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1

The measurement of wall shear stress  

NASA Astrophysics Data System (ADS)

The theoretical bases and implementation of state-of-the-art measurement techniques for wall shear stress are reviewed and illustrated with graphs and diagrams. The general structure of a turbulent boundary layer is described, and sections are devoted to local and global correlation methods (Clauser plots, Preston tubes, Stanton tubes, and heat-transfer methods), momentum-balance methods (pressure gradients and momentum thickness gradients), and direct methods (conventional and micromachined floating elements). The importance of accurate instrument calibration is stressed, and comparisons with theoretical predictions and near-wall velocity measurements are recommended when feasible.

Haritonidis, Joseph H.

2

Bed shear stress measurement using flexible micro-rods  

NASA Astrophysics Data System (ADS)

River morphodynamics is related to turbulent processes; and among influential factors shear stress is one of utter importance. Precise shear stress determination using direct and indirect methods has being investigated in recent years. Also, instantaneous fluctuation in wall shear stress distribution which can potentially affect erosion processes is being discussed currently. Here, a new method of indirect shear stress measurement using PDMS (Polydimethylsiloxan) flexible micro-rods is evaluated. Their flexural behavior in the flow within the boundary inner layer allows for the calculation of instantaneous shear stress values. The method is already used in medical fluid mechanics and now it is adopted for open channel investigations. A series of measurements were performed in order to investigate these micro-rods and their applicability to fluvial studies. This method may improve our understanding of boundary proximity region and enhance our insight into near bed momentum transfer mechanisms.

Farhadi, Alireza; Sindelar, Christine; Habersack, Helmut

2014-05-01

3

A new ultrasonographic instrument for measuring vessel wall shear stress.  

PubMed

A new ultrasonographic machine (FRP II) has been developed to measure vessel wall shear stress. A multigate ultrasound probe sends an ultrasound beam simultaneously focused in subsequent points 0.2 mm from each other along the transverse axis of a blood vessel. Blood velocity is measured by cross-correlation technique, which allows a rapid and economical analysis. Thus, the instantaneous (every 5 msec) blood velocity profile is reconstructed for the duration of the entire cardiac cycle. In order to verify the precision and sensitivity of the FRP II in measuring shear stress, 36 measurements were performed on the common carotid artery in 9 hypertensive subjects in different hemodynamic conditions. The FRP II-measured shear stress (the product of the shear rate and blood viscosity) was compared to that calculated by the Womersley's mathematical model (Y = 2K.Vcl/D, where Y = shear rate, Vcl = vessel center line blood velocity, D = vessel diameter). A good correlation (r = 0.77, p < 0.0001) was found between the peak systolic shear stresses measured by FRP II and that calculated by the Womersley's mathematical model, although an underestimation for higher values was observed with the latter method. In conclusion, we propose a new ultrasonographic instrument to measure "in vivo" the vessel wall shear stress. PMID:8086161

Bardelli, M; Carretta, R; Dotti, D; Fabris, B; Fischetti, F; Cominotto, F; Ussi, D; Calci, M; Candido, R

1994-04-01

4

Full Field Surface Shear Stress Measurements Using Liquid Crystals.  

National Technical Information Service (NTIS)

Surface shear stress measurement is of fundamental importance in aerodynamics for studying laminar to turbulent transition, surface vortical structures, drag determination and is a necessary diagnostic requirement for not only large aerodynamic surfaces b...

P. J. Disimile, N. Toy

1994-01-01

5

Measuring techniques for wall shearing stress in turbulent boundary layer  

NASA Astrophysics Data System (ADS)

This paper presents the calibration results and the comparison of various measuring techniques for wall shearing stress measurement, both in compressible and incompressible turbulent boundary layers. Techniques including Preston tube, Stanton tube, sublayer fence, surface hot-film and computational Preston tube method are discussed.

Dai, Changhui; Liu, Tianshu; Teng, Yongguang; Ming, Xiao

1988-05-01

6

Fluidic shear-stress measurement using surface-micromachined sensors  

Microsoft Academic Search

A poly-silicon hot-film shear-stress sensor insulated by a vacuum-chamber underneath has been designed and fabricated by the surface micromachining technology. The sensor is operated at both constant current and constant temperature modes. The dynamic performance (including time constant and cut-off frequency) measurement, calibration, and temperature compensation of the sensor have been realized

Jin-Biao Huang; Chang Liu; Fu-Kang Jiang; Steve Tung; Yu-Chong Tai; Chih-Ming Ho

1995-01-01

7

Wall shear stress measurements using a new transducer  

NASA Technical Reports Server (NTRS)

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

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

1986-01-01

8

SILICON MICROMACHINED SENSORS AND SENSOR ARRAYS FOR SHEAR-STRESS MEASUREMENTS IN AERODYNAMIC FLOWS  

E-print Network

the four orders of wall shear stress). In addition, the sensors have shown excellent repeatability, long-term1 SILICON MICROMACHINED SENSORS AND SENSOR ARRAYS FOR SHEAR- STRESS MEASUREMENTS IN AERODYNAMIC boundary layer research. Applications in low shear-stress environments such as turbulent boundary layers

Peraire, Jaime

9

Shear wave transducer for stress measurements in boreholes  

DOEpatents

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

Mao, Nai-Hsien (Castro Valley, CA)

1987-01-01

10

Experimental measurement of dynamic fluid shear stress on the aortic surface of the aortic valve leaflet  

PubMed Central

Aortic valve (AV) calcification is a highly prevalent disease with serious impact on mortality and morbidity. Although exact causes and mechanisms of AV calcification are unclear, previous studies suggest that mechanical forces play a role. Since calcium deposits occur almost exclusively on the aortic surfaces of AV leaflets, it has been hypothesized that adverse patterns of fluid shear stress on the aortic surface of AV leaflets promote calcification. The current study characterizes AV leaflet aortic surface fluid shear stresses using Laser Doppler velocimetry and an in vitro pulsatile flow loop. The valve model used was a native porcine valve mounted on a suturing ring and preserved using 0.15% glutaraldehyde solution. This valve model was inserted in a mounting chamber with sinus geometries, which is made of clear acrylic to provide optical access for measurements. To understand the effects of hemodynamics on fluid shear stress, shear stress was measured across a range of conditions: varying stroke volumes at the same heart rate and varying heart rates at the same stroke volume. Systolic shear stress magnitude was found to be much higher than diastolic shear stress magnitude due to the stronger flow in the sinuses during systole, reaching up to 20 dyn/cm2 at mid-systole. Upon increasing stroke volume, fluid shear stresses increased due to stronger sinus fluid motion. Upon increasing heart rate, fluid shear stresses decreased due to reduced systolic duration that restricted the formation of strong sinus flow. Significant changes in the shear stress waveform were observed at 90 beats/ min, most likely due to altered leaflet dynamics at this higher heart rate. Overall, this study represents the most well-resolved shear stress measurements to date across a range of conditions on the aortic side of the AV. The data presented can be used for further investigation to understand AV biological response to shear stresses. PMID:21416247

Yap, Choon Hwai; Saikrishnan, Neelakantan; Tamilselvan, Gowthami

2011-01-01

11

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

PubMed

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

Zoback, M D; Roller, J C

1979-10-26

12

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

NASA Technical Reports Server (NTRS)

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

Reda, Daniel C.; Wilder, Michael C.

1998-01-01

13

A Method of Measuring the Critical Shear Stress of Molten High-Density Polyethylene (HDPE) and the Relationship between the Critical Shear Stress and Molecular Weight or Molecular Weight Distribution  

Microsoft Academic Search

The flow curve of molten high-density polyethylene (HDPE) has the discontinuous point characterized by the critical shear stress. In this paper we propose a method for measuring the critical shear stress and discusse the relationship between the critical shear stress and molecular weight or molecular weight distribution of HDPE. During the experiment at a constant shear rate, the pressure increases

Takatoshi Shimura; Eiichi Takatori; Takeshi Yamamoto

2010-01-01

14

Direct measurement of wall shear stress in a reattaching flow with a photonic sensor  

NASA Astrophysics Data System (ADS)

Wall shear stress measurements are carried out in a planar backward-facing step flow using a micro-optical sensor. The sensor is essentially a floating element system and measures the shear stress directly. The transduction method to measure the floating element deflection is based on the whispering gallery optical mode (WGM) shifts of a dielectric microsphere. This method is capable of measuring floating element displacements of the order of a nanometer. The floating element surface is circular with a diameter of ˜960 µm, which is part of a beam that is in contact with the dielectric microsphere. The sensor is calibrated for shear stress as well as pressure sensitivity yielding 7.3 pm Pa-1 and 0.0236 pm Pa-1 for shear stress and pressure sensitivity, respectively. Hence, the contribution by the wall pressure is less than two orders of magnitude smaller than that of shear stress. Measurements are made for a Reynolds number range of 2000-5000 extending to 18 step heights from the step face. The results are in good agreement with those of earlier reports. An analysis is also carried out to evaluate the performance of the WGM sensor including measurement sensitivity and bandwidth.

Ayaz, U. K.; Ioppolo, T.; Ötügen, M. V.

2013-12-01

15

Measurement of turbulent wall shear-stress using micro-pillars  

NASA Astrophysics Data System (ADS)

In experimental fluid mechanics, measuring spatially and temporally resolved wall shear-stress (WSS) has proved a challenging problem. The micro-pillar shear-stress sensor (MPS3) has been developed with the goal of filling this gap in measurement techniques. The MPS3 comprises an array of flexible micro-pillars flush mounted on the wall of a wall-bounded flow field. The deflection of these micro-pillars in the presence of a shear field is a direct measure of the WSS. This paper presents the MPS3 development work carried out by RWTH Aachen University and Purdue University. The sensor concept, static and dynamic characterization and data reduction issues are discussed. Also presented are demonstrative experiments where the MPS3 was used to measure the WSS in both water and air. The salient features of the measurement technique, sensor development issues, current capabilities and areas for improvement are highlighted.

Gnanamanickam, E. P.; Nottebrock, B.; Große, S.; Sullivan, J. P.; Schröder, W.

2013-12-01

16

Silicon micromachined sensors and sensor arrays for shear-stress measurements in aerodynamic flows  

E-print Network

In this thesis we report on a new micromachined floating-element shear-stress sensor for turbulent boundary layer research. Applications in low shear-stress environments such as turbulent boundary layers require extremely ...

Padmanabhan, Aravind

17

Dynamic pressure and shear stress measurements on the stator wall of whirling annular seals  

E-print Network

Dynamic pressure and shear stress measurements on the stator wall of whirling annular seals are presented. Two flow conditions (Re=12,000 & 24,000), two seal speeds (Ta=3,300 & 6,600) and three eccentricity ratios (0, 10, & 50% of the clearance...

Winslow, Robert Bradley

2012-06-07

18

MEASUREMENT OF WALL SHEAR STRESS IN TURBULENT BOUNDARY LAYERS USING AN OPTICAL INTERFEROMETRY METHOD  

E-print Network

. The wall shear stress is calculated by measuring the velocity from a Pitot tube xed on the wall and a wall turbulent boundary layers. A calibration chart is required and the most commonly used is that of Pa- tel not depend on the law of the wall and preferably one which does not require calibration. A skin friction

Marusic, Ivan

19

Sensor for Direct Measurement of the Boundary Shear Stress in Fluid Flow  

NASA Technical Reports Server (NTRS)

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 and normal 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. In addition, available turbulence models cannot account accurately for the bed roughness effect. Direct measurement of the boundary shear and normal stress and their fluctuations are attractive alternatives. However, this approach is a challenging one especially for high spatial resolution and high fidelity measurements. The authors designed and fabricated a prototype miniature shear stress sensor including an EDM machined floating plate and a high-resolution laser optical encoder. Tests were performed both in air as well as operation in water with controlled flow. The sensor sensitivity, stability and signal-to-noise level were measured and evaluated. The detailed test results and a discussion of future work will be presented in this paper.

Bao, Xiaoqi; Badescu, Mircea; Bar-Cohen, Yoseph; Lih, Shyh-Shiuh; Sherrit, Stewart; Chang, Zensheu; Chen, Beck; Widholm, Scott; Ostlund, Patrick

2011-01-01

20

In vivo blood flow and wall shear stress measurements in the vitelline network  

NASA Astrophysics Data System (ADS)

The wall shear stress plays a key role in the interaction between blood flow and the surrounding tissue. To obtain quantitative information about this parameter, velocity measurements are required with sufficient spatial (and temporal) resolution. We present a methodology for the determination of the wall shear stress in vivo in the vitelline network of a chick embryo. Velocity data is obtained by microscopic particle image velocimetry using correlation ensemble averaging; the latter is used to increase the signal-to-noise ratio of the measurements. The temporal evolution of the pulsatile flow is reconstructed by sorting the image pairs based on a phase estimate. From these flow measurements, the wall shear stress can be derived either directly from the magnitude of the gradients or from fits to velocity profiles. Both methods give results that are in good agreement with each other, while the former method is significantly easier to implement. For more accurate studies, the full three-dimensional velocity field may be required. It is demonstrated how this velocity field can be obtained by scanning the measurement volume.

Poelma, C.; Vennemann, P.; Lindken, R.; Westerweel, J.

2008-10-01

21

Accurate and Independent Measurements of Wall-Shear Stress in Turbulent Flows  

Microsoft Academic Search

Oil Film Interferometry (OFI) is used to directly measure the wall-shear stress in the high Reynolds number turbulent boundary layers from three facilities used for ICET. Various optical arrangements were utilized to collect the digital images generated on transparent plugs integrated into the boundary layer surface. Test-section free stream velocities ranging from 10 to 60 m\\/s and development lengths from

2009-01-01

22

Yield shear stress and disaggregating shear stress of human blood  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

23

Estimates of Shear Stress and Measurements of Water Levels in the Lower Fox River near Green Bay, Wisconsin  

USGS Publications Warehouse

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.

Westenbroek, Stephen M.

2006-01-01

24

Shear Stress and Atherosclerosis  

PubMed Central

Hemodynamic shear stress, the frictional force acting on vascular endothelial cells, is crucial for endothelial homeostasis under normal physiological conditions. When discussing blood flow effects on various forms of endothelial (dys)function, one considers two flow patterns: steady laminar flow and disturbed flow because endothelial cells respond differently to these flow types both in vivo and in vitro. Laminar flow which exerts steady laminar shear stress is atheroprotective while disturbed flow creates an atheroprone environment. Emerging evidence has provided new insights into the cellular mechanisms of flow-dependent regulation of vascular function that leads to cardiovascular events such as atherosclerosis, atherothrombosis, and myocardial infarction. In order to study effects of shear stress and different types of flow, various models have been used. In this review, we will summarize our current views on how disturbed flow-mediated signaling pathways are involved in the development of atherosclerosis. PMID:24781409

Heo, Kyung-Sun; Fujiwara, Keigi; Abe, Jun-ichi

2014-01-01

25

Fluid wall shear stress measurements in a model of the human abdominal aorta: oscillatory behavior and relationship to atherosclerosis.  

PubMed

Clinically significant atherosclerosis in the human aorta is most common in the infrarenal segment. This study was initiated to test the hypothesis that flowfield properties are closely related to the localization of plaques in this segment of the arterial system. Wall shear stress was calculated from magnetic resonance velocity measurements of pulsatile flow in an anatomically accurate model of the human abdominal aorta. The wall shear stress values were compared with intimal thickening from 15 post-mortem aortas measured by quantitative morphometry of histological cross sections obtained at standard locations. Wall shear stress oscillated in direction throughout most of the infrarenal aorta, most prominently in the distal region. The time-averaged mean wall shear stress (-1.7 to 1.4 dyn/cm2) was lowest near the posterior wall in this region. These hemodynamic parameters coincided with the locations of maximal intimal thickening. Statistical correlation between oscillatory shear and intimal thickness yielded r = 0.79, P < 0.00001. Low mean shear stresses correlated nearly as well (r = -0.75, P < 0.00005). Comparison of our data with surface maps of Sudan Red staining and early lesions as reported by others revealed similar conclusions. In contrast, pulse and maximum shear stresses did not correlate with plaque localization as has been shown for other sites of selective involvement by atherosclerosis (r < 0.345). Simulated exercise conditions markedly changed the magnitude and pattern of wall shear stress in the distal abdominal aorta. These results demonstrate that in the infrarenal aorta, regions of low mean and oscillating wall shear stresses are predisposed to the development of plaque while regions of relatively high wall shear stress tend to be spared. PMID:7848371

Moore, J E; Xu, C; Glagov, S; Zarins, C K; Ku, D N

1994-10-01

26

Platelet Activation Due to Hemodynamic Shear Stresses: Damage Accumulation Model and Comparison to In Vitro Measurements  

PubMed Central

The need to optimize the thrombogenic performance of blood recirculating cardiovascular devices, e.g., prosthetic heart valves (PHV) and ventricular assist devices (VAD), is accentuated by the fact that most of them require lifelong anticoagulation therapy that does not eliminate the risk of thromboembolic complications. The formation of thromboemboli in the flow field of these devices is potentiated by contact with foreign surfaces and regional flow phenomena that stimulate blood clotting, especially platelets. With the lack of appropriate methodology, device manufacturers do not specifically optimize for thrombogenic performance. Such optimization can be facilitated by formulating a robust numerical methodology with predictive capabilities of flow-induced platelet activation. In this study, a phenomenological model for platelet cumulative damage, identified by means of genetic algorithms (GAs), was correlated with in vitro experiments conducted in a Hemodynamic Shearing Device (HSD). Platelets were uniformly exposed to flow shear representing the lower end of the stress levels encountered in devices, and platelet activity state (PAS) was measured in response to six dynamic shear stress waveforms representing repeated passages through a device, and correlated to the predictions of the damage accumulation model. Experimental results demonstrated an increase in PAS with a decrease in “relaxation” time between pulses. The model predictions were in very good agreement with the experimental results. PMID:18204318

Nobili, Matteo; Sheriff, Jawaad; Morbiducci, Umberto; Redaelli, Alberto; Bluestein, Danny

2009-01-01

27

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

NASA Technical Reports Server (NTRS)

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

Reda, Daniel C. (inventor.)

1995-01-01

28

Stress measurements at depth in the vicinity of the San Andreas Fault: Implications for the magnitude of shear stress at depth  

Microsoft Academic Search

Using the hydraulic fracturing technique, we have made a systematic series of in situ measurements in wells drilled near the San Andreas fault. In an attempt to provide constraints for the magnitude of shear stress on the San Andreas fault at depth we have measured both the variation of stress with distance from the fault in relatively shallow (~230 m)

Mark D. Zoback; Hiroaki Tsukahara; Stephen Hickman

1980-01-01

29

Development of a fiber Bragg grating sensor for in-shoe shear stress measurement: design and preliminary results  

NASA Astrophysics Data System (ADS)

In-shoe shear stress sensors are a required tool for the investigation of plantar ulcer development after the onset of diabetes. Recently, several transducers have been developed for measuring in-shoe shear stress using magneto- resistive technology, light intensity modulation, and copolymer piezoelectric materials. Common drawbacks in the previous methods are the relatively large size of the sensors and the difficulty in interrogating many sensors simultaneously in order to achieve distributed sensing. In this paper we demonstrate for the first time a shear stress sensor using Fiber Bragg gratings (FBGs). The small size and the multiplexing capability of FBGs enables quasi- distributed sensing of shear stress on the plantar surface by interrogating a large number of identical sensors. The sensor design is based on the theory of elastic bending of columns. The sensor consists of two FBGs fitted inside a metallic structure which is able to deform elastically under shear stress. This elastic deformation produces strain on the FBGs, which can be detected by measuring the Bragg wavelength shift of the reflected light of each FBG using a CCD spectrometer. Preliminary results on an enlarged version of the sensor have shown the applicability of FBGs for the implementation of the in-shoe sensor.

Koulaxouzidis, Andreas V.; Roberts, V. C.; Holmes, Melanie J.; Handerek, Vincent A.

2000-08-01

30

Accurate and Independent Measurements of Wall-Shear Stress in Turbulent Flows  

NASA Astrophysics Data System (ADS)

Oil Film Interferometry (OFI) is used to directly measure the wall-shear stress in the high Reynolds number turbulent boundary layers from three facilities used for ICET. Various optical arrangements were utilized to collect the digital images generated on transparent plugs integrated into the boundary layer surface. Test-section free stream velocities ranging from 10 to 60 m/s and development lengths from 5.5m to 21 m, resulted in friction velocities varying from 0.35 to 1.65 m/s, corresponding to boundary layer thicknesses varying by factors of nearly four. Silicon oils with viscosities from 20 to 1000 cSt were employed in the measurements, with multiple oils used for several of the test conditions. A reference temperature measurement was used in all three facilities and for the calibration of the oils as a function of temperature in four different laboratories using two types of viscometers. The processing of the images was carried out using several approaches and compared for consistency of the results. Results of the skin friction coefficient from the three wind tunnels are examined and compared as a function of the displacement thickness Reynolds number, as determined from hot-wire and Pitot probe profiles at comparable conditions, and are found to be accurately represented by the logarithmic Rotta relation. The various uncertainties and the final accuracy of this type of measurement are discussed.

R"Uedi, J.-D.; Duncan, R.; Imayama, S.; Chauhan, K.

2009-11-01

31

Measurements of the wall shear stress distribution in the outflow tract of an embryonic chicken heart  

PubMed Central

In order to study the role of blood–tissue interaction in the developing chicken embryo heart, detailed information about the haemodynamic forces is needed. In this study, we present the first in vivo measurements of the three-dimensional distribution of wall shear stress (WSS) in the outflow tract (OFT) of an embryonic chicken heart. The data are obtained in a two-step process: first, the three-dimensional flow fields are measured during the cardiac cycle using scanning microscopic particle image velocimetry; second, the location of the wall and the WSS are determined by post-processing flow velocity data (finding velocity gradients at locations where the flow approaches zero). The results are a three-dimensional reconstruction of the geometry, with a spatial resolution of 15–20 µm, and provides detailed information about the WSS in the OFT. The most significant error is the location of the wall, which results in an estimate of the uncertainty in the WSS values of 20 per cent. PMID:19401309

Poelma, C.; Van der Heiden, K.; Hierck, B. P.; Poelmann, R. E.; Westerweel, J.

2010-01-01

32

Friction ridges in cockroach climbing pads: anisotropy of shear stress measured on transparent, microstructured substrates  

Microsoft Academic Search

The contact of adhesive structures to rough surfaces has been difficult to investigate as rough surfaces are usually irregular\\u000a and opaque. Here we use transparent, microstructured surfaces to investigate the performance of tarsal euplantulae in cockroaches\\u000a (Nauphoeta cinerea). These pads are mainly used for generating pushing forces away from the body. Despite this biological function, shear stress\\u000a (force per unit

Christofer J. Clemente; Jan-Henning Dirks; David R. Barbero; Ullrich Steiner; Walter Federle

2009-01-01

33

Influence of wall roughness, adverse pressure gradient and heat transfer on wall shear stress measurements with Preston tubes  

NASA Astrophysics Data System (ADS)

Correction functions to the Preston tube calibration curve, accounting for wall roughness, adverse pressure gradient, and heat transfer, were derived, allowing quantitative measurements of local wall shear stress and skin friction in boundary layer flows modified by these parameters. The correction functions were calculated from analytical boundary layer laws and are compiled in tables. Experiments in rough tubes, on plates with positive pressure gradients, and on heated and cooled surfaces confirm theoretical results.

Nitsche, W.; Haberland, C.

1980-12-01

34

In situ measurements of erosion shear stress and geotechnical shear strength of the intertidal sediments of the experimental managed realignment scheme at Tollesbury, Essex, UK  

NASA Astrophysics Data System (ADS)

Managed realignment is one of several 'soft' engineering options which may reduce the costs of coastal defence, provide a more 'natural' response to the problem of rising sea levels and at the same time deliver environmental, specifically nature conservation, benefits. The success of this technique depends on the ability of the soils and sediments within the site to resist the erosive action of waves and tidal currents and allow sediment accretion to occur, at least at a rate equal to mean sea-level rise. Once a critical shear stress, ?0 crt exerted by the moving fluids over the bed, is exceeded erosion will occur. A cohesive strength meter (CSM) and the fall-cone method were used to gather data, in situ on the strength and stability of sediments from an experimental managed realignment site and an adjacent, established saltmarsh in south-east England. Following six years of regular tidal cover, the underlying agricultural soil appeared both very strong (mean surface shear strength, ? f=228 kPa) and highly resistant to erosion ( ? 0 crt=6.23 N m -2). During this period much of the site had been covered by sediment, and saltmarsh plants ( Salicornia europaea) had become established above the mean high water neap tide (MHWN) level. Above MHWN level (tidal cover time <15%) sediments had greater bulk densities and lower water contents which resulted in a moderate shear strength (? f=11.6 kPa) and resistance to erosion (? 0 crt=2.45 N m -2) . Below MHWN, where sediment accretion rates were greatest, poor consolidation resulted in very high water contents and low bulk densities. These areas were at the highest potential risk of erosion (? 0 crt=1.5 N m -2) and had very low shear strengths (? f=0.33 kPa) . Where sediment exceeded 25 cm depth, gullies formed allowing their banks and adjacent margins to drain faster than the surrounding sediment. This led to a significant increase in bed strength (? f=10.8 kPa) and stability (? 0 crt=4.3 N m -2) . These gullies were probably the early stages of the complex creek patterns characteristic of the adjacent, established saltmarsh. The established saltmarsh was rich in plants and had a well-developed (aggregated) soil structure. These soils had a moderate to high resistance to erosion (? 0 crt=2.45 N m -2) and shear strength (? f=25.6 kPa) . Undrained sediment shear strength, ?f, obtained with the fall-cone apparatus can also provide a useful indication of critical erosion shear stress, ?0 crt . Values of ?0 crt , measured across this site were all relatively large compared with computed bed stresses arising from locally generated waves. Thus this experimental managed realignment site was found to be primarily depositional and was thus successfully achieving the twin aims of protecting the coast from erosion and extending a rich ecosystem.

Watts, C. W.; Tolhurst, T. J.; Black, K. S.; Whitmore, A. P.

2003-11-01

35

Bottom shear stress in unsteady sewer flow.  

PubMed

The properties of unsteady open-channel turbulent flow were theoretically and experimentally investigated in a circular cross section channel with fixed sediment deposits. Velocity and turbulence distribution data were obtained using an ultrasonic velocity profiler (UVP). Different uniform flow conditions and triangular-shaped hydrographs were analysed. The hydrograph analysis revealed a dynamic wave behaviour, where the time lags of mean cross section velocity, friction velocity, discharge and flow depth were all evident. The bottom shear stress dynamic behaviour was estimated using four different approaches. Measurements of the velocity distribution in the inner region of the turbulent layer and of the Reynolds stress distribution in the turbulent flow provided the analysed data sets of the bottom shear stress. Furthermore, based on the Saint Venant equation, the bottom shear stress time behaviour was studied using both the kinematic and the dynamic flow principles. The dynamic values of the bottom shear stress were compared with those for the steady flow conditions. It is evident that bottom shear stress varies along the generated flood hydrograph and its variation is the function of the flow unsteadiness. Moreover, the kinematic flow principle is not an adequate type of approximation for presented flow conditions. PMID:17120638

Bares, V; Jirák, J; Pollert, J

2006-01-01

36

Stress measurements in high-shear granulators using calibrated “test” particles: application to scale-up  

Microsoft Academic Search

Granulation is a process by which fine powders are agglomerated into larger particles using a liquid binder. In high-shear granulation the powder–binder mix experiences intense agitation inside a mixing vessel as binder is dispersed and granules form and strengthen under the influence of shear and compacting forces in the device.It is an implicit assumption that in a “high shear” mixer,

Gabriel I Tardos; Karen P Hapgood; Oyabo O Ipadeola; James N Michaels

2004-01-01

37

Shear rejuvenation, aging and shear banding in yield stress fluids  

Microsoft Academic Search

The purpose of this work is to simulate shear rejuvenation and aging effects in shear thinning yield stress fluids in a typical rotational rheometer and to provide a common framework to describe the behavior of yield stress materials in general. This is particularly important in the determination of material constants under both steady and unsteady conditions. The breakdown and buildup

Andreas N. Alexandrou; Nicholas Constantinou; Georgios Georgiou

2009-01-01

38

Development of Hybrid Sensor Arrays for Sensor Arrays for Simultaneous Measurement of Pressure and Shear Stress Distribution  

NASA Technical Reports Server (NTRS)

This document reports on the progress in developing hybrid sensors for the simultaneous measurement of pressure and shear stress. The key feature for the success of the proposed hybrid sensor array is the ability to deposit Cu-Ni alloy with proper composition (55 - 45) on a silicon wafer to form a strain gage. This alloy strain gage replaces the normally used Si strain gages in MEMS, which are highly nonlinear and temperature dependent. The copper nickel, with proper composition (55 - 45), was successfully deposited on a silicon wafer with a few trials during this period of the project. Pictures of the Cu-Ni alloy strain gage and the x-ray spectra indicating the composition are shown. The planned tests are also reviewed.

2000-01-01

39

Quantification of Critical Shear Stress  

NSDL National Science Digital Library

à Photo of angle of repose experiment Provenance: Lonnie Leithold, North Carolina State University at Raleigh Reuse: This item is offered under a Creative Commons Attribution-NonCommercial-ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial purposes as long as you provide attribution and offer any derivative works under a similar license. This is a lab activity designed to give students experience with the concept and quantification of critical shear stress.

Leithold, Lonnie

40

Advances in endothelial shear stress proteomics.  

PubMed

The vascular endothelium lining the luminal surface of all blood vessels is constantly exposed to shear stress exerted by the flowing blood. Blood flow with high laminar shear stress confers protection by activation of antiatherogenic, antithrombotic and anti-inflammatory proteins, whereas low or oscillatory shear stress may promote endothelial dysfunction, thereby contributing to cardiovascular disease. Despite the usefulness of proteomic techniques in medical research, however, there are relatively few reports on proteome analysis of cultured vascular endothelial cells employing conditions that mimic in vivo shear stress attributes. This review focuses on the proteome studies that have utilized cultured endothelial cells to identify molecular mediators of shear stress and the roles they play in the regulation of endothelial function, and their ensuing effect on vascular function in general. It provides an overview on current strategies in shear stress-related proteomics and the key proteins mediating its effects which have been characterized so far. PMID:25017810

Firasat, Sabika; Hecker, Markus; Binder, Lutz; Asif, Abdul R

2014-10-01

41

Wall Shear Stress Measurement Using Phase Contrast Magnetic Resonance Imaging With Phase Contrast Magnetic Resonance Angiography in Arteriovenous Polytetrafluoroethylene Grafts  

Microsoft Academic Search

Purpose: The purpose of the present article was to determine the changes in luminal vessel area, blood flow, and wall shear stress in both the inflow artery and the venous stenosis of arteriovenous polytetrafluoroethylene (PTFE) grafts. Methods and materials: Polytetrafluoroethylene grafts were placed from the carotid artery to the ipsilateral jugular vein in 8 castrated juvenile male pigs. Contrast-enhanced magnetic

Sanjay Misra; Alex A. Fu; Khamal D. Misra; James F. Glockner; Debabrata Mukhopadhyay

2009-01-01

42

Anomalous stress profile in a sheared granular column.  

PubMed

We present measurements of the stress as a function of vertical position in a column of granular material sheared in a cylindrical Couette device. All three components of the stress tensor on the outer cylinder were measured as a function of distance from the free surface at shear rates low enough that the material was in the dense, slow flow regime. We find that the stress profile differs fundamentally from that of fluids, from the predictions of plasticity theories, and from intuitive expectation. We argue that the anomalous stress profile is due to an anisotropic fabric caused by the combined action of gravity and shear. PMID:23005989

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

2012-09-21

43

Computation of boundary shear stress distributions throughout river cross-sections: a comparison among four geometrical methods and aDcp measurements  

NASA Astrophysics Data System (ADS)

Despite the fundamental importance of assessing boundary shear stress distributions for hydraulic engineering purposes, comparison of calculated distributions with field measurements is rarely undertaken. Herein, we explore the ability of four geometrical methods to reproduce boundary shear stress distribution in rivers with arbitrary geometrical sections. Geometrical predictions are compared with numerical predictions obtained by the two-dimensional (2-D) depth averaged numerical model TELEMAC and field data derived from acoustic Doppler current profiler (aDcp) measurements conducted in the Saône river and in two trapezoidal open channels located in the Rhone river valley (France). Two approaches are used to derive the boundary shear stress distributions from the measured aDcp velocity profiles: a) mapping of the channel cross-section into sub-areas using velocity isovels and orthogonal lines, and b) fitting a logarithmic-law to the velocity profiles. Comparisons between the predicted and measured shear stresses indicate that Merged Perpendicular Method (MPM) and Wobus, Tucker, and Anderson (WTA) model behave similarly and yield better results compared to the Vertical Depth Method (VDM) and Normal Area Method (NAM), but some deviations remain. Potential sources of these discrepancies are discussed, showing that they are related to the assumptions of negligible secondary currents and of uniform flow conditions which are used for the four tested geometrical methods.

El Kadi Abderrezzak, K.; Le Coz, J.; Moore, S.

2010-12-01

44

Cycle-to-cycle variation effects on turbulent shear stress measurements in pulsatile flows  

NASA Astrophysics Data System (ADS)

Accurate evaluation of turbulent velocity statistics in pulsatile flows is important in estimating potential damage to blood constituents from prosthetic heart valves. Variations in the mean flow from one cycle to the next can result in artificially high estimates. Here we demonstrate a procedure using a digital, low-pass filter to remove the cycle-to-cycle variation from turbulence statistics. The results show that cycle-to-cycle variations can significantly affect estimates of turbulent Reynolds stress and should be either eliminated or demonstrated to be small when reporting pulsatile flow results.

Tiederman, W. G.; Privette, R. M.; Phillips, W. M.

1988-01-01

45

Time-resolved particle image velocimetry measurements with wall shear stress and uncertainty quantification for the FDA benchmark nozzle model  

E-print Network

We present validation of benchmark experimental data for computational fluid dynamics (CFD) analyses of medical devices using advanced Particle Image Velocimetry (PIV) processing and post-processing techniques. 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. Time-resolved PIV analysis was performed in five overlapping regions of the model for Reynolds numbers in the nozzle throat of 500, 2,000, 5,000, and 8,000. Images included a two-fold 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 comput...

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

2014-01-01

46

MEMS floating element sensor array for wall shear stress measurement under a turbulent boundary layer  

NASA Astrophysics Data System (ADS)

A novel electrochromic thin film transistor (EC-TFT) was fabricated and characterized in this work. This concept relies on ion transport to control gating. The channel material is tungsten oxide (WOx), produced by reactive magnetron sputtering. In its oxidized state WO3 is a transparent, wide band gap insulator (> 3 eV). However upon intercalation of light ions (H+, Li+) the material becomes both electrically conducting and opaque to visible light. This allows the EC-TFT to generate a complementary optical response. We optimized the fabrication of individual layers of the EC-TFT, and found that controlling the stoichiometry of WOx is a key step. Using RF magnetron sputtering, it was found that there is a narrow window to obtain material capable of reversible switching. Fully oxidized films proved it is difficult to intercalate ions efficiently. In contrast, insufficient oxygen produced films that were always in a metallic like state. Best results were obtained with the sputter power set at 200 W using O2 fractions of 42%-46% in argon. In the preliminary studies, the device was tested in a two-step fashion. First, devices were placed in solution and cyclic voltammetry was used to set the level of ion intercalation. Samples were then removed from the electrolyte, dried, and the source/drain current was measured on a probe station. This demonstrated the concept of the EC-TFT, showing that the transistor could be turned on and off reversely with a current ratio of Ion/Ioff ~ 1000, and showed significant color change during the intercalation. However, the conductivity in off state was too high to be a promising transistor, the two step approach made the data rather noisy, it was difficult to make good contacts and this was not an in situ measurement. In order to get the in situ measurement, macrodevices with large electrodes were fabricated and characterized, which made it easier to make electrical contacts. However, after the initial cycle the macrodevices were always in on state and could not be turned off because of the dielectric. In addition, it showed no significant color change. The top source/drain structure showed similar behavior as the bottom configuration. It was found that in these configurations the supply of electrons was insufficient to allow intercalation, and permanent defects in the Al2O3 dielectric developed to compensate. To address these issues, 3-terminal devices were fabricated and characterized simultaneously. These devices showed similar behaviors with the preliminary 2-step device. The IDS increased with decreasing gate biasing with a low threshold voltage -0.8 V. In addition, we observed the color changing. However, the conduction through the electrolyte resulted very low on/off ratio (<5). In conclusion, it is the conduction through the electrolyte contributes to the low on/off ratio.

Zong, Zhaowang

47

Endothelial gene regulation by laminar shear stress.  

PubMed

Endothelial cells, because of their unique localization, are constantly exposed to fluid mechanical forces derived by the flowing blood. These forces, and more specifically shear stresses; affect endothelial structure and function, both in vivo and in vitro, and are implicated as contributing factors in the development of cardiovascular diseases. We have demonstrated earlier that the shear stress selectively induces the transcription of several endothelial genes, and have defined a shear stress response element (SSRE) in the promoter of platelet-derived-growth-factor B (PDGF-B), that is shared by additional endothelial shear stress responsive genes. Here we further characterize this SSRE and the nuclear factors that bind to it, and imply the possible role of the endothelium cytoskeleton in transducing shear stress, leading to the expression of PDGF-B/SSRE constructs in transfected endothelial cells exposed to shear stress. We also present, yet a new shear stress response element in the Platelet Derived Growth Factor A promoter, that contains a binding site to the transcription factors egr1/sp1. These results further demonstrate the complexity of gene regulation by hemodynamic forces, and support the important part that these forces have in the physiology and pathophysiology of the vessel wall. PMID:9330726

Resnick, N; Yahav, H; Khachigian, L M; Collins, T; Anderson, K R; Dewey, F C; Gimbrone, M A

1997-01-01

48

Shear rate specific blood viscosity and shear stress of carotid artery duplex ultrasonography in patients with lacunar infarction  

PubMed Central

Background This study describes a new method for determining site-specific vascular shear stress using dynamic measures of shear rate and blood viscosity (BV) in the carotid arteries, and examines characteristics of carotid arterial shear stress among patients with lacunar infarction. Methods Vascular shear stress measurements were conducted in 37 patients (17 lacunar infarction patients and 20 control subjects) using duplex ultrasonography. Vessel wall diameters and velocities were measured in each arterial segment at peak-systolic (PS) and end-diastolic (ED) phases, for calculation of PS/ED shear rates. PS/ED shear stresses [dyne/cm2] were determined with PS/ED shear rates and shear-rate dependent BV values. For comparison, both values of hematocrit-derived BV and BV measurements at 300 s-1 were used for calculation of shear stress. Results All cardiovascular disease (CVD) risk factors including BV values were similar between the two groups. In both common carotid arteries, PS and ED shear stresses were significantly lower in the patients with lacunar infarction than in controls in multivariate models that included age, sex, and other major CVD risk factors. PS and ED shear stresses using the shear rate specific BV were 4.5% lower and 7.3% higher than those using the two other BVs, respectively. Conclusion Lacunar infarction was associated with reduced carotid arterial shear stress. The use of estimated BV for calculating carotid arterial shear stress provides more accurate assessment of the hemodynamic contribution of shear stress than previous models that have arbitrarily assigned a constant value to this dynamic flow property. PMID:23597083

2013-01-01

49

Determining Shear Stress Distribution in a Laminate  

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

50

Evaluation of the time dependent surface shear stress in turbulent flows  

NASA Technical Reports Server (NTRS)

The time dependent surface shear stress has been evaluated using surface heat transfer measurements. For fully developed turbulent pipe and open channel water flows, and incompressible and compressible turbulent boundary layer air flows the measurements indicate the absolute magnitude of the surface shear stress fluctuations will be greater than two times the mean values. The root-mean-square shear stress fluctuations were of the order of 0.2 to 0.4 times the mean surface shear values. Due to these large surface shear stress fluctuations and the nonlinear relation between heat transfer and shear stress, a special technique has been developed to evaluate the measurements. It was found that the non-linear averaging errors for a hot film-surface shear stress gauge in a fully developed pipe flow was of the order of 10 percent at low velocities. A hot wire-surface shear stress gauge was employed for measurements of turbulent boundary layers in air.

Sandborn, V. A.

1979-01-01

51

Micromachined thermal shear-stress sensor for underwater applications  

Microsoft Academic Search

This paper reports the development of micromachined thermal shear-stress sensors for underwater applications. The thermal shear-stress sensor is a polysilicon resistor sitting atop a vacuum-insulated nitride diaphragm. Special challenges for underwater measurements, such as the waterproof coating and minimization of pressure crosstalk, have been addressed. More rigid diaphragms than the aerial sensors are implemented to increase the operating range and

Yong Xu; Qiao Lin; Guoyu Lin; Rakesh B. Katragadda; Fukang Jiang; Steve Tung; Yu-Chong Tai

2005-01-01

52

Fluid shear stress threshold regulates angiogenic sprouting.  

PubMed

The density and architecture of capillary beds that form within a tissue depend on many factors, including local metabolic demand and blood flow. Here, using microfluidic control of local fluid mechanics, we show the existence of a previously unappreciated flow-induced shear stress threshold that triggers angiogenic sprouting. Both intraluminal shear stress over the endothelium and transmural flow through the endothelium above 10 dyn/cm(2) triggered endothelial cells to sprout and invade into the underlying matrix, and this threshold is not impacted by the maturation of cell-cell junctions or pressure gradient across the monolayer. Antagonizing VE-cadherin widened cell-cell junctions and reduced the applied shear stress for a given transmural flow rate, but did not affect the shear threshold for sprouting. Furthermore, both transmural and luminal flow induced expression of matrix metalloproteinase 1, and this up-regulation was required for the flow-induced sprouting. Once sprouting was initiated, continuous flow was needed to both sustain sprouting and prevent retraction. To explore the potential ramifications of a shear threshold on the spatial patterning of new sprouts, we used finite-element modeling to predict fluid shear in a variety of geometric settings and then experimentally demonstrated that transmural flow guided preferential sprouting toward paths of draining interstitial fluid flow as might occur to connect capillary beds to venules or lymphatics. In addition, we show that luminal shear increases in local narrowings of vessels to trigger sprouting, perhaps ultimately to normalize shear stress across the vasculature. Together, these studies highlight the role of shear stress in controlling angiogenic sprouting and offer a potential homeostatic mechanism for regulating vascular density. PMID:24843171

Galie, Peter A; Nguyen, Duc-Huy T; Choi, Colin K; Cohen, Daniel M; Janmey, Paul A; Chen, Christopher S

2014-06-01

53

Stress pulse attenuation in shear thickening fluid  

NASA Astrophysics Data System (ADS)

The stress pulse attenuation of the 62 vol/vol. % dense silica particle-ethylene glycol suspension was investigated by using a modified spilt Hopkinson pressure bar. In comparison to the neat ethylene glycol solution, the transmission pulse of the shear thickening is much weaker under the same impact condition. No energy loss is progressed for the neat ethylene glycol solution, thus it can be concluded that the energy dissipation behavior was happened in the silica particle based shear thickening fluid. In this work, the energy dissipation of the shear thickening fluid was reversible.

Jiang, Weifeng; Gong, Xinglong; Xuan, Shouhu; Jiang, Wanquan; Ye, Fang; Li, Xiaofeng; Liu, Taixiang

2013-03-01

54

Wall shear stress in accelerating and decelerating turbulent pipe flows  

Microsoft Academic Search

The unsteady wall shear stress in accelerating and decelerating turbulent flows of water in a smooth pipe was measured using two different approaches simultaneously : a “direct” mechanical approach, and a more conventional “indirect” approach involving measurement of the transient pressure gradient. The measurements covered a Reynolds number range of between 40,000 to 130,000, and values of acceleration parameter (?)

