Time-averaged Shear Stress Research Articles

The Effect of Coherent Structures in the Atmospheric Surface Layer on Blowing-Snow Transport

While turbulent bursts are considered critical for blowing-snow transport and initiation, the interaction of the airflow with the snow surface is not fully understood. To better characterize the coupling of turbulent structures and blowing-snow transport, observations collected in natural environments at the necessary high-resolution time scales are needed. To address this, high-frequency measurements of turbulence, blowing-snow density and particle velocity were made in the Canadian Rockies. During blowing-snow storms, modified variable-interval time averaging enabled identification of periods of near-surface blowing-snow coupling with shear-stress-producing motions in the lowest 2 m of the atmospheric surface layer. The identification of those turbulent motions responsible for blowing snow yields a better understanding of the event-driven mechanics of initiation and sustained transport. The type of coherent structures generating the Reynolds stress are just as important as the magnitude of the Reynolds stress in initiating and sustaining near-surface blowing snow. Our results suggest that blowing-snow models driven by merely the time-averaged shear stress lack physical realism in the near-surface region. The next phase of the development of blowing-snow models should incorporate parametrizations of coherent turbulent structures.

Numerical and Experimental Study of Wake Redirection Techniques in a Boundary Layer Wind Tunnel

The aim of the present paper is to validate a wind farm LES framework in the context of two distinct wake redirection techniques: yaw misalignment and individual cyclic pitch control. A test campaign was conducted using scaled wind turbine models in a boundary layer wind tunnel, where both particle image velocimetry and hot-wire thermo anemometers were used to obtain high quality measurements of the downstream flow. A LiDAR system was also employed to determine the non-uniformity of the inflow velocity field. A high-fidelity large-eddy simulation lifting-line model was used to simulate the aerodynamic behavior of the system, including the geometry of the wind turbine nacelle and tower. A tuning-free Lagrangian scale-dependent dynamic approach was adopted to improve the sub-grid scale modeling. Comparisons with experimental measurements are used to systematically validate the simulations. The LES results are in good agreement with the PIV and hot-wire data in terms of time-averaged wake profiles, turbulence intensity and Reynolds shear stresses. Discrepancies are also highlighted, to guide future improvements.

Time-averaged Turbulent Flow Characteristics over a Highly Spatially Heterogeneous Gravel-Bed

The present study focuses on the time-averaged turbulence characteristics over a highly spatially-heterogeneous gravel-bed. The timeaveraged streamwise velocity, Reynolds shear and normal stresses, turbulent kinetic energy, higher-order moments of velocity fluctuations, length scales, and the turbulent bursting were measured over a gravel-bed with an array of larger gravels. It was observed that the turbulence characteristics do not vary significantly above the crest level of the array as compared to those below the array. The nondimensional streamwise velocity decreases considerably with a decrease in depth below the array. Below the array, the Reynolds shear stress (RSS) deviates from the gravity- law of RSS distributions. Turbulence intensities reduce below the crest level of the gravel-bed. The third-order moments of velocity fluctuations increase below the crest level of the gravel-bed and give a clear indication of sweeps as the predominating event which were further verified with the quadrant analysis plots. The turbulent length scales values change significantly below the crest level of the gravel-bed.

Porous media approach for RANS simulation of compound open-channel flows with submerged vegetated floodplains

The main goal of this study is the 3D numerical simulation of river flows with submerged vegetated floodplains. Since, vegetation layers are usually dense and present a large spatial heterogeneity they are here represented as a porous media. Standard semi-empirical relations drawn for porous beds packed with non-spherical particles are used to estimate the porous media parameters based on the averaged geometry of the vegetation elements. Thus, eliminating the uncertainty arising from a bulk drag coefficient approach and allowing the use of a coarser mesh. The free flow is described by Reynolds-averaged Navier–Stokes (RANS) equations, whereas the porous media flow is described by the volumetric-average of RANS equations. The volume-of-fluid method and an anisotropic explicit algebraic Reynolds stress model are used for free-surface and turbulence closure, respectively. The simulation approach is validated against results by other authors featuring vegetated flows in horizontal and rectangular open-channel. The computed results show that the time-averaged streamwise velocity and Reynolds shear stress vertical profiles are properly simulated. The validated approach was applied to simulate compound open-channel flows with submerged vegetated floodplains and compared with data obtained in an experimental facility. The results show that the proposed porous media approach is adequate to simulate flows with submerged vegetation on the floodplains.

