bond two materials which are in a lapped relationship and which differ in value of modulus of elasticity. An insert placed between the adhesive and one of the two materials acts to cushion the discontinuity of material stiffness thereby reducing the peak shear stress in the adhesive bond.

A law of the wall for turbulent boundary layers with suction: Stevenson's formula revisited

NASA Astrophysics Data System (ADS)

Vigdorovich, Igor

2016-08-01

The turbulent velocity field in the viscous sublayer of the boundary layer with suction to a first approximation is homogeneous in any direction parallel to the wall and is determined by only three constant quantities — the wall shear stress, the suction velocity, and the fluid viscosity. This means that there exists a finite algebraic relation between the turbulent shear stress and the longitudinal mean-velocity gradient, using which as a closure condition for the equations of motion, we establish an exact asymptotic behavior of the velocity profile at the outer edge of the viscous sublayer. The obtained relationship provides a generalization of the logarithmic law to the case of wall suction.

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.

Experimental study of laminar blood flow through an artery treated by a stent implantation: characterisation of intra-stent wall shear stress.

PubMed

Benard, Nicolas; Coisne, Damien; Donal, Erwan; Perrault, Robert

2003-07-01

The stimulation of endothelial cells by arterial wall shear stress (WSS) plays a central role in restenosis. The fluid-structure interaction between stent wire and blood flow alters the WSS, particularly between stent struts. We have designed an in vitro model of struts of an intra-vascular prosthesis to study blood flow through a 'stented' section. The experimental artery consisted of a transparent square section test vein, which reproduced the strut design (100x magnifying power). A programmable pump was used to maintain a steady blood flow. Particle image velocimetry method was used to measure the flow between and over the stent branches, and to quantify WSS. Several prosthesis patterns that were representative of the total stent strut geometry were studied in a greater detail. We obtained WSS values of between -1.5 and 1.5Pa in a weak SS area which provided a source of endothelial stimulation propitious to restenosis. We also compared two similar patterns located in two different flow areas (one at the entry of the stent and one further downstream). We only detected a slight difference between the weakest SS levels at these two sites. As the endothelial proliferation is greatly influenced by the SS, knowledge of the SS modification induced by the stent implantation could be of importance for intra-vascular prostheses design optimisation and thus can help to reduce the restenosis incidence rate.

Modeling changes in rill erodibility and critical shear stress on native surface roads

Treesearch

Randy B. Foltz; Hakjun Rhee; William J. Elliot

2008-01-01

This study investigated the effect of cumulative overland flow on rill erodibility and critical shear stress on native surface roads in central Idaho. Rill erodibility decreased exponentially with increasing cumulative overland flow depth; however, critical shear stress did not change. The study demonstrated that road erodibility on the studied road changes over the...

Comparison of erythrocyte dynamics in shear flow under different stress-free configurations

NASA Astrophysics Data System (ADS)

Cordasco, Daniel; Yazdani, Alireza; Bagchi, Prosenjit

2014-04-01

An open question that has persisted for decades is whether the cytoskeleton of a red blood cell is stress-free or under a stress. This question is important in the context of theoretical modeling of cellular motion under a flowing condition where it is necessary to make an assumption about the stress-free state. Here, we present a 3D numerical study to compare the cell dynamics in a simple shear flow under two different stress-free states, a biconcave discocyte representing the resting shape of the cell, and a nearly spherical oblate shape. We find that whether the stress-free states make a significant difference or not depends on the viscosity of the suspending medium. If the viscosity is close to that of blood plasma, the two stress-free states do not show any significant difference in cell dynamics. However, when the suspending medium viscosity is well above that of the physiological range, as in many in vitro studies, the shear rate separating the tank-treading and tumbling dynamics is observed to be higher for the biconcave stress-free state than the spheroidal state. The former shows a strong shape oscillation with repeated departures from the biconcave shape, while the latter shows a nearly stable biconcave shape. It is found that the cell membrane in the biconcave stress-free state is under a compressive stress and a weaker bending force density, leading to a periodic compression of the cell. The shape oscillation then leads to a higher energy barrier against membrane tank-tread leading to an early transition to tumbling. However, if the cells are released with a large off-shear plane angle, the oscillations can be suppressed due to an azimuthal motion of the membrane along the vorticity direction leading to a redistribution of the membrane points and lowering of the energy barrier, which again results in a nearly similar behavior of the cells under the two different stress-free states. A variety of off-shear plane dynamics is observed, namely, rolling