E. B. Shuy

1996-01-01

55

Adjustable shear stress erosion and transport flume  

DOEpatents

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

Roberts, Jesse D. (Carlsbad, NM); Jepsen, Richard A. (Carlsbad, NM)

2002-01-01

56

Determining ow type, shear rate and shear stress in magmas from bubble shapes and orientations  

E-print Network

conduits, the origin of pyroclastic obsidian, and the emplacement history and dynamics of obsidian flows a juvenile obsidian clast from a pyroclastic fall deposit records predominantly simple shear. A third sample and shear stress are recorded by the pyroclastic obsidian (shear rate = 0.01 s31 , shear stress = 60 k

Manga, Michael

57

Microviscometry reveals reduced blood viscosity and altered shear rate and shear stress profiles  

E-print Network

Microviscometry reveals reduced blood viscosity and altered shear rate and shear stress profiles in diameter. Radial distributions in blood viscosity, shear stress, and shear rate are obtained and used that the microviscometric method consis- tently predicted a reduction in local and apparent blood viscosity after isovolemic

58

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 Central

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. PMID:25007248

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

2014-01-01

59

Effects of shear stress on the microalgae Chaetoceros muelleri  

Microsoft Academic Search

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\\u000a by varying the shear rates and the medium viscosities. It was possible to quantify the effect of shear stress over a wide\\u000a range, whilst preserving laminar flow conditions through the

Michiel H. A. MichelsAtze; Atze J. van der Goot; Niels-Henrik Norsker; René H. Wijffels

2010-01-01

60

Our recent strain-measurement-based shear modulus reconstruction  

Microsoft Academic Search

As a differential diagnosis technique of living soft tissues, we are developing ultrasonic strain measurement-based shear modulus reconstruction methods. In this report, to stabilize the 3D and 2D reconstruction of shear modulus, Poisson's ratio and density, we propose to deal with the mean normal stress as unknown. Moreover, we propose two new methods for measuring multidimensional displacement vector with high

Chikayoshi Sumi

2005-01-01

61

A tapered channel microfluidic device for comprehensive cell adhesion analysis, using measurements of detachment kinetics and shear stress-dependent motion  

PubMed Central

We have developed a method for studying cellular adhesion by using a custom-designed microfluidic device with parallel non-connected tapered channels. The design enables investigation of cellular responses to a large range of shear stress (ratio of 25) with a single input flow-rate. For each shear stress, a large number of cells are analyzed (500–1500 cells), providing statistically relevant data within a single experiment. Besides adhesion strength measurements, the microsystem presented in this paper enables in-depth analysis of cell detachment kinetics by real-time videomicroscopy. It offers the possibility to analyze adhesion-associated processes, such as migration or cell shape change, within the same experiment. To show the versatility of our device, we examined quantitatively cell adhesion by analyzing kinetics, adhesive strength and migration behaviour or cell shape modifications of the unicellular model cell organism Dictyostelium discoideum at 21?°C and of the human breast cancer cell line MDA-MB-231 at 37?°C. For both cell types, we found that the threshold stresses, which are necessary to detach the cells, follow lognormal distributions, and that the detachment process follows first order kinetics. In addition, for particular conditions’ cells are found to exhibit similar adhesion threshold stresses, but very different detachment kinetics, revealing the importance of dynamics analysis to fully describe cell adhesion. With its rapid implementation and potential for parallel sample processing, such microsystem offers a highly controllable platform for exploring cell adhesion characteristics in a large set of environmental conditions and cell types, and could have wide applications across cell biology, tissue engineering, and cell screening. PMID:22355300

Rupprecht, Peter; Golé, Laurent; Rieu, Jean-Paul; Vézy, Cyrille; Ferrigno, Rosaria; Mertani, Hichem C.; Rivière, Charlotte

2012-01-01

62

An evaluation of bed shear stress under turbid flows  

Microsoft Academic Search

This paper investigates the effect of various turbidity levels on the drag coefficient and bed shear stress in decelerating flows over a smooth bed. Turbid flows were simulated in a laboratory annular flume (Lab Carousel) using clay suspensions of different known mass concentrations. Flow velocity measurements were taken in the turbulent boundary layer and the values of bed drag coefficient

C. E. L. Thompson; C. L. Amos; M. Angelaki; T. E. R. Jones; C. E. Binks

2006-01-01

63

A Rotary Flow Channel for Shear Stress Sensor Calibration  

NASA Technical Reports Server (NTRS)

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.

Zuckerwar, Allan J.; Scott, Michael A.

2004-01-01

64

Yield stress and shear-banding in granular suspensions  

E-print Network

We study the emergence of a yield stress in dense suspensions of non-Brownian particles, by combining local velocity and concentration measurements using Magnetic Resonance Imaging with macroscopic rheometric experiments. We show that the competition between gravity and viscous stresses is at the origin of the development of a yield stress in these systems at relatively low volume fractions. Moreover, it is accompanied by a shear banding phenomenon that is the signature of this competition. However, if the system is carefully density matched, no yield stress is encountered until a volume fraction of 62.7 0.3%.

Abdoulaye Fall; Francois Bertrand; Guillaume Ovarlez; Daniel Bonn

2009-07-13

65

The role of dilation and confining stresses in shear thickening of dense suspensions  

E-print Network

Many densely packed suspensions and colloids exhibit a behavior known as Discontinuous Shear Thickening in which the shear stress jumps dramatically and reversibly as the shear rate is increased. We performed rheometry and video microscopy measurements on a variety of suspensions to determine the mechanism for this behavior. Shear profiles and normal stress measurements indicate that, in the shear thickening regime, stresses are transmitted through frictional rather than viscous interactions, and come to the surprising conclusion that the local constitutive relation between stress and shear rate is not necessarily shear thickening. If the suspended particles are heavy enough to settle we find the onset stress of shear thickening tau_min corresponds to a hydrostatic pressure from the weight of the particle packing where neighboring particles begin to shear relative to each other. Above tau_min, dilation is seen to cause particles to penetrate the liquid-air interface of the sheared sample. The upper stress boundary tau_max of the shear thickening regime is shown to roughly match the ratio of surface tension divided by a radius of curvature on the order of the particle size. These results suggest a new model in which the increased dissipation in the shear thickening regime comes from frictional stresses that emerge as dilation is frustrated by a confining stress from surface tension at the liquid-air interface. When instead the suspensions are confined by solid walls and have no liquid-air interface, we find tau_max is set by the stiffness of the most compliant boundary which frustrates dilation. This rheology can be described by a non-local constitutive relation in which the local relation between stress and shear rate is shear thinning, but where the stress increase comes from a normal stress term which depends on the global dilation.

Eric Brown; Heinrich M. Jaeger

2010-10-24

66

Sensor for Boundary Shear Stress in Fluid Flow  

NASA Technical Reports Server (NTRS)

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.

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

67

The role of critical shear stress on acute coronary syndrome.  

PubMed

Increased aggregation of RBC is associated with many vascular diseases, including acute coronary syndrome (ACS). Critical shear-stress (CSS) as in index of red cell aggregation is defined as either the minimum shear-stress required dispersing the aggregates. The objective of this study is to access the role of CSS in ACS comparing to SA, and to evaluate the correlation with usual biomarkers for atherosclerosis such as fibrinogen, hs-CRP. 169 SA and 223 ACS patients were finally enrolled. A detailed medical history and laboratory data were obtained for each participant from clinical records. CSS is measured by simultaneous measurement of shear stress and light backscattering using a small disposable kit with a microfluidic hemorheometer. We hypothesized that higher value of CSS might be associated increased thrombosis in ACS. As results, relatively younger age was shown and more male in ACS patients, and inflammatory markers (WBC, hs-CRP) were higher in ACS. Whole blood viscosities were significantly higher in ACS than SA along at all shear rates. CSS was 25.7% higher in ACS (333.8 ± 147.8) than in SA (265.4 ± 149.9 mPa) (p < 0.001). CSS was highly correlated white blood cell counts, hs-CRP, fibrinogen, and erythrocyte sedimentation rate (ESR). Among those variables, fibrinogen, and ESR were strongly correlated with CSS. We may suggest that CSS could be used as a novel risk marker for ACS. PMID:23445628

Kim, Jongyoun; Chung, Hyemoon; Cho, Minhee; Lee, Byoung-Kwon; Karimi, Ali; Shin, Sehyun

2013-01-01

68

Application and improvement of Raupach's shear stress partitioning model  

NASA Astrophysics Data System (ADS)

Aeolian processes such as the entrainment, transport and redeposition of sand, soil or snow are able to significantly reshape the earth's surface. In times of increasing desertification and land degradation, often driven by wind erosion, investigations of aeolian processes become more and more important in environmental sciences. The reliable prediction of the sheltering effect of vegetation canopies against sediment erosion, for instance, is a clear practical application of such investigations to identify suitable and sustainable counteractive measures against wind erosion. This study presents an application and improvement of a theoretical model presented by Raupach (Boundary-Layer Meteorology, 1992, Vol.60, 375-395 and Journal of Geophysical Research, 1993, Vol.98, 3023-3029) which allows for quantifying the sheltering effect of vegetation against sediment erosion. The model predicts the shear stress ratios ?S'/? and ?S''/?. Here, ?S is the part of the total shear stress ? that acts on the ground beneath the plants. The spatial peak ?S'' of the surface shear stress is responsible for the onset of particle entrainment whereas the spatial mean ?S' can be used to quantify particle mass fluxes. The precise and accurate prediction of these quantities is essential when modeling wind erosion. Measurements of the surface shear stress distributions ?S(x,y) on the ground beneath live vegetation canopies (plant species: lolium perenne) were performed in a controlled wind tunnel environment to determine the model parameters and to evaluate the model performance. Rigid, non-porous wooden blocks instead of the plants were additionally tested for the purpose of comparison, since previous wind tunnel studies used exclusively artificial plant imitations for their experiments on shear stress partitioning. The model constant c, which is needed to determine the total stress ? for a canopy of interest and which remained rather unspecified to date, was found to be c ? 0.27. It was also found that the model parameter m, which relates ?S'' with ?S' and which is needed to determine the peak surface shear stress ?S'', is rather impractically defined in the original model formulation, because m is identified to be a function of the wind velocity, the roughness element shape and the roughness density. We propose an alternative, more universal definition of an a-parameter as a substitute for the m-parameter simply linearly relating ?S'' with ?S'. This strong linear relation is supported by the measurements and can be made plausible with simple fluid dynamical arguments. The advantage of a over m is that a is solely a function of the roughness element shape. Finally, a method is presented to determine the new a-parameter for different kinds of roughness elements with relatively simple experimental methods.

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

2012-12-01

69

Dynamic strength measurement using magnetically applied pressure shear (MAPS)  

NASA Astrophysics Data System (ADS)

A newly developed experimental technique to measure dynamic material strength at high pressures on magneto-hydrodynamic (MHD) drive pulsed power platforms is demonstrated on aluminum. The application of an external magnetic field normal to the plane of the MHD drive current directly induces a shear stress wave in addition to the usual longitudinal stress wave. Strength is probed by passing this shear wave through the sample material where the transmissible shear stress is limited to the sample strength. The magnitude of the transmitted shear wave is measured via a transverse VISAR system from which the sample strength is determined. Details of the experimental approach will be presented along with results of initial experiments on 99.5% pure aluminum which demonstrate the utility of the technique.

Alexander, C. S.; Asay, J. R.; Haill, T. A.

2011-06-01

70

Shear stress effects on human embryonic kidney cells in Vitro.  

PubMed

Human embryonic kidney cells grown as an attached, confluent monolayer on a flat substrate were subjected to steady, uniform laminar flow of medium in a specially designed chamber in which flow patterns and shear stress are accurately defined and controlled. Experiments were performed for shear stress levels ranging from 0.2 to 6.0 N/m(2) with times of exposure to the shear stress ranging from 2 to 24 h. The influence of the shear field was slight at low shear stress (0.26 N/m(2)). Higher stress levels (0.65 N/m(2) and higher) had significant effects on cell morphology, and on the post-shear release of urokinase enzyme. Still higher stress levels (2.6 N/m(2) and higher) caused marked reduction in cell viability. These results may be of interest in addressing practical problems in developing commercial biosynthesis reactors. PMID:18553772

Stathopoulos, N A; Hellums, J D

1985-07-01

71

Shear stress induced stimulation of mammalian cell metabolism  

NASA Technical Reports Server (NTRS)

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.

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

1988-01-01

72

Critical shear stresses in cohesive soils  

E-print Network

Dimensions of the Test Apparatus. 12 12 Testing Procedure 15 Sample Preparation Performing the Scour Tests Summary of Scour Test Procedures. . . . . . . . . . Vane Shear Test Void Ratio and Per Cent Moisture Determinations 15 16 19 20 21 RESULTS... on Vane Shear Strength 32 Regression of Critical Shearing Force on Void Ratio 33 12. Critical Shearing Force at Liquid Limit versus Plasticity Index 39 13. Critical Shearing Force at Liquid Limit versus Per Cent Clay . . 39 Critical Shearing Force...

Rektorik, Robert James

2012-06-07

73

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

PubMed Central

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

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

2012-01-01

74

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

PubMed

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

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

2005-08-01

75

Instrumented Bolts Would Measure Shear Forces In Joints  

NASA Technical Reports Server (NTRS)

Bolts instrumented with strain gauges used to measure shear forces. Bolts installed in multiple-bolt lap joints to obtain data on distribution of stresses and deformations in and around joints. Strain gauges indicate share of applied load borne by each individual bolt. In original application, bolted panels made of advanced refractory composite materials designed to withstand use at temperatures up to 4,000 degrees F. Also applicable to other joint materials and measurement of shear loads in other connections such as, shear loads on shafts in pulleys or gears.

Sawyer, James Wayne; Mcwithey, Robert R.

1994-01-01

76

Evaluation of insulation shear stresses in the ITER central solenoid  

Microsoft Academic Search

CS insulation system binds the conductor jackets into a an important structural element of bucked ITER magnet system. The CS must accommodate bucking pressures, and the flexural motions of the TF case. Shear stresses in the CS are computed in a large ANSYS global model of the magnet system. The local structural response of the jacket also adds shear stresses

P. H. Titus; A. PAnin; A. I. Borovkov

1997-01-01

77

Influence of shear stress fluctuation on bed particle mobility  

Microsoft Academic Search

Whether or not a sediment particle is entrained from a channel bed is associated with both average bed shear stress and shear stress fluctuation, the latter being flow-dependent and also related to bed irregularities. In the first part of this study, a preliminary analysis of possible fluctuations induced by bed roughness is presented for the case of an immobile plane

Nian-Sheng Cheng

2006-01-01

78

On the relationship between uniaxial yield strength and resolved shear stress in polycrystalline materials  

Microsoft Academic Search

Attempts to correlate radiation-induced microstructural changes with changes in mechanical properties rely on a well-established theory to compute the resolved shear stress required to move dislocations through a field of obstacles. However, this microstructure-based shear stress must be converted to an equivalent uniaxial tensile stress in order to make comparisons with mechanical property measurements. A review of the radiation effects

R. E. Stoller; S. J. Zinkle

2000-01-01

79

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

SciTech Connect

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

Castillo, D. A., [Department of Geology and Geophysics, University of Adelaide (Australia); Younker, L.W. [Lawrence Livermore National Lab., CA (United States)

1997-01-30

80

Dynamic material strength measurement utilizing magnetically applied pressure-shear  

NASA Astrophysics Data System (ADS)

Magnetically applied pressure-shear (MAPS) is a recently developed technique used to measure dynamic material strength developed at Sandia National Laboratories utilizing magneto-hydrodynamic (MHD) drive pulsed power systems. MHD drive platforms generate high pressures by passing a large current through a pair of parallel plate conductors which, in essence, form a single turn magnet coil. Lorentz forces resulting from the interaction of the self-generated magnetic field and the drive current repel the plates and result in a high pressure ramp wave propagating in the conductors. This is the principle by which the Sandia Z Machine operates for dynamic material testing. MAPS relies on the addition of a second, external magnetic field applied orthogonally to both the drive current and the self-generated magnetic field. The interaction of the drive current and this external field results in a shear wave being induced directly in the conductors. Thus both longitudinal and shear stresses are generated. These stresses are coupled to a sample material of interest where shear strength is probed by determining the maximum transmissible shear stress in the state defined by the longitudinal compression. Both longitudinal and transverse velocities are measured via a specialized velocity interferometer system for any reflector (VISAR). Pressure and shear strength of the sample are calculated directly from the VISAR data. Results of tests on several materials at modest pressures (˜10GPa) will be presented and discussed.

Alexander, C. S.

2012-08-01

81

Wall shear stress in backward-facing step flow of a red blood cell suspension.  

PubMed

An experimental investigation of the wall shear stress distribution downstream of a backward-facing step is carried out. The wall shear stress distribution was determined by measuring the deformation of a gel layer, attached to the wall downstream of the step. Speckle pattern interferometry was applied to measure the deformation of the gel layer. The measured deformation, combined with the properties of the gel layer, served as an input for a finite element solid mechanics computation to determine the stress distribution in the gel layer. The wall shear stress, required to generate the measured deformation of the gel layer, was determined from these computations. A Newtonian buffer solution and a non-Newtonian red blood cell suspension were used as measuring fluids. The deformation of the gel layer was determined for a Newtonian buffer solution to evaluate the method and to obtain the properties of the gel layer. Subsequently, the wall shear stress distribution for the non-Newtonian red blood cell suspension was determined for three different flow rates. The inelastic non-Newtonian Carreau-Yasuda model served as constitutive model for the red blood cell suspension. Using this model, the velocity and wall shear stress distribution were computed by means of a finite element fluid mechanics computation. From the comparison between the numerical and the experimental results, it can be concluded that wall shear stresses, induced by the red blood cell suspension, can be modeled accurately by employing a Carreau-Yasuda model. PMID:10474654

Gijsen, F J; van de Vosse, F N; Janssen, J D

1998-01-01

82

Shear-Sensitive Liquid Crystal Coating Method: Surface-Inclination Effects on Shear Vector Measurements  

NASA Technical Reports Server (NTRS)

The shear-sensitive liquid crystal coating (SSLCC) method is an image-based technique for both visualizing dynamic surface-flow phenomena, such as transition and separation, and for measuring the continuous shear-stress vector distribution acting on an aerodynamic surface. Under proper lighting and viewing conditions (discussed below), the coating changes color in response to an applied aerodynamic shear. This color-change response is continuous and reversible, with a response time of milliseconds, and is a function of both the shear magnitude and the shear vector orientation relative to the observer. The liquid crystal phase of matter is a weakly-ordered, viscous, non-Newtonian fluid state that exists between the nonuniform liquid phase and the ordered solid phase of certain organic compounds. Cholesteric liquid crystal compounds possess a helical molecular arrangement that selectively scatters white light, incident along the helical axis, as a three-dimensional spectrum. This property is linked to the helical pitch length, which is within the range of wavelengths in the visible spectrum. The pitch length, and hence the wavelength of the scattered light, is influenced by shear stress normal to the helical axis. This unique optical property produces a measurable color change in response to an applied shearing force. The full-surface shear stress vector measurement method, developed at NASA-Ames, is schematically illustrated. As with the visualization method, the coated test surface is illuminated from the normal direction with white light and the camera is positioned at an above-plane view angle of approximately 30 deg. Experiments have been initiated at NASA Ames to begin the process of quantifying surface-inclination (surface-curvature) effects on shear vector measurement accuracy. In preliminary experiments, surface-inclination angles theta(sub x), theta(sub y) of 0, +/-5, +/-10, and +/-15 deg were employed. In this arrangement, white-light illumination was positioned normal to the untilted test surface, and the camera above-plane view angle was set at 30 deg relative to the untilted test surface. As can be seen, vector-aligned lambda(sub d) values showed no dependence on theta(sub x) or theta(sub y) for absolute values of these tilt angles is less than or equal to 15 deg. Acquisition and analyses of full-surface color images are presently underway to definitively document the insensitivity limits of the shear vector measurement methodology to surface-slope variations.

Reda, Daniel C.; Wilder, Michael C.; Nixon, David (Technical Monitor)

1998-01-01

83

Hemodynamic shear stress and the endothelium in cardiovascular pathophysiology  

PubMed Central

SUMMARY Endothelium lining the cardiovascular system is highly sensitive to hemodynamic shear stresses that act at the vessel luminal surface in the direction of blood flow. Physiological variations of shear stress regulate acute changes in vascular diameter and when sustained induce slow, adaptive, structural-wall remodeling. Both processes are endothelium-dependent and are systemically and regionally compromised by hyperlipidemia, hypertension, diabetes and inflammatory disorders. Shear stress spans a range of spatiotemporal scales and contributes to regional and focal heterogeneity of endothelial gene expression, which is important in vascular pathology. Regions of flow disturbances near arterial branches, bifurcations and curvatures result in complex spatiotemporal shear stresses and their characteristics can predict atherosclerosis susceptibility. Changes in local artery geometry during atherogenesis further modify shear stress characteristics at the endothelium. Intravascular devices can also influence flow-mediated endothelial responses. Endothelial flow-induced responses include a cell-signaling repertoire, collectively known as mechanotransduction, that ranges from instantaneous ion fluxes and biochemical pathways to gene and protein expression. A spatially decentralized mechanism of endothelial mechanotransduction is dominant, in which deformation at the cell surface induced by shear stress is transmitted as cytoskeletal tension changes to sites that are mechanically coupled to the cytoskeleton. A single shear stress mechanotransducer is unlikely to exist; rather, mechanotransduction occurs at multiple subcellular locations. PMID:19029993

Davies, Peter F

2010-01-01

84

Wall shear stress in intracranial aneurysms and adjacent arteries?  

PubMed Central

Hemodynamic parameters play an important role in aneurysm formation and growth. However, it is difficult to directly observe a rapidly growing de novo aneurysm in a patient. To investigate possible associations between hemodynamic parameters and the formation and growth of intracranial aneurysms, the present study constructed a computational model of a case with an internal carotid artery aneurysm and an anterior communicating artery aneurysm, based on the CT angiography findings of a patient. To simulate the formation of the anterior communicating artery aneurysm and the growth of the internal carotid artery aneurysm, we then constructed a model that virtually removed the anterior communicating artery aneurysm, and a further two models that also progressively decreased the size of the internal carotid artery aneurysm. Computational simulations of the fluid dynamics of the four models were performed under pulsatile flow conditions, and wall shear stress was compared among the different models. In the three aneurysm growth models, increasing size of the aneurysm was associated with an increased area of low wall shear stress, a significant decrease in wall shear stress at the dome of the aneurysm, and a significant change in the wall shear stress of the parent artery. The wall shear stress of the anterior communicating artery remained low, and was significantly lower than the wall shear stress at the bifurcation of the internal carotid artery or the bifurcation of the middle cerebral artery. After formation of the anterior communicating artery aneurysm, the wall shear stress at the dome of the internal carotid artery aneurysm increased significantly, and the wall shear stress in the upstream arteries also changed significantly. These findings indicate that low wall shear stress may be associated with the initiation and growth of aneurysms, and that aneurysm formation and growth may influence hemodynamic parameters in the local and adjacent arteries.

Wang, Fuyu; Xu, Bainan; Sun, Zhenghui; Wu, Chen; Zhang, Xiaojun

2013-01-01

85

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

NASA Astrophysics Data System (ADS)

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.

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

86

Influence of shear and deviatoric stress on the evolution of permeability in fractured rock  

NASA Astrophysics Data System (ADS)

The evolution of permeability in fractured rock as a function of effective normal stress, shear displacement, and damage remains a complex issue. In this contribution, we report on experiments in which rock surfaces were subject to direct shear under controlled pore pressure and true triaxial stress conditions while permeability was monitored continuously via flow parallel to the shear direction. Shear tests were performed in a pressure vessel under drained conditions on samples of novaculite (Arkansas) and diorite (Coso geothermal field, California). The sample pairs were sheared to 18 mm of total displacement at 5 ?m/s at room temperature and at effective normal stresses on the shear plane ranging from 5 to 20 MPa. Permeability evolution was measured throughout shearing via flow of distilled water from an upstream reservoir discharging downstream of the sample at atmospheric pressure. For diorite and novaculite, initial (preshear) fracture permeability is 0.5-1 × 10-14 m2 and largely independent of the applied effective normal stresses. These permeabilities correspond to equivalent hydraulic apertures of 15-20 ?m. Because of the progressive formation of gouge during shear, the postshear permeability of the diorite fracture drops to a final steady value of 0.5 × 10-17 m2. The behavior is similar in novaculite but the final permeability of 0.5 × 10-16 m2 is obtained only at an effective normal stress of 20 MPa.

Faoro, Igor; Niemeijer, Andre; Marone, Chris; Elsworth, Derek

2009-01-01

87

Production of functional proteins: balance of shear stress and gravity  

NASA Technical Reports Server (NTRS)

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.

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

2007-01-01

88

Production of functional proteins: balance of shear stress and gravity  

NASA Technical Reports Server (NTRS)

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.

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

2011-01-01

89

Shear-stress sensitive lenticular vesicles for targeted drug delivery.  

PubMed

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

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

90

Dynamic Calibration Technique for the Micro-Pillar Shear-Stress Sensor MPS^3  

Microsoft Academic Search

Based on magnetic excitation a dynamic calibration technique for the micro-pillar shear-stress sensor MPS^3, which allows to determine the local wall-shear stress in turbulent flows by optically measuring the velocity gradient within the viscous sublayer of turbulent flows, is described. The proposed dynamic calibration technique allows to assess the micro-pillar dynamic response for different flow media up to approximately 10kHz.

Wolfgang Schröder; Sebastian Große; Thomas Soodt

2008-01-01

91

Measurement of shear impedances of viscoelastic fluids  

SciTech Connect

Shear-wave reflection coefficients from a solid/fluid interface are derived for non-Newtonian fluids that can be described by Maxwell, Voigt, and power-law fluid models. Based on model calculations, we have identified the measurable effects on the reflection coefficients due to fluid non-Newtonian behavior. The models are used to interpret the viscosity data obtained by a technique based on shear impedance measurement.

Sheen, Shuh-Haw; Chien, Hual-Te; Raptis, A.C.

1996-12-31

92

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

NASA Astrophysics Data System (ADS)

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.

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

2011-12-01

93

IC-integrated flexible shear-stress sensor skin  

Microsoft Academic Search

This paper reports the successful development of the first IC-integrated flexible MEMS shear-stress sensor skin. The sensor skin is 1 cm wide, 2 cm long, and 70 ?m thick. It contains 16 shear-stress sensors, which are arranged in a 1-D array, with on-skin sensor bias, signal-conditioning, and multiplexing circuitry. We further demonstrated the application of the sensor skin by packaging

Yong Xu; Yu-Chong Tai; Adam Huang; Chih-Ming Ho

2003-01-01

94

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

NASA Technical Reports Server (NTRS)

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

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

2008-01-01

95

Shear induced drainage in foamy yield-stress fluids  

E-print Network

Shear induced drainage of a foamy yield stress fluid is investigated using MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes the system at rest, a fast drainage is observed when a horizontal shear is imposed. It is shown that the sheared interstitial material behaves as a viscous fluid in the direction of gravity, the effective viscosity of which is controlled by shear in transient foam films between bubbles. Results provided for several bubble sizes are not captured by the R^2 scaling classically observed for liquid flow in particulate systems, such as foams and thus constitute a remarkable demonstration of the strong coupling of drainage flow and shear induced interstitial flow. Furthermore, foam films are found to be responsible for the unexpected arrest of drainage, thus trapping irreversibly a significant amount of interstitial liquid.

Julie Goyon; François Bertrand; Olivier Pitois; Guillaume Ovarlez

2010-03-30

96

The Micro-Pillar Shear-Stress Sensor MPS3 for Turbulent Flow  

PubMed Central

Wall-shear stress results from the relative motion of a fluid over a body surface as a consequence of the no-slip condition of the fluid in the vicinity of the wall. To determine the two-dimensional wall-shear stress distribution is of utter importance in theoretical and applied turbulence research. In this article, characteristics of the Micro-Pillar Shear-Stress Sensor MPS3, which has been shown to offer the potential to measure the two-directional dynamic wall-shear stress distribution in turbulent flows, will be summarized. After a brief general description of the sensor concept, material characteristics, possible sensor-structure related error sources, various sensitivity and distinct sensor performance aspects will be addressed. Especially, pressure-sensitivity related aspects will be discussed. This discussion will serve as ‘design rules’ for possible new fields of applications of the sensor technology. PMID:22574010

Grosse, Sebastian; Schroder, Wolfgang

2009-01-01

97

Shear stress at the base of shield lithosphere  

NASA Astrophysics Data System (ADS)

One of the basic unresolved issues regarding plate tectonics is the degree of coupling between surface plates and convecting mantle below. Are the plates effectively decoupled from the mantle flow field by a low viscosity asthenosphere, or are they strongly coupled to mantle flow? While these two views are essentially incompatible, they both do a reasonably good job of predicting the motions of the surface plates, and cannot therefore be distinguished on this basis. The significant distinguishing feature for these models is the magnitude of basal shear stress that is applied to the base of the plate. While it is difficult to measure this stress directly, it is possible, in principle, to measure the corresponding deformation of the plate through observations of seismic anisotropy and to infer stress. Here we focus on the Canadian Shield, for which we expect strong plate-mantle interaction. We show that seismic anisotropy can be used to constrain the magnitude of the stress level applied to the base of the plate, and to document the level of interaction between tectonic plates and the mantle below.

Bokelmann, Götz H. R.; Silver, Paul G.

2002-12-01

98

Bounds on Surface Stress-Driven Shear Flow  

NASA Astrophysics Data System (ADS)

The background method is adapted to derive rigorous limits on surface speeds and bulk energy dissipation for shear stress-driven flow in two- and three-dimensional channels. By-products of the analysis are nonlinear energy stability results for plane Couette flow with a shear stress boundary condition: when the applied stress is gauged by a dimensionless Grashoff number , the critical for energy stability is 139.5 in two dimensions, and 51.73 in three dimensions. We derive upper bounds on the friction (a.k.a. dissipation) coefficient , where ? is the applied shear stress and is the mean velocity of the fluid at the surface, for flows at higher including developed turbulence: C f ?1/32 in two dimensions and C f ?1/8 in three dimensions. This analysis rigorously justifies previously computed numerical estimates.

Hagstrom, George I.; Doering, Charles R.

2014-02-01

99

Gyrokinetic Simulation of Residual Stress from Diamagnetic Velocity Shears  

NASA Astrophysics Data System (ADS)

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

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

2010-11-01

100

Some constraints on levels of shear stress in the crust from observations and theroc  

SciTech Connect

In situ stress determinations in North America, southern Africa, and Australia indicate that on the average the maximum shear stress increases lineary with depth to at least 5.1 km measured in soft rock, such as shale and sandstone, and to 3.7 km in hard rock, including granite and quartzite. Regression lines fitted to the data yield gradients of 3.8 MPa/km and 6.6 MPa/km for soft and hard rock, respectively. Generally, the maximum shear stress in compressional states of stress for which the least principal stress is oriented near vertically is substantially greater than in extensional stress regimes, with the greatest principal stress in a vertical direction. The equations of equilibrium and compatibility can be used to provide functional constraints on the state of stress. If the stress is assumed to vary only with depth z in a given region, then all nonzero components must have the form A+Bz, where A and B are constants which generally differ for various components. This implies that the deviatoric stress changes linearly with depth, and the general solution also allows the directions of the horizontal principal stresses to change monotonically with depth. Solutions to the equations, assuming stress to vary with both z and x, were fit to the observations of Zoback and Roller, who measured stress along a horizontal profile near Palmdale, California. The results indicate that the average shear stress in the upper 8 km of the fault zone is about 3.5 MPa less than the shear stress in the far field, but this far field term, which is part of the solution and has the form A+Bz, cannot be evaluated using the existing constant depth data.

McGarr, A.

1980-11-10

101

Measured kinematic fields in the biaxial shear of granular materials  

Microsoft Academic Search

Biaxial experiments are performed with rod assemblies to study the micro-mechanical deformation behavior of granular materials. The focus of these experiments is upon the micromechanical behavior under mixed boundary conditions, with stress-controlled lateral boundaries and displacement-controlled axial boundaries. Particle motions, i.e. displacements and rotations, are measured during the test. The particle motions are analyzed to study deformation patterns, including shear

A. Misra; H. Jiang

1997-01-01

102

Wind shear measuring on board an airliner  

NASA Technical Reports Server (NTRS)

A measurement technique which continuously determines the wind vector on board an airliner during takeoff and landing is introduced. Its implementation is intended to deliver sufficient statistical background concerning low frequency wind changes in the atmospheric boundary layer and extended knowledge about deterministic wind shear modeling. The wind measurement scheme is described and the adaptation of apparatus onboard an A300 airbus is shown. Preliminary measurements made during level flight demonstrate the validity of the method.

Krauspe, P.

1984-01-01

103

Exploring the Role of Shear Stress and Severe Turbulence in Downstream Fish Passage  

SciTech Connect

Fish may be exposed to damaging levels of fluid shear stress and turbulence while passing through hydroelectric power plants. The generally assumed locations for such potential damage are the turbine and draft tube passages, although it is possible that fish are also injured during passage over the spillway or through sluiceways and fish bypass outfalls. Unless mitigated, fluid-induced injuries and mortality could frustrate efforts to develop advanced, fish-friendly turbines or to provide safe alternate downstream passages. The effects of shear stress and turbulence on fish are poorly understood, in part because of the difficulties in conceptualizing these phenomena, determining their magnitudes and distribution within hydroelectric systems, and then recreating them in a controlled laboratory environment. We define the fluid phenomena that are relevant to the assessment of effects on fish. The magnitudes of fluid stresses associated with man-altered aquatic environments are often considerably higher than those found in natural environments (e.g., normal river flows). However, levels of shear stresses that occur during flash floods appear to be comparable to those expected within a turbine. Past studies of the effects of shear stress on fish are of limited value, mainly because of their narrow scope and lack of instrumentation to measure velocities on appropriately small scales. A laboratory experiment to study the effects of shear stress and turbulence on fish is described.

Cada, G.; Carlson, T.; Ferguson, J.; Richmond, M.; Sale, M.

1999-07-06

104

An evaluation of the Iosipescu specimen for composite materials shear property measurement  

NASA Technical Reports Server (NTRS)

A detailed evaluation of the suitability of the Iosipescu specimen tested in the modified Wyoming fixture is presented. A linear finite element model of the specimen is used to assess the uniformity of the shear stress field in the vicinity of the notch, and demonstrate the effect of the nonuniform stress field upon strain gage measurements used for the determination of composite shear moduli. Based upon test results from graphite-epoxy laminates, the proximity of the load introduction point to the test section and the material orthotropy greatly influence the individual gage readings, however, shear modulus determination is not significantly affected by the lack of pure shear. Correction factors are needed to allow for the nonuniformity of the strain field and the use of the average shear stress in the shear modulus evaluation. The correction factors are determined for the region occupied by the strain gage rosette. A comparison of the strain gage readings from one surface of a specimen with corresponding data from moire interferometry on the opposite face documented an extreme sensitivity of some fiber orientations to eccentric loading which induced twisting and spurious shear stress-strain curves. The discovery of specimen twisting explains the apparently inconsistent shear property data found in the literature. Recommendations for improving the reliability and accuracy of the shear modulus values are made, and the implications for shear strength measurement discussed.

Morton, J.; Ho, H.; Tsai, M. Y.; Farley, G. L.

1992-01-01

105

Shear modulus of porcine coronary artery in reference to a new strain measure.  

PubMed

To simplify the stress-strain relationship of blood vessels, we define a logarithmic-exponential (log-exp) strain measure to absorb the nonlinearity. As a result, the constitutive relation between the second Piola-Kirchhoff stress and the log-exp strain can be written as a generalized Hooke's law. In this work, the shear modulus of porcine coronary arteries is determined from the experimental data in inflation-stretch-torsion tests. It is found that the shear modulus with respect to the log-exp strain can be viewed as a material constant in the full range of elasticity, and the incremental shear modulus for Cauchy shear stress and small shear strain at various loading levels can be predicted by the proposed Hooke's law. This result further validates the linear constitutive relation for blood vessels when shear deformation is involved. PMID:17669488

Zhang, Wei; Lu, Xiao; Kassab, Ghassan S

2007-11-01

106

The Strength of the Earth's Crust under Horizontal Shear Stress  

Microsoft Academic Search

Recent theoretical expressions for the change in stress distribution caused by strike-slip faulting are applied to five real faults, and estimates are made of the maximum shear stress relieved in each case. The calculated values lie between 107 and 10 s dynes\\/cm , and reasonable refinements of the assumptions involved in the cMculation (particularly in the value of the coefficient

M. A. Chinnery

1964-01-01

107

Production of Functional Proteins: Balance of Shear Stress and Gravity  

NASA Technical Reports Server (NTRS)

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

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

2005-01-01

108

Dynamic response of wall shear stress on the stenosed artery  

Microsoft Academic Search

The present study deals with an appropriate mathematical model of an artery in the presence of constriction in which the generated wall shear stress due to blood flow is analysed. The geometry of the stenosed arterial segment in the diseased state, causing malfunction of the cardiovascular system, is formed mathematically. The flowing blood contained in the stenosed artery is treated

S. Sen; S. Chakravarty

2009-01-01

109

Flow resistance and critical shear stress in steep channels  

Microsoft Academic Search

The flow resistance and the critical shear stress in steep channels are investigated, using a velocity distribution law as elaborated by Tu, Tsujimoto et Graf. The friction factor can be predicted with the flow resistance relations together with numerical constants of integration. Two laboratory data and two field data sets are used to examine the validity. The experimental data from

B. Kironoto; W. H. Graf

1988-01-01

110

Shear stress-induced improvement of red blood cell deformability.  

PubMed

Classically, it is known that red blood cell (RBC) deformability is determined by the geometric and material properties of these cells. Experimental evidence accumulated during the last decade has introduced the concept of active regulation of RBC deformability. This regulation is mainly related to altered associations between membrane skeletal proteins and integral proteins, with the latter serving to anchor the skeleton to the lipid matrix. It has been hypothesized that shear stress induces alterations of RBC deformability: the current study investigated the dynamics of the transient improvement in deformability induced by shear stress at physiologically-relevant levels. RBC were exposed to various levels of shear stress (SS) in a Couette type shearing system that is part of an ektacytometer, thus permitting the changes in RBC deformability during the application of SS to be monitored. Initial studies showed that there is an increase in deformability of the RBC subjected to SS in the range of 5-20 Pa, with kinetics characterized by time constants of a few seconds. Such improvement in deformability, expressed by an elongation index (EI), was faster with higher levels of SS and hence yielded shorter time constants: absolute values of EI increased by 3-8% of the starting level. Upon the removal of the shear stress, this response by RBC was reversible with a slower time course compared to the increase in EI during application of SS. Increased calcium concentration in the RBC suspending medium prevented the improvement of deformability. It is suggested that the improvement of RBC deformability by shear forces may have significant effects on blood flow dynamics, at least in tissues supplied by blood vessels with impaired vasomotor reserve, and may therefore serve as a compensating mechanism for the maintenance of adequate microcirculatory perfusion. PMID:23863281

Meram, Ece; Yilmaz, Bahar D; Bas, Ceren; Atac, Nazl?; Yalcin, O; Meiselman, Herbert J; Baskurt, Oguz K

2013-01-01

111

Reynolds stress and effects of external and self-generated shear flows S. B. Korsholm. P. K. Michelsen, V. Naulin, and J. Juul Rasmussen  

E-print Network

Reynolds stress and effects of external and self-generated shear flows S. B. Korsholm. P. K interest. As presented analytically in [1] the so-called Reynolds stress gives a measure of the self. A measurement of the Reynolds stress can thus help to predict flows, e.g. shear flows in plasmas as demonstrated

112

Global shear stress pattern in main seismic active belts  

NASA Astrophysics Data System (ADS)

Based on the relationship between body wave magnitued m b and seismic moment M 0 presented by Pei-Shan CHEN (1981), by using the moment tensor and focal mechanism solution in the earthquake reports of EDR, a global shear stress pattern including shear stress values and directions of P and T principal axis was obatained. The distribution of ambient shear stress values in the globe is: the highest in intraplate followed by subduction zone, and the lowest in oceanic ridge. The horizontal directions of the maximum principal stresses we got are coincident very well with the result of Zoback (1989). The detail analysis of the stress state in Tonga region shows that: The subduction slab bends down in shallow by press and bends up in deep because of the resistance from deeper part. Between them, the slab is in an equilibrum state. After analysizing the global stress distribution, we got the result that: the plate is driven by a drag force from under its bottom, the plate motion results in its extensional state in oceanic ridge and compressive state in subduction zone.

Xiao, Lei; Chen, Pei-Shan

1997-03-01

113

Direct measurement of shear properties of microfibers.  

PubMed

As novel fibers with enhanced mechanical properties continue to be synthesized and developed, the ability to easily and accurately characterize these materials becomes increasingly important. Here we present a design for an inexpensive tabletop instrument to measure shear modulus (G) and other longitudinal shear properties of a micrometer-sized monofilament fiber sample, such as nonlinearities and hysteresis. This automated system applies twist to the sample and measures the resulting torque using a sensitive optical detector that tracks a torsion reference. The accuracy of the instrument was verified by measuring G for high purity copper and tungsten fibers, for which G is well known. Two industrially important fibers, IM7 carbon fiber and Kevlar(®) 119, were also characterized with this system and were found to have G = 16.5 ± 2.1 and 2.42 ± 0.32 GPa, respectively. PMID:25273783

Behlow, H; Saini, D; Oliveira, L; Durham, L; Simpson, J; Serkiz, S M; Skove, M J; Rao, A M

2014-09-01

114

Direct measurement of shear properties of microfibers  

NASA Astrophysics Data System (ADS)

As novel fibers with enhanced mechanical properties continue to be synthesized and developed, the ability to easily and accurately characterize these materials becomes increasingly important. Here we present a design for an inexpensive tabletop instrument to measure shear modulus (G) and other longitudinal shear properties of a micrometer-sized monofilament fiber sample, such as nonlinearities and hysteresis. This automated system applies twist to the sample and measures the resulting torque using a sensitive optical detector that tracks a torsion reference. The accuracy of the instrument was verified by measuring G for high purity copper and tungsten fibers, for which G is well known. Two industrially important fibers, IM7 carbon fiber and Kevlar® 119, were also characterized with this system and were found to have G = 16.5 ± 2.1 and 2.42 ± 0.32 GPa, respectively.

Behlow, H.; Saini, D.; Oliveira, L.; Durham, L.; Simpson, J.; Serkiz, S. M.; Skove, M. J.; Rao, A. M.

2014-09-01

115

The Need for a Shear Stress Calibration Standard  

NASA Technical Reports Server (NTRS)

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.

Scott, Michael A.

2004-01-01

116

Relationship between Concentration Difference of Different Density Lipoproteins and Shear Stress in Atherosclerosis  

PubMed Central

Previous research has observed concentration polarization in LDL and HDL in the arterial system. However, there is no report that links this concentration polarization to the development of vascular atherosclerosis (AS). Therefore, the purpose of this study is to establish the relationship between concentration difference of LDL and HDL and shear stress using a carotid bifurcation vascular model. PTFE was employed to create the carotid bifurcation model. Endothelial cells were coated on the inner wall of the graft. In a recirculation system, HDL and LDL concentration were measured under two different ICA flow velocities at 5 different locations within our model. We report the following: (1) LDL and HDL concentration difference was observed in both high flow and low flow environments; (2) the degree of LDL and HDL concentration polarization varied depending of high flow and low flow environment; (3) absolute values of concentration difference between LDL and HDL at the inner wall surface decreased with the increase in shear stress when shear stress was more than 1.5?Pa. This variation trend would be more pronounced if shear stress were less than 0.5?Pa. Our study suggests that under the action of shear stress, concentration differences of LDL or HDL create a disturbance in the balance of atherogenic factors and anti-As factors, resulting in the occurrence of AS. PMID:22481972

Meng, Wei; Yu, Fengxu; Chen, Huaiqing; Zhang, Jianmin; Zhang, Eryong; Guo, Yingqiang; Shi, Yingkang

2012-01-01

117

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

118

Flow separation and shear stress over angle-of-repose bed forms: A numerical investigation  

NASA Astrophysics Data System (ADS)

asymmetric bed forms commonly develop in rivers. The turbulence associated with flow separation that develops over their steep lee side is responsible for the form shear stress which can represent a substantial part of total shear stress in rivers. This paper uses the Delft3D modeling system to investigate the effects of bed form geometry and forcing conditions on flow separation length and associated turbulence, and bed form shear stress over angle-of-repose (30° lee side angle) bed forms. The model was validated with lab measurements that showed sufficient agreement to be used for a systematic analysis. The influence of flow velocity, bed roughness, relative height (bed form height/water depth), and aspect ratio (bed form height/length) on the variations of the normalized length of the flow separation zone, the extent of the wake region (where the turbulent kinetic energy (TKE) was more than 70% of the maximum TKE), the average TKE within the wake region and the form shear stress were investigated. Form shear stress was found not to scale with the size of the flow separation zone but to be related to the product of the normalized extent of the wake region (extent of the wake region/extent of water body above the bed form) and the average TKE within the wake region. The results add to understanding of the hydrodynamics of bed forms and may be used for the development of better parameterizations of small-scale processes for application in large-scale studies.