Abstract 19086: Pulsatile Shear Stress-mediated Fatty Acid Metabolites Promote Anti-oxidant Effects

Temporal and spatial variations in shear stress are intimately linked with vascular metabolic effects. While oscillatory shear stress (OSS) preferentially develops at the lateral wall of arterial bifurcations and induces oxidative stress and inflammatory states, pulsatile shear stress (PSS) develops in the straight portions of the vasculature to oppose the effects and promote “athero-protection”. In this context, we hypothesize that PSS activates PPAR-γ to produce athero-protective fatty acid metabolites to attenuate oxidative stress. Metabolomic analysis performed for 155 metabolites from human aortic endothelial cells exposed to PSS (τ avg = 23±8 dyne/cm 2 at 1 Hz for 4 hrs) revealed a significant increase in anti-inflammatory metabolites, including fatty acids such as oleic acid (OA) and dicarboxylic acids such as azelaic acid (AzA) (1.43±0.73-fold, 2.03±0.54-fold vs. control, respectively, p < 0.05, n=6), and up-regulated metabolism-related genes including SCD1 and PPAR-γ (1.42±0.14-fold, 1.53±0.32-fold vs. control, respectively, p < 0.001, n=6). SCD1 produces oleic acid, and SCD1 mRNA expression was down-regulated by siPPAR-γ (0.66±0.17-fold, vs. PSS, p < 0.05, n=4). AzA, a known PPAR-γ agonist and a breakdown product of oleic acid, down-regulated VCAM and ICAM (0.31±.09-fold, 0.18±.01-fold vs. control, respectively, p <0.05, n=4) when treated with AzA alone. AzA attenuated H 2 O 2 -induced VCAM and ICAM mRNA expression (0.36±0.10-fold, 0.38±0.20-fold, respectively, p <0.05, n=4). The role PSS on PPAR- γ activation was further recapitulated in a model for paroxysmal atrial fibrillation (Afib) in New Zealand White rabbits. The right atrium was paced at 800 bpm (cycle length (CL) = 75 ms) to produce an irregular ventricular response of a mean rate of 200 bpm (CL = 300 ms) to simulate atrial fibrillation for 4 hours. p-PPAR- γ staining in the athero-prone aortic arches vs. athero-protective descending aortas was decreased in response to a reduction in time-averaged shear stress (from τ avg = 32.0±2.4 dyne/cm 2 to τ avg = 22.7±3.5 dyne/cm 2 ). Overall, pulsatile shear stress-induced fatty acid metabolites impart anti-oxidant effects to maintain endothelial homeostasis, implicating the role of PPAR-γ-SCD1-OA-AzA pathway.

A microfluidic model to study fluid dynamics of mucus plug rupture in small lung airways.

Fluid dynamics of mucus plug rupture is important to understand mucus clearance in lung airways and potential effects of mucus plug rupture on epithelial cells at lung airway walls. We established a microfluidic model to study mucus plug rupture in a collapsed airway of the 12th generation. Mucus plugs were simulated using Carbopol 940 (C940) gels at concentrations of 0.15%, 0.2%, 0.25%, and 0.3%, which have non-Newtonian properties close to healthy and diseased lung mucus. The airway was modeled with a polydimethylsiloxane microfluidic channel. Plug motion was driven by pressurized air. Global strain rates and shear stress were defined to quantitatively describe plug deformation and rupture. Results show that a plug needs to overcome yield stress before deformation and rupture. The plug takes relatively long time to yield at the high Bingham number. Plug length shortening is the more significant deformation than shearing at gel concentration higher than 0.15%. Although strain rates increase dramatically at rupture, the transient shear stress drops due to the shear-thinning effect of the C940 gels. Dimensionless time-averaged shear stress, T xy , linearly increases from 3.7 to 5.6 times the Bingham number as the Bingham number varies from 0.018 to 0.1. The dimensionless time-averaged shear rate simply equals to T xy /2. In dimension, shear stress magnitude is about one order lower than the pressure drop, and one order higher than yield stress. Mucus with high yield stress leads to high shear stress, and therefore would be more likely to cause epithelial cell damage. Crackling sounds produced with plug rupture might be more detectable for gels with higher concentration.

Numerical Study of Turbulent Pulsatile Blood Flow through Stenosed Artery Using Fluid-Solid Interaction.

The turbulent pulsatile blood flow through stenosed arteries considering the elastic property of the wall is investigated numerically. During the numerical model validation both standard k-ε model and RNG K-ε model are used. Compared with the RNG K-ε model, the standard K-ε model shows better agreement with previous experimental results and is better able to show the reverse flow region. Also, compared with experimental data, the results show that, up to 70% stenosis, the flow is laminar and for 80% stenosis the flow becomes turbulent. Assuming laminar or turbulent flow and also rigid or elastic walls, the results are compared with each other. The investigation of time-averaged shear stress and the oscillatory shear index for 80% stenosis show that assuming laminar flow will cause more error than assuming a rigid wall. The results also show that, in turbulent flow compared with laminar flow, the importance of assuming a flexible artery wall is more than assuming a rigid artery wall.

Local surface shear stress measurements from oil streaks thinning rate

This paper presents an approach to evaluate the time-averaged local shear stresses based on the analysis of video images of oil streaks. The time-averaged deformation rate of thin oil streaks, deposited on a model exposed to an air stream, increases pro rata with the skin friction, while it is inversely proportional to the viscosity. Hence, the oil viscosity represents an important parameter in the method to be carefully characterized and selected for the level of aerodynamic skin friction, flow velocity, and time interval to be resolved. Post-processing image techniques were applied to reconstruct the rate of change in the oil thickness from oil streak length monitored by a high-speed camera. The present technique was demonstrated on a model exposed to subsonic and supersonic flows, and results were compared with theoretical calculations. The present measurement procedure offers ancillary information to the conventional qualitative oil dot visualizations, providing the wall shear stresses at a reduced cost and complexity.

See-saw rocking: an in vitro model for mechanotransduction research.

In vitro mechanotransduction studies, uncovering the basic science of the response of cells to mechanical forces, are essential for progress in tissue engineering and its clinical application. Many varying investigations have described a multitude of cell responses; however, as the precise nature and magnitude of the stresses applied are infrequently reported and rarely validated, the experiments are often not comparable, limiting research progress. This paper provides physical and biological validation of a widely available fluid stimulation device, a see-saw rocker, as an in vitro model for cyclic fluid shear stress mechanotransduction. This allows linkage between precisely characterized stimuli and cell monolayer response in a convenient six-well plate format. Models of one well were discretized and analysed extensively using computational fluid dynamics to generate convergent, stable and consistent predictions of the cyclic fluid velocity vectors at a rocking frequency of 0.5 Hz, accounting for the free surface. Validation was provided by comparison with flow velocities measured experimentally using particle image velocimetry. Qualitative flow behaviour was matched and quantitative analysis showed agreement at representative locations and time points. Maximum shear stress of 0.22 Pa was estimated near the well edge, and time-average shear stress ranged between 0.029 and 0.068 Pa. Human tenocytes stimulated using the system showed significant increases in collagen and GAG secretion at 2 and 7 day time points. This in vitro model for mechanotransduction provides a versatile, flexible and inexpensive method for the fluid shear stress impact on biological cells to be studied.

Abstract 19000: Shear Stress Responsive PKCe Modulates Mitochondrial Redox State With an Implication in Vascular Repair

Pulsatile (PSS) versus oscillatory (OSS) shear stress differentially modulates mechano-signal transduction with implications in atheroprotection and atherogenesis. In response to ischemia and reperfusion injury, Protein Kinase C isoform epsilon (PKCε) signaling in mitochondria confers cardioprotection. It is unknown whether PKCε was shear stress responsive via endothelial growth factor receptor (VEGFR) signaling with an implication in vascular endothelial repair. Both PSS (time-averaged shear stress (τave) = 25 dyne/cm2 at 1 Hz for 4 hr) and OSS (τave= 0.1±3 dyne/cm2 at 1 Hz for 4 hr) up-regulated PKCε mRNA by 1.7±0.2-fold and 1.5±0.1-fold in human aortic endothelial cells (HAEC), respectively, and protein expression by 1.5±0.1-fold and by 1.4±0.1-fold, respectively, which were inhibited in the presence of VEGFR-inhibitor, Cediranib (p < 0.05, n=4). Treatment with VEGF (100ng/ml) further up-regulated PKCε mRNA expression by 2.3±0.2-fold at 1 hour (p < 0.01, n=3). Matrigel assays revealed that both PKCε siRNA knockdown and VEGFR inhibitor disrupted endothelial tube formation in terms of reduction in tube lengths by 3.0±0.2-fold and by 3.8±0.2-fold, respectively, whereas over-expression of PKCε via adenovirus transfection rescued the formation in the presence and absence of VEGF inhibitor (p < 0.02, n=3). Furthermore, H2O2 increased MitoSox intensity for mitochondrial O2.- production (mtO2.-) by 1.4±0.2-fold, which was significantly attenuated by Adv-PKCε over-expression (p < 0.01, n=3). H2O2-induced mtO2.- production further disrupted tube formation, which was restored in the presence of Adv-PKCε over-expression. The phenotypic effect of PKCε was further recapitulated in the Tg(fli1:EGFP) transgenic zebrafish model, in which treatment with morpholinos to block either splicing or translation were used to inhibit PKCε protein expression, which interrupted repair of intersegmental vessels (ISV) after 3 days post tail amputation (n=6). Thus, our findings suggest that PKCε was shear stress responsive via endothelial growth factor receptor (VEGFR) signaling with an implication in vascular endothelial repair.