Lefebvre, Alice; Paarlberg, Andries J.; Winter, Christian

2014-02-01

119

The Origin of Persistent Shear Stress in Supercooled Liquids  

E-print Network

We show that the long time tail of the shear stress autocorrelation, whose growth at large supercooling is responsible for the apparent divergence of the shear viscosity, is a direct result of a residual shear stress in the structures associated with the local potential minima. We argue that the essential mechanical transition experienced by a liquid on cooling occurs at a temperature well above the glass transition temperature and corresponds to the crossover from the high temperature liquid to the viscous liquid, the latter characterised by stress relaxation dominated by the residual stress. Following on from this observation, as the density is decreased, the local potential minima become unable to sustain any persistent stress (and, hence, support a glass transition), in a manner that can be explicitly connected to the interactions between atoms. The reported crossover implies an associated change in the mechanism of dissipation in liquids and, hence, raises the prospect of a coherent microscopic treatment of nonlinear rheology and the relationship between self diffusion and viscosity in supercooled liquids.

Sneha Abraham; Peter Harrowell

2012-01-06

120

The Role of Shear Failure on Stress Characterization  

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

121

Shear stress prediction in shock loaded copper  

SciTech Connect

The stress-strain behavior during the shock rise of a 30 kbar and 54 kbar shock in copper is modeled using a plastic constitutive model that includes rate and temperature dependent hardening and accounts for the transition from thermally activated to viscous drag controlled deformation at high strain rates. A slight modification to the treatment of the mobile dislocation density within the model from that originally proposed leads to better agreement with the shock data than achieved previously. The results indicate that the deformation mechanism during the shock rise is a drag mechanism. 9 refs., 4 figs.

Follansbee, P.S.

1991-01-01

122

The mechanism for shear thickening in suspensions  

Microsoft Academic Search

Densely packed suspensions can shear thicken, in which the viscosity increases with shear rate. Video microscopy along with rheology measurements show the shear thickening regime is a transition from negligible particle motion at low stresses to fully developed shear flow at higher stresses. The onset of shear thickening occurs when the shear stress is sufficient to pull particles apart; for

Eric Brown; Heinrich Jaeger

2009-01-01

123

Combined effects of flow-induced shear stress and micropatterned surface morphology on neuronal differentiation of human mesenchymal stem cells.  

PubMed

This study investigated the combined effects of surface morphology and flow-induced shear stress on the neuronal differentiation of human mesenchymal stem cells. First, to examine the effect of surface morphology, three patterns were fabricated using photolithography and compared to the flat substrate. After selecting the most effective surface pattern, flow-induced shear stresses (0.10 and 0.25 Pa) were engaged parallel to the direction of the grooves. The degrees of alignment and neurite outgrowth were measured using digital image processing techniques for up to 10 days. Functional evaluations were also performed by monitoring the intracellular calcium concentration and the expression of synaptophysin, ?-tubulin III, and MAP2. Based on these analyses, the pattern of 5 ?m/5 ?m/3 ?m for groove/ridge/depth, respectively, was selected. Next, shear stresses (0.00, 0.10, 0.25 Pa) were applied to the cells on the selected substrate. The shear stresses affected the expression of those markers. The outgrowth measurements indicated that the shear stresses were effective at day 7. However, the effect of shear stresses tended to decrease at day 10. More cells showed higher calcium concentrations under 0.10 Pa. The alignment was also confirmed. Taken together, these results indicated that a shear stress of 0.10 Pa on the substrate of 5 ?m was most effective. Therefore, such combination of mechanical stimuli and surface pattern is expected to promote neuronal differentiation with regard to functional and morphological changes. PMID:23993713

Jeon, Kang Jin; Park, So Hee; Shin, Ji Won; Kang, Yun Gyeong; Hyun, Jin-Sook; Oh, Min Jae; Kim, Seon Yeon; Shin, Jung-Woog

2014-02-01

124

Effects of Oxygen Tension and Shear Stress on Human Endothelial Cell Prostacyclin Production  

Microsoft Academic Search

Underin vivoconditions of ischemia and reperfusion, vascular endothelium (EC) experience concurrent changes in oxygen tension, shear stress, and the local concentration of metabolites. These studies explored the combined effects of shear stress and oxygen tension on EC prostacyclin production. EC grown on microcarrier beads were exposed to 120 min of normoxia and basal shear stress by stirring at 20 rpm.

Hiram M. Soler; Michael T. Watkins; Hassan Albadawi; Hiroko Kadowaki; George M. Patton

1997-01-01

125

Shear Stress in Smooth Rectangular Open-Channel Flows and Pierre Y. Julien2  

E-print Network

Shear Stress in Smooth Rectangular Open-Channel Flows Junke Guo1 and Pierre Y. Julien2 Abstract: The average bed and sidewall shear stresses in smooth rectangular open-channel flows are determined after solving the continuity and momentum equations. The analysis shows that the shear stresses are function

Julien, Pierre Y.

126

Use of Axisymmetric Shearing as Technological Test Method to gather Flow Stress Data for Metals  

NASA Astrophysics Data System (ADS)

Cutting by shearing creates heavy shear deformation in a layer extending between the two applied shearing edges. Prediction of FEM-simulation is that effective strain and strain rates in the shear zone would reach very high levels even at mode rate shearing velocity. In this article experiments coupled with FEM-analysis are used to evaluate the potential of using a xisymmetric shearing for collecting flow stress data for metal forming purposes. It is shown that if accurate flow stress data are to be collected this way it is important to know how shearing occurs inside the shear zone.

Kandis, Janis; Valberg, Henry; Wenbin, Wu

2011-01-01

127

Insitu Rock Shear Device (Development of a New Insitu Shear Strength Measuring Device).  

National Technical Information Service (NTIS)

A new device has been designed to measure the insitu rock shear strength in cored holes. Limited field data indicate that shear strengths obtained by the Rock Shear Device are in good agreement with McVay's laboratory square root (sup 2) method.

C. Hay, D. Bloomquist, M. McVay

2008-01-01

128

Critical shear stress for erosion of cohesive soils subjected to temperatures typical of wildfires  

USGS Publications Warehouse

[1] Increased erosion is a well-known response after wildfire. To predict and to model erosion on a landscape scale requires knowledge of the critical shear stress for the initiation of motion of soil particles. As this soil property is temperature-dependent, a quantitative relation between critical shear stress and the temperatures to which the soils have been subjected during a wildfire is required. In this study the critical shear stress was measured in a recirculating flume using samples of forest soil exposed to different temperatures (40??-550??C) for 1 hour. Results were obtained for four replicates of soils derived from three different types of parent material (granitic bedrock, sandstone, and volcanic tuffs). In general, the relation between critical shear stress and temperature can be separated into three different temperature ranges (275??C), which are similar to those for water repellency and temperature. The critical shear stress was most variable (1.0-2.0 N m-2) for temperatures 2.0 N m-2) between 175?? and 275??C, and was essentially constant (0.5-0.8 N m-2) for temperatures >275??C. The changes in critical shear stress with temperature were found to be essentially independent of soil type and suggest that erosion processes in burned watersheds can be modeled more simply than erosion processes in unburned watersheds. Wildfire reduces the spatial variability of soil erodibility associated with unburned watersheds by eliminating the complex effects of vegetation in protecting soils and by reducing the range of cohesion associated with different types of unburned soils. Our results indicate that modeling the erosional response after a wildfire depends primarily on determining the spatial distribution of the maximum soil temperatures that were reached during the wildfire. Copyright 2005 by the American Geophysical Union.

Moody, J. A.; Dungan, Smith, J.; Ragan, B. W.

2005-01-01

129

Critical shear stress for erosion of cohesive soils subjected to temperatures typical of wildfires  

NASA Astrophysics Data System (ADS)

Increased erosion is a well-known response after wildfire. To predict and to model erosion on a landscape scale requires knowledge of the critical shear stress for the initiation of motion of soil particles. As this soil property is temperature-dependent, a quantitative relation between critical shear stress and the temperatures to which the soils have been subjected during a wildfire is required. In this study the critical shear stress was measured in a recirculating flume using samples of forest soil exposed to different temperatures (40°-550°C) for 1 hour. Results were obtained for four replicates of soils derived from three different types of parent material (granitic bedrock, sandstone, and volcanic tuffs). In general, the relation between critical shear stress and temperature can be separated into three different temperature ranges (<175°C 175°C-275°C; >275°C), which are similar to those for water repellency and temperature. The critical shear stress was most variable (1.0-2.0 N m-2) for temperatures <175°C, was a maximum (>2.0 N m-2) between 175° and 275°C, and was essentially constant (0.5-0.8 N m-2) for temperatures >275°C. The changes in critical shear stress with temperature were found to be essentially independent of soil type and suggest that erosion processes in burned watersheds can be modeled more simply than erosion processes in unburned watersheds. Wildfire reduces the spatial variability of soil erodibility associated with unburned watersheds by eliminating the complex effects of vegetation in protecting soils and by reducing the range of cohesion associated with different types of unburned soils. Our results indicate that modeling the erosional response after a wildfire depends primarily on determining the spatial distribution of the maximum soil temperatures that were reached during the wildfire.

Moody, John A.; Smith, J. Dungan; Ragan, B. W.

2005-03-01

130

Surface Shear Stress Around a Single Flexible Live Plant and a Rigid Cylinder  

NASA Astrophysics Data System (ADS)

The sheltering effect of vegetation against soil erosion and snow transport has direct implications on land degradation and local water storage as snow in many arid and semi arid regions. Plants influence the erosion, transport and redeposition of soil and snow by the wind through momentum absorption, local stress concentration, trapping particles in motion and reducing the area of sediment exposed to the wind. The shear stress distributions on the ground beneath plant canopies determine the onset and magnitude of differential soil and snow erosion on rough or vegetated surfaces, but this has been studied exclusively with artificial and rigid vegetation elements thus far. Real plants have highly irregular structures that can be extremely flexible and porous. They align with the flow at higher wind speeds, resulting in considerable changes to the drag and flow regimes relative to rigid imitations of comparable size. We present measurements in the SLF atmospheric boundary layer wind tunnel of the surface shear stress distribution around a live grass plant (Lolium Perenne) and a solid cylinder of comparable size. Irwin sensors are used to measure pressure differences close to the surface which can be calibrated with surface shear stress velocities. The basal to frontal area index of the plant and the cylinder as well as the Reynolds number of the two experimental setups have been checked for similarity and show good agreement. Distinctive differences between the shear stress pattern around the plant and the cylinder can be attributed to the influence of the plant’s porosity and flexibility. The sheltered zone behind the plant is narrower in cross-stream and longer in streamwise direction than that of the cylinder. For the plant, the lowest shear stresses in the sheltered zone are 50% lower than the mean surface shear stress (? = 0.15 N/m2) in the undisturbed flow. The sheltering was higher behind the cylinder with values reduced by 70% relative to background. “Speed-up” zones on both sides of the roughness elements experienced peak shear stress values 60% above background for the plant and almost 130% higher for the cylinder. While the integral sheltering effect of the plant is smaller in size and magnitude than that of the cylinder, the peak shear stresses in the lateral speed up zones are significantly lower. Since the onset of soil erosion occurs when a critical threshold shear stress is experienced, the lower peak shear stress means that plants provide better protection against soil erosion than rigid elements. This result suggests that parameterizations of flow over vegetated surfaces based on measurements of rigid elements may be incorrect. Further work will investigate sheltering and shear stress concentrations as a function of cylinder / plant density using real canopies instead of single objects.

Walter, B. A.; Gromke, C.; Leonard, K. C.; Clifton, A.; Lehning, M.

2010-12-01

131

Shear Deformation And Thickness Stress In Corner Fill  

NASA Astrophysics Data System (ADS)

Corner fill is a simple benchmark intended to gain knowledge of tube hydroforming and to evaluate hydroforming parameters. In corner fill, an originally round tube is put into a cylindrical die with square cross-sections. Under applied internal pressure, the tube expands in and fills the corners of the die. Corner fill has been studied by using finite element method. The analyses, however, failed to correctly predict the burst of the tube. In this paper, a two-dimensional implicit-static plane strain finite element model is used to study the important mechanical parameters of deformation, including shear deformation and thickness stress, of the tube. The purpose of this paper is to try to explain why the existing failure criteria, including the forming limit diagram and plastic strain criterion, cannot correctly predict burst failures in corner fill. In particular, quantitative values were obtained for shear deformation and thickness stress in a corner filled tube.

Chen, Kuo-Kuang

2005-08-01

132

Non-volcanic tremor driven by large transient shear stresses.  

PubMed

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

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

2007-08-01

133

ENaC regulation by proteases and shear stress  

PubMed Central

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

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

2013-01-01

134

Shear-Stress Partitioning in Live Plant Canopies and Modifications to Raupach's Model  

NASA Astrophysics Data System (ADS)

The spatial peak surface shear stress {tau _S^'' on the ground beneath vegetation canopies is responsible for the onset of particle entrainment and its precise and accurate prediction is essential when modelling soil, snow or sand erosion. This study investigates shear-stress partitioning, i.e. the fraction of the total fluid stress on the entire canopy that acts directly on the surface, for live vegetation canopies (plant species: Lolium perenne) using measurements in a controlled wind-tunnel environment. Rigid, non-porous wooden blocks instead of the plants were additionally tested for the purpose of comparison since previous wind-tunnel studies used exclusively artificial plant imitations for their experiments on shear-stress partitioning. The drag partitioning model presented by Raupach (Boundary-Layer Meteorol 60:375-395, 1992) and Raupach et al. (J Geophys Res 98:3023-3029, 1993), which allows the prediction of the total shear stress ? on the entire canopy as well as the peak {(tau _S ^''/tau )^{1/2}} and the average {(tau _S^'/tau )^{1/2}} shear-stress ratios, is tested against measurements to determine the model parameters and the model's ability to account for shape differences of various roughness elements. It was found that the constant c, needed to determine the total stress ? and which was unspecified to date, can be assumed a value of about c = 0.27. Values for the model parameter m, which accounts for the difference between the spatial surface average {tau _S^' and the peak {tau _S ^'' shear stress, are difficult to determine because m is a function of the roughness density, the wind velocity and the roughness element shape. A new definition for a parameter a is suggested as a substitute for m. This a parameter is found to be more closely universal and solely a function of the roughness element shape. It is able to predict the peak surface shear stress accurately. Finally, a method is presented to determine the new a parameter for different kinds of roughness elements.

Walter, Benjamin; Gromke, Christof; Lehning, Michael

2012-08-01

135

Rheological investigations of ferrofluids with a shear stress controlled rheometer.  

PubMed

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

Shahnazian, Hamid; Odenbach, Stefan

2008-05-21

136

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

PubMed Central

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

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

2012-01-01

137

Stress-structure relation in dense colloidal melt under forward and instantaneous reversal of shear  

E-print Network

Dense supercooled colloidal melt in forward shear from a quiescent state shows overshoot in shear stress at 10% strain with an unchanged fluid structure at equal stress before and after overshoot. In addition, we find overshoot in normal stress with a monotonic increase in osmotic pressure at an identical strain. The first and second normal stress become comparable in magnitude and opposite in sign. Functional dependence of the steady state stress and osmotic pressure with Peclet number demonstrate signature of crossover between Newtonian and nearly- Newtonian regime. Moreover, instantaneous shear reversal from steady state exhibit Bauschinger effect, where strong history dependence is observed depending on the time of flow reversal. The distribution of particulate stress and osmotic pressure at the point of flow reversal is shown to be a signature of the subsequent response. We link the history dependence of the stress-strain curves to changes in the fluid structure measured through the angular components of the radial distribution function. A uniform compression in transition from forward to reversed flowing state is found.

Amit Kumar Bhattacharjee

2014-10-29

138

Pulsatile flow and atherosclerosis in the human carotid bifurcation. Positive correlation between plaque location and low oscillating shear stress.  

PubMed

Fluid velocities were measured by laser Doppler velocimetry under conditions of pulsatile flow in a scale model of the human carotid bifurcation. Flow velocity and wall shear stress at five axial and four circumferential positions were compared with intimal plaque thickness at corresponding locations in carotid bifurcations obtained from cadavers. Velocities and wall shear stresses during diastole were similar to those found previously under steady flow conditions, but these quantities oscillated in both magnitude and direction during the systolic phase. At the inner wall of the internal carotid sinus, in the region of the flow divider, wall shear stress was highest (systole = 41 dynes/cm2, diastole = 10 dynes/cm2, mean = 17 dynes/cm2) and remained unidirectional during systole. Intimal thickening in this location was minimal. At the outer wall of the carotid sinus where intimal plaques were thickest, mean shear stress was low (-0.5 dynes/cm2) but the instantaneous shear stress oscillated between -7 and +4 dynes/cm2. Along the side walls of the sinus, intimal plaque thickness was greater than in the region of the flow divider and circumferential oscillations of shear stress were prominent. With all 20 axial and circumferential measurement locations considered, strong correlations were found between intimal thickness and the reciprocal of maximum shear stress (r = 0.90, p less than 0.0005) or the reciprocal of mean shear stress (r = 0.82, p less than 0.001). An index which takes into account oscillations of wall shear also correlated strongly with intimal thickness (r = 0.82, p less than 0.001). When only the inner wall and outer wall positions were taken into account, correlations of lesion thickness with the inverse of maximum wall shear and mean wall shear were 0.94 (p less than 0.001) and 0.95 (p less than 0.001), respectively, and with the oscillatory shear index, 0.93 (p less than 0.001). These studies confirm earlier findings under steady flow conditions that plaques tend to form in areas of low, rather than high, shear stress, but indicate in addition that marked oscillations in the direction of wall shear may enhance atherogenesis. PMID:3994585

Ku, D N; Giddens, D P; Zarins, C K; Glagov, S

1985-01-01

139

Wall shear stress on LDL accumulation in human RCAs.  

PubMed

The blood flow and transportation of molecules in the cardiovascular system plays crucial role in the genesis and progression of atherosclerosis. Atherosclerosis shows predilection in regions of the arterial tree with hemodynamic particularities, as local disturbances of wall shear stress in space, and locally high concentrations of lipoprotein. A semi-permeable nature of the arterial wall computational model is incorporated with hydraulic conductivity and permeability treated as wall shear stress dependent. Six image-based human diseased right coronary arteries (RCA) are used to elucidate the low-density lipoprotein (LDL) transport. The 3D reconstruction technique is a combination of angiography and IVUS. The numerical simulation couples the flow equations with the transport equation applying realistic boundary conditions at the wall. The coupling of fluid dynamics and solute dynamics at the endothelium is achieved by the Kedem-Katchalsky equation (water infiltration). The luminal surface LDL concentration at the arterial wall is flow-dependent with local variations due to geometric features. The relationship between WSS and luminal surface concentration of LDL indicates that LDL is elevated at locations where WSS is low. There is medium correlation (Pearson) between low WSS and high LDL. The degree of elevation in luminal surface LDL concentration is mostly affected by the water infiltration velocity at the vessel wall. Under constant water infiltration the shear dependent endothelial permeability effects, in comparison to those using constant value, are marginal. Area-averaged normalized LDL concentration over the RCAs, using constant water infiltration and endothelial permeability is 3.6% higher than that at the entrance. Area-averaged normalized LDL concentration over the RCAs, using shear dependent water infiltration and endothelial permeability is 9.6%. Perspective computational fluid dynamics users, incorporating mass transfer (LDL) within the blood flow, are forced to treat the problem using shear dependent endothelial values. PMID:20580302

Soulis, Johannes V; Fytanidis, Dimitrios K; Papaioannou, Vassilios C; Giannoglou, George D

2010-10-01

140

Adjusting Flow-Mediated Dilation for Shear Stress Stimulus Allows Demonstration of Endothelial Dysfunction in a Population with Moderate Cardiovascular Risk  

Microsoft Academic Search

Background\\/Aims: Although normalization of brachial artery flow-mediated dilation (FMD) to individual shear stress (FMD:shear stress ratio) has been proposed to improve this measure of endothelial function, the clinical utility of FMD normalization has not yet been demonstrated. We tested (1) whether following conventional 5-min forearm occlusion, the FMD:shear stress ratio would discriminate a population with moderate cardiovascular risk (MR) from

Jaume Padilla; Blair D. Johnson; Sean C. Newcomer; Daniel P. Wilhite; Timothy D. Mickleborough; Alyce D. Fly; Kieren J. Mather; Janet P. Wallace

2009-01-01

141

Identification of complex shear modulus from measured shear strains on a circular disc subjected to a transient torque  

Microsoft Academic Search

A method for identification of complex shear modulus from measured shear strains on a circular disc subjected to a transient torque at its centre has been established. It is based on the evolution of an outgoing shear wave between two radial positions at which the associated shear strains are measured. The two-dimensional shear wave solutions used are exact in the

S. Mousavi; L. Hillström; B. Lundberg

2008-01-01

142

Direct force wall shear measurements in pressure-driven three-dimensional turbulent boundary layers  

NASA Technical Reports Server (NTRS)

Unique, simultaneous direct measurements of the magnitude and direction of the local wall shear stress in a pressure-driven three-dimensional turbulent boundary layer are presented. The flow is also described with an oil streak wall flow pattern, a map of the wall shear stress-wall pressure gradient orientations, a comparison of the wall shear stress directions relative to the directions of the nearest wall velocity as measured with a typical, small boundary layer directionally sensitive claw probe, as well as limiting wall streamline directions from the oil streak patterns, and a comparison of the freestream streamlines and the wall flow streamlines. A review of corrections for direct force sensing shear meters for two-dimensional flows is presented with a brief discussion of their applicability to three-dimensional devices.

Mcallister, J. E.; Tennant, M. H.; Pierce, F. J.

1982-01-01

143

Some constraints on levels of shear stress in the crust from observations and theory.  

USGS Publications Warehouse

In situ stress determinations in North America, southern Africa, and Australia indicate that on the average the maximum shear stress increases linearly with depth to at least 5.1 km measured in soft rock, such as shale and sandstone, and to 3.7 km in hard rock, including granite and quartzite. Regression lines fitted to the data yield gradients of 3.8 MPa/km and 6.6 MPa/km for soft and hard rock, respectively. Generally, the maximum shear stress in compressional states of stress for which the least principal stress is oriented near vertically is substantially greater than in extensional stress regimes, with the greatest principal stress in a vertical direction. The equations of equilibrium and compatibility can be used to provide functional constrains on the state of stress. If the stress is assumed to vary only with depth z in a given region, then all nonzero components must have the form A + Bz, where A and B are constants which generally differ for the various components. - Author

McGarr, A.

1980-01-01

144

SPH SIMULATIONS OF A LOBE PUMP: PREDICTION OF PROTEIN SHEAR STRESS AT DIFFERENT PUMP EFFICIENCIES  

Microsoft Academic Search

Using Smoothed Particle Hydrodynamics (SPH), the motion of a Lobe Pump under load was simulated in order to predict variations in shear stress and pump efficiency with varying gap size (1, 2, 4 mm) between the lobes and pump housing. The simulations indicated that pump shear was dependent on gap size, with shear stress levels (0 - 40 Pa) correlating

Mahesh PRAKASH; Nick STOKES; Joseph BERTOLINI; Owen TATFORD; Peter GOMME

145

The effect of roughness elements on wind erosion: The importance of surface shear stress distribution  

NASA Astrophysics Data System (ADS)

of surface roughness effects on aeolian sediment transport is a key source of uncertainty in wind erosion models. Drag partitioning schemes are used to account for roughness by scaling the soil entrainment threshold by the ratio of shear stress on roughness elements to that on the vegetated land surface. This approach does not explicitly account for the effects of roughness configuration, which may be important for sediment flux. Here we investigate the significance of roughness configuration for aeolian sediment transport, the ability of drag partitioning approaches to represent roughness configuration effects, and the implications for model accuracy. We use wind tunnel measurements of surface shear stress distributions to calculate sediment flux for a suite of roughness configurations, roughness densities, and wind velocities. Roughness configuration has a significant effect on sediment flux, influencing estimates by more than 1 order of magnitude. Measured and modeled drag partitioning approaches overestimate the predicted flux by 2 to 3 orders of magnitude. The drag partition is sensitive to roughness configuration, but current models cannot effectively represent this sensitivity. The effectiveness of drag partitioning approaches is also affected by estimates of the aerodynamic roughness height used to calculate wind shear velocity. Unless the roughness height is consistent with the drag partition, resulting fluxes can show physically implausible patterns. These results should make us question current assessments of the magnitude of vegetated dryland dust emissions. Representing roughness effects on surface shear stress distributions will reduce uncertainty in quantifying wind erosion, enabling better assessment of its impacts and management solutions.

Webb, Nicholas P.; Okin, Gregory S.; Brown, Shannon

2014-05-01

146

Low wall shear stress in carotid arteries in subjects with left ventricular hypertrophy.  

PubMed

Left ventricular hypertrophy (LVH) is an independent risk factor for cardiovascular complications including atherosclerosis. The close linkage between LVH and carotid atherosclerosis has been the focus of much research. However, the underlying mechanism linking the two conditions is not fully understood. Low wall shear stress contributes to intimal thickening and atherosclerosis development as a local mechanism. In the present study, we investigated the relationship between wall shear stress and LVH in subjects with risk factors for atherosclerosis. Eighty subjects with at least one risk factor for atherosclerosis; ie, hypertension, diabetes mellitus, hyperlipidemia, or smoking, were enrolled. Intimal-medial thickness (IMT), number of plaques, internal dimensions, and blood flow velocity in the common carotid artery were evaluated. Wall shear stress was calculated using a Poiseuillean parabolic model of velocity distribution: shear stress = 4 x blood viscosity x central flow velocity/internal dimension. Subjects were divided into two groups; LVH(-) (n = 36) and LVH(+) (n = 44), according to their left ventricular mass index (LVMI). Mean shear stress and systolic peak shear stress were significantly lower in subjects with LVH compared with subjects without LVH. Furthermore, mean shear stress (r = -0.42, P < .0001) and peak shear stress (r = -0.31, P < 0.01) were significantly inversely related to LVMI. Stepwise regression analysis revealed that wall shear stress independently correlated with LVMI as well as IMT. These results indicate that low shear stress could function as a local factor in the development of atherosclerosis in subjects with LVH. PMID:10950397

Jiang, Y; Kohara, K; Hiwada, K

2000-08-01

147

Nanoscale Stress Measurements and Standards  

E-print Network

Nanoscale Stress Measurements and Standards SEMICONDUCTORS Our objective is to develop accurate measurement methods for the nanoscale stress distributions and surface defects that control device performance % CAGR and 41 % US share. · Measurement of stress distributions around transistors in semiconductor

Magee, Joseph W.

148

A New High-Pressure, High-Shear Stress Viscometer and Results for Lubricants  

Microsoft Academic Search

A new high-pressure, high-shear stress viscometer has been developed with a pressure capability of 300 MPa and shear stress of 26 MPa for shear which is essentially isothermal in the context of the Theological characterization of shear-thinning lubricants. Four liquid lubricants have been investigated. Newtonian and rate-independent behavior were observed, and it appears that molecular weight may have a role

Scott Bair; Ward O. Winer

1993-01-01

149

Viscosity, Shear Waves, and Atomic-Level Stress-Stress Correlations  

SciTech Connect

The Green-Kubo equation relates the macroscopic stress-stress correlation function to a liquid's viscosity. The concept of the atomic-level stresses allows the macroscopic stress-stress correlation function in the equation to be expressed in terms of the space-time correlations among the atomic-level stresses. Molecular dynamics studies show surprisingly long spatial extension of stress-stress correlations and also longitudinal and transverse waves propagating in liquids over ranges which could exceed the system size. The results reveal that the range of propagation of shear waves corresponds to the range of distances relevant for viscosity. Thus our results show that viscosity is a fundamentally nonlocal quantity. We also show that the periodic boundary conditions play a nontrivial role in molecular dynamics simulations, effectively masking the long-range nature of viscosity.

Levashov, Valentin A [ORNL; Morris, James R [ORNL; Egami, Takeshi [ORNL

2011-01-01

150

Spatial Stress and Strain Distributions of Viscoelastic Layers in Oscillatory Shear  

PubMed Central

One of the standard experimental probes of a viscoelastic material is to measure the response of a layer trapped between parallel surfaces, imposing either periodic stress or strain at one boundary and measuring the other. The relative phase between stress and strain yields solid-like and liquid-like properties, called the storage and loss moduli, respectively, which are then captured over a range of imposed frequencies. Rarely are the full spatial distributions of shear and normal stresses considered, primarily because they cannot be measured except at boundaries and the information was not deemed of particular interest in theoretical studies. Likewise, strain distributions throughout the layer were traditionally ignored except in a classical protocol of Ferry, Adler and Sawyer, based on snapshots of standing shear waves. Recent investigations of thin lung mucus layers exposed to oscillatory stress (breathing) and strain (coordinated cilia), however, suggest that the wide range of healthy conditions and environmental or disease assaults lead to conditions that are quite disparate from the “surface loading” and “gap loading” conditions that characterize classical rheometers. In this article, we extend our previous linear and nonlinear models of boundary stresses in controlled oscillatory strain to the entire layer. To illustrate non-intuitive heterogeneous responses, we characterize experimental conditions and material parameter ranges where the maximum stresses migrate into the channel interior. PMID:22547900

Lindley, Brandon S.; Forest, M. Gregory; Smith, Breannan D.; Mitran, Sorin M.; Hill, David B.

2010-01-01

151

Role of xanthine oxidoreductase and NAD(P)H oxidase in endothelial superoxide production in response to oscillatory shear stress  

NASA Technical Reports Server (NTRS)

Oscillatory shear stress occurs at sites of the circulation that are vulnerable to atherosclerosis. Because oxidative stress contributes to atherosclerosis, we sought to determine whether oscillatory shear stress increases endothelial production of reactive oxygen species and to define the enzymes responsible for this phenomenon. Bovine aortic endothelial cells were exposed to static, laminar (15 dyn/cm2), and oscillatory shear stress (+/-15 dyn/cm2). Oscillatory shear increased superoxide (O2.-) production by more than threefold over static and laminar conditions as detected using electron spin resonance (ESR). This increase in O2*- was inhibited by oxypurinol and culture of endothelial cells with tungsten but not by inhibitors of other enzymatic sources. Oxypurinol also prevented H2O2 production in response to oscillatory shear stress as measured by dichlorofluorescin diacetate and Amplex Red fluorescence. Xanthine-dependent O2*- production was increased in homogenates of endothelial cells exposed to oscillatory shear stress. This was associated with decreased xanthine dehydrogenase (XDH) protein levels and enzymatic activity resulting in an elevated ratio of xanthine oxidase (XO) to XDH. We also studied endothelial cells lacking the p47phox subunit of the NAD(P)H oxidase. These cells exhibited dramatically depressed O2*- production and had minimal XO protein and activity. Transfection of these cells with p47phox restored XO protein levels. Finally, in bovine aortic endothelial cells, prolonged inhibition of the NAD(P)H oxidase with apocynin decreased XO protein levels and prevented endothelial cell stimulation of O2*- production in response to oscillatory shear stress. These data suggest that the NAD(P)H oxidase maintains endothelial cell XO levels and that XO is responsible for increased reactive oxygen species production in response to oscillatory shear stress.

McNally, J. Scott; Davis, Michael E.; Giddens, Don P.; Saha, Aniket; Hwang, Jinah; Dikalov, Sergey; Jo, Hanjoong; Harrison, David G.

2003-01-01

152

Endothelial cell dynamics under pulsating flows: significance of high versus low shear stress slew rates (d(tau)/dt).  

PubMed

Shear stress modulates endothelial cell (EC) remodeling via realignment and elongation. We provide the first evidence that the upstroke slopes of pulsatile flow, defined as shear stress slew rates (positive d(tau)/dt), affect significantly the rates at which ECs remodel. We designed a novel flow system to isolate various shear stress slew rates by precisely controlling the frequency, amplitude, and time-averaged shear stress (tau(ave)) of pulsatile flow. Bovine aortic endothelial cell (BAEC) monolayers were exposed to three conditions: (1) pulsatile flow (1 Hz) at high slew rate (293 dyn/cm2 s), (2) pulsatile flow (1 Hz) at low slew rate (71 dyn/cm2s), and (3) steady laminar flow at d(tau)/dt = 0. All of the three conditions were operated at tau(ave) = 50 dyn/cm2. BAEC elongation and alignment were measured over 17 h. We were able to demonstrate the effects of shear stress slew rates ((tau)/dt) on EC remodeling at a fixed spatial shear stress gradient (d(tau)/dx). We found that pulsatile flow significantly increased the rates at which EC elongated and realigned, compared to steady flow at d(tau)/dt = 0. Furthermore, EC remodeling was faster in response to high than to low slew rates at a given tau(ave). PMID:12108839

Hsiai, Tzung K; Cho, Sung K; Honda, Henry M; Hama, Susan; Navab, Mohamad; Demer, Linda L; Ho, Chih-Ming

2002-05-01

153

Haemoconcentration, shear-stress increase and carotid artery diameter regulation after furosemide administration in older hypertensives.  

PubMed

The aim of the present study was to determine whether changes of carotid wall shear stress induced by changes in blood viscosity after diuretic administration cause carotid arterial dilatation in elderly hypertensives, as reported in the cat. Arterial wall shear rate (ultrasound technique, profilmeter FRP III), the systo-diastolic diameter (echotracking technique) and the mean blood flow velocity and volume of the common carotid artery, the blood viscosity (rotational viscometer) and the finger arterial blood pressure (Finapress Ohmeda) were measured in 12 young volunteers (aged 25+/-2 years) and in 12 elderly hypertensives (aged 80+/-4 years) treated with short-acting calcium antagonists up to 24h before the study, both at baseline and after intravenous furosemide infusion (0.5mg/min), when the haematocrit had increased by at least two percentage points. After furosemide administration the mean arterial blood pressure decreased and blood viscosity and carotid systolic shear stress increased in both groups. However, common carotid artery diameter increased only in the young controls but not in the elderly hypertensives. These data show that an increase in carotid shear stress caused by haemoconcentration induces carotid vasodilatation only in young healthy subjects, and not in elderly hypertensives. This effect may be related to impaired endothelium function and/or arterial wall mechanics. PMID:11250127

Fazio, M; Bardelli, M; Cominotto, F; Fiammengo, F; Fabris, B; Fischetti, F; Candido, R; Pascazio, L; Lapasin, R; Carretta, R

2001-03-01

154

A review of Reynolds stress models for turbulent shear flows  

NASA Technical Reports Server (NTRS)

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

Speziale, Charles G.

1995-01-01

155

Dynamic response of wall shear stress on the stenosed artery.  

PubMed

The present study deals with an appropriate mathematical model of an artery in the presence of constriction in which the generated wall shear stress due to blood flow is analysed. The geometry of the stenosed arterial segment in the diseased state, causing malfunction of the cardiovascular system, is formed mathematically. The flowing blood contained in the stenosed artery is treated as non-Newtonian and the flow is considered to be two-dimensional. The motion of the arterial wall and its effect on local fluid mechanics is not ruled out from the present pursuit. The flow analysis applies the time-dependent, two-dimensional incompressible nonlinear Navier-Stokes equations for non-Newtonian fluids. The flow-field can be obtained primarily following the radial coordinate transformation, using the appropriate boundary conditions and finally adopting a suitable finite difference scheme numerically. The influences of flow unsteadiness, the arterial wall distensibility and the presence of stenosis on the flow-field and the wall shear stresses are quantified in order to indicate the susceptibility to atherosclerotic lesions and thereby to validate the applicability of the present theoretical model. PMID:19294542

Sen, S; Chakravarty, S

2009-10-01

156

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

Microsoft Academic Search

The unsteady low Reynolds number aerodynamics of flapping flight was investigated experimentally through flow visualization\\u000a by suspended particle imagery and wall shear stress measurement from micro-array hot-film anemometry. In conjunction, a mechanism\\u000a was developed to create a flapping motion with three degrees of freedom and adjustable flapping frequency. The flapping kinematics\\u000a and wing shape were selected for dynamic similarity to

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

2010-01-01

157

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

NASA Technical Reports Server (NTRS)

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.

Srinivas, S.

1975-01-01

158

Proximal stenosis may induce initiation of cerebral aneurysms by increasing wall shear stress and wall shear stress gradient.  

PubMed

Hemodynamic parameters, such as wall shear stress (WSS), WSS gradient (WSSG), aneurysm formation indicator (AFI), or gradient oscillatory number (GON), have been proposed to be linked to initiation of cerebral aneurysms. However, how such conditions occur in humans is unclear. We encountered a rare and interesting case to address this issue. A patient had a newly formed aneurysm with proximal stenosis, which was confirmed by serial imagings. We made two pre-aneurysm models: one with stenosis and the other without stenosis. We performed computational fluid dynamics simulations for these models. Owing to jet flow caused by the stenosis, the maximum WSS and WSSG on the aneurysm initiation site were approximately doubled and tripled, respectively. However, the oscillatory shear index (OSI), AFI, and GON did not change substantially by the stenosis. Computer simulations using artificial vascular models with different degrees of proximal stenosis at different distances demonstrated that oscillatory shear index, AFI, and GON did not change substantially by the stenosis. These results showed that proximal stenosis caused high WSS and high WSSG at the aneurysm initiation site, possibly leading to aneurysm initiation. Proximal stenosis may be a potential factor to induce initiation of one class of cerebral aneurysms by increasing WSS and WSSG. Copyright © 2014 John Wiley & Sons, Ltd. PMID:24706583

Kono, Kenichi; Fujimoto, Takeshi; Terada, Tomoaki

2014-10-01

159

Modeling flow and shear stress fields over unsteady three dimensional dunes  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

160

Effects of shear stress cultivation on cell membrane disruption and intracellular calcium concentration in sonoporation of endothelial cells.  

PubMed

Microbubble facilitated ultrasound (US) application can enhance intracellular delivery of drugs and genes in endothelial cells cultured in static condition by transiently disrupting the cell membrane, or sonoporation. However, endothelial cells in vivo that are constantly exposed to blood flow may exhibit different sonoporation characteristics. This study investigates the effects of shear stress cultivation on sonoporation of endothelial cells in terms of membrane disruption and changes in the intracellular calcium concentration ([Ca(2+)](i)). Sonoporation experiments were conducted using murine brain microvascular endothelial (bEnd.3) cells and human umbilical vein endothelial cells (HUVECs) cultured under static or shear stress (5 dyne/cm(2) for 5 days) condition in a microchannel environment. The cells were exposed to a short US tone burst (1.25 MHz, 8 ?s duration, 0.24 MPa) in the presence of Definity™ microbubbles to facilitate sonoporation. Membrane disruption was assessed by propidium iodide (PI) and changes in [Ca(2+)](i) measured by fura-2AM. Results from this study show that shear stress cultivation significantly reduced the impact of ultrasound-driven microbubbles activities on endothelial cells. Cells cultured under shear stress condition exhibited much lower percentage with membrane disruption and changes in [Ca(2+)](i) compared to statically cultured cells. The maximum increases of PI uptake and [Ca(2+)](i) were also significantly lower in the shear stress cultured cells. In addition, the extent of [Ca(2+)](i) waves in shear cultured HUVECs was reduced compared to the statically cultured cells. PMID:20863503

Park, Juyoung; Fan, Zhenzhen; Deng, Cheri X

2011-01-01

161

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

PubMed Central

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

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

2013-01-01

162

Bed Forms Modulating Temporal Peaks on Near-Bank Shear Stresses, the Wabash River Case  

NASA Astrophysics Data System (ADS)

There is a great body of experimental work showing how bed forms modulate bed roughness, flow field structure, and sediment transport rates in straight flumes. Recently, it was observed that migrating bed forms produce temporal and spatial peaks of shear stresses along the outer bank of an experimental meandering channel. These stresses are about 50% larger than the shear stresses exerted by the mean near-bank flow. As fluvial erosion bank erosion rates are typically linearly related to applied shear stress, the migration rate of the bend may be significantly increased. However, this hypothesis has never been tested in the field, where bed forms could be more complex than those found in experimental cases. Herein, only fluvial erosion is considered, while geotechnical processes occurring at the outer bank are not accounted for. Detailed measurements of hydrodynamics (using acoustic Doppler profiler), bed morphology (using multibeam and RTK GPS) and bank morphology (using laser scanner) were conducted at two bends on the Wabash River along the Illinois and Indiana Stateline. The bed morphology exhibited different scales of bed forms, ranging from dunes to ripples. Using Wavelet analysis to discriminate the bed morphology it was possible to separate the ripples and dunes structures resulting in a bed without bed forms, which shows the typical erosion (outer bank)/deposition (inner bank) arrangement in meandering channels. Using a fully three-dimensional Reynolds-Averaged Navier-Stokes (RANS) numerical model, two cases are simulated: [1] bend with bed forms, and [2] bend without bed forms to test the above hypothesis. The results show that the three-dimensional flow field is compares well to that observed for both scenarios. Further, peaks in shear stresses along the outer bank are indeed observed, which are correlated to the location of the bed forms with respect to the bend. Further conclusion and its importance for long-term morphodynamics of meandering channels are described.

Abad, J. D.; Frias, C. E.; Langendoen, E. J.; Best, J.; Rhoads, B. L.; Konsoer, K. M.; Garcia, M. H.