Time-averaged shear stress and velocity distribution in an open channel junction

The present study reports the effect of transcritical flow on velocity and shear stress distribution over a partially rough bed. Four series of runs have been performed in a right-angled open channel junction. Experimental data obtained from compound channel are used to compare with the simple ones. The physical size and geometry of roughness elements are chosen to encompass transcritical flow regimes. Using a small portion of the data recorded at each location using an acoustic Doppler velocimeter over a grid defined, an evaluation of transcritical flow in open channel junction is presented. The approach flow is subcritical followed by a transition to supercritical flow in the downstream branch channel is realized. Results show that the flow features around the junction are related to the strength of the secondary currents and turbulence quantities in shear layer increases with discharge ratio. Energy loss is associated with the bed shear stress exhibited is more in the simple channel than that of compound channel. The complete data-set describing transcritical flow at a 90° open channel junction is presented as a resource for the validation of 3D computational fluid dynamics codes that utilize a free surface model.

Effects of wall distensibility in hemodynamic simulations of an arteriovenous fistula

Arteriovenous fistulae are created surgically to provide adequate access for dialysis patients suffering from end-stage renal disease. It has long been hypothesized that the rapid blood vessel remodeling occurring after fistula creation is in part a process to restore the mechanical stresses to some preferred level, i.e., mechanical homeostasis. The current study presents fluid-structure interaction (FSI) simulations of a patient-specific model of a mature arteriovenous fistula reconstructed from 3D ultrasound scans. The FSI results are compared with previously published data of the same model but with rigid walls. Ultrasound-derived wall motion measurements are also used to validate the FSI simulations of the wall motion. Very large time-averaged shear stresses, 10-15 Pa, are calculated at the fistula anastomosis in the FSI simulations, values which are much larger than what is typically thought to be the normal homeostatic shear stress in the peripheral vasculature. Although this result is systematically lower by as much as 50% compared to the analogous rigid-walled simulations, the inclusion of distensible vessel walls in hemodynamic simulations does not reduce the high anastomotic shear stresses to "normal" values. Therefore, rigid-walled analyses may be acceptable for identifying high shear regions of arteriovenous fistulae.

A laboratory study of irregular shoaling waves

The present research aims to investigate the dynamics of a single laboratory irregular wave, characterized by a narrow-banded spectrum and developing on a sloping sand bottom, in intermediate waters up to the surf zone. Experiments focused on the wave shoaling region, in order to examine how the wave is affected by breaking induced turbulence offshore the surf zone. A 3D acoustic Doppler velocimeter was used to measure the three wave velocity components, which were all processed to evaluate the time-averaged vertical distributions of orbital velocities, wave and turbulent Reynolds shear stresses and turbulent intensities. The vertical distributions of the phase-averaged velocity components, turbulent kinetic energy and transport of turbulence were also analysed. The adopted phase-averaging technique was applied to each investigated measurement point. Therefore, the crucial element of the study is that all the analysed values derive directly from real measurements and are not approximated by any kind of interpolation. The study confirmed some dynamic behaviour in the shoaling zone already known in the literature, such as the typical cell-type flow pattern of the mean flow and the necessity to evaluate the turbulent kinetic energy with all the three velocity components, when available, which would otherwise be underestimated. Referring to the time-averaged wave and Reynolds shear stresses, a contribution was added to the open debate on their order of magnitude. The measured wave Reynolds shear stresses were also compared with the results of the model by Zou et al. (J Geophys Res 111:C09032, 2006), confirming the behaviour typical of dissipative breaking waves. The analysis of turbulence transport in the shoaling zone revealed that it is seaward directed close to the surface and landward directed close to the bottom. The results presented in the paper can be extended only to other analogous flow conditions.