2013-12-01

163

A shear test method to measure shear strength of metallic materials and solder joints using small specimens  

Microsoft Academic Search

The asymmetrical four-point bend (AFPB) shear test, which is the subject of this paper, was originally developed by Slepetz et al. to evaluate the shear properties of large metallic specimens and polymer composites. Similar to the Iosipescu test, the AFPB test also provides a nearly pure-shear stress state with an unnotched specimen. However, since the AFPB test involves point loading

Ö Ünal; D. J. Barnard; I. E. Anderson

1999-01-01

164

Shear induced drainage in foamy yield-stress fluids J. Goyon1,2,*  

E-print Network

1 Shear induced drainage in foamy yield-stress fluids J. Goyon1,2,* , F. Bertrand1 , O. Pitois2 Paris Est, Laboratoire LPMDI, Marne la Vallée, France Abstract: Shear induced drainage of a foamy yield stabilizes the system at rest, a fast drainage is observed when a horizontal shear is imposed. It is shown

Paris-Sud XI, Université de

165

The Effect of Shear and Deviatoric Stress on Permeability Evolution in Fractured Diorite and Novaculite  

NASA Astrophysics Data System (ADS)

The evolution of the permeability in fractured rocks, subject to changing stress conditions is a complex issue. In this contribution, we report on experiments in which rock surfaces were sheared under controlled pore pressure and true-triaxial stress conditions while permeability was monitored via flow parallel to the shear direction. Direct shear tests were performed in a pressure vessel under drained conditions on samples of novaculite and of diorite (Coso Geothermal field, CA). Smoothed-faced, prismatic blocks were roughened by grinding with 60# grit, assembled face-to-face, and jacketed within a sealed membrane. Normal stress was applied by a hydraulic ram, and confining- and pore-pressures were maintained via high precision servo- controlled pressure intensifiers. The sample pairs are sheared to 18 mm of final displacement at 5 ?m/sec, at room temperature, and at effective normal stresses ranging from 5 to 20 MPa. Permeability evolution is measured throughout shearing via flow of distilled water from an upstream reservoir at 0.1 MPa and discharged at atmospheric pressure. The permeability of the smooth fracture in novaculite varied only slightly with applied effective normal stress (0.5×10^{-14} m2 at 5 MPa, 10^{-14} m2 at 10 MPa and 10^{-14} m2 at 20 MPa). The corresponding hydraulic apertures were respectively 20 ?m, 15 ?m and 13 ?m. In all three tests, permeability dropped one order of magnitude upon shearing. Steady state values for permeability and shear strength were typically reached after ~ 10 mm of load point displacement. An associated reduction of ~10 ?m was observed in the hydraulic apertures. Similar behavior was recorded in the diorite sample but only at the highest effective normal stresses. The initial permeability of ~10^{-13 m2 dropped four orders of magnitude before reaching a steady state value. This corresponds to a reduction in hydraulic aperture of ~23 ?m, reaching a steady state magnitude of ~1.5 ?m. At lower effective normal stresses diorite shows different behavior. At 5 MPa the initial permeability of 0.5×10^{-14} m2 increases to 10^{-14} m2 at peak strength and drops to 0.5×10^{-14} m2 during steady state frictional sliding. This corresponds to a hydraulic aperture of 20 ?m. At 10 MPa the initial permeability of 10^{-14} m2 (hydraulic aperture 14 ?m) reduces to 10-16 m2 (hydraulic aperture of 2.8 ?m) at peak shear strength, and then partly recovers by one order of magnitude to reach a final steady permeability of 10^{-15} m2 (final hydraulic aperture of 5 ?m). We observe that permeability decreases with an increase in the applied normal load. Moreover, shearing results in a permeability reduction in all cases, and is more pronounced at higher normal stress levels. This is presumably the result of mechanical removal (wear or fracturing) of asperities along the sliding contacts. The removed material subsequently clogs the fracture, reducing the permeability. Chemical analyses of pore fluids will be reported.

Faoro, I.; Elsworth, D.; Marone, C.; Niemeijer, A.

2006-12-01

166

Modeling the relation between suction, effective stress and shear strength in partially saturated granular media  

E-print Network

Decades of geotechnical research firmly established that the mechanical properties (shear strength and deformation characteristics) of soils are related to soil's "effective stress", i.e. the stress carried by the solid ...

Toker, Nabi Kartal, 1979-

2007-01-01

167

Disbond monitoring in adhesive joints using shear stress optical fiber sensors  

NASA Astrophysics Data System (ADS)

We present dedicated shear stress optical fiber sensors for in situ disbond monitoring of adhesive bonds. The shear stress sensitivity of these sensors is about 60 pm MPa-1, which corresponds to a shear strain sensing resolution of 50 ??. By integrating a combination of three such sensors in the adhesive bond line of a single lap joint, we can assess the internal shear stress distribution when the joint is tensile loaded. Disbonding of this joint was initiated by cyclic tensile loading, and the sensor responses were monitored during this process. Our results show that this sensing system can detect disbonds as small as 100 ?m.

Sulejmani, Sanne; Sonnenfeld, Camille; Geernaert, Thomas; Luyckx, Geert; Mergo, Pawel; Urbanczyk, Waclaw; Chah, Karima; Thienpont, Hugo; Berghmans, Francis

2014-07-01

168

Dynamic variation of ambient shear stress field before and after three strong earthquakes in Yunnan  

NASA Astrophysics Data System (ADS)

A method calculating relative shear stress values in seismic source areas using data of seismic wave is used in this paper which is introduced by Pei-Shan CHEN, based on a rupture model studying seismic rupture process from theory of fracture mechanics. The shear stress values ? 0 in mid-small earthquakes occurred in 1986 June of 1997 in Yunnan and its adjacent areas are used to trace the variation process of ambient shear stress field before and after three strong earthquakes in Yunnan and China-Myanmar border areas. The results show that there exists a background of high values in ambient shear stress field. In the development process of a strong earthquake, the ambient shear stress field in and nearby its potential source area increases obviously, for a long time, experiences a process of multiple down — up — down — up. The pattern of dynamic variation of ambient shear stress field has reflected the multi-source fields of seismogenesis in and nearby Yunnan. There exist multi-increased areas of ambient shear stress fields synchronously in southwest, northwest, west and east of Yunnan, and three strong earthquakes of magnitude about 7 occurred in the most obviously increased areas of ambient shear stress fields.

Wang, Shao-Jin; Long, Xiao-Fan

1999-03-01

169

Temporal evolution of cell focal adhesions: experimental observations and shear stress profiles  

E-print Network

Temporal evolution of cell focal adhesions: experimental observations and shear stress profiles D as the growth rate increases. Using a composite mechanics model by which the evolution of the shear stress804643n Live cells create adhering contacts with substrates via focal adhesion (FA) sites associated

Zaidel-Bar, Ronen

170

Monocyte recruitment to endothelial cells in response to oscillatory shear stress  

E-print Network

for direct correlations of shear stress with distri- bution of focal atherosclerotic lesions (12Monocyte recruitment to endothelial cells in response to oscillatory shear stress TZUNG K. HSIAI,,1 of Engineering and Keck School of Medicine, Los Angeles; § Department of Mechanical and Aerospace Engineering

Wong, Pak Kin

171

Shear stress regulation of nitric oxide production in uterine and placental artery endothelial cells  

E-print Network

culture models of shear stress. In order to apply the results of these studies more effectively, we need: pregnancy, shear stress, nitric oxide, blood flow, rheology Introduction Substantial increases in uterine in pregnancy. Because there is no innervation of fetal placental vessels, vasodilation has been thought

Chesler, Naomi C.

172

Critical shear stress for erosion of cohesive soils subjected to temperatures typical of wildfires  

Microsoft Academic Search

Increased erosion is a well-known response after wildfire. To predict and to model erosion on a landscape scale requires knowledge of the critical shear stress for the initiation of motion of soil particles. As this soil property is temperature-dependent, a quantitative relation between critical shear stress and the temperatures to which the soils have been subjected during a wildfire is

John A. Moody; J. Dungan Smith; B. W. Ragan

2005-01-01

173

The role of death-associated protein kinase (DAPK) in endothelial apoptosis under fluid shear stress.  

PubMed

Endothelial cells are the interface between hemodynamic fluid flow and vascular tissue contact. They actively translate physical and chemical stimuli into intracellular signaling cascades which in turn regulate cell function, and endothelial dysfunction leads to inflammation and diseased conditions. For example, atherosclerosis, a chronic vascular disease, favorably develops in regions of disturbed fluid flow and low shear stress. Apoptosis, or programmed cell death, must be properly regulated to maintain homeostasis in the vascular wall. The loss of apoptosis control, as seen in low shear stress regions, is implicated in various diseases such as atherosclerosis and cancer. Death-associated protein kinase (DAPK) is a pro-apoptotic regulator for various cell types that is localized in the cell cytoskeleton and regulates changes in cytoplasm associated with apoptosis. Yet its role in endothelial cells remains unclear. Laminar shear stress inhibits cytokine, oxidative stress, and serum starvation induced endothelial apoptosis, while extended shearing elicit structural changes and cell alignment. We hypothesize that DAPK potentially plays a role in attenuating endothelial apoptosis in response to shear stress. We found that shear stress regulates DAPK expression and apoptotic activity in endothelial cells. In fact, shear stress alone induces DAPK and apoptosis, but has the opposite effect in the presence of apoptotic triggers such as tissue necrosis factor ? (TNF?). This review summarizes mechanisms of endothelial mechanotransduction and apoptosis, and explores the potential of DAPK as a novel signaling pathway involved in mediating protective effects of shear stress on the vasculature. PMID:23806751

Rennier, Keith; Ji, Julie Y

2013-08-14

174

Calculations of the critical shear stress for motion of uniform and heterogeneous sediments  

Microsoft Academic Search

An expression for the critical shear stress noncohesive sediment is derived from the balance of forces on individual particles at the surface of a bed. The resulting equation, for a given grain size and density, depends on the near-bed drag force, lift force to drag force ratio, and particle angle of repose. Calculated values of the critical shear stress for

Patricia L. Wiberg; J. Dungan Smith

1987-01-01

175

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

NASA Astrophysics Data System (ADS)

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.

Mitchell, Michael J.; King, Michael R.

2013-01-01

176

DEM Simulation of Direct Shear: 1. Rupture Under Constant Normal Stress Boundary Conditions  

NASA Astrophysics Data System (ADS)

A particle-based distinct element method and its grain-based method are used to generate and simulate a synthetic specimen calibrated to the rupture characteristics of an intact (non-jointed) low-porosity brittle rock deformed in direct shear. The simulations are compared to the laboratory-generated ruptures and used to investigate rupture at various normal stress magnitudes. The fracturing processes leading to shear rupture zone creation and the rupture mechanism are found to be normal stress dependent (progressing from tensile splitting to shear rupture) and show partial confirmation of rupture zone creation in nature and in experiments from other materials. The normal stress dependent change is found to be due to the orientation of the major principal stress and local stress concentrations internal to the synthetic specimens being deformed. The normal stress dependent rupture creation process results in a change to the rupture zone's geometry, shear stress versus horizontal displacement response, and thus ultimate strength.

Bewick, R. P.; Kaiser, P. K.; Bawden, W. F.; Bahrani, N.

2014-09-01

177

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

NASA Technical Reports Server (NTRS)

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

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

1986-01-01

178

Brachial artery adaptation to lower limb exercise training: role of shear stress.  

PubMed

Lower limb exercise increases upper limb conduit artery blood flow and shear stress, and leg exercise training can enhance upper limb vascular function. We therefore examined the contribution of shear stress to changes in vascular function in the nonexercising upper limbs in response to lower limb cycling exercise training. Initially, five male subjects underwent bilateral brachial artery duplex ultrasound to measure blood flow and shear responses to 30-min cycling exercise at 80% of maximal heart rate. Responses in one forearm were significantly (P < 0.05) attenuated via cuff inflation throughout the exercise bout. An additional 11 subjects participated in an 8-wk cycle training study undertaken at a similar intensity, with unilateral cuff inflation around the forearm during each exercise bout. Bilateral brachial artery flow-mediated dilation responses to a 5-min ischemic stimulus (FMD%), an ischemic handgrip exercise stimulus (iEX), and endothelium-independent NO donor administration [glyceryl trinitrate (GTN)] were measured at 2, 4, and 8 wk. Cycle training increased FMD% in the noncuffed limb at week 2, after which time responses returned toward baseline levels (5.8 ± 4.1, 8.6 ± 3.8, 7.4 ± 3.5, 6.0 ± 2.3 at 0, 2, 4 and 8 wk, respectively; ANOVA: P = 0.04). No changes in FMD% were observed in the cuffed arm. No changes were evident in response to iEX or GTN in either the cuffed or noncuffed arms (P > 0.05) across the 8-wk intervention period. Our data suggest that lower limb cycle training induces a transient increase in upper limb vascular function in healthy young humans, which is, at least partly, mediated via shear stress. PMID:22403347

Birk, Gurpreet K; Dawson, Ellen A; Atkinson, Ceri; Haynes, Andrew; Cable, N Timothy; Thijssen, Dick H J; Green, Daniel J

2012-05-01

179

Development of In-Fiber Reflective Bragg Gratings as Shear Stress Monitors in Aerodynamic Facilities  

NASA Technical Reports Server (NTRS)

Bragg gratings centered at nominal wavelengths of 1290 nm and 1300 run were inscribed in a 9/125 microns germano-silicate optical fiber, using continuous wave frequency doubled Ar+ laser radiation at 244 nm. Such gratings have been used extensively as temperature and strain monitors in smart structures. They have, however, never been used for measuring aerodynamic shear stresses. As a test of their sensitivity as shear stress monitors, a Bragg fiber attached to a metal plate was subjected to laminar flows in a glass pipe. An easily measurable large flow-induced wavelength shift (Delta Lambda(sub B)) was observed in the Bragg reflected wavelength. Thereafter, the grating was calibrated by making one time, simultaneous measurements of Delta Lambda(sub B) and the coefficient of skin friction (C(sub f)) with a skin friction balance, as a function of flow rates in a subsonic wind tunnel. Onset of fan-induced transition in the tunnel flow provided a unique flow rate for correlating Delta Lambda(sub B) and (C(sub f) values needed for computing effective modulus of rigidity (N(sub eff)) of the fiber attached to the metal plate. This value Of N(sub eff) is expected to remain constant throughout the elastic stress range expected during the Bragg grating aerodynamic tests. It has been used for calculating the value of Cf at various tunnel speeds, on the basis of measured values of Bragg wavelength shifts at those speeds.

Parmar, Devendra S.; Sprinkle, Danny R.; Singh, Jag J.

1998-01-01

180

Stress driven shear bands and the effect of confinement on their structures--a rheological, flow visualization, and Rheo-SALS study.  

PubMed

An equimolar mixture of a cationic surfactant, cetylperidinium chloride (CPyCl), and salt sodium salicylate (NaSal) forms wormlike micelles in aqueous solutions. Under shear, the solution shows a pronounced shear-thickening behavior, which is coupled with oscillations in shear rate and the apparent viscosity. In this shear-thickening regime shear bands form, which also oscillate in position and intensity. These shear bands are visualized by direct imaging and Rheo-small angle light scattering methods. Temporal intensity fluctuations of the shear bands were evaluated using image analysis. Fourier transformations (FT) of the oscillating shear rate and intensity of the shear bands showed a single dominating frequency in the power spectrum analysis. This characteristic frequency as well as the amplitude of shear rate fluctuation was found to increase with stress. From the rheological and optical measurements, we propose that a stress driven mechanism is responsible for the formation of shear bands. Experiments performed in transparent parallel-plate geometry show dampening of the shear rate oscillations and increase in the characteristic frequency with decrease in the gap. Power spectrum analysis and the SALS measurements confirm the formation of different structures as a function of gap size in the parallel-plate geometry. PMID:16171332

Herle, Vishweshwara; Fischer, Peter; Windhab, Erich J

2005-09-27

181

Experimental assessment of Owen's second hypothesis on surface shear stress induced by a fluid during sediment saltation  

NASA Astrophysics Data System (ADS)

widely used, yet thus far unproven, fluid dynamical hypothesis originally presented by P. R. Owen 50 years ago, states that the surface shear stress induced by a fluid on the ground during equilibrium sediment saltation is constant and independent of the magnitude of the fluid velocity and consequently the particle mass flux. This hypothesis is one of the key elements in almost all current model descriptions of sediment erosion. We measured the surface shear stress in a drifting-sand wind tunnel and found Owen's hypothesis being merely an approximation of the real situation. A significant decrease of the fluid stress with increasing wind velocities was measured for low to intermediate particle mass fluxes. For high particle mass fluxes, Owen's hypothesis essentially holds, although a slight increase of the fluid stress was measured.

Walter, B.; Horender, S.; Voegeli, C.; Lehning, M.

2014-09-01

182

An implementation of Bayesian lensing shear measurement  

NASA Astrophysics Data System (ADS)

The Bayesian gravitational shear estimation algorithm developed by Bernstein & Armstrong can potentially be used to overcome multiplicative noise bias and recover shear using very low signal-to-noise ratio (S/N) galaxy images. In that work, the authors confirmed that the method is nearly unbiased in a simplified demonstration, but no test was performed on images with realistic pixel noise. Here, I present a full implementation for fitting models to galaxy images, including the effects of a point spread function (PSF) and pixelization. I tested the implementation using simulated galaxy images modelled as Sérsic profiles with n = 1 (exponential) and n = 4 (De Vaucouleurs'), convolved with a PSF and a flat pixel response function. I used a round Gaussian model for the PSF to avoid potential PSF-fitting errors. I simulated galaxies with mean observed, post-PSF full width at half-maximum equal to approximately 1.2 times that of the PSF, with lognormal scatter. I also drew fluxes from a lognormal distribution. I produced independent simulations, each with pixel noise tuned to produce different mean S/N ranging from 10-1000. I applied a constant shear to all images. I fitted the simulated images to a model with the true Sérsic index to avoid modelling biases. I recovered the input shear with fractional error ?g/g < 2 × 10-3 in all cases. In these controlled conditions, and in the absence of other multiplicative errors, this implementation is sufficiently unbiased for current surveys and approaches the requirements for planned surveys.

Sheldon, Erin S.

2014-10-01

183

Localized ?4 Integrin Phosphorylation Directs Shear Stress-Induced Endothelial Cell Alignment  

PubMed Central

Vascular endothelial cells respond to laminar shear stress by aligning in the direction of flow, a process which may contribute to athero-protection. Here we report that localized ?4 integrin phosphorylation is a mechanism for establishing the directionality of shear stress-induced alignment in microvascular endothelial cells. Within 5 minutes of exposure to a physiological level of shear stress, endothelial ?4 integrins became phosphorylated on Ser988. In wounded monolayers, phosphorylation was enhanced at the downstream edges of cells relative to the source of flow. The shear-induced ?4 integrin phosphorylation was blocked by inhibitors of cAMP-dependent protein kinase A (PKA), an enzyme involved in the alignment of endothelial cells under prolonged shear. Moreover, shear-induced localized activation of the small GTPase Rac1, which specifies the directionality of endothelial alignment, was similarly blocked by PKA inhibitors. Furthermore, endothelial cells bearing a non-phosphorylatable ?4(S988A) mutation failed to align in response to shear stress, thus establishing ?4 as a relevant PKA substrate. We thereby show that shear-induced PKA-dependent ?4 integrin phosphorylation at the downstream edge of endothelial cells promotes localized Rac1 activation, which in turn directs cytoskeletal alignment in response to shear stress. PMID:18583710

Goldfinger, Lawrence E.; Tzima, Eleni; Stockton, Rebecca; Kiosses, William B.; Kinbara, Kayoko; Tkachenko, Eugene; Gutierrez, Edgar; Groisman, Alex; Nguyen, Phu; Chien, Shu; Ginsberg1, Mark H.

2009-01-01

184

Studies on stress distribution in pavements subjected to surface shear forces  

PubMed Central

It has been pointed out by some researchers1,2) that road pavements are subjected to vertical stress due to vehicles on them as well as shear stress at the time of braking or acceleration of vehicles. In this paper, the results of elastic analysis to obtain the rigorous solution for an elastic two-layer system subjected to surface shear stress are described and it is shown that the effect of shear stresses applied at the surface gives rise to fairly large stresses in the system. On the basis of these findings, the author attempts to explain why pavement failure takes place frequently at places such as crossings and curved parts where pavements are subjected to high magnitude of surface shear stresses. PMID:24522154

KIMURA, Tsutomu

2014-01-01

185

Studies on stress distribution in pavements subjected to surface shear forces.  

PubMed

It has been pointed out by some researchers that road pavements are subjected to vertical stress due to vehicles on them as well as shear stress at the time of braking or acceleration of vehicles. In this paper, the results of elastic analysis to obtain the rigorous solution for an elastic two-layer system subjected to surface shear stress are described and it is shown that the effect of shear stresses applied at the surface gives rise to fairly large stresses in the system. On the basis of these findings, the author attempts to explain why pavement failure takes place frequently at places such as crossings and curved parts where pavements are subjected to high magnitude of surface shear stresses. PMID:24522154

Kimura, Tsutomu

2014-01-01

186

Modeling shear wave splitting due to stress-induced anisotropy, with an application to Mount Asama Volcano, Japan  

NASA Astrophysics Data System (ADS)

We use numerical modeling to investigate the proposed stress-based origin for changing anisotropy at Mount Asama Volcano, Japan. Stress-induced anisotropy occurs when deviatoric stress conditions are applied to rocks which are permeated by microcracks and compliant pore space, leading to an anisotropic distribution of open crack features. Changes to the local stress field around volcanoes can thus affect the anisotropy of the region. The 2004 eruption of Mount Asama Volcano coincided with time-varying shear wave splitting measurements, revealing changes in anisotropy that were attributed to stress changes associated with the eruption. To test this assertion, we create a model that incorporates knowledge of the volcanic stress, ray tracing, and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model. Anisotropy is calculated in two ways, by considering a basic case of having uniform crack density and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress. Our results show that this approach is sensitive to crack density, crack compliance, and the regional stress field, all of which are poorly constrained parameters. In the case of dry crack closure, results show that modeled stress conditions produce a much smaller degree of anisotropy than indicated by measurements. We propose that the source of anisotropy changes at Asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes, such as the movement of pore fluid.

Shelley, Adrian; Savage, Martha; Williams, Charles; Aoki, Yosuke; Gurevich, Boris

2014-05-01

187

In Vivo Wall Shear Measurements within the Developing Zebrafish Heart  

PubMed Central

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. PMID:24124507

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

2013-01-01

188

Response of mesenchymal stem cells to shear stress in tissue-engineered vascular grafts  

PubMed Central

Aim: Recent studies have demonstrated that mesenchymal stem cells (MSCs) can differentiate into endothelial cells. The effect of shear stress on MSC differentiation is incompletely understood, and most studies have been based on two-dimensional systems. We used a model of tissue-engineered vascular grafts (TEVGs) to investigate the effects of shear stress on MSC differentiation. Methods: MSCs were isolated from canine bone marrow. The TEVG was constructed by seeding MSCs onto poly-?-caprolactone and lactic acid (PCLA) scaffolds and subjecting them to shear stress provided by a pulsatile bioreactor for four days (two days at 1 dyne/cm2 to 15 dyne/cm2 and two days at 15 dyne/cm2). Results: Shear stress significantly increased the expression of endothelial cell markers, such as platelet-endothelial cell adhesion molecule-1 (PECAM-1), VE-cadherin, and CD34, at both the mRNA and protein levels as compared with static control cells. Protein levels of alpha-smooth muscle actin (?-SMA) and calponin were substantially reduced in shear stress-cultured cells. There was no significant change in the expression of ?-SMA, smooth muscle myosin heavy chain (SMMHC) or calponin at the mRNA level. Conclusion: Shear stress upregulated the expression of endothelial cell-related markers and downregulated smooth muscle-related markers in canine MSCs. This study may serve as a basis for further investigation of the effects of shear stress on MSC differentiation in TEVGs. PMID:19417732

Dong, Jian-de; Gu, Yong-quan; Li, Chun-min; Wang, Chun-ren; Feng, Zeng-guo; Qiu, Rong-xin; Chen, Bing; Li, Jian-xin; Zhang, Shu-wen; Wang, Zhong-gao; Zhang, Jian

2009-01-01

189

Wall shear stress modulation of ATP/ADP concentration at the endothelium.  

PubMed

A mathematical model of ATP/ADP reaction at the surface of the endothelium for any two-dimensional or axisymmetric nonsingular functional form of the wall shear stress has been presented. Excellent agreement is obtained with the numerical solution for the parallel plate case. For spatially varying wall shear stress, such as the stagnation point flow and a backward facing step the ATP concentrations are shown to have a maximum at the stagnation point streamline and the reattachment point respectively. Increasing the Reynolds number increases both the ATP and ADP concentrations. For the backward facing step significant spatial variations occur in the concentration. Hence, a strong controlling factor for physiological kinetic values is the geometry of the arterial vessel since this determines the wall shear stress and thence the transport to the reactive surface. The area of high concentration also occurs where the wall shear stress is low (limiting case is zero shear stress). Atherosclerotic plaques are known to occur in areas of low wall shear stress and at vessel bifurcations where the wall shear stress is spatially varying. The ATP/ADP concentrations at these particular points may very well contribute to the formation of plaques. PMID:14649496

David, Tim

2003-11-01

190

Mechanics of Materials 26 (1997) 6380 On the Use of Shear-Lag Methods for Analysis of Stress Transfer in  

E-print Network

. The shear-lag method does a much worse job of predicting shear stresses and energy release rates for Analysis of Stress Transfer in Unidirectional Composites John A. Nairn Material Science and Engineering The "shear-lag" analysis method is frequently used for analysis of stress transfer between the fiber

Nairn, John A.

191

Surface chemistry modulates osteoblasts sensitivity to low fluid shear stress.  

PubMed

Low fluid shear stress (FSS) is the mechanical environment encountered by osteoblasts in implanted bones or native bones of bed rest patients. High sensitivity of osteoblasts to low FSS is beneficial to osteogenesis. We hypothesize that this sensitivity might be regulated by chemical microenvironment provided by scaffolds. To confirm this hypothesis, self-assembled monolayers (SAMs) were used to provide various surface chemistries including OH, CH3 , and NH2 while parallel-plate fluid flow system produced low FSS (5 dynes/cm(2) ). Alterations in S-phase cell fraction, alkaline phosphatase activity, fibronectin (Fn), and collagen type I (COL I) secretion compared to those without FSS exposure were detected to characterize the sensitivity. Osteoblasts on OH and CH3 SAMs demonstrated obvious sensitivity while on NH2 SAMs negligible sensitivity was observed. Examination of the cell aspect ratio, orientation, and focal adhesions before and after FSS exposure indicates that the full spreading and robust focal adhesions on NH2 SAMs should be responsible for the negligible sensitivity through increasing the cell tolerance to low FSS. Despite the higher sensitivity, the Fn and COL I depositions on both OH and CH3 SAMs after FSS exposure were still less than on NH2 SAMs without FSS exposure. These results suggest that elaborate design of surface chemical compositions is essential for orchestration of surface chemistry with low FSS to realize both high sensitivity and high matrix secretion, facilitating the formation of functional bone tissues in implanted bone. PMID:24443183

Xing, Juan; Li, Yan; Lin, Manping; Wang, Jinfeng; Wu, Jinchuan; Ma, Yufei; Wang, Yuanliang; Yang, Li; Luo, Yanfeng

2014-11-01

192

Wall shear stress in normal left coronary artery tree.  

PubMed

Despite the fact that the role of wall shear stress (WSS) as a local mechanical factor in atherogenesis is well established, its distribution over the entire normal human left coronary artery (LCA) tree has not yet been studied. A three-dimensional computer generated model of the epicardial LCA tree, based on averaged human data set extracted from angiographies, was adopted for finite-element analysis of the Navier-Stokes flow equations treating blood as non-Newtonian fluid. The LCA tree includes the left main coronary artery (LMCA), the left anterior descending (LAD), the left circumflex artery (LCxA) and their major branches. In proximal LCA tree regions where atherosclerosis frequently occurs, low WSS appears. Low WSS regions occur at bifurcations in regions opposite the flow dividers, which are anatomic sites predisposed for atherosclerotic development. On the LMCA bifurcation, at regions opposite to the flow divider, dominant low WSS values occur ranging from 0.75 to 2.25 N/m2. High WSS values are encountered at all flow dividers. This work determines, probably for the first time, the topography of the WSS in the entire normal human LCA epicardial tree. It is also the first work determining the spatial WSS differentiation between proximal and distal normal human LCA parts. The haemodynamic analysis of the entire epicardial LCA tree further verifies the implications of the WSS in atherosclerosis mechanisms. PMID:16439244

Soulis, Johannes V; Farmakis, Thomas M; Giannoglou, George D; Louridas, George E

2006-01-01

193

Flow instability and wall shear stress variation in intracranial aneurysms  

PubMed Central

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

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

2010-01-01

194

Implantation of a Carotid Cuff for Triggering Shear-stress Induced Atherosclerosis in Mice  

PubMed Central

It is widely accepted that alterations in vascular shear stress trigger the expression of inflammatory genes in endothelial cells and thereby induce atherosclerosis (reviewed in 1 and 2). The role of shear stress has been extensively studied in vitro investigating the influence of flow dynamics on cultured endothelial cells 1,3,4 and in vivo in larger animals and humans 1,5,6,7,8. However, highly reproducible small animal models allowing systematic investigation of the influence of shear stress on plaque development are rare. Recently, Nam et al. 9 introduced a mouse model in which the ligation of branches of the carotid artery creates a region of low and oscillatory flow. Although this model causes endothelial dysfunction and rapid formation of atherosclerotic lesions in hyperlipidemic mice, it cannot be excluded that the observed inflammatory response is, at least in part, a consequence of endothelial and/or vessel damage due to ligation. In order to avoid such limitations, a shear stress modifying cuff has been developed based upon calculated fluid dynamics, whose cone shaped inner lumen was selected to create defined regions of low, high and oscillatory shear stress within the common carotid artery 10. By applying this model in Apolipoprotein E (ApoE) knockout mice fed a high cholesterol western type diet, vascular lesions develop upstream and downstream from the cuff. Their phenotype is correlated with the regional flow dynamics 11 as confirmed by in vivo Magnetic Resonance Imaging (MRI) 12: Low and laminar shear stress upstream of the cuff causes the formation of extensive plaques of a more vulnerable phenotype, whereas oscillatory shear stress downstream of the cuff induces stable atherosclerotic lesions 11. In those regions of high shear stress and high laminar flow within the cuff, typically no atherosclerotic plaques are observed. In conclusion, the shear stress-modifying cuff procedure is a reliable surgical approach to produce phenotypically different atherosclerotic lesions in ApoE-deficient mice. PMID:22294044

Kuhlmann, Michael T.; Cuhlmann, Simon; Hoppe, Irmgard; Krams, Rob; Evans, Paul C.; Strijkers, Gustav J.; Nicolay, Klaas; Hermann, Sven; Schafers, Michael

2012-01-01

195

An Implementation of Bayesian Lensing Shear Measurement  

E-print Network

The Bayesian gravitational shear estimation algorithm developed by Bernstein and Armstrong (2014) can potentially be used to overcome noise bias and recover shear using very low signal-to-noise ratio (S/N) galaxy images. In that work the authors confirmed the method is sufficiently unbiased for planned surveys (fractional error less than 2 x 10^{-3}) in a simplified demonstration, but no test was performed on images. Here I present a full implementation for fitting models to galaxy images, including the effects of a point spread function (PSF) and pixelization. I tested the implementation using simulated galaxy images modeled as Sersic profiles with n=1 (exponential) and n=4 (De Vaucouleurs'), convolved with a PSF and a flat pixel response function. I used a round Gaussian model for the PSF to avoid potential PSF-fitting errors. I simulated galaxies with mean observed, post-PSF full-width at half maximum equal to approximately 1.2 times that of the PSF, with log-normal scatter. I also drew fluxes from a log-n...

Sheldon, Erin S

2014-01-01

196

A Micromachined Geometric Moire Interferometric Floating-Element Shear Stress Sensor  

NASA Technical Reports Server (NTRS)

This paper presents the development of a floating-element shear stress sensor that permits the direct measurement of skin friction based on geometric Moir interferometry. The sensor was fabricated using an aligned wafer-bond/thin-back process producing optical gratings on the backside of a floating element and on the top surface of the support wafer. Experimental characterization indicates a static sensitivity of 0.26 microns/Pa, a resonant frequency of 1.7 kHz, and a noise floor of 6.2 mPa/(square root)Hz.

Horowitz, S.; Chen, T.; Chandrasekaran, V.; Tedjojuwono, K.; Nishida, T.; Cattafesta, L.; Sheplak, M.

2004-01-01

197

An Octahedral Shear Strain Based measure of SNR for 3D MR Elastography  

PubMed Central

A signal to noise ratio (SNR) measure based on the octahedral shear strain (the maximum shear strain in any plane for a 3D state of strain) is presented for MR elastography, where motion-based SNR measures are commonly used. The shear strain, ?, is directly related to the shear modulus, ?, through the definition of shear stress, ? = ??. Therefore, noise in the strain is the important factor in determining the quality of motion data, rather than the noise in the motion. Motion and strain SNR measures were found to be correlated for MRE of gelatin phantoms and human breast. Analysis of the stiffness distributions of phantoms reconstructed from the measured motion data revealed a threshold for both strain and motion SNR where MRE stiffness estimates match independent mechanical testing. MRE of the feline brain showed significantly less correlation between the two SNR measures. The strain SNR measure had a threshold above which the reconstructed stiffness values were consistent between cases, whereas the motion SNR measure did not provide a useful threshold, primarily due to rigid body motion effects. PMID:21654044

McGarry, MDJ; Van Houten, EEW; Perrinez, PR; Pattison, AJ; Weaver, JB; Paulsen, KD

2011-01-01

198

An Approach to Estimate Interface Shear Stress of Ceramic Matrix Composites from Hysteresis Loops  

Microsoft Academic Search

An approach to estimate interface shear stress of ceramic matrix composites during fatigue loading has been developed in this\\u000a paper. By adopting a shear-lag model which includes the matrix shear deformation in the bonded region and friction in the\\u000a debonded region, the matrix crack space and interface debonding length are obtained by matrix statistical cracking model and\\u000a fracture mechanics interface

Longbiao Li; Yingdong Song

2010-01-01

199

epsnoise: Pixel noise in ellipticity and shear measurements  

NASA Astrophysics Data System (ADS)

epsnoise simulates pixel noise in weak-lensing ellipticity and shear measurements. This open-source python code can efficiently create an intrinsic ellipticity distribution, shear it, and add noise, thereby mimicking a "perfect" measurement that is not affected by shape-measurement biases. For theoretical studies, we provide the Marsaglia distribution, which describes the ratio of normal variables in the general case of non-zero mean and correlation. We also added a convenience method that evaluates the Marsaglia distribution for the ratio of moments of a Gaussian-shaped brightness distribution, which gives a very good approximation of the measured ellipticity distribution also for galaxies with different radial profiles. We provide four shear estimators, two based on the ? ellipticity measure, two on ?. While three of them are essentially plain averages, we introduce a new estimator which requires a functional minimization.

Melchior, Peter; Viola, Massimo

2012-04-01

200

Stress response and structural transitions in sheared gyroidal and lamellar amphiphilic mesophases: Lattice-Boltzmann simulations  

E-print Network

Stress response and structural transitions in sheared gyroidal and lamellar amphiphilic mesophases and lamellar amphiphilic mesophases to steady shear simulated using a bottom-up lattice-Boltzmann model for amphiphilic fluids and sliding periodic Lees-Edwards bound- ary conditions. We study the gyroid per se above

Harting, Jens

201

Figure 1: Multiplex logarithmic microfluidic perfusion array for probing shear stress effects on stem cells. (A)  

E-print Network

on stem cells. (A) Microfluidic perfusion systems exhibit more defined shear stress profiles and consume) for a typical soluble factor (MW~20 kDa) secreted by mouse embryonic stem cells (mESCs) investigated EMBRYONIC STEM CELLS Y.C. Toh1 and J. Voldman1* 1 Massachusetts Institute of Technology, USA ABSTRACT Shear

Voldman, Joel

202

Evidence That Release of Adenosine Triphosphate From Endothelial Cells During Increased Shear Stress Is Vesicular  

E-print Network

Evidence That Release of Adenosine Triphosphate From Endothelial Cells During Increased Shear: In response to increased shear stress, vascular endothelial cells release adenosine triphosphate (ATP, like that of nerve cells, is probably by vesicular exocytosis. Key Words: Endo- thelial cells--Quinacrine--Adenosine

Burnstock, Geoffrey

203

Hemodynamic Shear Stress and Endothelial Dysfunction in Hemodialysis Access  

PubMed Central

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

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

2014-01-01

204

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

SciTech Connect

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

Waltz, R. E.; Staebler, G. M. [General Atomics, P.O. Box 85608, San Diego, California 92186-5608 (United States); Solomon, W. M. [Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543-0451 (United States)

2011-04-15

205

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

NASA Astrophysics Data System (ADS)

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

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

2011-04-01

206

Numerical study of a dynamic resonant wall shear stress sensor with spectral element method  

NASA Astrophysics Data System (ADS)

A numerical study with spectral element method for a dynamic resonant shear stress sensor concept is presented in this paper, in which the numerical model of the sensor consisted of an unsteady three-dimensional boundary layer model for the flow and a simple mechanical model for the sensor itself. Spectral element method was used to explore fluid flow properties around the sensor in the three-dimensional boundary layer model. The three-dimensional unsteady spectral element method code was first verified with Blasius solutions in a flat plate boundary layer flow. The sensor's sensitivity to wall shear stress was then numerically determined in a laminar boundary layer. Finally, the physical mechanism of the dynamic resonant shear stress sensor was analyzed by using the verified model. The results showed that the sensitivity of the dynamic resonant shear stress sensor was due to the energy lost produced by the oscillating interaction between the sensor and fluid flow.

Zhang, Xu

2013-07-01

207

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

NASA Astrophysics Data System (ADS)

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

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

208

Effect of Fluid Shear Stress on Endocytosis of Heparan Sulfate and Low-density Lipoproteins  

PubMed Central

Hemodynamic stress is a critical factor in the onset of atherosclerosis such that reduced rates of shear stress occurring at regions of high curvature are more prone to disease. The level of shear stress has direct influence on the thickness and integrity of the glycocalyx layer. Here we show that heparan sulfate, the main component of the glycocalyx layer, forms an intact layer only on cell surfaces subjected to shear, and not under static conditions. Furthermore, receptor-mediated endocytosis of heparan sulfate and low-density liporoteins is not detectable in cells exposed to shear stress. The internalized heparan sulfate and low-density lipoproteins are colocalized as shown by confocal imaging. PMID:18309375

Barkefors, Irmeli; Aidun, Cyrus K.; Ulrika Egertsdotter, E. M.

2007-01-01

209

Experimental and computational validation of Hele-Shaw stagnation flow with varying shear stress  

NASA Astrophysics Data System (ADS)

An in vitro flow model system with continuous variation of fluid shear stress can be used to test cell responses to a range of shear stresses. In this investigation, we validated such a flow system computationally for steady and unsteady flow conditions and experimentally for steady flow conditions. The unsteady flow validation is important for studying cells such as endothelial cells that experience unsteady flow conditions in their native environment. The system is capable of exposing cells in different regions of the chamber to steady or unsteady shear stress conditions with average values ranging linearly from 0 to 30 dyn/cm. These tests and analyses demonstrate that the variable-width parallel plate flow system can be used to test the influence of a range of steady and unsteady fluid shear stress levels on cell activities.

Tefft, Brandon J.; Kopacz, Adrian M.; Liu, Wing Kam; Liu, Shu Q.

2013-12-01

210

A vortex-based model of velocity and shear stress in a partially vegetated shallow channel  

E-print Network

This paper presents a method for predicting the distributions of velocity and shear stress in shallow channels with a boundary of emergent vegetation. Experiments in a laboratory channel with model vegetation show that the ...

White, Brian L.

211

Theory to Predict Shear Stress on Cells in Turbulent Blood Flow  

PubMed Central

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

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

2014-01-01

212

Shear Stress in Nickel and Ni-60Co under One-Dimensional Shock Loading  

SciTech Connect

The dynamic response of pure nickel (Ni), and its alloy, Ni-60Co (by weight %), has been investigated during one-dimensional shock loading. Few materials' properties are different and the only significantly altered feature is the reduced stacking fault energy (SFE) for the Ni-60Co. This paper considers the effect of this reduced SFE on the shear strength. Data (in terms of shock stress, particle velocity and shock velocity) are also presented. The influence on the shear stress, {tau} of cobalt additions in nickel are then investigated and presented. Results indicate that the lateral stress is increasing in both materials with the increasing impact stress. The shear stress was found to be higher in the nickel than in the Ni-60Co. The progressive decrease of the lateral stress noted during loading indicates a complex mechanism of deformation behind the shock front.

Workman, A.; Wallwork, A. [AWE, Aldermaston, Reading, RG7 4PR (United Kingdom); Meziere, Y. J. E.; Millett, J. C. F. [Defence Academy of the UK, Cranfield University, Shrivenham, Swindon, SN6 8LA (United Kingdom); Bourne, N. K. [University of Manchester, Sackville Street, Manchester, M60 1QD (United Kingdom)

2006-07-28

213

Air flow and shear stress modifications resulting from annual wind barriers  

E-print Network

AIR FLOW AND SHEAR STRESS MODIFICATIONS RESULTING FROM ANNUAL WIND BARRIERS A Thesis by ROBERT CRAIG SCHWARTZ Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree... of MASTER OF SCIENCE May 1992 Major Subject: Soil Science AIR FLOW AND SHEAR STRESS MODIFICATIONS RESULTING FROM ANNUAL WIND BARRIERS A Thesis by ROBERT CRAIG SCHWARTZ Approved as to style and content by. Anthony S. . uo (Co-Chair of Committee...

Schwartz, Robert Craig

2012-06-07

214

DNA microarray reveals changes in gene expression of shear stressed human umbilical vein endothelial cells  

Microsoft Academic Search

Using DNA microarray screening (GeneFilter 211, Research Genetics, Huntsville, AL) of mRNA from primary human umbilical vein endothelial cells (HUVEC), we identified 52 genes with significantly altered expression under shear stress [25 dynes\\/cm2 for 6 or 24 h (1 dyne = 10 muN), compared with matched stationary controls]; including several genes not heretofore recognized to be shear stress responsive. We

Susan M. McCormick; Suzanne G. Eskin; Larry V. McIntire; Christina L. Teng; Chiung-Mei Lu; Christopher G. Russell; Krishnan K. Chittur

2001-01-01

215

Shear Stress Regulates Endothelial Nitric-oxide Synthase Promoter Activity through Nuclear Factor B Binding  

Microsoft Academic Search

We have previously demonstrated that shear stress increases transcription of the endothelial nitric-oxide synthase (eNOS) by a pathway involving activation of the tyrosine kinase c-Src and extracellular signal-re- lated kinase 1\\/2 (ERK1\\/2). In the present study sought to determine the events downstream of this pathway. Shear stress activated a human eNOS promoter chlor- amphenicol acetyl-CoA transferase chimeric construct in a

Michael E. Davis; Isabella M. Grumbach; Tohru Fukai; Alexis Cutchins; David G. Harrison

2004-01-01

216

Shear stress-induced apoptosis of adherent neutrophils: A mechanism for persistence of cardiovascular device infections  

PubMed Central

The mechanisms underlying problematic cardiovascular device-associated infections are not understood. Because the outcome of the acute response to infection is largely dependent on the function of neutrophils, the persistence of these infections suggests that neutrophil function may be compromised because of cellular responses to shear stress. A rotating disk system was used to generate physiologically relevant shear stress levels (0–18 dynes/cm2; 1 dyne = 10 ?N) at the surface of a polyetherurethane urea film. We demonstrate that shear stress diminishes phagocytic ability in neutrophils adherent to a cardiovascular device material, and causes morphological and biochemical alterations that are consistent with those described for apoptosis. Complete neutrophil apoptosis occurred at shear stress levels above 6 dynes/cm2 after only 1 h. Morphologically, these cells displayed irreversible cytoplasmic and nuclear condensation while maintaining intact membranes. Analysis of neutrophil area and filamentous actin content demonstrated concomitant decreases in both cell area and actin content with increasing levels of shear stress. Neutrophil phagocytosis of adherent bacteria decreased with increasing shear stress. Biochemical alterations included membrane phosphatidylserine exposure and DNA fragmentation, as evaluated by in situ annexin V and terminal deoxynucleotidyltransferase-mediated dUTP end labeling (TUNEL) assays, respectively. The potency of the shear-stress effect was emphasized by comparative inductive studies with adherent neutrophils under static conditions. The combination of tumor necrosis factor-? and cycloheximide was ineffective in inducing >21% apoptosis after 3 h. These findings suggest a mechanism through which shear stress plays an important role in the development of bacterial infections at the sites of cardiovascular device implantation. PMID:10823909

Shive, Matthew S.; Salloum, Mariah L.; Anderson, James M.