Cardiac-like flow generator for long-term imaging of endothelial cell responses to circulatory pulsatile flow at microscale

In vitro models of circulatory hemodynamics are required to mimic the microcirculation for study of endothelial cell responses to pulsatile shear stress by live cell imaging. This study reports the design, fabrication and characterisation of a microfluidic device that generates cardiac-like flow in a continuous culture system with a circulatory volume of only 2-3 μL. The device mimics a single chamber heart, with the following cardiac phases: (1) closure of the ventricle inlet valve, (2) contraction of the ventricle (systole) followed by opening of the outlet valve and (3) relaxation of the ventricle (diastole) with opening of the inlet valve whilst the outlet valve remains closed. Periodic valve states and ventricular contractions were actuated by microprocessor controlled pneumatics. The time-dependent velocity-field was characterised by micro-particle image velocimetry (μ-PIV). μ-PIV observations were used to help tune electronic timing of valve states and ventricular contractions for synthesis of an arterial pulse waveform to study the effect of pulsatile shear stress on bovine artery endothelial cells (BAECs). BAECs elongated and aligned with the direction of shear stress after 48 h of exposure to a pulsatile waveform with a maximum shear stress of 0.42 Pa. The threshold for BAECs alignment and elongation under steady (non-pulsatile) flow reported by Kadohama et al. (2006) is 0.7-1.4 Pa. These cells respond to transient shear stress because the time average shear stress of the pulse waveform to generate this morphological response was only 0.09 Pa, well below the steady flow threshold. The microfluidic pulse generator can simulate circulatory hemodynamics for live cell imaging of shear-induced signalling pathways.

Atrial fibrillation pacing decreases intravascular shear stress in a New Zealand white rabbit model: implications in endothelial function.

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 rabbits (n = 4) at rapid and irregular intervals. Surface electrical cardiograms were recorded for atrial and ventricular rhythm, and intravascular convective heat transfer was measured by microthermal sensors, from which ISS was inferred. Rapid and irregular pacing decreased arterial systolic and diastolic pressures (baseline, 99/75mmHg; rapid regular pacing, 92/73; rapid irregular pacing, 90/68; p < 0.001, n = 4), temporal gradients ([Formula: see text] from 1,275 ± 80 to 1,056 ± 180 dyne/cm(2) s), and reduced ISS (from baseline at 32.0 ± 2.4 to 22.7 ± 3.5 dyne/cm(2)). Computational fluid dynamics code demonstrated that experimentally inferred ISS provided a close approximation to the computed wall shear stress 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 [Formula: see text] , we further demonstrated that rapid pulse rates at 150bpm down-regulated endothelial nitric oxide, promoted superoxide (O 2 (.-) ) production, and increased monocyte binding to endothelial cells. These findings suggest that rapid pacing reduces ISS and [Formula: see text] , and rapid pulse rates modulate endothelial responses.

Three-Dimensional Flow Characteristics within the Scour Hole around Circular Uniform and Compound Piers

AbstractThe experimental observations are presented on flow patterns and turbulence characteristics measured by an acoustic Doppler velocimeter within the developing (transient stage) scour hole around circular uniform and compound piers. Four series of experimental runs were conducted under the clear-water approach flow conditions. One experimental run was conducted around a uniform circular pier of diameter 114 mm, whereas the other three runs were conducted around a circular compound pier of diameter 114 mm and footing diameter 210 mm. In the series with a circular compound pier, the top surface of the footing was placed at three different elevations with respect to the general level of the channel bed, i.e., above the bed level, at the bed level, and below the level of the channel bed. Detailed measurements are presented on components of time-averaged velocity, turbulence intensity, and Reynolds shear stress around the pier in radial planes at 0°, 90°, and 180° from the flow axis. Flow structure aroun...