2000-01-01

217

Association between Ischemic Stroke and Vascular Shear Stress in the Carotid Artery  

PubMed Central

Background and Purpose Vascular shear stress is essential for maintaining the morphology and function of endothelial cells. We hypothesized that shear stress in the internal carotid artery (ICA) may differ between patients with ischemic stroke and healthy control subjects. Methods ICA shear stress was calculated in 143 controls and 122 patients with ischemic stroke who had a normal ICA or an ICA with <50% stenosis. The stroke group included patients who presented with a first-ever or recurrent ischemic stroke but excluded cardioembolic stroke and uncertain etiologies. Of the 122 patients, 107 (87.7%) and 15 (12.3%) patients were categorized as first-ever and recurrent stroke, respectively. Results Carotid diameters were significantly larger, and both peak-systolic and end-diastolic velocities were significantly lower in patients with ischemic stroke than in controls (all p values <0.05). Mean values of peak-systolic and end-diastolic shear stress in both ICAs were significantly lower in patients with ischemic stroke in models that adjusted for age, sex, and vascular risk factors (p for trend <0.05). The ICA shear stress was lowest in patients with recurrent stroke or the subtype of small-vessel occlusion. Higher peak-systolic and end-diastolic shear stresses in both ICAs were independently and negatively associated with ischemic stroke after adjusting for potential confounders (all p values <0.05). Conclusions ICA shear stresses were significantly lower in patients with ischemic stroke than in control subjects. Future studies should attempt to define the causal relationship between carotid arterial shear stress and ischemic stroke. PMID:24829599

Lee, Jun-Young; Rosenson, Robert S.

2014-01-01

218

Shearing Interferometry: A Flexible Technique For Wavefront Measurement  

NASA Astrophysics Data System (ADS)

Shearing interferometry has been under development at Itek for well over a decade for a variety of applications. Principal among these has been the high speed measurement of optical beams transmitted through atmospheric turbulence. The technique has also been applied successfully to measurement of complex highly aspheric optical surfaces, control or camera systems, and diagnostic measurement of laser beams. Shearing interferometry has several features that make it highly advantageous for these applications. First, it is self referencing and so can be used to measure wavefronts from remote sources without the usual interferometric requirement of a separate high quality coherent reference beam. Second, it can be configured to operate with broadband or 'white light' sources. Third, provided the source has some spatial coherence, it can operate with extended sources. Fourthly, it can provide linear wavefront measurement over as large a dynamic range as required (hence its value in measuring steep aspherics), and finally, it can give highly accurate wavefront measurement with the bare minimum of available light. These features make shearing interferometry a flexible and valuable technique. Itek has developed several different implementations of shearing interferometers. A particularly successful one for atmospheric turbulence measurement has been the rotating grating shearing interferometer. However in recent years, applications involving pulsed sources have led to the development of some DC (direct current) techniques, including the use of spatial carrier frequency gratings, and the use of polarization. In this paper, we describe the technique and principals of shearing interferometry and give examples of the various implementations, as well as providing results and data on their performance.

Hardy, John W.; MacGovern, Alan J.

1987-03-01

219

Lysophosphatidic acid induces shear stress-dependent contraction in mouse aortic strip in situ.  

PubMed

We previously reported that lysophosphatidic acid (LPA) regulates Ca²? influx of fluid flow in stimulated endothelial cells and that LPA and shear stress showed increment and suppressive effects on phenylephrine-induced vasoconstriction and acetylcholine-induced vasodilatation, respectively. However, a vasoconstrictive effect of LPA alone in the presence of shear stress was not found. The present study examined the effect of LPA alone in the presence of shear stress on Ca²? responses in endothelial and smooth muscle cells and contraction in mouse aortic strip using real-time 2-photon laser scanning microscopy and a custom-made parallel-plate flow chamber. Application of micromolar LPA and high shear stress elicited movement of endothelial cells after Ca²? responses. The endothelial cells moved along the major axis of smooth muscle cells, a direction that was identical to that found during vasoconstriction evoked by the application of phenylephrine. The frequency of Ca²? oscillations in smooth muscle cells was highest according to endothelial movement. Vasoconstriction evoked by LPA and shear stress was significantly reduced by the application of a thromboxane A? receptor antagonist, a cyclooxygenase inhibitor, and a thromboxane synthase inhibitor. These results suggest that micromolar LPA and high shear stress elicit vasoconstriction that is caused by Ca²?-dependent contraction in medial smooth muscle cells. Thromboxane A? may be involved in that response. PMID:24084212

Niioka, Takeharu; Ohata, Hisayuki; Momose, Kazutaka; Honda, Kazuo

2013-12-01

220

Monocyte recruitment to endothelial cells in response to oscillatory shear stress  

PubMed Central

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 (?) at ± 2.6 dyn/cm2 at a time-averaged shear stress (?ave) = 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

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

221

Heterogeneity and the Role of Normal Stresses during the Extensional Thinning of Non-Brownian Shear-Thickening Fluids  

Microsoft Academic Search

We contrast the extensional and shear dynamics of non-Brownian suspensions as a function of particle concentration. We show that the thinning rate selected during the viscoelastic pinch-off of a liquid bridge is related to the shear rate at which normal stresses become positive, which differs from the shear rate at the onset of shear thickening. By tracking particles, we demonstrate

Matthieu Roché; Hamid Kellay; Howard A. Stone

2011-01-01

222

Glycocalyx acting as a mechanotransducer of fluid shear stress  

E-print Network

It is widely recognized that fluid shearing forces acting on endothelial cells (ECs) have a profound effect on EC morphology, structure and function. Recent investigations in vivo have indicated the presence of a thick ...

Yao, Yu, Ph. D. Massachusetts Institute of Technology

2005-01-01

223

Stress response and structural transitions in sheared gyroidal and lamellar amphiphilic mesophases: lattice-Boltzmann simulations  

E-print Network

We report on the stress response of gyroidal and lamellar amphiphilic mesophases to steady shear simulated using a bottom-up lattice-Boltzmann model for amphiphilic fluids and sliding periodic (Lees-Edwards) boundary conditions. We study the gyroid per se (above the sponge-gyroid transition, of high crystallinity) and the molten gyroid (within such a transition, of shorter-range order). We find that both mesophases exhibit shear-thinning, more pronounced and at lower strain rates for the molten gyroid. At late times after the onset of shear, the skeleton of the crystalline gyroid becomes a structure of interconnected irregular tubes and toroidal rings, mostly oriented along the velocity ramp imposed by the shear, in contradistinction with free-energy Langevin-diffusion studies which yield a much simpler structure of disentangled tubes. We also compare the shear stress and deformation of lamellar mesophases with and without amphiphile when subjected to the same shear flow applied normal to the lamellae. We find that the presence of amphiphile allows (a) the shear stress at late times to be higher than in the case without amphiphile, and (b) the formation of rich patterns on the sheared interface, characterised by alternating regions of high and low curvature.

Nelido Gonzalez-Segredo; Jens Harting; Giovanni Giupponi; Peter V. Coveney

2006-01-20

224

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

PubMed

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

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

2013-08-26

225

Natural convection flow near a vertical plate that applies a shear stress to a viscous fluid.  

PubMed

The unsteady natural convection flow of an incompressible viscous fluid near a vertical plate that applies an arbitrary shear stress to the fluid is studied using the Laplace transform technique. The fluid flow is due to both the shear and the heating of the plate. Closed-form expressions for velocity and temperature are established under the usual Boussinesq approximation. For illustration purposes, two special cases are considered and the influence of pertinent parameters on the fluid motion is graphically underlined. The required time to reach the steady state in the case of oscillating shear stresses on the boundary is also determined. PMID:24278110

Rubbab, Qammar; Vieru, Dumitru; Fetecau, Corina; Fetecau, Constantin

2013-01-01

226

Natural Convection Flow near a Vertical Plate that Applies a Shear Stress to a Viscous Fluid  

PubMed Central

The unsteady natural convection flow of an incompressible viscous fluid near a vertical plate that applies an arbitrary shear stress to the fluid is studied using the Laplace transform technique. The fluid flow is due to both the shear and the heating of the plate. Closed-form expressions for velocity and temperature are established under the usual Boussinesq approximation. For illustration purposes, two special cases are considered and the influence of pertinent parameters on the fluid motion is graphically underlined. The required time to reach the steady state in the case of oscillating shear stresses on the boundary is also determined. PMID:24278110

Rubbab, Qammar; Vieru, Dumitru; Fetecau, Corina; Fetecau, Constantin

2013-01-01

227

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

Microsoft Academic Search

The results of optical and rheological experiments performed on a viscoelastic solution (cetyltrimethylammonium bromide + sodium salicylate in water) are reported. The flow curve has a horizontal plateau extending between two critical shear rates characteristic of heterogeneous flows formed by two layers of fluid with different viscosities. These two bands which also have different optical anisotropy are clearly seen by

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

2005-01-01

228

Proteomic Analysis of Shear Stress-Mediated Protection from TNF-alpha in Endothelial Cells  

PubMed Central

Previous studies have shown that physiological levels of shear stress can protect endothelial cells (ECs) from apoptotic stimuli. Here we differentiate between acute and chronic protection and demonstrate the use of proteomic technologies to uncover mechanisms associated with chronic protection of endothelial cells. We hypothesized that changes in abundance of proteins associated with the TNF-alpha signaling cascade orchestrate shear stress-mediated protection from TNF-alpha when cells are pre-conditioned with shear prior to the exposure of apoptotic stimuli. Detection of cleaved caspase 3 through Western blot analysis confirmed chronic shear-stress mediated protection from TNF-alpha. In the presence of the nitric oxide synthase (NOS) inhibitor, LMNA, chronic protection remained. Treatment with a de novo protein synthesis inhibitor, cycloheximide, eliminated this protective effect. Isotopic labeling experiments, coupled with liquid chromatography tandem mass spectrometry, (LC MS/MS) of isolated components of the TNF-alpha pathway revealed that CARD9, a known activator of the NF-?B pathway, was increased (60%) in sheared cells versus non-sheared cells. This result was confirmed through Western blot analysis. Our data suggests that de novo formation of proteins is required for protection from TNF-alpha in endothelial cells chronically exposed to shear stress, and that CARD9 is a candidate protein in this response. PMID:20536739

Freed, Julie K.; Greene, Andrew S.

2013-01-01

229

Diastolic Wall Shear Stress in the Internal Carotid Artery Is Associated with Different Cardiovascular Risk Factors than Systolic Wall Shear Stress  

Microsoft Academic Search

Background and Purpose: Wall shear stress (WSS) is the frictional force exerted by the circulating blood on the endothelium. Low systolic WSS is identified as an atherosclerotic risk factor. Recently, also the importance of diastolic WSS has been described. Still, it is unknown whether diastolic WSS carries similar cardiovascular risk factors compared to systolic WSS. Methods: Of 379 subjects (70–82

Inge H. Palm-Meinders; Frieke M. A. Box; Anton J. M. de Craen; Gerard J. Blauw; Mark A. van Buchem; Jeroen van der Grond

2009-01-01

230

Autonomous Effects of Shear Stress and Cyclic Circumferential Stretch regarding Endothelial Dysfunction and Oxidative Stress: An ex vivo Arterial Model  

Microsoft Academic Search

Cyclic circumferential stretch and shear stress caused by pulsatile blood flow work in concert, yet are very different stimuli capable of independently mediating endothelial function by modulating eNOS expression, oxidative stress (via production of superoxide anion) and NO bioavailability. Porcine carotid arteries were perfused using an ex vivo arterial support system for 72 h. Groups we created by combining normal

Tyler N. Thacher; Paolo Silacci; Nikos Stergiopulos; Rafaela F. da Silva

2010-01-01

231

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

PubMed Central

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

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

2008-01-01

232

Fibronectin and F-actin redistribution in cultured endothelial cells exposed to shear stress.  

PubMed

Cultured endothelial cells exposed to shear stresses in vitro undergo a reorganization of their F-actin-containing cytoskeletons which culminates in realignment with flow direction. Since a close transmembrane association exists between actin microfilaments and extracellular fibronectin, this study was undertaken to examine whether the actin reorganization induced by shear stress is accompanied by perturbations in the underlying fibronectin matrix. In a closed circulatory loop, bovine endothelial monolayers were exposed to steady, laminar flows corresponding to shear stress levels of 6 and 26 dynes/cm2 for 2, 6, 12, and 24 hours. The co-distribution of fibronectin and F-actin was determined in specimens which were double-labeled with antiserum to fibronectin and rhodamine phalloidin, respectively. Under the influence of shear stress, cells underwent coordinate shape changes resulting in varying degrees of alignment with flow direction. Reorientation at these shear stress levels was dependent on both the time of exposure and the magnitude of shear stress and was accompanied by a reorganization in cellular fibronectin and F-actin. In controls (no flow) correspondence between the two proteins was limited to similarly arranged, radial foci of fibronectin and F-actin filaments at the basal cell surfaces. In flow specimens, coincidence was detected only between occasional fibronectin fibrils and F-actin stress fibers. As a consequence of shear stress, fibronectin became more uniformly distributed beneath monolayers and frequently was organized into bands of densely packed fibrils. Despite this extensive reorganization, rearrangement of fibronectin did not result in the formation of identical, linear structures with F-. PMID:4068668

Wechezak, A R; Viggers, R F; Sauvage, L R

1985-12-01

233

Shear stress gradients remodel endothelial monolayers in vitro via a cell proliferation-migration-loss cycle.  

PubMed

Wall shear stress has been implicated in the genesis of atherosclerosis because a strong correlation exists between the location of developing arterial lesions and regions where particular gradients in stress occur. Studying the behavior of endothelial cells in such regions may contribute to our understanding of the disease etiology. We report the detailed migratory history of endothelial cells subjected to large shear stress gradients caused by a surface protuberance in an in vitro model system. The history of cell migration, cell division, and cell loss from the surface was continuously monitored in confluent human umbilical vein endothelial cell monolayers for 48 hours after the onset of flow. Individual cells were tracked using time-lapse video microscopy. In contrast to a uniform laminar flow field in which cells were observed to continually rearrange their relative position with no net migration, in a disturbed flow field there was a net migration directed away from the region of high shear gradient. This organized migration pattern under disturbed flow conditions was accompanied by more than a twofold increase in cell motility. In addition, cell division increased in the vicinity of the flow separation (maximum shear stress gradient of 34 dyne/cm2 per mm) whereas cell loss was increased upstream and downstream in the regions where the shear gradient diminishes. These data suggest a steady cell proliferation-migration-loss cycle and indicate that local shear stress gradient may play a key role in the morphological remodeling of the vascular endothelium in vivo. PMID:9409299

Tardy, Y; Resnick, N; Nagel, T; Gimbrone, M A; Dewey, C F

1997-11-01

234

Novel double path shearing interferometer in corneal topography measurements  

NASA Astrophysics Data System (ADS)

The paper presents an approach for measurements of corneal topography by use of a patent pending double path shearing interferometer (DPSI). Laser light reflected from the surface of the cornea is divided and directed to the inputs of two interferometers. The interferometers use lateral shearing of wavefronts in two orthogonal directions. A tilt of one of the mirrors in each interferometric setup perpendicularly to the lateral shear introduces parallel carrier frequency fringes at the output of each interferometer. There is orthogonal linear polarization of the laser light used in two DPSI. Two images of fringe patters are recorded by a high resolution digital camera. The obtained fringe patterns are used for phase difference reconstruction. The phase of the wavefront was reconstructed by use of algorithms for a large grid based on discrete integration. The in vivo method can also be used for tear film stability measurement, artificial tears and contact lens tests.

Licznerski, Tomasz J.; Jaronski, Jaroslaw; Kosz, Dariusz

2005-09-01

235

A flexible micromachined optical sensor for simultaneous measurement of pressure and shear force distribution on foot  

NASA Astrophysics Data System (ADS)

Lower limb complications associated with diabetes include the development of plantar ulcers that can lead to infection and subsequent amputation. While it is known from force plate analyses that there are medial/lateral and anterior/posterior shear components of the ground reaction force, there is little known about the actual distribution of this force during daily activities, nor about the role that shear plays in causing plantar ulceration. Furthermore, one critical reason why these data have not been obtained previously is the lack of a validated, widely used, commercially available shear sensor, in part because of the various technical issues associated with shear measurement. Here we have developed novel means of tranducing plantar shear and pressure stress via a new microfabricated optical system. The pressure/shear sensor consists of an array of optical waveguides lying in perpendicular rows and columns separated by elastomeric pads. A map of pressure and shear stress is constructed based on observed macro bending through the intensity attenuation from the physical deformation of two adjacent perpendicular optical waveguides. The uniqueness of the sensor is in its batch fabrication process, which involves injection molding and embossing techniques with Polydimethylsiloxane (PDMS) as the optical medium. Here we present the preliminary results of the prototype. The sensor has been shown to have low noise and responds linearly to applied loads. The smallest detectable force on each sensor element based on the current setup is ~0.1 N. The smallest area we have resolved in our mesh sensor is currently 950x950?m2

Wang, Wei-Chih; Panergo, Reynold R.; Galvanin, Christopher M.; Ledoux, William; Sangeorzan, Bruce; Reinhall, Per G.

2003-07-01

236

Sensor for Viscosity and Shear Strength Measurement  

SciTech Connect

Measurement of the physical properties (viscosity and density) of waste slurries is critical in evaluating transport parameters to ensure turbulent flow through transport pipes. The environment for measurement and sensor exposure is extremely harsh; therefore, reliability and ruggedness are critical in the sensor design. The work for this project will be performed in three phases. The first phase, carried out in FY96, involved (1) an evaluation of acoustic and other methods for viscosity measurement; (2) measurement of the parameters of slurries over the range of percent solids found in tanks and transport systems; (3) a comparison of physical properties (e.g., viscosity and density) to percent solids found composition; and (4) the design of a prototype sensor. The second phase (FY97) will involve the fabrication of a prototype hybrid sensor to measure the viscosity and mechanical properties of slurries in remote, high-radiation environments. Two different viscometer designs are being investigated in this study: a magnetostrictive pulse wave guide viscometer; an oscillating cylinder viscometer. In FY97, the Hemispheric Center for Environmental Technology (HCET) at Florida International University (FIU), which has printed circuit, thick film, thin film, and co-fired ceramic fabrication capability, will fabricate five probes for demonstration after technology selection and evaluation.

Dillon, J.; Moore, J.E. Jr.; Ebadian, M.A.; Jones, W.K.

1998-10-20

237

An analysis of riverbed shear stress in the context of plugging in bank filtration systems  

NASA Astrophysics Data System (ADS)

The Louisville Water Company (LWC) has investigated the alluvial aquifer upstream of the Falls of the Ohio River since the late 1940's, with an interest in securing an adequate volume of high quality water. A high-yield horizontal bank filtration collection well was put into operation at the B.E.Payne Water Treatment Plant in 1999, with a design capacity of 57,000 cubic meters/day and an initial yield of greater than 87,000 cubic meters/day. In the first 3 years of operation, the yield has decreased to a stable yield of 68,000 cubic meters/day. LWC desired a better understanding of the factors that affect long-term yield from riverbank filtation systems, and initiated a research program to provide data for better modeling and prediction of yield from such systems. Data were assembled from various agencies having information on flow, velocity profiles, slope, and sediment transport on the river, and particle size distribution of riverbed sediments and the aquifer. These data were used to calculate shear velocity, boundary shear stress, and friction slope. Riverbed shear stress was calculated at 7.5 Newtons/square meter at maximum river flow, and less than 1 Newton/square meter under average flow conditions. These data, along with aquifer characteristics and well yield, were used as a baseline for comparison with other river systems. Additional work is planned that will provide a direct measure of flux in the riverbed. This will allow the mapping of riverbed hydraulic conductivity in the vicinity of the well, and allow a spatial and temporal comparison to piezometric and well yield data.

Hubbs, S.

2003-04-01

238

Rac1 mediates laminar shear stress-induced vascular endothelial cell migration  

PubMed Central

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

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

2013-01-01

239

The FASEB Journal Research Communication Fluid shear stress primes mouse embryonic stem cells  

E-print Network

The FASEB Journal · Research Communication Fluid shear stress primes mouse embryonic stem cells the increasing use of perfusion culture in stem cell research, it is unclear how changes in the stem cell stress is a ubiquitous environmen- tal cue experienced by stem cells when they are being differentiated

Voldman, Joel

240

Measurement of mechanical properties of homogeneous tissue with ultrasonically induced shear waves  

NASA Astrophysics Data System (ADS)

Fundamental mechanical properties of tissue are altered by many diseases. Regional and systemic diseases can cause changes in tissue properties. Liver stiffness is caused by cirrhosis and fibrosis. Vascular wall stiffness and tone are altered by smoking, diabetes and other diseases. Measurement of tissue mechanical properties has historically been done with palpation. However palpation is subjective, relative, and not quantitative or reproducible. Elastography in which strain is measured due to stress application gives a qualitative estimate of Young's modulus at low frequency. We have developed a method that takes advantage of the fact that the wave equation is local and shear wave propagation depends only on storage and loss moduli in addition to density, which does not vary much in soft tissues. Our method is called shearwave dispersion ultrasonic velocity measurement (SDUV). The method uses ultrasonic radiation force to produce repeated motion in tissue that induces shear waves to propagate. The shear wave propagation speed is measured with pulse echo ultrasound as a function of frequency of the shear wave. The resulting velocity dispersion curve is fit with a Voight model to determine the elastic and viscous moduli of the tissue. Results indicate accurate and precise measurements are possible using this "noninvasive biopsy" method. Measurements in beef along and across the fibers are consistent with the literature values.

Greenleaf, James F.; Chen, Shigao

2007-03-01

241

Granular Shear Zone Formation: Acoustic Emission Measurements and Fiber-bundle Models  

NASA Astrophysics Data System (ADS)

We couple the acoustic emissions method with conceptual models of granular material behavior for investigation of granular shear zone formation and to assess eminence of landslide hazard. When granular materials are mechanically loaded or sheared, they tend to produce discrete events of force network restructuring, and frictional interaction at grain contacts. Such abrupt perturbations within the granular lattice release part of the elastic energy stored in the strained material. Elastic waves generated by such events can be measured as acoustic emissions (AE) and may be used as surrogates for intermittent structural transitions associated with shear zone formation. To experimentally investigate the connection between granular shearing and acoustic signals we performed an array of strain-controlled shear-frame tests using glass beads. AE were measured with two different systems operating at two frequency ranges. High temporal resolution measurements of the shear stresses revealed the presence of small fluctuations typically associated with low-frequency (< 20 kHz) acoustic bursts. Shear stress jumps and linked acoustic signals give account of discrete events of grain network rearrangements and obey characteristic exponential frequency-size distributions. We found that statistical features of force jumps and AE events depend on mechanical boundary conditions and evolve during the straining process. Activity characteristics of high-frequency (> 30 kHz) AE events is linked to friction between grains. To interpret failure associated AE signals, we adapted a conceptual fiber-bundle model (FBM) that describes some of the salient statistical features of failure and associated energy production. Using FBMs for the abrupt mechanical response of the granular medium and an associated grain and force chain AE generation model provides us with a full description of the mechanical-acoustical granular shearing process. Highly resolved AE may serve as a diagnostic tool not only for detection of shear zone development and straining in granular matter and but also for investigating internal grain scale mechanical processes. The AE method could be integrated into monitoring networks of landslide-prone slopes and other early warning systems for abrupt mass release (snow avalanches).

Michlmayr, Gernot; Or, Dani

2013-04-01

242

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

USGS Publications Warehouse

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

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

2003-01-01

243

Sensor for viscosity and shear strength measurement  

SciTech Connect

Measurement of the physical properties (viscosity and density) of waste slurries is critical in evaluating transport parameters to ensure turbulent flow through transport pipes. The environment for measurement and sensor exposure is extremely harsh; therefore, reliability and ruggedness are critical in the sensor design. Two different viscometer techniques are being investigated in this study, based on: magnetostrictive pulse generated acoustic waves; and an oscillating cylinder. Prototype sensors have been built and tested which are based on both techniques. A base capability instrumentation system has been designed, constructed, and tested which incorporates both of these sensors. It requires manual data acquisition and off-line calculation. A broad range of viscous media has been tested using this system. Extensive test results appear in this report. The concept for each technique has been validated by these test results. This base capability system will need to be refined further before it is appropriate for field tests. The mass of the oscillating system structure will need to be reduced. A robust acoustic probe assembly will need to be developed. In addition, in March 1997 it was made known for the first time that the requirement was for a deliverable automated viscosity instrumentation system. Since then such a system has been designed, and the hardware has been constructed so that the automated concept can be proved. The rest of the hardware, which interfaced to a computer, has also been constructed and tested as far as possible. However, for both techniques the computer software for automated data acquisition, calculation, and logging had not been completed before funding and time ran out.

Ebadian, M.A.; Dillion, J.; Moore, J.; Jones, K.

1998-01-01

244

Wall shear stress determination in boundary layers with unknown law of the wall by a modified Preston tube method  

NASA Astrophysics Data System (ADS)

Based on the concept of an extended Preston tube method for known law of the wall which permits consideration of additional boundary layer parameters, a modified, computer-aided measuring technique has been developed which appears to be capable of providing reasonable results for arbitrary 2-D flows with unknown law of the wall. Theoretical and experimental investigations are performed which result in a set of calibration curves as well as an iteration method for measuring wall shear stress in nonclassical boundary layer flows. Correction functions are derived leading to a simplified method of wall shear stress determination by means of iteration, and while simultaneously considering the influence of several boundary layer parameters. The new method is verified in the linear-turbulent transition flow of a flat plate and in the entrance flow of a pipe.

Haberland, C.; Nitsche, W.

245

A microfluidic shear device that accommodates parallel high and low stress zones within the same culturing chamber.  

PubMed

Fluid shear stress (FSS) plays a critical role in regulating endothelium function and maintaining vascular homeostasis. Current microfluidic devices for studying FSS effects on cells either separate high shear stress zone and low shear stress zone into different culturing chambers, or arranging the zones serially along the flow direction, which complicates subsequent data interpretation. In this paper, we report a diamond shaped microfluidic shear device where the high shear stress zone and the low shear stress zone are arranged in parallel within one culturing chamber. Since the zones with different shear stress magnitudes are aligned normal to the flow direction, the cells in one stress group are not substantially affected by the flow-induced cytokine/chemokine releases by cells in the other group. Cell loading experiments using human umbilical vein endothelial cells show that the device is able to reveal stress magnitude-dependent and loading duration-dependent cell responses. The co-existence of shear stress zones with varied magnitudes within the same culturing chamber not only ensures that all the cells are subject to the identical culturing conditions, but also allows the resemblance of the differential shear stress pattern in natural arterial conditions. The device is expected to provide a new solution for studying the effects of heterogeneous hemodynamic patterns in the onset and progression of various vascular diseases. PMID:25332743

Zhang, X; Huk, D J; Wang, Q; Lincoln, J; Zhao, Y

2014-09-01

246

Shear stress and VEGF enhance endothelial differentiation of human adipose-derived stem cells.  

PubMed

Herein we combine chemical and mechanical stimulation to investigate the effects of vascular endothelial growth factor (VEGF) and physiological shear stress in promoting the differentiation human adipose derived stem cells (ADSCs) into endothelial cells. ADSCs were isolated and characterized; endothelial differentiation was promoted by culturing confluent cells in 50?ng/ml VEGF under physiological shear stress for up to 14 days. Afterwards, endothelial cells were seeded onto collagen or acellular aortic valve matrices and exposed to four culture conditions: shear stress + VEGF; shear stress - VEGF; static + VEGF and static - VEGF. After 7 days, phenotype was investigated. ADSCs subjected to shear stress and VEGF express a comprehensive range of specific endothelial markers (vWF, eNOS and FLT-1 after 7 days and CD31, FLk-1 and VE-cadherin after 14 days) and maintain the phenotype when seeded onto scaffolds. Our protocol proved to be an efficient source of endothelial-like cells for tissue engineering based on autologous ADSC. PMID:25112491

Colazzo, Francesca; Alrashed, Fahad; Saratchandra, Padmini; Carubelli, Ivan; Chester, Adrian H; Yacoub, Magdi H; Taylor, Patricia M; Somers, Pamela

2014-10-01

247

Spatial Regulation of Inflammation by Human Aortic Endothelial Cells in a Linear Gradient of Shear Stress  

PubMed Central

Objective Atherosclerosis is a focal disease that develops at sites of low and oscillatory shear stress in arteries. This study aimed to understand how endothelial cells sense a gradient of fluid shear stress and transduce signals that regulate membrane expression of cell adhesion molecules and monocyte recruitment. Methods Human aortic endothelial cells were stimulated with TNF-? and simultaneously exposed to a linear gradient of shear stress that increased from 0 to 16 dyne/cm2. Cell adhesion molecule expression and activation of NF?B were quantified by immunofluorescence microscopy with resolution at the level of a single endothelial cell. Monocyte recruitment was imaged using custom microfluidic flow chambers. Results VCAM-1 and E-selectin upregulation was greatest between 2–4 dyne/cm2 (6 and 4-fold, respectively) and above 8 dyne/cm2 expression was suppressed below that of untreated endothelial cells. In contrast, ICAM-1 expression and NF?B nuclear translocation increased with shear stress up to a maximum at 9 dyne/cm2. Monocyte recruitment was most efficient in regions where E-selectin and VCAM-1 expression was greatest. Conclusions We found that the endothelium can sense a change in shear stress on the order of 0.25 dyne/cm2 over a length of ~10 cells, regulating the level of protein transcription, cellular adhesion molecule expression, and leukocyte recruitment during inflammation. PMID:18464160

TSOU, JEAN K.; GOWER, R. MICHAEL; TING, HAROLD J.; SCHAFF, ULRICH Y.; INSANA, MICHAEL F.; PASSERINI, ANTHONY G.; SIMON, SCOTT I.

2009-01-01

248

Fluid Shear Stress Alters the Hemostatic Properties of Endothelial Outgrowth Cells  

PubMed Central

Surface endothelialization is an attractive means to improve the performance of small diameter vascular grafts. While endothelial outgrowth cells (EOCs) are considered a promising source of autologous endothelium, the ability of EOCs to modulate coagulation-related blood activities is not well understood. The goal of this study was to assess the role of arterial flow conditions on the thrombogenic phenotype of EOCs. EOCs derived from baboon peripheral blood, as well as mature arterial endothelial cells from baboons, were seeded onto adsorbed collagen, then exposed to physiologic levels of fluid shear stress. For important hemostatic pathways, cellular responses to shear stress were characterized at the gene and protein level and confirmed with a functional assay for activated protein C (APC) activity. For EOCs, fluid shear stress upregulated gene and protein expression of anticoagulant and platelet inhibitory factors, including thrombomodulin, tissue factor pathway inhibitor, and nitric oxide synthase 3 (eNOS). Fluid shear stress significantly altered the functional activity of EOCs by increasing APC levels. This study demonstrates that fluid shear stress is an important determinant of EOC hemostatic properties. Accordingly, manipulation of EOC phenotype by mechanical forces may be important for the development of thrombo-resistant surfaces on engineered vascular implants. PMID:21787250

Ensley, Ann E.; Nerem, Robert M.; Anderson, Deirdre E.J.; Hanson, Stephen R.

2012-01-01

249

Pro-atherogenic shear stress and HIV proteins synergistically upregulate cathepsin K in endothelial cells.  

PubMed

Major advances in highly active antiretroviral therapies (HAART) have extended the lives of people living with HIV, but there still remains an increased risk of death by cardiovascular diseases (CVD). HIV proteins have been shown to contribute to cardiovascular dysfunction with effects on the different cell types that comprise the arterial wall. In particular, HIV-1 transactivating factor (Tat) has been shown to bind to endothelial cells inducing a range of responses that contribute to vascular dysfunction. It is well established that hemodynamics also play an important role in endothelial cell mediated atherosclerotic development. When exposed to low or oscillatory shear stress, such as that found at branches and bifurcations, endothelial cells contribute to proteolytic vascular remodeling by upregulating cathepsins, potent elastases and collagenases that contribute to altered biomechanics and plaque formation. Mechanisms to understand the influence of Tat on shear stress mediated vascular remodeling have not been fully elucidated. Using an in vivo HIV-Tg mouse model and an in vitro cone and plate shear stress bioreactor to actuate physiologically relevant pro-atherogenic or atheroprotective shear stress on human aortic endothelial cells, we have shown synergism between HIV proteins and pro-atherogenic shear stress to increase endothelial cell expression of the powerful protease cathepsin K, and may implicate this protease in accelerated CVD in people living with HIV. PMID:24719048

Parker, Ivana Kennedy; Roberts, Ladeidra Monet; Hansen, Laura; Gleason, Rudolph L; Sutliff, Roy L; Platt, Manu O

2014-06-01

250

Shear stress-induced mechanotransduction protein deregulation and vasculopathy in a mouse model of progeria  

PubMed Central

Introduction A mouse model of progeria derived by insertion of the human mutant LMNA gene (mLMNA), producing mutant lamin A, shows loss of smooth muscle cells in the media of the ascending aorta. We hypothesized that high shear stress, in the presence of mutant lamin A, induces this vasculopathy and tried to define the molecular and cellular basis for aortic vasculopathy. Methods Ascending and descending aortas from wild type (WT) and mLMNA+ mice were compared using proteomics, Western blots, PCR and immunostaining. To determine whether high fluidic shear stress, known to occur in the ascending aorta, contributed to the vasculopathy, we exposed descending aortas of mLMNA+ mice, with no apparent vasculopathy, to 75 dynes/cm2 shear stress for 30 minutes using a microfluidic system. Results When the mice were one year of age, expression of several mechanotransduction proteins in the ascending aorta, including vimentin, decreased in mLMNA+ mice but no decrease occurred in the descending aorta. High fluidic shear stress produced a significant reduction in vimentin of mLMNA+ mice but not in similarly treated WT mice. Conclusions The occurrence of mutant lamin A and high shear stress correlate with a reduction in the level of mechanotransduction proteins in smooth muscle cells of the media. Reduction of these proteins may contribute over time to development of vasculopathy in the ascending aorta in progeria syndrome. PMID:24661531

2014-01-01

251

In-situ shear stress indicator using heated strain gages at the flow boundary  

NASA Astrophysics Data System (ADS)

This work borrows the concept of hot-wire anemometry and sketch a technique that uses local heat transfer to infer the flow field and the corresponding stress. Conventional strain gages were mounted at the flow solid boundary as the heat source and acrylic boundary was chosen for its low thermal conductivity ensuring heat accumulation when a gage is energized. The gage would now work in slightly overheated state and its self-heating leads to an additional thermal strain. When exposed to a flow field, heat is brought away by local forced convection, resulting in deviations in gage signal from that developed in quiescent liquid. We have developed a facility to achieve synchronous gage measurements at different locations on a solid boundary. Three steady flow motions were considered: circular Couette flow, rectilinear uniform flow, and rectilinear oscillating flow. Preliminary tests show the gage reading does respond to the imposed flow through thermal effects and greater deviation was measured in flows of higher shear strain rates. The correlation between the gage signals and the imposed flow field is further examined by theoretical analysis. We also introduced a second solid boundary to the vicinity of the gage in the two rectilinear flows. The gage readings demonstrate rises in its magnitudes indicating wall amplification effect on the local shear strain, agreeing to the drag augmentation by a second solid boundary reported in many multiphase flow literatures.

Yeh, Chi-An; Yang, Fuling

2011-11-01

252

Shear stresses on megathrusts: Implications for mountain building behind subduction zones  

NASA Astrophysics Data System (ADS)

Shear stresses ? on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu-Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia (northern Sumatra) have been determined. The main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses, in the plane of a trench- or arc-normal section. For a typical brittle and ductile megathrust rheology, frictional shear stress ? = ??gz, for depth z, and ductile shear stress ? = A exp (B/RT) at temperature T, where ?, A, B are rheological parameters treated as constants. Rheological constants common to all the megathrusts (?crust, ?mantle, B) are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure, using a simplex minimization algorithm, taking account of the induced mantle corner flow at depth (65 ± 15 km (2?)) and constant radiogenic heating (0.65 ± 0.3 ?W m-3 (2?)) throughout the crust. The A constants are solved individually for each subduction zone, assuming that the maximum depth of interplate slip earthquakes marks the brittle-ductile transition. The best fit solution shows two groupings of megathrusts, with most subduction zones having a low mean shear stress in the range 7-15 MPa (?crust = 0.032 ± 0.006, ?mantle = 0.019 ± 0.004) and unable to support elevations >2.5 km. For a typical frictional sliding coefficient ˜0.5, the low effective coefficients of friction suggest high pore fluid pressures at ˜95% lithostatic pressure. Tonga and northern Chile require higher shear stresses with ?crust = 0.095 ± 0.024, ?mantle = 0.026 ± 0.007, suggesting slightly lower pore fluid pressures, at ˜81% lithostatic. Ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency, with B = 36 ± 18 kJ mol-1. Amantle values are sensitive to the precise value of B but are in the range 1-20 kPa. The power law exponent n for mantle flow is poorly constrained but is likely to be large (n > 4). The brittle-ductile transition in the crust occurs at temperatures in the range 370°C-512°C, usually close to the base of the crust and in the mantle at much lower temperatures (180°C-300°C), possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties. In subduction zones where the subducted slab is older than 50 Ma, a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are ?300°C. In much younger subduction zones, the stress transmission is confined mainly to the crust. The shear stresses, particularly in the crust, may be kept low by some sort of lubricant such as abundant water-rich trench fill, which lowers the frictional sliding coefficient or effective viscosity and/or raises pore fluid pressure. The unusual high stress subduction zone in northern Chile lacks significant trench fill and may be poorly lubricated, with a mean shear stress ˜37 MPa required to support elevations >4 km in the high Andes. However, where the crust is thin in sediment-starved and poorly lubricated subduction zones, such as Tonga, the mean shear stress will still be low. Sediment may lubricate megathrusts accommodating underthrusting of continental crust, such as in the Himalayas or eastern central Andes, which have a low mean shear stress ˜15 MPa.

Lamb, Simon

2006-07-01

253

Localized shear deformation and softening of bulk metallic glass: stress or temperature driven?  

PubMed

Metallic glasses due to their unique combination of physical and chemical properties have a great potential in various applications: materials for construction, medical, MEMs devices and so on. The deformation mechanism in metallic glasses is very much different from that in conventional crystalline materials and not yet fully understood. Here we are trying to find out what drives shear deformation in metallic glasses. The compression experiments of the bulk metallic glassy (BMG) samples coated with tin, Rose metal and indium were performed. There were no melting sites of the coating observed near individual shear bands. Melting occurred only near fracture surface, near microcracks and in the places of shear band concentrations. The results indicate that shear banding is rather a stress driven process while the temperature rise that was observed takes place due to friction forces in the viscous supercooled liquid thin layer in the shear bands. PMID:24100784

Ketov, S V; Louzguine-Luzgin, D V

2013-01-01

254

Transverse shear and normal stresses in a laminated anisotropic curved beam  

E-print Network

; tezial properties. The model used is n curved beam discretized into an arbitrary number of layers each of which can be treated as a homogeneous orthotropic lamina. The inside radius is taken to be the reference surface and 1' or generality each layer...' the beam or plate increases. the transverse normal stress will also increase and surpass the value ot' the transverse shear stress, becoming a, dominate mechanism in the delamination process. These stresses are often calculated using formulations based...

Ellington, Gerald Canada

2012-06-07

255

A novel shear reduction insole effect on the thermal response to walking stress, balance, and gait.  

PubMed

Shear stresses have been implicated in the formation of diabetes-related foot ulcers. The aim of this study was to evaluate the effect of a novel shear-reducing insole on the thermal response to walking, balance, and gait. Twenty-seven diabetes peripheral neuropathy patients were enrolled and asked to take 200 steps in both intervention and standard insoles. Thermal foot images of the feet were taken at baseline (1) following a 5-minute temperature acclimatization and (2) after walking. Testing order was randomized, and a 5-minute washout period was used between testing each insole condition. Sudomotor function was also assessed. Gait and balance were measured under single and dual task conditions using a validated body worn sensor system. The mean age was 65.1 years, height was 67.3 inches, weight was 218 pounds, and body mass index was 33.9, 48% were female, and 82% had type 2 diabetes. After walking in both insole conditions, foot temperatures increased significantly in standard insoles. The intervention insole significantly reduced forefoot and midfoot temperature increases (64.1%, P = .008; 48%, P = .046) compared to standard insoles. There were significant negative correlations with sudomotor function and baseline temperatures (r = .53-.57). The intervention demonstrated 10.4% less gait initiation double support time compared to standard insoles (P = .05). There were no differences in static balance measures. We found significantly lower forefoot and midfoot temperature increases following walking with shear-reducing insoles compared to standard insoles. We also found improvements in gait. These findings merit future study for the prevention of foot ulcer. PMID:25107709

Wrobel, James S; Ammanath, Peethambaran; Le, Tima; Luring, Christopher; Wensman, Jeffrey; Grewal, Gurtej S; Najafi, Bijan; Pop-Busui, Rodica

2014-11-01

256

Enhanced External Counterpulsation Inhibits Intimal Hyperplasia by Modifying Shear Stress-Responsive Gene Expression in Hypercholesterolemic Pigs  

Microsoft Academic Search

Background—Enhanced external counterpulsation (EECP) is a circulation assist device that may improve endothelial dysfunction by increasing shear stress. Chronic exposure of vascular endothelial cells and vascular smooth muscle cells to relatively high physiological shear stress has antiproliferative and vasoprotective effects. The present study hypothesizes that EECP inhibits intimal hyperplasia and atherogenesis by modifying shear stress-responsive gene expression. Methods and Results—Thirty-five

Yan Zhang; Xiaohong He; Xiaolin Chen; Hong Ma; Donghong Liu; Jinyun Luo; Zhimin Du; Yafei Jin; Yan Xiong; Jiangui He; Kuijian Wang; William E. Lawson; John C. K. Hui; Zhensheng Zheng; Guifu Wu

2010-01-01

257

Shear Stress Regulates Endothelial Nitric Oxide Synthase Expression Through c-Src by Divergent Signaling Pathways  

Microsoft Academic Search

In this study, we defined the signaling cascade responsible for increased eNOS mRNA expression in response to laminar shear stress. This pathway depends on the tyrosine kinase c-Src because shear induction of eNOS mRNA is blocked by the c-Src inhibitors PP1 and PP2, as well as an adenovirus encoding kinase inactive c-Src. After activation of c-Src, this pathway diverges. One

Michael E. Davis; Hua Cai; Grant R. Drummond; David G. Harrison

258

Viscosity, granular-temperature, and stress calculations for shearing assemblies of inelastic, frictional disks  

Microsoft Academic Search

Employing nonequilibrium molecular-dynamics methods the effects of two energy loss mechanisms on viscosity, stress, and granular-temperature in assemblies of nearly rigid, inelastic frictional disks undergoing steady-state shearing are calculated. Energy introduced into the system through forced shearing is dissipated by inelastic normal forces or through frictional sliding during collisions resulting in a natural steady-state kinetic energy density (granular-temperature) that depends

Otis R. Walton; ROBERT L. BRAUN

1986-01-01

259

Exercise-mediated changes in conduit artery wall thickness in humans: role of shear stress  

Microsoft Academic Search

Episodic increases in shear stress have been proposed as a mechanism that induces training-induced adaptation in arterial wall remodeling in humans. To address this hypothesis in humans, we examined bilateral brachial artery wall thickness using high-resolution ultrasound in healthy men across an 8-wk period of bilateral handgrip training. Unilaterally, shear rate was attenuated by cuff inflation around the forearm to

D. H. J. Thijssen; E. A. Dawson; I. C. van den Munckhof; T. M. Tinken; E. den Drijver; N. Hopkins; N. T. Cable; D. J. Green

2011-01-01

260

Hydrostatic and shear consolidation tests with permeability measurements on Waste Isolation Pilot Plant crushed salt  

SciTech Connect

Crushed natural rock salt is a primary candidate for use as backfill and barrier material at the Waste Isolation Pilot Plant (WIPP) and therefore Sandia National Laboratories (SNL) has been pursuing a laboratory program designed to quantify its consolidation properties and permeability. Variables that influence consolidation rate that have been examined include stress state and moisture content. The experimental results presented in this report complement existing studies and work in progress conducted by SNL. The experiments described in this report were designed to (1) measure permeabilities of consolidated specimens of crushed salt, (2) determine the influence of brine saturation on consolidation under hydrostatic loads, and 3) measure the effects of small applied shear stresses on consolidation properties. The laboratory effort consisted of 18 individual tests: three permeability tests conducted on specimens that had been consolidated at Sandia, six hydrostatic consolidation and permeability tests conducted on specimens of brine-saturated crushed WIPP salt, and nine shear consolidation and permeability tests performed on crushed WIPP salt specimens containing 3 percent brine by weight. For hydrostatic consolidation tests, pressures ranged from 1.72 MPa to 6.90 MPa. For the shear consolidation tests, confining pressures were between 3.45 MPa and 6.90 MPa and applied axial stress differences were between 0.69 and 4.14 MPa. All tests were run under drained conditions at 25{degrees}C.

Brodsky, N.S. [RE/SPEC, Inc., Rapid City, SD (United States)

1994-03-01

261

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

NASA Astrophysics Data System (ADS)

Numerical simulations are conducted to determine microstructure and rheology of sheared suspensions of charged colloidal particles at a volume fraction of ?=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 (?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 ?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.

Kumar, Amit; Higdon, Jonathan J. L.

2010-11-01

262

Exercise-Mediated Wall Shear Stress Increases Mitochondrial Biogenesis in Vascular Endothelium  

PubMed Central

Objective Enhancing structural and functional integrity of mitochondria is an emerging therapeutic option against endothelial dysfunction. In this study, we sought to investigate the effect of fluid shear stress on mitochondrial biogenesis and mitochondrial respiratory function in endothelial cells (ECs) using in vitro and in vivo complementary studies. Methods and Results Human aortic- or umbilical vein-derived ECs were exposed to laminar shear stress (20 dyne/cm2) for various durations using a cone-and-plate shear apparatus. We observed significant increases in the expression of key genes related to mitochondrial biogenesis and mitochondrial quality control as well as mtDNA content and mitochondrial mass under the shear stress conditions. Mitochondrial respiratory function was enhanced when cells were intermittently exposed to laminar shear stress for 72 hrs. Also, shear-exposed cells showed diminished glycolysis and decreased mitochondrial membrane potential (??m). Likewise, in in vivo experiments, mice that were subjected to a voluntary wheel running exercise for 5 weeks showed significantly higher mitochondrial content determined by en face staining in the conduit (greater and lesser curvature of the aortic arch and thoracic aorta) and muscle feed (femoral artery) arteries compared to the sedentary control mice. Interestingly, however, the mitochondrial biogenesis was not observed in the mesenteric artery. This region-specific adaptation is likely due to the differential blood flow redistribution during exercise in the different vessel beds. Conclusion Taken together, our findings suggest that exercise enhances mitochondrial biogenesis in vascular endothelium through a shear stress-dependent mechanism. Our findings may suggest a novel mitochondrial pathway by which a chronic exercise may be beneficial for vascular function. PMID:25375175

Kim, Boa; Lee, Hojun; Kawata, Keisuke; Park, Joon-Young

2014-01-01

263

Dynamic Shear Stress Regulation of Inflammatory and Thrombotic Pathways in Baboon Endothelial Outgrowth Cells  

PubMed Central

Endothelial outgrowth cells (EOCs) have garnered much attention as a potential autologous endothelial source for vascular implants or in tissue engineering applications due to their ease of isolation and proliferative ability; however, how these cells respond to different hemodynamic cues is ill-defined. This study investigates the inflammatory and thrombotic response of baboon EOCs (BaEOCs) to four hemodynamic conditions using the cone and plate shear apparatus: steady, laminar shear stress (SS); pulsatile, nonreversing laminar shear stress (PS); oscillatory, laminar shear stress (OS); and net positive, pulsatile, reversing laminar shear stress (RS). In summary, endothelial nitric oxide synthase (eNOS) mRNA was significantly upregulated by SS compared to OS. No differences were found in the mRNA levels of the inflammatory markers intercellular adhesion molecule-1 (ICAM-1), E-selectin, and vascular cell adhesion molecule-1 (VCAM-1) between the shear conditions; however, OS significantly increased the number of monocytes bound when compared to SS. Next, SS increased the anti-thrombogenic mRNA levels of CD39, thrombomodulin, and endothelial protein-C receptor (EPCR) compared to OS. SS also significantly increased CD39 and EPCR mRNA levels compared to RS. Finally, no significant differences were detected when comparing pro-thrombotic tissue factor mRNA or its activity levels. These results indicate that shear stress can have beneficial (SS) or adverse (OS, RS) effects on the inflammatory or thrombotic potential of EOCs. Further, these results suggest SS hemodynamic preconditioning may be optimal in increasing the efficacy of a vascular implant or in tissue-engineered applications that have incorporated EOCs. PMID:23406430

Hinds, Monica T.; Nerem, Robert M.

2013-01-01

264

Fracture transmissivity as a function of normal and shear stress: First results in Opalinus Clay  

NASA Astrophysics Data System (ADS)

Fracture transmissivity has been investigated along an idealised fracture for the influence of normal stress and for the transient behaviour during a slow shear experiment. A linear trend for the relationship between effective stress and transmissivity has been proposed for normal loads between 1 and 5 MPa; as effective stress increases transmissivity decreases. Transmissivity was very low throughout the complete spectrum of effective stresses examined and was close to the permeability for intact Opalinus Clay, suggesting that the fracture had effectively closed. During active shearing at a constant normal load, fracture transmissivity was seen to initially reduce, probably due to clear smearing. A series of flux events were seen, with transmissivity increasing by a factor of four. Some of the flux events corresponded with dilation, whilst others did not. This suggests that the opening flow paths were localised and did not result in bulk dilatancy. During the course of the shear test the sample formed its own series of fractures and a complex pattern of deformation occurred along the fracture surface to a depth of less than 1 mm. The impression of the end of the injection hole clearly shows that the block underwent at least 5 mm of the total 6 mm of shear displacement. The injection of fluorescein showed that flow along the fracture was not uniformly radial, as one might expect for such an experimental geometry. At the time of injection there were a number of dominant flow features, mainly in the direction of shear and only perpendicular on one side of the fracture surface. Flow occurred along the original fracture surface as well as the newly formed shear surface, indicating multiple pathways in a complex manner. The evolution of fracture transmissivity is very complex, even along initially planar surfaces. Fracture transmissivity has been seen to be a function of normal stress and porewater pressure, and has also been seen to be a dynamic feature during shear.

Cuss, Robert J.; Milodowski, Antoni; Harrington, Jon F.

265

Novel high bandwidth wall shear stress sensor for ultrasonic cleaning applications  

NASA Astrophysics Data System (ADS)

Ultrasonic cleaning is due to the action of cavitation bubbles. The details of the cleaning mechanisms are not revealed or confirmed experimentally, yet several studies suggest that the wall shear stresses generated are very high, i.e. of the order of several thousand Pascal. Ultrasonic cleaning applications span a wide range from semiconductor manufacturing, to low pressure membrane cleaning, and the in the medical field cleaning of surgical instruments. We have developed a novel sensor to monitor and quantify cleaning activity which is (1) very sturdy, (2) has a high bandwidth of several megahertz, (3) is cheap in manufacturing costs, and (4) of very small size. We analyze the sensor signal by comparing its response time correlated to single laser induced cavitation bubbles using high-speed photography. Additionally, we will present first measurements in ultrasonic cleaning bathes using again high-speed photography. A preliminary discussion on the working mechanism of the sensor will be presented.

Gonzalez-Avila, S. Roberto; Prabowo, Firdaus; Ohl, Claus-Dieter

2010-11-01

266

Integrating satellite observations with modelling: basal shear stress of the Filcher-Ronne ice streams, Antarctica.  

PubMed

Using inverse methods constrained by recent satellite observations, we have produced a comprehensive estimate of the basal shear stress beneath the Filchner-Ronne ice streams. The inversions indicate that a weak bed (approx. 4-20kPa) underlies much of these ice streams. Compared to the Ross ice streams, the distribution of weak subglacial till is more heterogeneous, with 'sticky spots' providing much of the resistance to flow. A weak bed beneath Recovery ice stream extends several hundred kilometres inland with flow. Along this ice stream, discrepancies between thickness measurements and flux estimates suggest the existence of a deep (-1400m) trough not resolved by existing maps of subglacial topography. We hypothesize that the presence of this and other deep troughs is a major influence on this sector of the ice sheet that is not fully incorporated in current models of ice-sheet evolution. PMID:16782610

Joughin, Ian; Bamber, Jonathan L; Scambos, Ted; Tulaczyk, Slawek; Fahnestock, Mark; MacAyeal, Douglas R

2006-07-15

267

Estimate Interface Shear Stress of Unidirectional C/SiC Ceramic Matrix Composites from Hysteresis Loops  

NASA Astrophysics Data System (ADS)

The tensile-tensile fatigue behavior of unidirectional C/SiC ceramic matrix composites at room and elevated temperature has been investigated. An approach to estimate the interface shear stress of ceramic matrix composites under fatigue loading has been developed. Based on the damage mechanisms of fiber sliding relative to matrix in the interface debonded region upon unloading and subsequent reloading, the unloading interface reverse slip length and reloading interface new slip length are determined by the fracture mechanics approach. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulatd in terms of interface shear stress. By comparing the experimental hysteresis loss energy with the computational values, the interface shear stress of unidirectional C/SiC ceramic composites corresponding to different cycles at room and elevated temperatures has been predicted.

Longbiao, Li; Yingdong, Song; Youchao, Sun

2013-08-01

268

Critical Shear Stress and Erosion Rates on the Western Adriatic Shelf  

NASA Astrophysics Data System (ADS)

One of the fundamental limitations on our ability to predict sediment transport rates in fine-grained marine settings is our poor understanding of spatial and temporal changes in critical shear stress and erosion rates. Temporal variations in these quantities can be the result of consolidation as well as seasonal and storm-related changes in deposition (e.g., degree of flocculation of settling particles) and bed surface properties (e.g., roughness, biogenic coatings, organic content). Spatial variations may be related to changes in sediment composition, grain size, water depth, distance from sediment sources and changes in hydrodynamic conditions. The western Adriatic shelf provides a low-to-moderate energy, fine-grained shelf setting with a large sediment source (the Po River) and a number of smaller sources along the Apennine coast. Critical shear stress and erosion rates were measured along the 20-m isobath between the Po River mouth and the Gargano Promontory during the winter (February) and spring/summer (June) of 2003 to investigate spatial and temporal effects on their values. Measurements were made using a Gust erosion chamber fitted onto the top of 10-cm-diameter core collected using a hydraulically damped short (~1 m) piston corer, which minimized disturbance to the bed surface. The erosion chamber applied a range of known stresses (0.01 - 0.4 Pa) to the sediment surface of the core, each of which was maintained for 20-30 min. Water in the chamber was replaced by pumping seawater into the chamber; water leaving the chambers passed through a turbidity sensor that recorded turbidity levels every 0.5s. The water was then collected in bottles, filtered, dried and weighed to obtain the mass of eroded sediment. The results were used to calibrate the turbidity measurements. Filtered material was combusted to obtain organic fraction. Two to four replicates were made at each site. The measurements show a clear seasonal signal with higher erosion rates during winter when organic content was lower. Erosion rates vary spatially, but not with any consistent alongshelf trend. Erosion rates are higher near several river mouths and toward the southern end of the sampling line. Parameterizations of erosion rates based on the measurements are being used in a shelf-sediment transport model and compared to near-bed tripod measurements of suspended-sediment concentration.

Wiberg, P.

2003-12-01

269

Plastic Coupling and Stress Relaxation During Nonproportional Axial-Shear Strain-Controlled Loading  

NASA Technical Reports Server (NTRS)

A nonproportional strain-controlled load path consisting of two segments was applied to the cobalt-based alloy Haynes 188 at 650 C. The first segment was purely axial; the aria1 strain was then held constant while the shear strain was increased during the second segment. The alloy exhibited about a 95-percent reduction in axial stress (298 to 15 MPa during shear straining. This reduction was due primarily to plastic coupling, but time-dependent stress relaxation also occurred. A rate-independent plasticity model approximated the stress reduction due to plastic coupling reasonably well, but as expected was unable to account for time-dependent stress relaxation. A viscoplasticity model capable of predicting the interaction between stress relaxation and plastic coupling also predicted the plastic coupling reasonably well. The accuracy of the viscoplastic model is shown to depend greatly upon the set of nonunique material parameters, which must be characterized from a sufficiently large range of load histories.

Lissenden, Cliff J.; Arnold, Steven M.; Saleeb, Atef F.

2001-01-01

270

Analyses of Failure Mechanisms and Residual Stresses in Graphite/Polyimide Composites Subjected to Shear Dominated Biaxial Loads  

NASA Technical Reports Server (NTRS)

This research contributes to the understanding of macro- and micro-failure mechanisms in woven fabric polyimide matrix composites based on medium and high modulus graphite fibers tested under biaxial, shear dominated stress conditions over a temperature range of -50 C to 315 C. The goal of this research is also to provide a testing methodology for determining residual stress distributions in unidirectional, cross/ply and fabric graphite/polyimide composites using the concept of embedded metallic inclusions and X-ray diffraction (XRD) measurements.

Kumosa, M.; Predecki, P. K.; Armentrout, D.; Benedikt, B.; Rupnowski, P.; Gentz, M.; Kumosa, L.; Sutter, J. K.

2002-01-01

271

A new parallel plate shear cell for in situ real-space measurements of complex fluids under shear flow  

NASA Astrophysics Data System (ADS)

We developed and tested a parallel plate shear cell that can be mounted on top of an inverted microscope to perform confocal real-space measurements on complex fluids under shear. To follow structural changes in time, a plane of zero velocity is created by letting the plates move in opposite directions. The location of this plane is varied by changing the relative velocities of the plates. The gap width is variable between 20 and 200?m with parallelism better than 1?m. Such a small gap width enables us to examine the total sample thickness using high numerical aperture objective lenses. The achieved shear rates cover the range of 0.02-103s-1. This shear cell can apply an oscillatory shear with adjustable amplitude and frequency. The maximum travel of each plate equals 1cm, so that strains up to 500 can be applied. For most complex fluids, an oscillatory shear with such a large amplitude can be regarded as a continuous shear. We measured the flow profile of a suspension of silica colloids in this shear cell. It was linear except for a small deviation caused by sedimentation. To demonstrate the excellent performance and capabilities of this new setup we examined shear induced crystallization and melting of concentrated suspensions of 1?m diameter silica colloids.

Wu, Yu Ling; Brand, Joost H. J.; van Gemert, Josephus L. A.; Verkerk, Jaap; Wisman, Hans; van Blaaderen, Alfons; Imhof, Arnout

2007-10-01

272

Quantifying turbulent wall shear stress in a subject specific human aorta using large eddy simulation.  

PubMed

In this study, large-eddy simulation (LES) is employed to calculate the disturbed flow field and the wall shear stress (WSS) in a subject specific human aorta. Velocity and geometry measurements using magnetic resonance imaging (MRI) are taken as input to the model to provide accurate boundary conditions and to assure the physiological relevance. In total, 50 consecutive cardiac cycles were simulated from which a phase average was computed to get a statistically reliable result. A decomposition similar to Reynolds decomposition is introduced, where the WSS signal is divided into a pulsating part (due to the mass flow rate) and a fluctuating part (originating from the disturbed flow). Oscillatory shear index (OSI) is plotted against time-averaged WSS in a novel way, and locations on the aortic wall where elevated values existed could easily be found. In general, high and oscillating WSS values were found in the vicinity of the branches in the aortic arch, while low and oscillating WSS were present in the inner curvature of the descending aorta. The decomposition of WSS into a pulsating and a fluctuating part increases the understanding of how WSS affects the aortic wall, which enables both qualitative and quantitative comparisons. PMID:22209366

Lantz, Jonas; Gårdhagen, Roland; Karlsson, Matts

2012-10-01

273

Rac1 and Cdc42 GTPases regulate shear stress-driven ?-catenin signaling in osteoblasts  

SciTech Connect

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

Wan, Qiaoqiao; Cho, Eunhye [Department of Biomedical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202 (United States)] [Department of Biomedical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202 (United States); Yokota, Hiroki [Department of Biomedical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202 (United States) [Department of Biomedical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202 (United States); Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, IN 46202 (United States); Na, Sungsoo, E-mail: sungna@iupui.edu [Department of Biomedical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202 (United States)] [Department of Biomedical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202 (United States)

2013-04-19

274

Representation of turbulent shear stress by a product of mean velocity differences  

NASA Technical Reports Server (NTRS)

A quadratic form in the mean velocity for the turbulent shear stress is presented. It is expressed as the product of two velocity differences whose roots are the maximum velocity in the flow and a cutoff velocity below which the turbulent shear stress vanishes. Application to pipe and channel flows yields the centerline velocity as a function of pressure gradient, as well as the velocity profile. The flat plate, boundary-layer problem is solved by a system of integral equations to obtain friction coefficient, displacement thickness, and momentum-loss thickness. Comparisons are made with experiment.

Braun, W. H.

1977-01-01

275

On-Chip Evaluation of Shear Stress Effect on Cytotoxicity of Mesoporous Silica Nanoparticles  

PubMed Central

In this work, nanotoxicity in the bloodstream was modeled and the cytotoxicity of sub-50 nm mesoporous silica nanoparticles to human endothelial cells was investigated under microfluidic flow conditions. Compared to traditional in vitro cytotoxicity assays performed under static conditions, unmodified mesoporous silica nanoparticles show higher and shear stress-dependent toxicity to endothelial cells under flow conditions. Interestingly, even under flow conditions, highly organo-modified mesoporous silica nanoparticles show no significant toxicity to endothelial cells. This paper clearly demonstrates that shear stress is an important factor to be considered in in vitro nanotoxicology assessments and provides a simple device for pursuing this consideration. PMID:22032307

Kim, Donghyuk; Lin, Yu-Shen; Haynes, Christy L.

2011-01-01

276

Spatial relationships between shearing stresses and pressure on the plantar skin surface during gait.  

PubMed

Based on the hypothesis that diabetic foot lesions have a mechanical etiology, extensive efforts have sought to establish a relationship between ulcer occurrence and plantar pressure distribution. However, these factors are still not fully understood. The purpose of this study was to simultaneously record shear and pressure distributions in the heel and forefoot and to answer whether: (i) peak pressure and peak shear for anterior-posterior (AP) and medio-lateral (ML) occur at different locations, and if (ii) peak pressure is always centrally located between sites of maximum AP and ML shear stresses. A custom built system was used to collect shear and pressure data simultaneously on 11 subjects using the 2-step method. The peak pressure was found to be 362 kPa ± 106 in the heel and 527 kPa ± 123 in the forefoot. In addition, the average peak shear values were higher in the forefoot than in the heel. The greatest shear on the plantar surface of the forefoot occurred in the anterior direction (mean and std. dev.: 37.7 ± 7.6 kPa), whereas for the heel, peak shear the foot was in the posterior direction (21.2 ± 5 kPa). The results of this study suggest that the interactions of the shear forces caused greater "spreading" in the forefoot and greater tissue "dragging" in the heel. The results also showed that peak shear stresses do not occur at the same site or time as peak pressure. This may be an important factor in locating where skin breakdown occurs in patients at high-risk for ulceration. PMID:22169152

Stucke, Samantha; McFarland, Daniel; Goss, Larry; Fonov, Sergey; McMillan, Grant R; Tucker, Amy; Berme, Necip; Cenk Guler, Hasan; Bigelow, Chris; Davis, Brian L

2012-02-01

277

Shear Stress Transmission Model for the Flagellar Rotary Motor  

PubMed Central

Most bacteria that swim are propelled by flagellar filaments, which are driven by a rotary motor powered by proton flux. The mechanism of the flagellar motor is discussed by reforming the model proposed by the present authors in 2005. It is shown that the mean strength of Coulomb field produced by a proton passing the channel is very strong in the Mot assembly so that the Mot assembly can be a shear force generator and induce the flagellar rotation. The model gives clear calculation results in agreement with experimental observations, e g., for the charasteristic torque-velocity relationship of the flagellar rotation. PMID:19325821

Mitsui, Toshio; Ohshima, Hiroyuki

2008-01-01

278

Theory and Practice of Shear/Stress Strain Gage Hygrometry  

NASA Technical Reports Server (NTRS)

Mechanical hygrometry has progressed during the last several decades from crude hygroscopes to state-of-the art strain-gage sensors. The strain-gage devices vary from different metallic beams to strain-gage sensors using cellulose crystallite elements, held in full shear restraint. This old technique is still in use but several companies are now actively pursuing development of MEMS miniaturized humidity sensors. These new sensors use polyimide thin film for water vapor adsorption and desorption. This paper will provide overview about modern humidity sensors.

Shams, Qamar A.; Fenner, Ralph L.

2006-01-01

279

Application of speckle shearing interferometry to strain measurement of composites  

SciTech Connect

The demand of experimental data for designing composite structure is increased. This paper reports the in-plane strain measurement of composite materials using speckle shearing interferometric technique. The fringe patterns depicting the derivatives of the displacements are shown. The extracted strain data are used to determine the material properties, Young`s modulus and Poisson`s ratio. The experimental result compares well with available existing data.

Chen, T.Y.; Fan, I.L. [National Cheng Kung Univ., Tainan (Taiwan, Province of China). Dept. of Mechanical Engineering

1993-12-31

280

Wall shear stress oscillation and its gradient in the normal left coronary artery tree bifurcations  

PubMed Central

Background: It is known that blood flow properties such as low/ oscillatory wall shear stress (WSS), high blood viscosity, low blood velocity and high concentration of low density lipoprotein (LDL) macromolecules, are some of the main flow parameters causing atherosclerosis. Limited research has been undertaken on the pulsatile WSS and WSS gradient (WSSG) analysis focusing in the differentiation between the bifurcation itself and the lateral to it walls in a normal left coronary artery (LCA). The results obtained show the flow characteristics and qualify the spatial and temporal distribution of WSS ant its gradient in regions close to the LCA tree flow dividers and in opposite to them areas. Methods: A 3D computer generated model of the LCA tree based on averaged human data extracted from angiographies was developed for computational fluid dynamics analysis. Physiological phasic flow velocity is incorporated as entrance boundary condition. Results: The instantaneous min wall shear stress oscillates from 0.45 to 2.84 N/m2 at the flow divider and from 0.25 to 1.28 N/m2 at the lateral walls of the main bifurcation. However, for the D1-S1 bifurcation (first diagonal-first septal), the instantaneous min wall shear stress oscillates from 0.6 to 3.85 N/m2 at the flow divider and from 0.6 to 2.65 N/m2 at the lateral walls. Mean wall shear stress, from max systole to max diastole, experiences a 129.0 % increase at the main bifurcation flow divider. The difference between max and min wall shear stress for the flow divider of the main bifurcation, as it is compared with the max wall shear stress over the entire cardiac pulse, attains a maximum value of 81.1 % for the lateral walls and 60.0 % at the peak of diastole. At the D1-S1 bifurcation, the corresponding difference values are 69.0% and 57.0 % for the lateral walls and flow divider, respectively. The mean wall shear stress gradient experiences a 123.0 % increase from max systole to max diastole at the main bifurcation flow divider and 153.0 % at main bifurcation lateral walls. Conclusions: Proximal LCA bifurcation exhibit lower spatial wall shear stress and lower wall shear stress gradient values compared to distal bifurcations. The lateral walls compared to the bifurcation itself are exposed to low WSS and WSSG. With regards to the temporal variation, wall shear stress and its gradient exhibited lower values throughout systole as compared to diastole, suggesting a possible atherogenic effect of both the systolic phase by itself as well as the phasic oscillation of wall shear stress and its gradient from systole to diastole. PMID:25125945

Soulis, JV; Fytanidis, DK; Seralidou, KV; Giannoglou, GD

2014-01-01

281

Shear and transverse stress in a thin superconducting layer in simplified coated conductor architecture with a pre-existing detachment  

NASA Astrophysics Data System (ADS)

The thermal-electro-mechanical behavior of second generation YBa2Cu3O7-? coated conductors subjected to thermal cooling and electric charging is investigated in this paper. Shear stress at the interface and transverse normal stress in the film are analyzed through the elasticity theory. The plane strain approach is assumed, and a singular integral equation governing the problem is derived in terms of the interfacial shear stress, thermal strain, and the electromagnetic force. After that, we evaluate the shear and transverse stress distribution in the conductor during cool down and electric charging, respectively. The results show that large residual compressive stress (about 150 MPa) and interfacial shear stress (about 400 MPa) remain in the conductor during cool down. In the electric charging process while small compared to the thermal one destructive tensile stress present near the edges of the conductor. Effects of the stiffness of substrate on the stress distribution in the conductor are investigated also.

Jing, Ze; Yong, Huadong; Zhou, Youhe

2013-07-01

282

Proteomic analysis of Staphylococcus aureus biofilm cells grown under physiologically relevant fluid shear stress conditions  

PubMed Central

Background The biofilm forming bacterium Staphylococcus aureus is responsible for maladies ranging from severe skin infection to major diseases such as bacteremia, endocarditis and osteomyelitis. A flow displacement system was used to grow S. aureus biofilms in four physiologically relevant fluid shear rates (50, 100, 500 and 1000 s-1) to identify proteins that are associated with biofilm. Results Global protein expressions from the membrane and cytosolic fractions of S. aureus biofilm cells grown under the above shear rate conditions are reported. Sixteen proteins in the membrane-enriched fraction and eight proteins in the cytosolic fraction showed significantly altered expression (p?shear. These 24 proteins were identified using nano-LC-ESI-MS/MS. They were found to be associated with various metabolic functions such as glycolysis / TCA pathways, protein synthesis and stress tolerance. Increased fluid shear stress did not influence the expression of two important surface binding proteins: fibronectin-binding and collagen-binding proteins. Conclusions The reported data suggest that while the general metabolic function of the sessile bacteria is minimal under high fluid shear stress conditions, they seem to retain the binding capacity to initiate new infections. PMID:24855455

2014-01-01

283

X-ray Scattering Measurements of Particle Orientation in a Sheared Polymer/Clay Dispersion  

SciTech Connect

We report steady and transient measurements of particle orientation in a clay dispersion subjected to shear flow. An organically modified clay is dispersed in a Newtonian polymer matrix at a volume fraction of 0.02, using methods previously reported by Mobuchon et al. (Rheol Acta 46: 1045, 2007). In accord with prior studies, mechanical rheometry shows yield stress-like behavior in steady shear, while time dependent growth of modulus is observed following flow cessation. Measurements of flow-induced orientation in the flow-gradient plane of simple shear flow using small-angle and wide-angle X-ray scattering (SAXS and WAXS) are reported. Both SAXS and WAXS reveal increasing particle orientation as shear rate is increased. Partial relaxation of nanoparticle orientation upon flow cessation is well correlated with time-dependent changes in complex modulus. SAXS and WAXS data provide qualitatively similar results; however, some quantitative differences are attributed to differences in the length scales probed by these techniques.

Pujari, Saswati; Dougherty, Leah; Mobuchon, Christoph; Carreau, Pierre J.; Heuzey, Marie-Claude; Burghardt, Wesley R. (Ecole); (NWU); (Ecole)

2012-01-20

284

Modified Shear Box Test Apparatus for Measuring Shear Strength of Unsaturated Residual Soil  

Microsoft Academic Search

Abstract: Residual soils occur in most countries of the world but the greater areas and depths are normally found in tropical humid areas. Most of these soils exhibit high suctions for most of the year. The shear strength parameters, c’ and ? ’, of soil can be obtained using conventional shear strength tests. However the conventional shear strength test equipments

Bujang B. k. Huat; Faisal Hj. Ali; S. Hashim

2005-01-01

285

Dipole shear anisotropy logging  

SciTech Connect

Sonic-scale shear anisotropy of formations are obtained from dipole-shear measurements acquired by orthogonal source and receiver pairs. These measurements resemble miniature 4-component shear VSP surveys inside the borehole. Data from two orthogonal sources and an array of orthogonal receivers are processed providing three main logs; the fast and slow shear slownesses, and the fast shear polarization angle. Potential applications include: shale anisotropy for better seismic models, maximum stress and fracture/microcrack strike direction for horizontal well drilling and fracture height and azimuth determination in hydraulic fracturing.

Esmersoy, C.; Kane, M. [Schlumberger-Doll Research, Ridgefield, CT (United States); Koster, K.; Williams, M. [Amoco Production, Denver, CO (United States); Boyd, A. [GeoQuest, Aurora, CO (United States)

1994-12-31

286

Endothelial Cell Membrane Sensitivity to Shear Stress is Lipid Domain Dependent  

PubMed Central

Blood flow-associated shear stress causes physiological and pathophysiological biochemical processes in endothelial cells that may be initiated by alterations in plasma membrane lipid domains characterized as liquid-ordered (lo), such as rafts or caveolae, or liquid-disordered (ld). To test for domain–dependent shear sensitivity, we used time-correlated single photon counting instrumentation to assess the photophysics and dynamics of the domain-selective lipid analogues DiI-C12 and DiI-C18 in endothelial cells subjected to physiological fluid shear stress. Under static conditions, DiI-C12 fluorescence lifetime was less than DiI-C18 lifetime and the diffusion coefficient of DiI-C12 was greater than the DiI-C18 diffusion coefficient, confirming that DiI-C12 probes ld, a more fluid membrane environment, and DiI-C18 probes the lo phase. Domains probed by DiI-C12 exhibited an early (10 s) and transient decrease of fluorescence lifetime after the onset of shear while domains probed by DiI-C18 exhibited a delayed decrease of fluorescence lifetime that was sustained for the 2 min the cells were subjected to flow. The diffusion coefficient of DiI-C18 increased after shear imposition, while that of DiI-C12 remained constant. Determination of the number of molecules (N) in the control volume suggested that DiI-C12-labeled domains increased in N immediately after step-shear, while N for DiI-C18-stained membrane transiently decreased. These results demonstrate that membrane microdomains are differentially sensitive to fluid shear stress. PMID:22247740

Tabouillot, Tristan; Muddana, Hari S.; Butler, Peter J.

2012-01-01

287

Atrial Fibrillation Pacing Decreases Intravascular Shear Stress in a New Zealand White Rabbit Model: Implications in Endothelial Function  

PubMed Central

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 (??/?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 ?avg=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 ??/?t, and rapid pulse rates modulate endothelial responses. PMID:22983703

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

288

Evolution of shear stress, protein expression, and vessel area in an animal model of arterial dilatation in hemodialysis grafts  

PubMed Central

Purpose To evaluate the wall shear stress, protein expression of matrix metalloproteinases-2 (MMP-2), -9 (MMP-9), and the inhibitors (tissue inhibitor of matrix metalloproteinases-1 (TIMP-1), and -2 (TIMP-2)), and vessel area over time in a porcine model for hemodialysis polytetrafluoroethylene (PTFE) grafts. Materials and methods In 21 pigs, subtotal renal infarction was performed and 28 days later, a PTFE graft was placed to connect the carotid artery to the ipsilateral jugular vein. Phase contrast MR was used to measure blood flow and vessel area at 1, 3, 7, and 14 days after graft placement. Wall shear stress was estimated from Poiseuille’s law. Animals were sacrificed at day 3 (N=7), day 7 (N=7), and day 14 (N=7) and expression of MMP-2, MMP-9, TIMP-1, and TIMP-2 were determined at the grafted and control arteries. Results The mean wall shear stress of the grafted artery was higher than the control artery at all time points (P<0.05). It peaked by day 3 and decreased by days 7–14 as the vessel area nearly doubled. By days 7–14, there was a significant increase in active MMP-2 followed by a significant increase in pro and active MMP-9 by day 14 (P<0.05, grafted artery versus control). TIMP-1 expression peaked by day 7 and then decreased while TIMP-2 expression was decreased at days 7–14. Conclusions The wall shear stress of the grafted artery peaks by day 3 with increased MMP-2 activity by days 7–14 followed by pro and active MMP-9 by day 14 and the vessel area nearly doubled. PMID:20123196

Misra, Sanjay; Fu, Alex A.; Misra, Khamal D.; Glockner, James F.; Mukhopadyay, Debabrata

2010-01-01

289

Ageing under Shear: Effect of Stress and Temperature Field  

NASA Astrophysics Data System (ADS)

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

Shukla, Asheesh; Joshi, Yogesh M.

2008-07-01

290

Review of the shear-stress transport turbulence model experience from an industrial perspective  

Microsoft Academic Search

The present author was asked to provide an update on the status and the more recent developments around the shear-stress transport (SST) turbulence model for this special issue of the journal. The article is therefore not intended as a comprehensive overview of the status of engineering turbulence modelling in general, nor on the overall turbulence modelling strategy for ANSYS computational

Florian R. Menter

2009-01-01

291

Cell-matrix adhesion characterization using multiple shear stress zones in single stepwise microchannel  

NASA Astrophysics Data System (ADS)

This paper presents a cell chip capable to characterize cell-matrix adhesion by monitoring cell detachment rate. The proposed cell chip can supply multiple levels of shear stress in single stepwise microchannel. As epithelial-mesenchymal transition (EMT), one of hallmarks of cancer metastasis is closely associated to the interaction with extracelluar matrix (ECM), we took advantage of two lung cancer cell models with different adhesion properties to ECM depending their epithelial or mesenchymal properties, including the pair of lung cancer cells with (A549sh) or without E-cadherin expression (A549sh-Ecad), which would be optimal model to examine the alteration of adhesion properties after EMT induction. The cell-matrix adhesion resisting to shear stress appeared to be remarkably differed between lung cancer cells. The detachment rate of epithelial-like H358 and mesenchymal-like H460 cells was 53%-80% and 25%-66% in the shear stress range of 34-60 dyn/cm2, respectively. A549sh-Ecad cells exhibits lower detachment rate (5%-9%) compared to A549sh cells (14%-40%). By direct comparison of adhesion between A549sh and A549sh-Ecad, we demonstrated that A549shE-cad to mimic EMT were more favorable to the ECM attachment under the various levels of shear stress. The present method can be applied to quantitative analysis of tumor cell-ECM adhesion.

Kim, Min-Ji; Doh, Il; Bae, Gab-Yong; Cha, Hyuk-Jin; Cho, Young-Ho

2014-08-01

292

Quantitative morphodynamics of endothelial cells within confluent cultures in response to fluid shear stress.  

PubMed Central

To evaluate shear stress-induced effects on cultured cells we have extended the mechanical setup of a multichannel in vitro rheological system and developed software allowing entire processing control and image data analysis. The values of cell motility, degree of orientation (alignment), and cell elongation were correlated as a function of time (morphodynamics). Collective and individual endothelial cells within confluent cultures displayed a shear stress-dependent characteristic phase behavior of the following time course: resting conditions (phase I), change of motility (phase II), onset of alignment (phase III), and finally cell elongation (phase IV). Especially cell motility was characterized by a randomized zigzag movement around mean trajectories (fluctuations) together with mean cell locomotion. Onset of shear stress caused a down-regulation of fluctuations of 30% within <10 min and simultaneously increased locomotion velocities preferring the flow direction (phase II). After a lag period of 10 to 20 min cells orientated in the direction of flow (phase III) without significant cell elongation, which finally occurs within hours (phase IV). These data provide first evidence that cells within confluent endothelial monolayers respond to shear stress with a characteristic phase behavior. PMID:10968992

Dieterich, P; Odenthal-Schnittler, M; Mrowietz, C; Kramer, M; Sasse, L; Oberleithner, H; Schnittler, H J

2000-01-01

293

On the effects of stenting on wall shear stress Nicoud Franck1  

E-print Network

On the effects of stenting on wall shear stress Nicoud Franck1 , Vernhet Hélène2 1 University-renal aorta of five New-Zealand white rabbits. With a fundamental frequency of 4 Hz (heart rate close to 240 arteries. The use of intravascular stents tends to lower the complication rate, although restenosis rates

Nicoud, Franck

294

The calculation of stress intensity factors for combined tensile and shear loading  

Microsoft Academic Search

Stress intensity calculations are presented for cases of combined tensile and shear loading for a linear elastic material. Using functions of a complex variable, a theory is developed to determine the direction of maximum energy release rate. A finite element method using virtual crack extensions is also used to determine the energy release rate for crack extensions in various directions

T. K. Hellen; W. S. Blackburn

1975-01-01

295

Laminar shear stress regulates mitochondrial dynamics, bioenergetics responses and PRX3 activation in endothelial cells.  

PubMed

Endothelial cells in the vascular system are constantly subjected to the frictional force of shear stress due to the pulsatile nature of blood flow. Although several proteins form part of the shear stress mechano-sensing pathway, the identification of mechano-transducing pathways is largely unknown. Given the increasing evidence for a signaling function of mitochondria in endothelial cells, the aim of this study was to investigate their role as mechano-sensor organelles during laminar shear stress (LSS). We demonstrated that LSS activates intracellular signaling pathways that modulate not only mitochondrial dynamics but also mitochondrial function. At early time points of LSS, the fission-related protein Drp1 was recruited from the cytosol to mitochondria and activated mitochondrial fission. LSS-dependent increase in intracellular Ca(2+) concentration was indispensable for mitochondrial fission. As alterations in mitochondrial dynamics have been related to changes in bioenergetics profiles, we studied mitochondrial function after LSS. We found that LSS decreased respiration rate, increased mitochondrial membrane potential and promoted the mitochondrial generation of ROS with the subsequent oxidation and activation of the antioxidant enzyme PRX3. Our data support a novel and active role for mitochondria in endothelial cells as active players, able to transduce the mechanical force of shear stress in the vascular endothelium into a biological response. PMID:25038307

Bretón-Romero, Rosa; Acín-Perez, Rebeca; Rodríguez-Pascual, Fernando; Martínez-Molledo, María; Brandes, Ralf P; Rial, Eduardo; Enríquez, José A; Lamas, Santiago

2014-11-01

296

ORAL/POSTER REFERENCE: ICF100374OR SHEAR LAG MODELLING OF THERMAL STRESSES IN  

E-print Network

structures that transfer loads to one another through shear stresses within the matrix material. The original of the Hedgepeth[1] model was proposed by Beyerlein and Landis[3] to include the effects of matrix stiffness and temperature along any given fiber or matrix region are developed. The derivation of the governing equations

297

The control of endothelial cell adhesion and migration by shear stress and matrix-substrate anchorage.  

PubMed

Endothelial cells constitute the natural inner lining of blood vessels and possess anti-thrombogenic properties. This characteristic is frequently used by seeding endothelial cells on vascular prostheses. As the type of anchorage of adhesion ligands to materials surfaces is known to determine the mechanical balance of adherent cells, we investigated herein the behaviour of endothelial cells under physiological shear stress conditions. The adhesion ligand fibronectin was anchored to polymer surfaces of four physicochemical characteristics exhibiting covalent and non-covalent attachment as well as high and low hydrophobicity. The in situ analysis combined with cell tracking of shear stress-induced effects on cultured isolated cells and monolayers under venous (0.5 dyn/cm(2)) and arterial (12 dyn/cm(2)) shear stress over a time period of 24 h revealed distinct differences in their morphological and migratory features. Most pronounced, unidirectional and bimodal migration patterns of endothelial cells in or against flow direction were found in dependence on the type of substrate-matrix anchorage. Combined by an immunofluorescent analysis of the actin cytoskeleton, cell-cell junctions, cell-matrix adhesions, and matrix reorganization these results revealed a distinct balance of laminar shear stress, cell-cell contacts and substrate-matrix anchorage in affecting endothelial cell fate under flow conditions. This analysis underlines the importance of materials surface parameters as well as primary and secondary adhesion ligand anchorage in the context of artificial blood vessels for future therapeutic devices. PMID:22154622

Teichmann, Juliane; Morgenstern, Alexander; Seebach, Jochen; Schnittler, Hans-Joachim; Werner, Carsten; Pompe, Tilo

2012-03-01

298

Helical flows of second grade fluid due to constantly accelerated shear stresses  

Microsoft Academic Search

The helical flows of second grade fluid between two infinite coaxial circular cylinders is considered. The motion is produced by the inner cylinder that at the initial moment applies torsional and longitudinal constantly accelerated shear stresses to the fluid. The exact analytic solutions, obtained by employing the Laplace and finite Hankel transforms and presented in series form in term of

M. Jamil; A. Rauf; C. Fetecau; N. A. Khan

2011-01-01

299

In vitro shear stress-induced platelet activation: sensitivity of human and bovine blood.  

PubMed

As platelet activation plays a critical role in physiological hemostasis and pathological thrombosis, it is important in the overall hemocompatibility evaluation of new medical devices and biomaterials to assess their effects on platelet function. However, there are currently no widely accepted in vitro test methods to perform this assessment. In an effort to develop effective platelet tests for potential use in medical device evaluation, this study compared the sensitivity of platelet responses to shear stress stimulation of human and bovine blood using multiple platelet activation markers. Fresh whole blood samples anticoagulated with heparin or anticoagulant citrate dextrose, solution A (ACDA) were exposed to shear stresses up to 40?Pa for 2?min using a cone-and-plate rheometer model. Platelet activation was characterized by platelet counts, platelet surface P-selectin expression, and serotonin release into blood plasma. The results indicated that exposure to shear stresses above 20?Pa caused significant changes in all three of the platelet markers for human blood and that the changes were usually greater with ACDA anticoagulation than with heparin. In contrast, for bovine blood, the markers did not change with shear stress stimulation except for plasma serotonin in heparin anticoagulated blood. The differences observed between human and bovine platelet responses suggest that the value of using bovine blood for in vitro platelet testing to evaluate devices may be limited. PMID:23738621

Lu, Qijin; Hofferbert, Bryan V; Koo, Grace; Malinauskas, Richard A

2013-10-01

300

American Institute of Aeronautics and Astronautics A MEMS Floating Element with Bump Shear Stress Sensor  

E-print Network

and temporal resolution. In the last two decades, MEMS-based shear stress sensors have emerged as a possible-9]. The previous MEMS-based sensors to appear in the literature have two common features: 1) all MEMS wafer and calibrated only up to 1~2 Pa; 2) A majority of MEMS-based sensors besides floating element

White, Robert D.

301

Role of subcellular shear-stress distributions in endothelial cell mechanotransduction.  

PubMed

The endothelium of blood vessels presents a wavy surface to the flowing blood. The subcellular distribution of shear stress depends on the shape and orientation of the cells and on their spatial arrangement within the monolayer. By studying details of the distribution of stress at this scale and the morphological responses that serve to modify the distribution, we can gain insight into the physical mechanisms by which the cell senses its fluid mechanical environment. The rapidly growing body of evidence indicates that endothelial cells discriminate between subtle variations in the exact loading conditions including differences in temporal and spatial gradients of shear stress, steady and pulsatile laminar flow, and laminar and turbulent flows. While in a few studies the effects of these individual flow characteristics have been carefully isolated, it is difficult to assess the relative importance of any one parameter. To interpret the relationships between isolated flow characteristics or the integrated effects of combined loading conditions and the biochemical signaling events that mediate the cell response, a full stress analysis of the cell is needed. The microscopic distribution of shear stress acting upon the cell surface provides the boundary condition for such an analysis. Experimental and analytical tools are being developed to assess the stress distribution throughout the cellular structures that might be involved in mechanotransduction. PMID:12085999

Barbee, Kenneth A

2002-04-01

302

Method and apparatus for measuring stress  

DOEpatents

A method and apparatus for determining stress in a material independent of micro-structural variations and anisotropies. The method comprises comparing the velocities of two horizontally polarized and horizontally propagating ultrasonic shear waves with interchanged directions of propagation and polarization. The apparatus for carrying out the method comprises periodic permanent magnet-electromagnetic acoustic transducers for generating and detecting the shear waves and means for determining the wave velocities.

Thompson, R. Bruce (Ames, IA)

1985-06-11

303

Identification of two novel shear stress responsive elements in rat angiotensin I converting enzyme promoter.  

PubMed

Mechanical forces contribute to maintenance of cardiovascular homeostasis via the control of release and production of vasoactive substances. We demonstrated previously that shear stress decreases rat ACE activity and expression. Using a reporter gene approach and mutagenesis, we show now that the classic shear stress responsive element or SSRE (GAGACC) contained within 1,274 bp of this promoter is not functional in response to shear stress (15 dyn/cm2, 18 h) [for the wild-type ACE promoter (WLuc), static control (C) = 107 +/- 6.5%, shear stress (SS) = 65.9 +/- 9.4%, n = 8; for the promoter with the classic SSRE mutated (WSS-mut), C = 100 +/- 8.2%, SS = 60.2 +/- 5.2%, n = 10, respectively]. Analysis of progressive deletion mutants unraveled a 57-bp fragment, position -251 to -195, from the transcription start site, containing functional SSRE (for WLuc, C = 107 +/- 6.5%, SS = 65.9 +/- 9.4%, n = 8; for 378, C = 100 +/- 6.4%, SS = 60.4 +/- 4.3%, n = 11; for 251, C = 99.7 +/- 2.6%, SS = 63.2 +/- 5.5%, n = 7; for 194, C = 104.6 +/- 8.1%, SS = 92.4 +/- 6.9%, n = 9). This fragment responded to shear stress even in the context of a heterologous promoter. Finally, functional analysis of mutated candidate regulatory elements identified by gel shift, DNase I footprint, and conservation of aligned sequences revealed that only the double mutant (Barbie/GAGA-mut) but not isolated disruption of the Barbie (WBarbie-mut) or the GAGA (WGAGA-mut) prevented the shear-stress-induced response (for Barbie/GAGA-mut, C = 97.9 +/- 5%, SS = 99.4 +/- 7.2%, n = 6; for WBarbie-mut, C = 106.1 +/- 8.6%, SS = 65.9 +/- 9.4%, n = 6; for WGAGA-mut, C = 100.1 +/- 2.9%, SS = 66.7 +/- 1.6, n = 6;). Taken together, these data provide direct evidence for the new role of Barbie and GAGA boxes in mediating the shear-stress-induced downregulation of rat ACE expression and demonstrate that the classic SSRE (GAGACC) is not functional under the experimental conditions tested. PMID:14872008

Miyakawa, Ayumi Aurea; de Lourdes Junqueira, Maria; Krieger, José Eduardo

2004-04-13

304

Free convection in parallelogram-shaped enclosures with isothermal active walls: viscous shear stress in active systems  

NASA Astrophysics Data System (ADS)

Thermocouples are often used for thermoregulation of active thermal systems. When the junctions of these sensors are under a natural convection flow, it is necessary to take into account the viscous stress that can affect the measurement of temperature and therefore the regulation set points. The main objective of this work is to study the viscous shear stress taking place close to the active hot wall in closed air-filled cavities of parallelogrammic shape. The influence of shear stress is examined for different inclination angles of the cavity and large Rayleigh numbers which are usual in thermal applications. The local stress distributions are presented for the steady state for all the geometric configurations considered. The Nusselt number at the hot wall as well as the temperature and stream function distributions in the cavities are also included. The findings obtained from the numerical simulation using the finite volume method are validated by thermal measurements on an experimental cavity. This study confirms the need to properly choose the location of thermocouples in the reference cell used for controlling the active system.

Baïri, A.; Zarco-Pernia, E.; García de María, J.-M.; Laraqi, N.

2012-10-01

305

Magnetoacoustic stress measurements in steel  

NASA Technical Reports Server (NTRS)

Uniaxial stress effects on the low-field magnetoacoustic interaction have been studied using bulk compressional waves and Rayleigh surface waves in numerous steel samples having various impurity concentrations (Namkung et al., 1984). The results invariably showed that the initial slope of acoustic natural velocity variations, with respect to net induced magnetization parallel to the stress axis, is positive under tension and negative under compression. The results of current measurements in railroad rail steel having about 0.68 wt percent carbon content are typical for medium range carbon steels. The low-field natural velocity slope in this particular type of steel, which is almost zero when unstressed, becomes steeper with increased magnitude of stress in both directions. Hence, the nondestructive determination of the sign of residual stress in railroad wheels and rails is possible using this technique. This paper discusses the basic physical mechanism underlying the experimental observations and presents the results obtained in railroad rail steel.

Namkung, M.; Utrata, D.; Allison, S. G.; Heyman, J. S.

1985-01-01

306

Sox18 preserves the pulmonary endothelial barrier under conditions of increased shear stress.  

PubMed

Shear stress secondary to increased pulmonary blood flow (PBF) is elevated in some children born with congenital cardiac abnormalities. However, the majority of these patients do not develop pulmonary edema, despite high levels of permeability inducing factors. Previous studies have suggested that laminar fluid shear stress can enhance pulmonary vascular barrier integrity. However, little is known about the mechanisms by which this occurs. Using microarray analysis, we have previously shown that Sox18, a transcription factor involved in blood vessel development and endothelial barrier integrity, is up-regulated in an ovine model of congenital heart disease with increased PBF (shunt). By subjecting ovine pulmonary arterial endothelial cells (PAEC) to laminar flow (20?dyn/cm(2) ), we identified an increase in trans-endothelial resistance (TER) across the PAEC monolayer that correlated with an increase in Sox18 expression. Further, the TER was also enhanced when Sox18 was over-expressed and attenuated when Sox18 expression was reduced, suggesting that Sox18 maintains the endothelial barrier integrity in response to shear stress. Further, we found that shear stress up-regulates the cellular tight junction protein, Claudin-5, in a Sox18 dependent manner, and Claudin-5 depletion abolished the Sox18 mediated increase in TER in response to shear stress. Finally, utilizing peripheral lung tissue of 4 week old shunt lambs with increased PBF, we found that both Sox18 and Claudin-5 mRNA and protein levels were elevated. In conclusion, these novel findings suggest that increased laminar flow protects endothelial barrier function via Sox18 dependent up-regulation of Claudin-5 expression. PMID:24677020

Gross, Christine M; Aggarwal, Saurabh; Kumar, Sanjiv; Tian, Jing; Kasa, Anita; Bogatcheva, Natalia; Datar, Sanjeev A; Verin, Alexander D; Fineman, Jeffrey R; Black, Stephen M

2014-11-01

307

Evaluation of shear stress accumulation on blood components in normal and dysfunctional bileaflet mechanical heart valves using smoothed particle hydrodynamics.  

PubMed

Evaluating shear induced hemodynamic complications is one of the major concerns in design of the mechanical heart valves (MHVs). The monitoring of these events relies on both numerical simulations and experimental measurements. Currently, numerical approaches are mainly based on a combined Eulerian-Lagrangian approach. A more straightforward evaluation can be based on the Lagrangian analysis of the whole blood. As a consequence, Lagrangian meshfree methods are more adapted to such evaluation. In this study, smoothed particle hydrodynamics (SPH), a fully meshfree particle method originated to simulate compressible astrophysical flows, is applied to study the flow through a normal and a dysfunctional bileaflet mechanical heart valves (BMHVs). The SPH results are compared with the reference data. The accumulation of shear stress patterns on blood components illustrates the important role played by non-physiological flow patterns and mainly vortical structures in this issue. The statistical distribution of particles with respect to shear stress loading history provides important information regarding the relative number of blood components that can be damaged. This can be used as a measure of the response of blood components to the presence of the valve implant or any implantable medical device. This work presents the first attempt to simulate pulsatile flow through BMHVs using SPH method. PMID:22980575

Shahriari, S; Maleki, H; Hassan, I; Kadem, L

2012-10-11

308

A Wafer-Bonded, Floating Element Shear-Stress Sensor Using a Geometric Moire Optical Transduction Technique  

NASA Technical Reports Server (NTRS)

This paper presents a geometric Moir optical-based floating-element shear stress sensor for wind tunnel turbulence measurements. The sensor was fabricated using an aligned wafer-bond/thin-back process producing optical gratings on the backside of a floating element and on the top surface of the support wafer. Measured results indicate a static sensitivity of 0.26 microns/Pa, a resonant frequency of 1.7 kHz, and a noise floor of 6.2 mPa/(square root)Hz.

Horowitz, Stephen; Chen, Tai-An; Chandrasekaran, Venkataraman; Tedjojuwono, Ken; Cattafesta, Louis; Nishida, Toshikazu; Sheplak, Mark

2004-01-01

309

Application of a Reynolds stress turbulence model to the compressible shear layer  

NASA Technical Reports Server (NTRS)

Theoretically based turbulence models have had success in predicting many features of incompressible, free shear layers. However, attempts to extend these models to the high-speed, compressible shear layer have been less effective. In the present work, the compressible shear layer was studied with a second-order turbulence closure, which initially used only variable density extensions of incompressible models for the Reynolds stress transport equation and the dissipation rate transport equation. The quasi-incompressible closure was unsuccessful; the predicted effect of the convective Mach number on the shear layer growth rate was significantly smaller than that observed in experiments. Having thus confirmed that compressibility effects have to be explicitly considered, a new model for the compressible dissipation was introduced into the closure. This model is based on a low Mach number, asymptotic analysis of the Navier-Stokes equations, and on direct numerical simulation of compressible, isotropic turbulence. The use of the new model for the compressible dissipation led to good agreement of the computed growth rates with the experimental data. Both the computations and the experiments indicate a dramatic reduction in the growth rate when the convective Mach number is increased. Experimental data on the normalized maximum turbulence intensities and shear stress also show a reduction with increasing Mach number.

Sarkar, S.; Balakrishnan, L.

1990-01-01

310

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

NASA Astrophysics Data System (ADS)

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, kayaking, precession, and a new dynamics termed "hovering." For the physiological viscosity range, the shear-plane tumbling appears to be relatively less common, while the rolling is observed to be more stable.

Cordasco, Daniel; Yazdani, Alireza; Bagchi, Prosenjit

2014-04-01

311

Effect of shear stress on electromagnetic behaviors in superconductor-ferromagnetic bilayer structure  

NASA Astrophysics Data System (ADS)

In this paper, the electromagnetic response and shielding behaviour of superconductor-ferromagnetic bilayer structure are studied. The magnetomechanical coupling in ferromagnetic materials is also considered. Based on the linear piezomagnetic coupling model and anti-plane shear deformation, the current density and magnetic field in superconducting strip are obtained firstly. The effect of shear stress on the magnetization of strip is discussed. Then, we consider the magnetic cloak for superconductor-ferromagnetic bilayer structure. The magnetic permeability of ferromagnetic material is obtained for perfect cloaking in uniform magnetic field with magnetomechanical coupling in ferromagnet. The simulation results show that the electromagnetic response in superconductors will change by applying the stress only to the ferromagnetic material. In addition, the performance of invisibility of structure for non-uniform field will be affected by mechanical stress. It may provide a method to achieve tunability of superconducting properties with mechanical loadings.

Yong, Huadong; Zhao, Meng; Jing, Ze; Zhou, Youhe

2014-09-01

312

Hydraulic fracturing in situ stress measurements to 2.1 km depth at Cajon Pass, California  

USGS Publications Warehouse

Stress measurements to 2.1 km reveal stress changes with depth that cannot be explained by an elastic response to uniform crustal strain. The data at about 1 km depth suggest that the stress is limited by the frictional strength of rock and is perturbed at greater depths by faults which intersect the borehole. The stress data indicate that there is little or no right-lateral shear stress acting on planes parallel to the San Andreas Fault. -Authors

Healy, J.H.; Zoback, M.D.

1988-01-01

313

Minimizing shear and compressive stress during pancreaticojejunostomy: rationale of a new technical modification.  

PubMed

The failure of the pancreaticojejunal anastomosis remains an important and potentially lethal postoperative complication after pancreaticoduodenectomy. During the pancreaticojejunostomy, creation of compressive and shear forces during suture placement and knot tying may cause deformation of and cutting through the fragile pancreatic parenchyma. We sought to understand the mechanics of needle-pancreas interaction and make adjustments to our pancreaticojejunostomy technique so that the creation of shear and compressive stress could be minimized. We provide a detailed description, a mathematical model, and analysis of the outcomes of our new technical modifications. PMID:24369374

Neychev, Vladimir K; Sladinger, Pierre F

2014-02-01

314

Yield stress measurements with the vane  

Microsoft Academic Search

Yield stress measurements were performed on a TiO2 pigment suspension with the vane in both a rate controlled and a stress controlled mode. In the rate controlled mode, a constant rotational speed is applied to the vane immersed in the suspension, and the resulting stress is measured as a function of time. In the stress controlled mode, a constant stress

Petra V. Liddel; David V. Boger

1996-01-01

315

The relationship between wall shear stress distributions and intimal thickening in the human abdominal aorta  

PubMed Central

Purpose The goal of this work was to determine wall shear stress (WSS) patterns in the human abdominal aorta and to compare these patterns to measurements of intimal thickness (IT) from autopsy samples. Methods The WSS was experimentally measured using the laser photochromic dye tracer technique in an anatomically faithful in vitro model based on CT scans of the abdominal aorta in a healthy 35-year-old subject. IT was quantified as a function of circumferential and axial position using light microscopy in ten human autopsy specimens. Results The histomorphometric analysis suggests that IT increases with age and that the distribution of intimal thickening changes with age. The lowest WSS in the flow model was found on the posterior wall inferior to the inferior mesenteric artery, and coincided with the region of most prominent IT in the autopsy samples. Local geometrical features in the flow model, such as the expansion at the inferior mesenteric artery (common in younger individuals), strongly influenced WSS patterns. The WSS was found to correlate negatively with IT (r2 = 0.3099; P = 0.0047). Conclusion Low WSS in the abdominal aorta is co-localized with IT and may be related to atherogenesis. Also, rates of IT in the abdominal aorta are possibly influenced by age-related geometrical changes. PMID:14641919

Bonert, Michael; Leask, Richard L; Butany, Jagdish; Ethier, C Ross; Myers, Jerry G; Johnston, K Wayne; Ojha, Matadial

2003-01-01

316

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

NASA Astrophysics Data System (ADS)

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.

Gnanamanickam, Ebenezer P.; Sullivan, John P.

2012-12-01

317

Estimate Interface Shear Stress of Woven Ceramic Matrix Composites from Hysteresis Loops  

NASA Astrophysics Data System (ADS)

An approach to estimate the fiber/matrix interface shear stress of woven ceramic matrix composites during fatigue loading has been developed in this paper. Based on the analysis of the microstructure, the woven ceramic matrix composites were divided into four elements of 0o warp yarns, 90o weft yarns, matrix outside of the yarns and the open porosity. When matrix cracking and fiber/matrix interface debonding occur upon first loading to the peak stress, it is assumed that fiber slipping relative to matrix in the interface debonded region of the 0o warp yarns is the mainly reason for the occurrence of the hysteresis loops of woven ceramic matrix composiets during unloading and subsequent reloading. The unloading interface reverse slip length and reloading interface new slip length are determined by the interface slip mechanisms. The hysteresis loops of three different cases have been derived. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulated in terms of the fiber/matrix interface shear stress. By comparing the experimental hysteresis loss energy with the computational values, the fiber/matrix interface shear stress of woven ceramic matrix composites corresponding to different cycles can then be derived. The theoretical results have been compared with experimental data of two different woven ceramic composites.

Li, Longbiao; Song, Yingdong

2013-12-01

318

Resistance to Fluid Shear Stress Is a Conserved Biophysical Property of Malignant Cells  

PubMed Central

During metastasis, cancer cells enter the circulation in order to gain access to distant tissues, but how this fluid microenvironment influences cancer cell biology is poorly understood. A longstanding view is that circulating cancer cells derived from solid tissues may be susceptible to damage from hemodynamic shear forces, contributing to metastatic inefficiency. Here we report that compared to non-transformed epithelial cells, transformed cells are remarkably resistant to fluid shear stress (FSS) in a microfluidic protocol, exhibiting a biphasic decrease in viability when subjected to a series of millisecond pulses of high FSS. We show that magnitude of FSS resistance is influenced by several oncogenes, is an adaptive and transient response triggered by plasma membrane damage and requires extracellular calcium and actin cytoskeletal dynamics. This novel property of malignant cancer cells may facilitate hematogenous metastasis and indicates, contrary to expectations, that cancer cells are quite resistant to destruction by hemodynamic shear forces. PMID:23226552

Henry, Michael D.

2012-01-01

319

Importance of wind, fetch and water levels on waves and wave-generated shear stresses in a shallow coastal lagoon  

NASA Astrophysics Data System (ADS)

Wind waves and the bed shear stresses they produce are critical for the morphological and ecological equilibrium of shallow tidal basins. Wave-generated shear stresses are the main mechanism responsible for sediment erosion on tidal flats, and regulate both sediment concentrations in the water column and, together with tidal currents, sediment export to salt marshes and to the ocean. We analyze the response of a system of shallow tidal basins along the Eastern Shore of Virginia, U.S., to wind-wave events, with a specific focus on the interplay of basin morphology, tidal elevation and wind direction on depth, fetch and the resulting wave- generated shear stresses. Our analysis indicates that the potential for erosion is the highest when the salt marshes are submerged. Under these conditions the direction of the wind is critical, with maximum wave heights and erosion potential occurring for winds blowing along the barrier islands of the basin (NNE-SSW). We identify four bottom shear stress regimes produced by wind waves in the Virginia Coastal Reserve as a function of water elevation. For elevations between MLLW and MSL the increase in water depth dominates the increase in wave height thus reducing the bottom shear stresses. For elevations between MSL and MHHW the flooding of the salt marshes increases fetch, wave height and bottom shear stresses, producing the largest resuspesion events in the bay. Finally, for elevations above MHHW, the increase in depth reduces the average bottom shear stresses, thus reducing possible erosion in the tidal flats.

Wiberg, P.; Fagherazzi, S.

2008-12-01

320

A comparative study of the shear stress induced in the leakage backflow produced by four types of heart valve prostheses.  

PubMed

The clinical selection of a prosthetic heart valve requires the consideration of many factors. These include the pressure drop across the prosthesis, the amount of leakage backflow when the valve is closed as well as the associated shear stress within this flow for a particular valve design. Major emphasis has been placed upon the documentation of the pressure drop across prosthetic valves. Consideration of the shear stress induced in the leakage backflow associated with prosthetic valves, however, has received less attention. The purpose of this investigation, therefore, is to determine theoretically the extent of this particular shear stress for four types of currently used mechanical prosthetic cardiac valves. All valves have the same tissue annulus diameter of 27 mm. The leakage backflow is relatively small compared to the main forward flow through the orifice. However, it is important from the haemolysis point of view. The effect of the shear stress on the suspected occurrence of haemolysis is analysed by two theoretical approaches: (a) the laminar flow approach, and (b) the entry flow approach, and then compared with the findings of previous investigators dealing with the critical shear stresses which may damage the red blood cells. It was found that both approaches give practically equal shear stresses under the same pressure differential for all valves investigated. Some haemolysis is expected to occur but it is generally compensated by bone marrow hyperfunction, since that type of shear stress is sustained for only a short duration of time. PMID:2095141

Haggag, Y A

1990-01-01

321

Nature of stress accommodation in sheared granular material: Insights from 3D numerical modeling  

NASA Astrophysics Data System (ADS)

Active faults often contain distinct accumulations of granular wear material. During shear, this granular material accommodates stress and strain in a heterogeneous manner that may influence fault stability. We present new work to visualize the nature of contact force distributions during 3D granular shear. Our 3D discrete numerical models consist of granular layers subjected to normal loading and direct shear, where gouge particles are simulated by individual spheres interacting at points of contact according to simple laws. During shear, we observe the transient microscopic processes and resulting macroscopic mechanical behavior that emerge from interactions of thousands of particles. We track particle translations and contact forces to determine the nature of internal stress accommodation with accumulated slip for different initial configurations. We view model outputs using novel 3D visualization techniques. Our results highlight the prevalence of transient directed contact force networks that preferentially transmit enhanced stresses across our granular layers. We demonstrate that particle size distribution (psd) controls the nature of the force networks. Models having a narrow (i.e. relatively uniform) psd exhibit discrete pipe-like force clusters with a dominant and focussed orientation oblique to but in the plane of shear. Wider psd models (e.g. power law size distributions D = 2.6) also show a directed contact force network oblique to shear but enjoy a wider range of orientations and show more out-of-plane linkages perpendicular to shear. Macroscopic friction level, is insensitive to these distinct force network morphologies, however, force network evolution appears to be linked to fluctuations in macroscopic friction. Our results are consistent with predictions, based on recent laboratory observations, that force network morphologies are sensitive to grain characteristics such as particle size distribution of a sheared granular layer. Our numerical approach offers the potential to investigate correlations between contact force geometry, evolution and resulting macroscopic friction, thus allowing us to explore ideas that heterogeneous force distributions in gouge material may exert an important control on fault stability and hence the seismic potential of active faults.

Mair, Karen; Hazzard, James F.

2007-07-01

322

On the relations between notch stress and crack stress intensity in plane shear and mixed mode loading  

NASA Astrophysics Data System (ADS)

The relations between notch stress and crack stress intensity in the case of plane shear loading (mode II) are established based on Muskhelishvili's solution for the elliptical hole and conforming with Creager's solution for the blunt crack. Convergence investigations for the limit value formulae are included. Formulae for fictitious crack tip rounding according to Neuber's microstructural support hypothesis are given for mode II loading and extended to mixed mode loading. Short cracks are included. The findings are applied to the slit tips of cruciform welded joints.

Radaj, D.; Zhang, S.

1993-03-01

323

Characterizing Wave- and Current-Induced Bottom Shear Stress: U.S. Middle Atlantic Bight  

NASA Astrophysics Data System (ADS)

The combined action of waves and currents at the seabed creates bottom shear stress, impacting local geology, habitat, and anthropogenic use. In this study, a methodology is developed to characterize the magnitude of benthic disturbance based on spatially and seasonally-resolved statistics (mean, standard deviation, 95th percentile) of wave-current bottom shear stress. The frequency of stress forcing is used to distinguish regions dominated by storms (return interval longer than 33 hours) from those dominated by the tides (periods shorter than 33 hours). In addition, the relative magnitude of the contribution to stress from waves, tides, and storm-driven currents is investigated by comparing wave stress, tidal current stress, and stress from the residual current (currents with tides removed), as well as through cross-correlation of wave and current stress. The methodology is applied to numerical model time-series data for the Middle Atlantic Bight (MAB) off the U.S. East Coast for April 2010 to April 2011; currents are provided from the Integrated Ocean Observing System (IOOS) operational hydrodynamic forecast Experimental System for Predicting Shelf and Slope Optics (ESPreSSO) and waves are provided from a Simulating WAves Nearshore (SWAN) hindcast developed for this project. Spatial resolution of the model is about 5 km and time-series wave and current data are at 1 and 2-hours respectively. Regions of the MAB delineated by stress characteristics include a tidally-dominated shallow region with relative high stress southeast of Massachusetts over Nantucket Shoals; a coastal band extending offshore to about 30 m water depth dominated by waves; a region dominated by waves and wind-driven currents offshore of the Outer Banks of North Carolina; and a low stress region southeast of Long Island, approximately coincident with an area of fine-grained sediments called the "Mud Patch". Comparison of the stress distribution with surface sediment texture data shows that coarser sediments are typically found in regions of higher stress forcing and finer sediments in regions of weaker stress, consistent with the conceptual model of finer sediments being winnowed away in areas of stronger stress. Estimates of sea floor mobility across the MAB are attained using the observed sediment texture data to establish critical stress thresholds for initiation of sediment movement. Relatively high mobility (critical stress exceeded 30-40% of the year) is observed over Nantucket Shoals and in a band along the coast out to a depth of 30-40 m. These results may have application for placement or design of offshore structures such as wind turbines, and assist in habitat delineation for benthic species.

Dalyander, S.; Butman, B.

2011-12-01

324

Measuring stress: Uses and limitations  

SciTech Connect

The topic of the uses and limitations of measuring stress in the oceans is addressed. The use of the term stress in this context is ambiguous. Rather than assuming that stress is an actual measurable property of oceans, one may assume that the term refers to the complex of harmful things that is going on in the oceans. that is, it is a cover term for a complex of processes and states, many of which are unknown or undefined. The appeal of the term stress used in that way is very tempting. Considerable complexity results from the fact that the ocean consists of numerous and diverse species and ecosystems each of which have various properties that have some claim to protection. Another source of complexity is the diversity of factors which constitute threats to the environment. In evaluating the concepts of ecological risk assessment, there are sources of hazardous agents, there are receptors that are affected, and there is a process by which these interact termed exposure. As a result of exposure there is some probability that the actions of the sources have caused or will cause some effect on the receptor, the risk. Assessments may be source driven; they may attempt to determine the risks associated with a waste outfall. Assessments may also be effects driven; they may attempt to determine the cause of a die-off of marine mammals or determine the likelihood of a particular rise in sea-level. Finally, they may be exposure driven. Each of these components of the causal chain might serve as measures of stress.

Suter, G.W. II [Oak Ridge National Lab., TN (United States). Environmental Sciences Div.

1994-12-31

325

Characterizing wave- and current- induced bottom shear stress: U.S. middle Atlantic continental shelf  

USGS Publications Warehouse

Waves and currents create bottom shear stress, a force at the seabed that influences sediment texture distribution, micro-topography, habitat, and anthropogenic use. This paper presents a methodology for assessing the magnitude, variability, and driving mechanisms of bottom stress and resultant sediment mobility on regional scales using numerical model output. The analysis was applied to the Middle Atlantic Bight (MAB), off the U.S. East Coast, and identified a tidally-dominated shallow region with relatively high stress southeast of Massachusetts over Nantucket Shoals, where sediment mobility thresholds are exceeded over 50% of the time; a coastal band extending offshore to about 30 m water depth dominated by waves, where mobility occurs more than 20% of the time; and a quiescent low stress region southeast of Long Island, approximately coincident with an area of fine-grained sediments called the “Mud Patch”. The regional high in stress and mobility over Nantucket Shoals supports the hypothesis that fine grain sediment winnowed away in this region maintains the Mud Patch to the southwest. The analysis identified waves as the driving mechanism for stress throughout most of the MAB, excluding Nantucket Shoals and sheltered coastal bays where tides dominate; however, the relative dominance of low-frequency events varied regionally, and increased southward toward Cape Hatteras. The correlation between wave stress and local wind stress was lowest in the central MAB, indicating a relatively high contribution of swell to bottom stress in this area, rather than locally generated waves. Accurate prediction of the wave energy spectrum was critical to produce good estimates of bottom shear stress, which was sensitive to energy in the long period waves.

Dalyander, P. Soupy; Butman, Bradford; Sherwood, Christopher R.; Signell, Richard P.; Wilkin, John L.

2013-01-01

326

Characterizing wave- and current- induced bottom shear stress: U.S. middle Atlantic continental shelf  

NASA Astrophysics Data System (ADS)

Waves and currents create bottom shear stress, a force at the seabed that influences sediment texture distribution, micro-topography, habitat, and anthropogenic use. This paper presents a methodology for assessing the magnitude, variability, and driving mechanisms of bottom stress and resultant sediment mobility on regional scales using numerical model output. The analysis was applied to the Middle Atlantic Bight (MAB), off the U.S. East Coast, and identified a tidally-dominated shallow region with relatively high stress southeast of Massachusetts over Nantucket Shoals, where sediment mobility thresholds are exceeded over 50% of the time; a coastal band extending offshore to about 30 m water depth dominated by waves, where mobility occurs more than 20% of the time; and a quiescent low stress region southeast of Long Island, approximately coincident with an area of fine-grained sediments called the "Mud Patch". The regional high in stress and mobility over Nantucket Shoals supports the hypothesis that fine grain sediment winnowed away in this region maintains the Mud Patch to the southwest. The analysis identified waves as the driving mechanism for stress throughout most of the MAB, excluding Nantucket Shoals and sheltered coastal bays where tides dominate; however, the relative dominance of low-frequency events varied regionally, and increased southward toward Cape Hatteras. The correlation between wave stress and local wind stress was lowest in the central MAB, indicating a relatively high contribution of swell to bottom stress in this area, rather than locally generated waves. Accurate prediction of the wave energy spectrum was critical to produce good estimates of bottom shear stress, which was sensitive to energy in the long period waves.

Dalyander, P. Soupy; Butman, Bradford; Sherwood, Christopher R.; Signell, Richard P.; Wilkin, John L.

2013-01-01

327

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

PubMed Central

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

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

2013-01-01

328

Direct measurement of strain rates in ductile shear zones: A new method based on syntectonic dikes  

Microsoft Academic Search

We describe a new method to estimate directly ductile strain rates at an outcrop scale from the deformation of dikes emplaced within a shear zone. The method is tested in a well-constrained shear zone: the Ailao Shan–Red River shear zone, for which global strain rates can be calculated from published fault rates. The strain rate was determined by measuring independently

C. Sassier; P. H. Leloup; D. Rubatto; O. Galland; Y. Yue; Ding Lin

2009-01-01

329

A Triaxial-Measurement Shear-Test Device for Soft Biological Tissues  

Microsoft Academic Search

A novel shear-test device for soft biological tissue, capable of applying simple shear deformations simultaneously in two orthogonal directions while measuring the resulting forces generated in three axes, is described. We validated the device using a synthetic gel, the properties of which were ascertained from independent tensile and rotational shear tests. Material parameters for the gel were fitted using neo-Hookean

Socrates Dokos; Ian J. LeGrice; Bruce H. Smaill; Alistair A. Young

2000-01-01

330

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

PubMed Central

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

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

2013-01-01

331

Shear stress influences the pluripotency of murine embryonic stem cells in stirred suspension bioreactors.  

PubMed

Pluripotent embryonic stem cells (ESCs) have been used increasingly in research as primary material for various tissue-engineering applications. Pluripotency, or the ability to give rise to all cells of the body, is an important characteristic of ESCs. Traditional methods use leukaemia inhibitory factor (LIF) to maintain murine embryonic stem cell (mESC) pluripotency in static and bioreactor cultures. When LIF is removed from mESCs in static cultures, pluripotency genes are downregulated and the cultures will spontaneously differentiate. Recently we have shown the maintenance of pluripotency gene expression of mESCs in stirred suspension bioreactors during differentiation experiments in the absence of LIF. This is undesired in a differentiation experiment, where the goal is downregulation of pluripotency gene expression and upregulation of gene expression characteristic to the differentiation. Thus, the objective of this study was to examine how effectively different levels of shear stress [100 rpm (6 dyne/cm(2) ), 60 rpm (3 dyne/cm(2) )] maintained and influenced pluripotency in suspension bioreactors. The pluripotency markers Oct-4, Nanog, Sox-2 and Rex-1 were assessed using gene expression profiles and flow-cytometry analysis and showed that shear stress does maintain and influence the gene expression of certain pluripotency markers. Some significant differences between the two levels of shear stress were seen and the combination of shear stress and LIF was observed to synergistically increase the expression of certain pluripotency markers. Overall, this study provides a better understanding of the environmental conditions within suspension bioreactors and how these conditions affect the pluripotency of mESCs. PMID:22653738

Gareau, Tia; Lara, Giovanna G; Shepherd, Robert D; Krawetz, Roman; Rancourt, Derrick E; Rinker, Kristina D; Kallos, Michael S

2014-04-01

332

A transient shear stress model for the analysis of laminar water-hammer problems  

Microsoft Academic Search

A transient shear stress model for the solution of water-hammer problems for laminar flow in pipes is presented. The model is based on the polynomial expansion of the radial profiles of axial velocities, and the solution of the resulting set of equations by the method of characteristics.This approach, as compared to the usual quasi-steady model (which can be regarded as

Ricardo A. Prado; Axel E. Larreteguy

2002-01-01

333

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

Microsoft Academic Search

Entrainment of sediment particles from channel beds into the channel flow is influenced by the characteristics of the flow\\u000a turbulence which produces stochastic shear stress fluctuations at the bed. Recent studies of the structure of turbulent flow\\u000a has recognized the importance of bursting processes as important mechanisms for the transfer of momentum into the laminar\\u000a boundary layer. Of these processes,

A. Keshavarzy; J. E. Ball

1997-01-01

334

Shear Stress and Sediment Resuspension in Canopy and Meadow-Forming Submersed Macrophyte Communities  

Microsoft Academic Search

PURPOSE: This technical note examines the impact of differing biomass levels and plant architectural types on bottom shear stress and sediment resuspension in shallow systems. Studies were conducted at Lake Christina, Minnesota, in late August-early September 1998, when macrophyte biomass levels exceeded 200 g\\/m2 and in June 2000, when biomass was greatly reduced (< 20 g\\/m2). The macrophyte beds that

William F. James; John W. Barko; Malcolm G. Butler

335

Shear stress and sediment resuspension in relation to submersed macrophyte biomass  

Microsoft Academic Search

We examined the impacts of macrophyte beds dominated by a canopy-forming (Myriophyllum sibiricum) and a meadow-forming (Chara canescens) species on bottom shear stress (t) and resuspension in shallow Lake Christina, Minnesota (U.S.A.). Studies were conducted in late summer, 1998, when macrophyte biomass levels exceeded 200 g m-2, and in early summer, 2000, when biomass was greatly reduced (-2) in both plant beds.

William F. James; John W. Barko; Malcolm G. Butler

2004-01-01

336

Visualization of Simulated Endothelial Shear Stress and Blood Flow in Coronary Arteries  

NASA Astrophysics Data System (ADS)

Low endothelial shear stress (ESS) identifies areas of atherosclerotic disease lesion formation in the coronary arteries. However, it is impossible to directly measure ESS in vivo for an entire arterial tree. As part of the Multiscale Hemodynamics Project, computed tomography angiography (CTA) data is being used to obtain patient specific heart and coronary system geometries and then MUPHY, a multi-physics and multi-scale simulation code combining microscopic Molecular Dynamics (MD) with a hydro-kinetic Lattice Boltzmann (LB) method, is applied in order to simulate blood flow through the coronary arteries. Having effective visualizations of the simulation's multidimensional output, including ESS, is vital for the quick and thorough non-invasive evaluation of the patient. To this end, we have developed new visualization tools and techniques to make the simulation's output useful in a clinical diagnostic setting, examined the effectiveness of 2D versus 3D representations, and explored blood flow representations. The visualization methods developed are also applicable to other areas of fluid dynamics.

Borkin, Michelle; Feldman, Charles L.; Pfister, Hanspeter; Melchionna, Simone; Kaxiras, Efthimios

2010-11-01

337

Steady and transient fluid shear stress stimulate NO release in osteoblasts through distinct biochemical pathways  

NASA Technical Reports Server (NTRS)

Fluid flow has been shown to be a potent stimulus in osteoblasts and osteocytes and may therefore play an important role in load-induced bone remodeling. The objective of this study was to investigate the characteristics of flow-activated pathways. Previously we reported that fluid flow stimulates rapid and continuous release of nitric oxide (NO) in primary rat calvarial osteoblasts. Here we demonstrate that flow-induced NO release is mediated by shear stress and that this response is distinctly biphasic. Transients in shear stress associated with the onset of flow stimulated a burst in NO production (8.2 nmol/mg of protein/h), while steady flow stimulated sustained NO production (2.2 nmol/mg of protein/h). Both G-protein inhibition and calcium chelation abolished the burst phase but had no effect on sustained production. Activation of G-proteins stimulated dose-dependent NO release in static cultures of both calvarial osteoblasts and UMR-106 osteoblast-like cells. Pertussis toxin had no effect on NO release. Calcium ionophore stimulated low levels of NO production within 15 minutes but had no effect on sustained production. Taken together, these data suggest that fluid shear stress stimulates NO release by two distinct pathways: a G-protein and calcium-dependent phase sensitive to flow transients, and a G-protein and calcium-independent pathway stimulated by sustained flow.

McAllister, T. N.; Frangos, J. A.

1999-01-01

338

DNA microarray reveals changes in gene expression of shear stressed human umbilical vein endothelial cells  

PubMed Central

Using DNA microarray screening (GeneFilter 211, Research Genetics, Huntsville, AL) of mRNA from primary human umbilical vein endothelial cells (HUVEC), we identified 52 genes with significantly altered expression under shear stress [25 dynes/cm2 for 6 or 24 h (1 dyne = 10 ?N), compared with matched stationary controls]; including several genes not heretofore recognized to be shear stress responsive. We examined mRNA expression of nine genes by Northern blot analysis, which confirmed the results obtained on DNA microarrays. Thirty-two genes were up-regulated (by more than 2-fold), the most enhanced being cytochromes P450 1A1 and 1B1, zinc finger protein EZF/GKLF, glucocorticoid-induced leucine zipper protein, argininosuccinate synthase, and human prostaglandin transporter. Most dramatically decreased (by more than 2-fold) were connective tissue growth factor, endothelin-1, monocyte chemotactic protein-1, and spermidine/spermine N1-acetyltransferase. The changes observed suggest several potential mechanisms for increased NO production under shear stress in endothelial cells. PMID:11481467

McCormick, Susan M.; Eskin, Suzanne G.; McIntire, Larry V.; Teng, Christina L.; Lu, Chiung-Mei; Russell, Christopher G.; Chittur, Krishnan K.

2001-01-01

339

Effect of the glycocalyx layer on transmission of interstitial flow shear stress to embedded cells  

PubMed Central

In this paper, a simple theoretical model is developed to describe the transmission of force from interstitial fluid flow to the surface of a cell covered by a proteoglycan / glycoprotein layer (glycocalyx) and embedded in an extracellular matrix. Brinkman equations are used to describe flow through the extracellular matrix and glycocalyx layers and the solid mechanical stress developed in the glycocalyx by the fluid flow loading is determined. Using reasonable values for the Darcy permeability of extracellular matrix and glycocalyx layers and interstitial flow velocity, we are able to estimate the fluid and solid shear stresses imposed on the surface of embedded vascular, cartilage and tumor cells in vivo and in vitro. The principal finding is that the surface solid stress is typically one to two orders of magnitude larger than the surface fluid stress. This indicates that interstitial flow shear stress can be sensed by the cell surface glycocalyx, supporting numerous recent observations that interstitial flow can induce mechanotransduction in embedded cells. This study may contribute to understanding of interstitial flow-related mechanobiology in embryogenesis, tumorigenesis, tissue physiology and diseases and has implications in tissue engineering. PMID:22411016

Shi, Zhong-Dong

2012-01-01

340

Stress field and seismic anisotropy around Mt. Fuji deduced from temporal consistency and spatial distribution of shear wave splitting  

NASA Astrophysics Data System (ADS)

Shear wave splitting (SWS) has been used not only to examine structures deduced from strain fields of deep part of Earth but also to interpret subsurface structures at shallow depths of crust as well as stress and structural interactions. We measure SWS in the Mt. Fuji volcanic region whose seismicity was low before the 2011 Tohoku-Oki earthquake, but a Mw 5.9 event on 15 March 2011, four days after the mainshock, triggered the regional seismicity. We measure SWS from seismic data between 2009 and 2013 at 16 stations near the summit. We also measure SWS from the NIED Hi-net data in 2011 at 8 stations to interpret regional seismic anisotropy. We apply an automatic shear wave splitting measurement technique (Savage et al., 2010) to process numerous data with a careful verification of the measurement results to avoid various disturbances. The measured delay times range from 0.04 to 0.09s. Both fast polarization directions and delay times do not change due to the Mw 5.9 event. In addition, SWS measurements are stable at each station. Spatial distribution of fast polarization directions has two characteristics. Fast polarization directions close to the summit follow a radial pattern from the summit. By contrast, fast polarization directions that are more than approximately 15 km away from the summit are parallel to the NW-SE regional compression although a few stations do not entirely follow this pattern. Since the delay times do not have depth dependency, the anisotropy around Mt. Fuji is presumed to locate at shallow depths less than approximately 4 km. Loading of the edifice and regional stress field is responsible for the two distinctive characteristics of the spatial distribution of fast polarization directions. After the 2011 Tohoku-Oki earthquake, change of stresses can be inferred from increase of seismicity. Since the SWS measurement shows temporal consistency, we infer that the seismic anisotropy is caused by the seismic structure whose magnitude of anisotropy is so large that they are not sensitive to the stresses or stress perturbation of Mw 5.9 event are not significant on anisotropic structure of the area. Further analysis is required for quantitative analysis of magnitude of anisotropy and its sensitivity to the stresses that govern the Mt. Fuji volcanic region.

Araragi, Kohtaro; Savage, Martha; Ohminato, Takao; Aoki, Yosuke

2014-05-01

341

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

PubMed

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. PMID:17349339

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

2007-01-01

342

Measurement of interfacial shear strength in SiC-fiber/Si sub 3 N sub 4 composites  

SciTech Connect

This paper reports an indentation method for measuring shear strength in brittle matrix composites applied to SiC-fiber/Si{sub 3}N{sub 4}-matrix samples. Three methods were used to manufacture the composites: reaction bonding of a Si/SiC preform, hot-pressing, and nitrogen- overpressure sintering. An indentation technique developed by Marshall for thin specimens was used to measure the shear strength of the interface and the interfacial friction stresses. This was done by inverting the sample after the initial push through and retesting the pushed fibers. SEM observations showed that the shear strength was determined by the degree of reaction between the fiber and the matrix unless the fiber was pushed out of its (well-bonded) sheath.

Laughner, J.W. (New York State College of Ceramics at Alfred Univ., Alfred, NY (US)); Bhatt, R.T. (NASA-Lewis Research Center, Cleveland, OH (US))

1989-10-01

343

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

NASA Astrophysics Data System (ADS)

The majority of crustal faults host earthquakes when the ratio of average background shear stress ?b to effective normal stress ?eff is ?b/?eff?0.6. In contrast, mature plate-boundary faults like the San Andreas Fault (SAF) operate at ?b/?eff?0.2. Dynamic weakening, the dramatic reduction in frictional resistance at coseismic slip velocities that is commonly observed in laboratory experiments, provides a leading explanation for low stress levels on mature faults. Strongly velocity-weakening friction laws permit rupture propagation on flat faults above a critical stress level ?pulse/?eff?0.25. Provided that dynamic weakening is not restricted to mature faults, the higher stress levels on most faults are puzzling. In this work, we present a self-consistent explanation for the relatively high stress levels on immature faults that is compatible with low coseismic frictional resistance, from dynamic weakening, for all faults. We appeal to differences in structural complexity with the premise that geometric irregularities introduce resistance to slip in addition to frictional resistance. This general idea is quantified for the special case of self-similar fractal roughness of the fault surface. Natural faults have roughness characterized by amplitude-to-wavelength ratios ? between 10-3 and 10-2. Through a second-order boundary perturbation analysis of quasi-static frictionless sliding across a band-limited self-similar interface in an ideally elastic solid, we demonstrate that roughness induces an additional shear resistance to slip, or roughness drag, given by ?drag=8?3?2G??/?min, for G?=G/(1-?) with shear modulus Gand Poisson's ratio ?, slip ?, and minimum roughness wavelength ?min. The influence of roughness drag on fault mechanics is verified through an extensive set of dynamic rupture simulations of earthquakes on strongly rate-weakening fractal faults with elastic-plastic off-fault response. The simulations suggest that fault rupture, in the form of self-healing slip pulses, becomes probable above a background stress level ?b??pulse+?drag. For the smoothest faults (?˜10-3), ?drag is negligible compared to frictional resistance, so that ?b??pulse?0.25?eff. However, on rougher faults (?˜10-2), roughness drag can exceed frictional resistance. We expect that ?drag ultimately departs from the predicted scaling when roughness-induced stress perturbations activate pervasive off-fault inelastic deformation, such that background stress saturates at a limit (?b?0.6?eff) determined by the finite strength of the off-fault material. We speculate that this strength, and not the much smaller dynamically weakened frictional strength, determines the stress levels at which the majority of faults operate.

Fang, Zijun; Dunham, Eric M.

2013-07-01

344

Characterizaton of the Vessel Geometry, Flow Mechanics and Wall Shear Stress in the Great Arteries of Wildtype Prenatal Mouse  

PubMed Central

Introduction Abnormal fluid mechanical environment in the pre-natal cardiovascular system is hypothesized to play a significant role in causing structural heart malformations. It is thus important to improve our understanding of the prenatal cardiovascular fluid mechanical environment at multiple developmental time-points and vascular morphologies. We present such a study on fetal great arteries on the wildtype mouse from embryonic day 14.5 (E14.5) to near-term (E18.5). Methods Ultrasound bio-microscopy (UBM) was used to measure blood velocity of the great arteries. Subsequently, specimens were cryo-embedded and sectioned using episcopic fluorescent image capture (EFIC) to obtain high-resolution 2D serial image stacks, which were used for 3D reconstructions and quantitative measurement of great artery and aortic arch dimensions. EFIC and UBM data were input into subject-specific computational fluid dynamics (CFD) for modeling hemodynamics. Results In normal mouse fetuses between E14.5–18.5, ultrasound imaging showed gradual but statistically significant increase in blood velocity in the aorta, pulmonary trunk (with the ductus arteriosus), and descending aorta. Measurement by EFIC imaging displayed a similar increase in cross sectional area of these vessels. However, CFD modeling showed great artery average wall shear stress and wall shear rate remain relatively constant with age and with vessel size, indicating that hemodynamic shear had a relative constancy over gestational period considered here. Conclusion Our EFIC-UBM-CFD method allowed reasonably detailed characterization of fetal mouse vascular geometry and fluid mechanics. Our results suggest that a homeostatic mechanism for restoring vascular wall shear magnitudes may exist during normal embryonic development. We speculate that this mechanism regulates the growth of the great vessels. PMID:24475188

Yap, Choon Hwai; Liu, Xiaoqin; Pekkan, Kerem

2014-01-01

345

Direct Experimental Measurement of the Speed-Stress Relation for Dislocations in a Plasma Crystal  

SciTech Connect

The speed-stress relation for gliding edge dislocations was experimentally measured for the first time. The experimental system used, a two-dimensional plasma crystal, allowed observation of individual dislocations at the ''atomistic'' level and in real time. At low applied stress dislocations moved subsonically, at higher stress their speed abruptly increased to 1.9 times the speed of shear waves, then slowly grew with stress. There is evidence that immediately after nucleation dislocations can move faster than pressure waves.

Nosenko, V.; Morfill, G. E.; Rosakis, P. [Max-Planck-Institut fuer extraterrestrische Physik, D-85741 Garching (Germany); Department of Applied Mathematics, University of Crete, Heraklion 71409 (Greece)

2011-04-15

346

Effect of surface shear stress on the attachment of Pseudomonas fluorescens to stainless steel under defined flow conditions  

SciTech Connect

The ease with which microorganisms grow on submerged surfaces is the cause of certain industrial problems (i.e. fouling and corrosion of pipelines, heat exchangers, cooling towers, etc.). Application of the radical-flow growth chamber to the study of the initial stages of bacterial adhesion to surfaces under flowing conditions is reported. The adhesive properties of the bacterium Pseudomonas fluorescens (NCIB9046) to stainless steel (type AISI 316 were found to highly dependent on surface shear stress and the time and concentration of cells used in the incubation procedure. Maximum levels of adhesion occurred in zones of lowest surface shear stress, particularly less than 6-8 Nm/sup -2/, Adhesion was still noticeable to shear stresses even up to 130 Nm/sup -2/. Significant detachment of cells from a monolayer attached under static conditions was found to occur at surface shear stresses in excess of 10-12 Nm/sup -2/. (JMT)

Duddridge, J.E.; Kent, C.A.; Laws, J.F.

1982-01-01

347

Mis-sizing of stent promotes intimal hyperplasia: impact of endothelial shear and intramural stress.  

PubMed

Stent can cause flow disturbances on the endothelium and compliance mismatch and increased stress on the vessel wall. These effects can cause low wall shear stress (WSS), high wall shear stress gradient (WSSG), oscillatory shear index (OSI), and circumferential wall stress (CWS), which may promote neointimal hyperplasia (IH). The hypothesis is that stent-induced abnormal fluid and solid mechanics contribute to IH. To vary the range of WSS, WSSG, OSI, and CWS, we intentionally mismatched the size of stents to that of the vessel lumen. Stents were implanted in coronary arteries of 10 swine. Intravascular ultrasound (IVUS) was used to size the coronary arteries and stents. After 4 wk of stent implantation, IVUS was performed again to determine the extent of IH. In conjunction, computational models of actual stents, the artery, and non-Newtonian blood were created in a computer simulation to yield the distribution of WSS, WSSG, OSI, and CWS in the stented vessel wall. An inverse relation (R(2) = 0.59, P < 0.005) between WSS and IH was found based on a linear regression analysis. Linear relations between WSSG, OSI, and IH were observed (R(2) = 0.48 and 0.50, respectively, P < 0.005). A linear relation (R(2) = 0.58, P < 0.005) between CWS and IH was also found. More statistically significant linear relations between the ratio of CWS to WSS (CWS/WSS), the products CWS × WSSG and CWS × OSI, and IH were observed (R(2) = 0.67, 0.54, and 0.56, respectively, P < 0.005), suggesting that both fluid and solid mechanics influence the extent of IH. Stents create endothelial flow disturbances and intramural wall stress concentrations, which correlate with the extent of IH formation, and these effects were exaggerated with mismatch of stent/vessel size. These findings reveal the importance of reliable vessel and stent sizing to improve the mechanics on the vessel wall and minimize IH. PMID:21926337

Chen, Henry Y; Sinha, Anjan K; Choy, Jenny S; Zheng, Hai; Sturek, Michael; Bigelow, Brian; Bhatt, Deepak L; Kassab, Ghassan S

2011-12-01

348

Mis-sizing of stent promotes intimal hyperplasia: impact of endothelial shear and intramural stress  

PubMed Central

Stent can cause flow disturbances on the endothelium and compliance mismatch and increased stress on the vessel wall. These effects can cause low wall shear stress (WSS), high wall shear stress gradient (WSSG), oscillatory shear index (OSI), and circumferential wall stress (CWS), which may promote neointimal hyperplasia (IH). The hypothesis is that stent-induced abnormal fluid and solid mechanics contribute to IH. To vary the range of WSS, WSSG, OSI, and CWS, we intentionally mismatched the size of stents to that of the vessel lumen. Stents were implanted in coronary arteries of 10 swine. Intravascular ultrasound (IVUS) was used to size the coronary arteries and stents. After 4 wk of stent implantation, IVUS was performed again to determine the extent of IH. In conjunction, computational models of actual stents, the artery, and non-Newtonian blood were created in a computer simulation to yield the distribution of WSS, WSSG, OSI, and CWS in the stented vessel wall. An inverse relation (R2 = 0.59, P < 0.005) between WSS and IH was found based on a linear regression analysis. Linear relations between WSSG, OSI, and IH were observed (R2 = 0.48 and 0.50, respectively, P < 0.005). A linear relation (R2 = 0.58, P < 0.005) between CWS and IH was also found. More statistically significant linear relations between the ratio of CWS to WSS (CWS/WSS), the products CWS × WSSG and CWS × OSI, and IH were observed (R2 = 0.67, 0.54, and 0.56, respectively, P < 0.005), suggesting that both fluid and solid mechanics influence the extent of IH. Stents create endothelial flow disturbances and intramural wall stress concentrations, which correlate with the extent of IH formation, and these effects were exaggerated with mismatch of stent/vessel size. These findings reveal the importance of reliable vessel and stent sizing to improve the mechanics on the vessel wall and minimize IH. PMID:21926337

Chen, Henry Y.; Sinha, Anjan K.; Choy, Jenny S.; Zheng, Hai; Sturek, Michael; Bigelow, Brian; Bhatt, Deepak L.

2011-01-01

349

Caveolin-1 regulates shear stress-dependent activation of extracellular signal-regulated kinase  

NASA Technical Reports Server (NTRS)

Fluid shear stress activates a member of the mitogen-activated protein (MAP) kinase family, extracellular signal-regulated kinase (ERK), by mechanisms dependent on cholesterol in the plasma membrane in bovine aortic endothelial cells (BAEC). Caveolae are microdomains of the plasma membrane that are enriched with cholesterol, caveolin, and signaling molecules. We hypothesized that caveolin-1 regulates shear activation of ERK. Because caveolin-1 is not exposed to the outside, cells were minimally permeabilized by Triton X-100 (0.01%) to deliver a neutralizing, polyclonal caveolin-1 antibody (pCav-1) inside the cells. pCav-1 then bound to caveolin-1 and inhibited shear activation of ERK but not c-Jun NH(2)-terminal kinase. Epitope mapping studies showed that pCav-1 binds to caveolin-1 at two regions (residues 1-21 and 61-101). When the recombinant proteins containing the epitopes fused to glutathione-S-transferase (GST-Cav(1-21) or GST-Cav(61-101)) were preincubated with pCav-1, only GST-Cav(61-101) reversed the inhibitory effect of the antibody on shear activation of ERK. Other antibodies, including m2234, which binds to caveolin-1 residues 1-21, had no effect on shear activation of ERK. Caveolin-1 residues 61-101 contain the scaffolding and oligomerization domains, suggesting that binding of pCav-1 to these regions likely disrupts the clustering of caveolin-1 or its interaction with signaling molecules involved in the shear-sensitive ERK pathway. We suggest that caveolae-like domains play a critical role in the mechanosensing and/or mechanosignal transduction of the ERK pathway.

Park, H.; Go, Y. M.; Darji, R.; Choi, J. W.; Lisanti, M. P.; Maland, M. C.; Jo, H.

2000-01-01

350

Rubbing salt into wounded endothelium: sodium potentiates proatherogenic effects of TNF-? under non-uniform shear stress.  

PubMed

Increased consumption of sodium is a risk factor for hypertension and cardiovascular diseases. In vivo studies indicated that high dietary sodium may have a direct negative influence on endothelium. We investigated the effects of high sodium on the endothelial activation during early steps of atherogenesis. Endothelial cells (HUVECs) grown in a model of arterial bifurcations were exposed to shear stress in the presence of normal or high (+ 30 mmol/l) sodium. Adherent THP-1 cells, and the adhesion molecule expression were quantified. Sodium channel blockers, pathways' inhibitors, and siRNA against tonicity-responsive enhancer binding protein (TonEBP) were used to identify the mechanisms of sodium effects on endothelium. ApoE-deficient mice on low-fat diet received water containing normal or high salt (8% w/v) for four weeks, and the influence of dietary salt on inflammatory cell adhesion in the common carotid artery and carotid bifurcation was measured by intravital microscopy. In vitro, high sodium dramatically increased the endothelial responsiveness to tumour necrosis factor-? under non-uniform shear stress. Sodium-induced increase in monocytic cell adhesion was mediated by reactive oxygen species and the endothelial nitric oxygen synthase, and was sensitive to the knockdown of TonEBP. The results were subsequently confirmed in the ApoE-deficient mice. As compared with normal-salt group, high-salt intake significantly enhanced the adhesion of circulating CD11b+ cells to carotid bifurcations, but not to the straight segment of common carotid artery. In conclusion, elevated sodium has a direct effect on endothelial activation under atherogenic shear stress in vitro and in vivo, and promotes the endothelial-leukocyte interactions even in the absence of increased lipid concentrations. PMID:24573382

Wild, J; Soehnlein, O; Dietel, B; Urschel, K; Garlichs, C D; Cicha, I

2014-07-01

351

Differential regulation of protease activated receptor-1 and tissue plasminogen activator expression by shear stress in vascular smooth muscle cells  

NASA Technical Reports Server (NTRS)

Recent studies have demonstrated that vascular smooth muscle cells are responsive to changes in their local hemodynamic environment. The effects of shear stress on the expression of human protease activated receptor-1 (PAR-1) and tissue plasminogen activator (tPA) mRNA and protein were investigated in human aortic smooth muscle cells (HASMCs). Under conditions of low shear stress (5 dyn/cm2), PAR-1 mRNA expression was increased transiently at 2 hours compared with stationary control values, whereas at high shear stress (25 dyn/cm2), mRNA expression was decreased (to 29% of stationary control; P<0.05) at all examined time points (2 to 24 hours). mRNA half-life studies showed that this response was not due to increased mRNA instability. tPA mRNA expression was decreased (to 10% of stationary control; P<0.05) by low shear stress after 12 hours of exposure and was increased (to 250% of stationary control; P<0.05) after 24 hours at high shear stress. The same trends in PAR-1 mRNA levels were observed in rat smooth muscle cells, indicating that the effects of shear stress on human PAR-1 were not species-specific. Flow cytometry and ELISA techniques using rat smooth muscle cells and HASMCs, respectively, provided evidence that shear stress exerted similar effects on cell surface-associated PAR-1 and tPA protein released into the conditioned media. The decrease in PAR-1 mRNA and protein had functional consequences for HASMCs, such as inhibition of [Ca2+] mobilization in response to thrombin stimulation. These data indicate that human PAR-1 and tPA gene expression are regulated differentially by shear stress, in a pattern consistent with their putative roles in several arterial vascular pathologies.

Papadaki, M.; Ruef, J.; Nguyen, K. T.; Li, F.; Patterson, C.; Eskin, S. G.; McIntire, L. V.; Runge, M. S.

1998-01-01

352

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

NASA Technical Reports Server (NTRS)

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.

Abid, R.; Speziale, C. G.

1992-01-01

353

Heterogeneity and the Role of Normal Stresses during the Extensional Thinning of Non-Brownian Shear-Thickening Fluids  

NASA Astrophysics Data System (ADS)

We contrast the extensional and shear dynamics of non-Brownian suspensions as a function of particle concentration. We show that the thinning rate selected during the viscoelastic pinch-off of a liquid bridge is related to the shear rate at which normal stresses become positive, which differs from the shear rate at the onset of shear thickening. By tracking particles, we demonstrate that the extensional flow is heterogeneous, with local variations of the volume fraction consistent with self-dilution. This nonuniform structure is the cause of the buckling of the threads formed after breakup.

Roché, Matthieu; Kellay, Hamid; Stone, Howard A.

2011-09-01

354

Variation in bed level shear stress on surfaces sheltered by nonerodible roughness elements  

NASA Astrophysics Data System (ADS)

Direct bed level observations of surface shear stress, pressure gradient variability, turbulence intensity, and fluid flow patterns were carried out in the vicinity of cylindrical roughness elements mounted in a boundary layer wind tunnel. Paired corkscrew vortices shed from each of the elements result in elevated shear stress and increased potential for the initiation of particle transport within the far wake. While the size and shape of these trailing vortices change with the element spacing, they persist even for large roughness densities. Wake interference coincides with the impingement of the upwind horseshoe vortices upon one another at a point when their diameter approaches half the distance between the roughness elements. While the erosive capability of the horseshoe vortex has been suggested for a variety of settings, the present study shows that the fluid stress immediately beneath this coherent structure is actually small in comparison to that caused by compression of the incident flow as it is deflected around the element and attached vortex. Observations such as these are required for further refinement of models of stress partitioning on rough surfaces.

Sutton, Stephen L. F.; McKenna-Neuman, Cheryl

2008-09-01

355

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

PubMed

Understanding the role of mechanical forces on cell behavior is critical for tissue engineering, regenerative medicine, and disease initiation studies. Current hemodynamic bioreactors are largely limited to 2D substrates or the application of general flow conditions at a tissue level, which eliminates the investigation of some essential physiological and pathological responses. One example is the mesenchymal transformation of endothelial cells in response to shear stress. Endothelial to mesenchymal transformation (EndMT) is a valve morphogenic mechanism associated with aortic valve disease initiation. The aortic valve experiences oscillatory shear on the disease-susceptible fibrosa, and the role of hemodynamics on adult EndMT is unknown. The goal of this work was to develop and characterize a microfluidic bioreactor that applies physiologically relevant laminar or oscillatory shear stresses to endothelial cells and permits the quantitative analysis of 3D cell-extracellular matrix (ECM) interactions. In this study, porcine aortic valve endothelial cells were seeded onto 3D collagen I gels and exposed to different magnitudes of steady or oscillatory shear stress for 48?h. Cells elongated and aligned perpendicular to laminar, but not oscillatory shear. Low steady shear stress (2?dyne/cm(2) ) and oscillatory shear stress upregulated EndMT (ACTA2, Snail, TGFB1) and inflammation (ICAM1, NFKB1) related gene expression, EndMT-related (?SMA) protein expression, and matrix invasion when compared with static controls or cells exposed to high steady shear (10 and 20?dyne/cm(2) ). Our system enables direct testing of the role of shear stress on endothelial cell mesenchymal transformation in a dynamic, 3D environment and shows that hemodynamics regulate EndMT in adult valve endothelial cells. Biotechnol. Bioeng. 2014;111: 2326-2337. © 2014 Wiley Periodicals, Inc. PMID:24898772

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

2014-11-01

356

Regulation of shear stress on rolling behaviors of HL-60 cells on P-selectin  

NASA Astrophysics Data System (ADS)

Circulating leukocytes in trafficking to the inflammatory sites, will be first tether to, and then roll on the vascular surface. This event is mediated through specific interaction of P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1), and regulated by hemodynamics. Poor data were reported in understanding P-selectin-mediated rolling. With the flow chamber technique, we herein observed HL-60 cell rolling on P-selectin with or without 3% Ficoll at various wall shear stresses from 0.05 to 0.4 dyn/cm2. The results demonstrated that force rather than transport regulated the rolling, similar to rolling on L- and E-selectin. The rolling was accelerated quickly by an increasing force below the optimal shear threshold of 0.15 dyn/cm2 first and then followed by a slowly decelerating phase starting at the optimum, showing a catch-slip transition and serving as a mechanism for the rolling. The catch-slip transition was completely reflected to the tether lifetime and other rolling parameters, such as the mean and fractional stop time. The narrow catch bond regime stabilized the rolling quickly, through steeply increasing fractional stop time to a plateau of about 0.85. Data presented here suggest that the low shear stress threshold serves as a mechanism for most cell rolling events through P-selectin.

Ling, YingChen; Fang, Ying; Yang, XiaoFang; Li, QuHuan; Lin, QinYong; Wu, JianHua

2014-10-01

357

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

NASA Technical Reports Server (NTRS)

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.

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

2001-01-01

358

Vascular cell adhesion molecule-1 expression in endothelial cells exposed to physiological coronary wall shear stresses.  

PubMed

Atherosclerosis is consistently found in bifurcations and curved segments of the circulatory system, indicating disturbed hemodynamics may participate in disease development. In vivo and in vitro studies have shown that endothelial cells (ECs) alter their gene expression in response to their hemodynamic environment, in a manner that is highly dependent on the exact nature of the applied forces. This research exposes cultured ECs to flow patterns present in the coronary arterial network, in order to determine the role of hemodynamic forces in plaque initiation. Vascular cell adhesion molecule-1 (VCAM-1) was examined as an indicator of plaque growth, as it participates in monocyte adhesion, which is one of the initial steps in the formation of fatty lesions. The hemodynamics of a healthy right and left coronary artery were determined by reconstructing 3D models from cineangiograms and employing computational fluid dynamic models to establish physiological coronary flow patterns. Wall shear stress (WSS) profiles selected from these studies were applied to ECs in a cone and plate bioreactor. The cone and plate system was specifically designed to be capable of reproducing the high frequency harmonics present in physiological waveforms. The shear stresses chosen represent those from regions prone to disease development and healthier arterial segments. The levels of the transcriptional and cell surface anchored VCAM-1 were quantified by flow cytometry and real time RT-PCR over a number of timepoints to obtain a complete picture of the relationship between this adhesion molecule and the applied shear stress. The WSS profiles from regions consistently displaying a higher incidence of plaques in vivo, induced greater levels of VCAM-1, particularly at the earlier timepoints. Conversely, the WSS profile from a straight section of vessel with undisturbed flow indicated no upregulation in VCAM-1 and a significant downregulation after 24 h, when compared with static controls. Low shear stress from the outer wall of a bifurcation induced four times the levels of VCAM-1 messenger ribonucleic acid (mRNA) after four hours when compared with levels of mRNA induced by WSS from a straight arterial section. This shear profile also induced prolonged expression of the surface protein of this molecule. The current study has provided insight into the possible influences of coronary hemodynamics on plaque localization, with VCAM-1 only significantly induced by the WSS from disease prone regions. PMID:19604015

O'Keeffe, Lucy M; Muir, Gordon; Piterina, Anna V; McGloughlin, Tim

2009-08-01

359

Measurements of Shear Reduction of 2D Vortex Diffusion  

Microsoft Academic Search

Experiments with magnetized ion columns in the 2-dimensional regime demonstrate shear reduction of vortex diffusion, in close correspondence with recent theory.(D.H.E. Dubin, Phys. Lett. A 284), 112 (2001). Here, the ions move in ( r, theta ) as point vortices, and we can accurately control the vorticity zeta (r), fluid rotation Omega (r), and shear S (r) ? r ;

C. F. Driscoll; F. Anderegg; D. H. E. Dubin

2001-01-01

360

Modeling effects of bank friction and woody bank vegetation on channel flow and boundary shear stress in the Rio Puerco, New Mexico  

USGS Publications Warehouse

[1] We have applied a physically based model for steady, horizontally uniform flow to calculate reach-averaged velocity and boundary shear-stress distributions in a natural stream with woody vegetation on the channel banks. The model calculates explicitly the form drag on woody plant stems and includes the effects of vegetation on the boundary shear stress, velocity, and turbulence fields. Average channel shapes, bed gradients, and shrub characteristics were determined for four long, internally similar reaches covering an 81 -km segment of the lower Rio Puerco, in north-central New Mexico. Hydraulic geometries for each of three flow events were determined from reach-average channel shapes, slopes, and measured silt-line heights above the bed. Model results show that friction on the lateral boundaries reduced the boundary shear stress in the center of the Rio Puerco channel by as much as 20%. In reaches with moderate to dense bank shrubs, perimeter-averaged boundary shear stresses were reduced by almost 40% during near bankfull flows. Model-calculated discharges for all three flow events indicate there was a loss of about 40% of the upstream discharge through the 81-km river segment. Infiltration rates estimated from discharge losses during the quasi-steady peak flows and areas of the bed and banks suggest rates of flow loss into the silty sand of the upper banks are about 8 times greater than rates of loss into the clayey silt-covered bed and lower banks.

Griffin, E.R.; Kean, J.W.; Vincent, K.R.; Smith, J.D.; Friedman, J.M.

2005-01-01

361

Differential inhibition by hyperglycaemia of shear stress- but not acetylcholine-mediated dilatation in the iliac artery of the anaesthetized pig  

PubMed Central

Clinical hyperglycaemia affects vascular endothelial function, but the effect on shear stress-induced arterial dilatation has not yet been established. We hypothesized that hyperglycaemia would inhibit this response via impaired glycocalyx mechanotransduction. Experiments were carried out in the anaesthetized pig in which pressure, blood flow and diameter of the left iliac artery were measured at two sites: proximal (d1) and distal (d2). Infusion of glucose, sufficient to raise blood glucose to 16–30 mm along the whole length of the artery, attenuated the shear stress-dependent dilatation in both sections of the artery with preservation of the responses to acetylcholine. The distal site was then isolated using snares and the lumen exposed to blood containing 25–35 mm glucose for 20 min. In the control situation, after exposure of both sections to normoglycaemia (5.7 mm glucose), both sections of artery showed increases in diameter in response to shear stress and acetylcholine. Hyperglycaemia attenuated the shear stress-dependent dilatation in the distal section only (P < 0.25), but not the response to acetylcholine. It is concluded from these results that the hyperglycaemia-impaired dilatation is consistent with loss of mechanotransducing properties of the endothelial glycocalyx by hyperglycaemia. These findings offer a possible explanation for the increased incidence of vascular disease in diabetic patients. PMID:16543269

Kelly, R; Ruane-O'Hora, T; Noble, M I M; Drake-Holland, A J; Snow, H M

2006-01-01

362

Finite size analysis of zero-temperature jamming transition under applied shear stress  

E-print Network

By finding local minima of an enthalpy-like energy, we can generate jammed packings of frictionless spheres under constant shear stress $\\sigma$ and obtain the yield stress $\\sigma_y$ by sampling the potential energy landscape. For three-dimensional systems with harmonic repulsion, $\\sigma_y$ satisfies the finite size scaling with the limiting scaling relation $\\sigma_y\\sim\\phi - \\phi_{_{c,\\infty}}$, where $\\phi_{_{c,\\infty}}$ is the critical volume fraction of the jamming transition at $\\sigma=0$ in the thermodynamic limit. The width or uncertainty of the yield stress decreases with decreasing $\\phi$ and decays to zero in the thermodynamic limit. The finite size scaling implies a length $\\xi\\sim (\\phi-\\phi_{_{c,\\infty}})^{-\

Hao Liu; Xiaoyi Xie; Ning Xu

2013-12-10

363

Wall shear stress variations and unsteadiness of pulsatile blood-like flows in 90-degree bifurcations.  

PubMed

Complex and slow interaction of different mechanical and biochemical processes in hemodynamics is believed to govern atherogenesis. Over the last decades studies have shown that fluid mechanical factors such as the Wall Shear Stress (WSS) and WSS gradients can play an important role in the pathological changes of the endothelium. This study provides further indications that the effects of fluid mechanical aspects are correlated with the diseased regions of the larger arteries. Unsteady high temporal WSS gradients (TWSSG), a function of the shear-thinning property of the non-Newtonian viscosity, move with the separation bubble. Red Blood Cell (RBC) dilution due to the secondary flows determines the magnitudes of the WSS and TWSSG. The results indicate that the focal nature of the TWSSG may have implications on the response of the endothelium. PMID:23816175

van Wyk, Stevin; Prahl Wittberg, Lisa; Fuchs, Laszlo

2013-09-01

364

Behavior of Three Metallic Alloys under Combined Axial-Shear Stresses at Elevated Temperature  

NASA Technical Reports Server (NTRS)

Type 316 stainless steel, Haynes 188, and Inconel 718 samples were subjected to an axial-shear strain controlled loading history while the specimen temperature was held at 650 C to quantify the evolution of material state under a complex biaxial load path when the material is in the viscoplastic domain. Yield surfaces were constructed in the axial-shear stress plane using a sensitive, 30 x 10(exp -6)m/m, equivalent offset strain definition for the yield strain. Subsequent yield surfaces were constructed at various points along the strain path to define the material evolution. These subsequent yield surface translated, expanded, and distorted relative to the initial yield surface. Each of these very different materials exhibited components of isotropic, kinematic and distortional hardening. Furthermore, subsequent yield surfaces for each material have a very well defined front face and a poorly defined, flattened, back side.

Colaiuta, J. F.; Lissenden, C. J.; Lerch, B. A.

2003-01-01

365

Deposit membrane fouling: influence of specific cake layer resistance and tangential shear stresses.  

PubMed

Cake fouling is the leading cause of membrane permeability decrease when filtering mixed liquor suspension containing high suspended solid concentrations. A simple model is proposed to simulate the cake resistance evolution with time by considering a macro-scale fouling linked only to the accumulation of particles on the membrane surface. This accumulation appears as the difference between the flux of deposited particles due to the filtration and the flux of particles detached from the membrane surface due to the tangential shear stresses caused by recirculation flow in the sidestream membrane bioreactor (MBR) or gas sparging close to the membrane surface for submerged MBR configuration. Two determining parameters were then highlighted: the specific cake resistance and the 'shear parameter'. Based on these parameters it is possible to predict model outputs as cake resistance and permeate flux evolution for short-time filtration periods. PMID:25026577

Charfi, A; Harmand, J; Ben Amar, N; Grasmick, A; Heran, M

2014-01-01

366

Residual stress Part 1 Measurement techniques  

E-print Network

Overview Residual stress Part 1 ­ Measurement techniques P. J. Withers and H. K. D. H. Bhadeshia Residual stress is that which remains in a body that is stationary and at equilibrium with its surroundings, beneficial residual stresses can be introduced deliberately. Residual stresses are more difficult to predict

Cambridge, University of

367

Coseismic and postseismic stress in the northern "Eastern California Shear Zone" over the past 150 years.  

NASA Astrophysics Data System (ADS)

Diffuse plate boundary regions are characterized by a high rate of deformation, but distributed over a wider zone when compared to plate boundary faults like the San Andreas and the North Anatolian faults. In diffuse plate boundaries the space-time relationship between activity on specific faults and the location of previous and subsequent events in the region are not clear. The Eastern California Shear Zone (ECSZ) is part of such a diffuse boundary. The purpose of our work is to determine how faults in the northern ECSZ interact in terms of static stress transfer at different spatial and temporal scales. Here we calculate the evolution of Coulomb stress in the northern ECSZ due to coseismic and postseismic redistribution of static stress induced by moderate and large (Mw × 6) earthquakes, beginning with the 1872 Mw 7.8 Owens Valley event. Our preliminary results show that the 1872 Owens Valley earthquake, the largest event to have occurred in the region over the past 150 years, has a significant influence in terms of coseismic and postseismic stress changes. This event produced significant stress increase in northern Owens Valley, where seven Mw × 6 earthquakes struck between 1980 and 1986. Stress changes were significantly amplified by postseismic relaxation. The total stress increase may have influenced the renewed activity of the Long Valley Caldera resurgent dome. In turn, magmatic activity in the caldera increased stress in the region of the 1980-1986 earthquake sequences. Several of the 1980-1986 events also appear to be connected to one another in terms of Coulomb stress loading. Finally, we find that significant stress, in the order of 3-4 bars, has by now accumulated on the White Mountains fault and the northern Death Valley fault, where no historical events have been recorded. Both these faults are capable of producing Mw × 7 earthquakes. We are now working on improving our model in terms of fault geometry and kinematics, and on extending it to the western Basin and Range.

Verdecchia, Alessandro; Carena, Sara

2013-04-01

368

Purinergic Signaling is Required for Fluid Shear Stress-Induced NF-kB Translocation in Osteoblasts  

SciTech Connect

Fluid shear stress regulates gene expression in osteoblasts, in part by activation of the transcription factor NF-kB. We examined whether this process was under control of purinoceptor activation. MC3T3-E1 osteoblasts under static conditions expressed the NF-kB inhibitory protein IkB alpha and exhibited cytosolic localization of NF-kB. Under fluid shear stress, I?B? levels decreased, and concomitant nuclear localization of NF-kB was observed. Cells exposed to fluid shear stress in ATP-depleted medium exhibited no significant reduction in I?B?, and NF-kB remained within the cytosol. Similar results were found using oxidized ATP or Brilliant Blue G, P2X7 receptor antagonists, indicating that the P2X7 receptor is responsible for fluid shear-stress-induced I?B? degradation and nuclear accumulation of NF-kB. Pharmacologic blockage of the P2Y6 receptor also prevented shear-induced IkB alpha degradation. These phenomena involved neither ERK1/2 signaling nor autocrine activation by P2X7-generated lysophosphatidic acid. Our results suggest that fluid shear stress regulates NF-kB activity through the P2Y6 and P2X7 receptor.

Genetos, Damian C.; Karin, Norman J.; Geist, Derik J.; Donahue, Henry J.; Duncan, Randall L.

2011-04-01

369

Use of shear-stress-sensitive, temperature-insensitive liquid crystals for hypersonic boundary-layer transition detection  

SciTech Connect

The use of shear-stress-sensitive, temperature-insensitive (SSS/TI) liquid crystals (LCs) has been evaluated as a boundary-layer transition detection technique for hypersonic flows. Experiments were conducted at Mach 8 in the Sandia National Laboratories Hypersonic Wind Tunnel using a flat plate model at near zero-degree angle of attack over the freestream unit Reynolds number range 1.2-5.8x10{sup 6}/ft. Standard 35mm color photography and Super VHS color video were used to record LC color changes due to varying surface shear stress during the transition process for a range of commercial SSS liquid crystals. Visual transition data were compared to an established method using calorimetric surface heat-transfer measurements to evaluate the LC technique. It is concluded that the use of SSS/TI LCs can be an inexpensive, safe, and easy to use boundary-layer transition detection method for hypersonic flows. However, a valid interpretation of the visual records requires careful attention to illumination intensity levels and uniformity, lighting and viewing angles, some prior understanding of the general character of the flow, and the selection of the appropriate liquid crystal for the particular flow conditions.

Aeschliman, D.P.; Croll, R.H.; Kuntz, D.W.

1997-04-01

370

Flow through internal elastic lamina affects shear stress on smooth muscle cells (3D simulations).  

PubMed

We describe a three-dimensional numerical simulation of interstitial flow through the medial layer of an artery accounting for the complex entrance condition associated with fenestral pores in the internal elastic lamina (IEL) to investigate the fluid mechanical environment around the smooth muscle cells (SMCs) right beneath the IEL. The IEL was modeled as an impermeable barrier to water flow except for the fenestral pores, which were assumed to be uniformly distributed over the IEL. The medial layer was modeled as a heterogeneous medium composed of a periodic array of cylindrical SMCs embedded in a continuous porous medium representing the interstitial proteoglycan and collagen matrix. Depending on the distance between the IEL bottom surface and the upstream end of the proximal layer of SMCs, the local shear stress on SMCs right beneath the fenestral pore could be more than 10 times higher than that on the cells far removed from the IEL under the conditions that the fenestral pore diameter and area fraction of pores were kept constant at 1.4 microm and 0.05, respectively. Thus these proximal SMCs may experience shear stress levels that are even higher than endothelial cells exposed to normal blood flow (order of 10 dyn/cm(2)). Furthermore, entrance flow through fenestral pores alters considerably the interstitial flow field in the medial layer over a spatial length scale of the order of the fenestral pore diameter. Thus the spatial gradient of shear stress on the most superficial SMC is noticeably higher than computed for endothelial cell surfaces. PMID:11788405

Tada, Shigeru; Tarbell, John M

2002-02-01

371

Influence of flow shear-stress and bed aggradation rate on grain-fabric formation  

NASA Astrophysics Data System (ADS)

In this study, we theoretically investigated the influence of hydraulic parameters on depositional grain-fabric formation that can be measured by method of anisotropy of magnetic susceptibility (AMS), and performed preliminary flume experiments to determine the model parameters. It has been suggested that the grain fabric (preferred orientation of grain long axes) of sand/sandstone provides significant sedimentological information such as paleocurrent direction, sediment-transport processes, and depositional environments. Grains orient along preferred directions because of complicated interactions between fluids and sediment particles; however, the influence of hydraulic parameters on grain-fabric formation has hardly been studied. There are two types of preferred grain orientations—a(p)a(i) and a(t)b(i). a(t)b(i)- and a(p)a(i)-type fabrics are preferred grain orientations that are perpendicular and parallel to the flow direction, respectively. River gravels tend to exhibit the a(t)b(i) fabric, whereas turbidite sandstones often exhibit the a(p)a(i) fabric; nonetheless, there exist many exceptions, and the grain-fabric tendency often fluctuates even in a single bed. The cause of the two types of grain fabric currently unknown, and the flow parameters that influence the fabric type have not yet been determined. Toward this end, we tried to establish an active-layer-type model for grain-fabric formation. Instead of numerical solutions for the movement of each grain, probabilistic density functions (PDFs) of the orientations of both influx and outflux grains to the control volume were considered. Preliminary flume experiments using rice grains indicated that the PDFs of grain orientation can be well approximated by the von Mises distribution, and on average, the orientations of influx and outflux grains were both perpendicular to the flow direction. This is because the projection area of a grain to the flow direction is maximized when the grain orients perpendicular to the flow. The proposed model can describe the influence of flow parameters for grain-fabric formation. First, if the sediment grain size, grain shape and the bed-aggradation rate are constant, it is predicted that (1) a relatively weak flow (low Shields dimensionless stress) produces the a(t)b(i) fabric whereas (2) an intense flow (high Shields dimensionless stress) produces the a(p)a(i) fabric. Generally, gravel rivers exhibit a lower Shields stress whereas a sand river and turbid currents exhibit a higher Shields stress. Thus, the result of this study agrees with empirical fact that larger grains (river gravels) tend to exhibit the a(t)b(i) fabric. Next, if the sediment grain size and shape and the flow shear stress are constant, it is predicted that (3) a low bed-aggradation rate produces the a(p)a(i) fabric whereas (4) a high bed-aggradation rate produces the a(t)b(i) or bi-modal fabric. This result agrees with experimental results reported by Nishio (2007). Our model can be applied to various types of low-density flows in which grains are transported without collisions, such as river flows and turbidity currents. Although model parameters need to be determined by carrying out additional flume experiments, this model is useful for estimating the flow properties from the grain fabric.

Naruse, H.

2009-12-01

372

Measuring the Shear-Tension Coupling of Engineering Fabrics  

SciTech Connect

Modelling the forming process of engineering fabrics and textile composites using a mechanical approach, such as FEM, requires characterisation of material behaviour. Using Picture Frame (PF) tests, several previous studies have reported a coupling between in-plane tension and fabric shear compliance. However, characterising this behaviour accurately has proven problematic due to the sensitivity of the PF test to small fabric misalignments in the test rig, prompting innovative solutions such as the use of load-cells mounted on the side bars of the PF rig to measure in-plane tension during testing. This paper focuses on an alternative testing technique, the Biaxial Bias Extension test, as a means to investigate this coupling. The approach has several benefits including simple equipment requirements, the ability to vary sample dimensions and boundary conditions. The main difficulty lies in extracting the material contribution to the recorded signal. To do this, an experimental method is demonstrated using two very different textiles; glass fabric and self-reinforced polypropylene both plain weaves. The latter is challenging to characterise and was chosen due to its high propensity to wrinkle at room temperature.

Abdiwi, F.; Harrison, P. [Department of Mechanical Engineering, James Watt Building (South) University of Glasgow, Glasgow G12 8QQ (United Kingdom); Guo, Z. [School of Civil Engineering and Geosciences Newcastle University, Newcastle upon Tyne, NE1 7RU UK (United Kingdom); Yu, W. R. [Department of Materials Science and Engineering, Seoul National University, San 56-1, Shillim-dong, Gwanak-gu, Seoul 151-742 (Korea, Republic of); Potluri, P.

2011-05-04

373

Measuring the Shear-Tension Coupling of Engineering Fabrics  

NASA Astrophysics Data System (ADS)

Modelling the forming process of engineering fabrics and textile composites using a mechanical approach, such as FEM, requires characterisation of material behaviour. Using Picture Frame (PF) tests, several previous studies have reported a coupling between in-plane tension and fabric shear compliance. However, characterising this behaviour accurately has proven problematic due to the sensitivity of the PF test to small fabric misalignments in the test rig, prompting innovative solutions such as the use of load-cells mounted on the side bars of the PF rig to measure in-plane tension during testing. This paper focuses on an alternative testing technique, the Biaxial Bias Extension test, as a means to investigate this coupling. The approach has several benefits including simple equipment requirements, the ability to vary sample dimensions and boundary conditions. The main difficulty lies in extracting the material contribution to the recorded signal. To do this, an experimental method is demonstrated using two very different textiles; glass fabric and self-reinforced polypropylene both plain weaves. The latter is challenging to characterise and was chosen due to its high propensity to wrinkle at room temperature.

Abdiwi, F.; Harrison, P.; Guo, Z.; Potluri, P.; Yu, W. R.

2011-05-01

374

On Measuring the Third Dimension of Cultured Endothelial Cells in Shear Flow  

NASA Astrophysics Data System (ADS)

The stress in the endothelial cells induced by blood flow depends on the waviness of the blood-endothelium interface and the slopes at the junctions of neighboring cells in the direction of flow. The height and slope in the third dimension of the living endothelial cells cannot be measured by ordinary optical and electron microscopy. Here we show that interference microscopy meets the challenge. We measured the geometry of cultured confluent human vascular endothelial cells in a flow, and we found that in a normal section parallel to the flow, the absolute values of the surface slopes at the cell junctions were 0.70 ± 0.02 (SE) and 0.80 ± 0.02 (SE) at the leading and trailing edges of the cells, respectively, in a culture medium of osmolarity 310 mosM with a shear stress of approximately 1 N/m^2. A reversal of the flow direction led to a reversal of the slope pattern. An increase in medium osmolarity above 310 mosM induced an initial decrease in the slopes followed by a return to normal, whereas a decrease in the osmolarity had a reversed effect. These results, in light of our previous theoretical analyses, show that tensile stress exists in the endothelial cell membrane, and that the mechanism of tension accumulation is a reality. The accumulation is not 100% because the membranes are not smooth at the cell junctions.

Liu, S. Q.; Yen, Morris; Fung, Y. C.

1994-09-01

375

On measuring the third dimension of cultured endothelial cells in shear flow.  

PubMed Central

The stress in the endothelial cells induced by blood flow depends on the waviness of the blood-endothelium interface and the slopes at the junctions of neighboring cells in the direction of flow. The height and slope in the third dimension of the living endothelial cells cannot be measured by ordinary optical and electron microscopy. Here we show that interference microscopy meets the challenge. We measured the geometry of cultured confluent human vascular endothelial cells in a flow, and we found that in a normal section parallel to the flow, the absolute values of the surface slopes at the cell junctions were 0.70 +/- 0.02 (SE) and 0.80 +/- 0.02 (SE) at the leading and trailing edges of the cells, respectively, in a culture medium of osmolarity 310 mosM with a shear stress of approximately 1 N/m2. A reversal of the flow direction led to a reversal of the slope pattern. An increase in medium osmolarity above 310 mosM induced an initial decrease in the slopes followed by a return to normal, whereas a decrease in the osmolarity had a reversed effect. These results, in light of our previous theoretical analyses, show that tensile stress exists in the endothelial cell membrane, and that the mechanism of tension accumulation is a reality. The accumulation is not 100% because the membranes are not smooth at the cell junctions. Images PMID:8090723

Liu, S Q; Yen, M; Fung, Y C

1994-01-01

376

Method of determining shear stress employing a monomer-polymer laminate structure  

NASA Technical Reports Server (NTRS)

A laminate structure attached to the test surface of an article is presented. The laminate structure is comprised of a liquid crystal polymer substrate. A light absorbing coating is applied to the substrate and is thin enough to permit bonding steric interaction between the liquid crystal polymer substrate and an overlying liquid crystal monomer thin film. Light is directed through and reflected by the liquid crystal monomer thin film and unreflected light is absorbed by the underlying coating. The wavelength of the reflected light is indicative of the shear stress experienced by the test surface.

Singh, Jag J. (inventor); Eftekhari, Abe (inventor); Parmar, Devendra S. (inventor)

1993-01-01

377

Real-Time Intravascular Shear Stress in the Rabbit Abdominal Aorta  

PubMed Central

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

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

2012-01-01

378

Synergistic Regulation of Angiogenic Sprouting by Biochemical Factors and Wall Shear Stress  

PubMed Central

The process of sprouting angiogenesis involves activating endothelial cells in a quiescent monolayer of an existing vessel to degrade and migrate into the underlying matrix to form new blood vessels. While the roles of biochemical factors in angiogenic sprouting have been well characterized, the roles of fluid forces have received much less attention. This review summarizes results that support a role for wall shear stress in post-capillary venules as a mechanical factor capable of synergizing with biochemical factors to stimulate pro-angiogenic signaling in endothelial cells and promote sprout formation. PMID:22247741

Kaunas, Roland; Kang, Hojin; Bayless, Kayla J.

2011-01-01

379

Glycated collagen alters endothelial cell actin alignment and nitric oxide release in response to fluid shear stress.  

PubMed

People with diabetes suffer from early accelerated atherosclerosis, which contributes to morbidity and mortality from myocardial infarction, stroke, and peripheral vascular disease. Atherosclerosis is thought to initiate at sites of endothelial cell injury. Hyperglycemia, a hallmark of diabetes, leads to non-enzymatic glycosylation (or glycation) of extracellular matrix proteins. Glycated collagen alters endothelial cell function and could be an important factor in atherosclerotic plaque development. This study examined the effect of collagen glycation on endothelial cell response to fluid shear stress. Porcine aortic endothelial cells were grown on native or glycated collagen and exposed to shear stress using an in vitro parallel plate system. Cells on native collagen elongated and aligned in the flow direction after 24 h of 20 dynes/cm(2) shear stress, as indicated by a 13% decrease in actin fiber angle distribution standard deviation. However, cells on glycated collagen did not align. Shear stress-mediated nitric oxide release by cells on glycated collagen was half that of cells on native collagen, which correlated with decreased endothelial nitric oxide synthase (eNOS) phosphorylation. Glycated collagen likely inhibited cell shear stress response through altered cell-matrix interactions, since glycated collagen attenuated focal adhesion kinase activation with shear stress. When focal adhesion kinase was pharmacologically blocked in cells on native collagen, eNOS phosphorylation with flow was reduced in a manner similar to that of glycated collagen. These detrimental effects of glycated collagen on endothelial cell response to shear stress may be an important contributor to accelerated atherosclerosis in people with diabetes. PMID:21555127

Kemeny, Steven F; Figueroa, Dannielle S; Andrews, Allison M; Barbee, Kenneth A; Clyne, Alisa Morss

2011-07-01

380

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)

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.

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

1979-01-01