Simulation of membrane fouling considering mixed liquor viscosity and variation of shear stress on membrane surface

Simulation of membrane fouling in MBR was conducted considering accumulation, detachment and consolidation of extracellular polymeric substances accumulated on membrane surface. The fluctuation of shear stress working on membrane surface and the influence of the viscosity of mixed liquor were considered for the evaluation of shear stress. A flat-sheet-type membrane module was used and the change of trans-membrane pressure was measured in a laboratory-scale MBR reactor. Shear stress working on membrane surface caused by aeration was measured by a shear force sensor changing viscosity of bulk liquid. Effective shear stress on membrane surface was defined in the model as the sum of time-averaged shear stress and three times of standard deviation. The increase in the trans-membrane pressure was accurately simulated by the developed model suggesting validity of the developed fouling model and the idea of the effective shear stress on membrane surface.

Discussion of “Protecting Vertical-Wall Abutments with Riprap Mattresses” by A. H. Cardoso and C. M. S. Fael

The authors are to be commended for their experimental program and for the attempt to provide guidelines for safe design of scour countermeasures by means of launching aprons. This is a technical field in which the existing literature is still relatively scarce. In this discussion, some remarks will be given about the upstream width wu of the launching apron refer to Fig. 12 of the paper under discussion. The authors did not report specific experimentation in this respect. Indeed, the upstream width of the mattress has received relatively scarce attention in works on riprap countermeasures. Experimental evidence has shown that for short abutments as well, the values of the time-averaged shear stress distribution in the corner region upstream of the abutment face are lower than that in the incoming reach Rajaratnam and Nwachukwu 1983; Molinas et al. 1998; Dey and Barbhuiya 2005. If the phenomenon is modeled at threshold conditions as is typically done in clear-water scour experiments, no scour would be expected upstream of the abutment face in all the regions with stress lower than that in the upstream reach. Experimental evidence on sediment motion during the scour process at short and intermediate abutments Radice et al. 2006,2008 has shown that at the beginning of the erosion process, the particle motion is most intense at the abutment nose where scour starts. In contrast, grain motion is almost negligible at the junction between the duct and abutment walls junction region; here, an appreciable rate of scour depth is observed only after the scour hole whose point of maximum depth is typically at the obstacle nose has reached a significant transverse slope. In terms of design of scour countermeasures, protecting the sediment bed around the abutment nose might be sufficient to prevent the scouring process. An experimental test of local scour at a protected abutment was performed to check this issue. A rectangular duct with a length of 5.8 m and a width of 0.40 m was used. The recess section of the duct was filled with uniform plastic grains with a characteristic size of D = 3.6 mm and a relative density of 1.43. Water discharge was 18.5 l / s, corresponding to the threshold value for incipient particle motion in the undisturbed bed. The abutment model was a vertical plate with a length of 10 cm and a thickness of 1 cm Fig. 1. The experimental installation has been described in detail by Radice et al. 2009, who performed an identical scour experiment without abutment protection. In that study, the possibility of comparing scour data taken under freesurface and covered flows was also demonstrated. In the present experiment, the sediment bed around the abutment nose was protected by means of a square plate as shown in Fig. 1. The plate dimension was 20 cm along both the flow and transverse directions; plate thickness was 1 cm. The plate was roughened gluing on its upper surface sediments identical to those of the loose bed; thus the resulting thickness of the countermeasure was 1.4 mm. The described protection device is actually a rough collar installed at the level of the original sediment bed. However, at scour initiation the collar is likely to behave similarly to a launching apron with an elevation identical to that of the protected sediment bed. Therefore, the following observations on scour initiation should not be significantly affected by the different configuration of the countermeasure. At the beginning of the process, some sediment motion was evident in the junction region Fig. 2. In this region the timeaverage shear stress is expected to be smaller than the threshold value; it is therefore reasonable to assume the motion events as due to the peak stress values. During the motion events, the grains assumed directions opposite to that of the incoming flow, consistent with the structure of the principal vortex. The sediment particles moved downstream of the abutment passing over the protection plate. The grain motion in the junction region resulted in a weak but nonnegligible scour rate. Some particles coming from the upstream reach can be recognized. This is consistent with the definition used here of incipient particle motion, which corresponds to a weak sediment transport condition dimensionless sediment transport rate per unit width= 6 10 �5 . After about 3 min of run time Fig. 3, the junction-region scour depth exceeded the thickness of the protection plate and the sediments

The influence of heart stress on the brain: a preliminary study

The interaction between the cardiac hemodynamics and brain activity has been extensively studied in the past. however these studies have involved not only an increase in heart rate but also an increase in blood pressure. One method to evaluate for underlying coronary artery disease is to perform stress testing. Often, myocardial stress is achieved by the patient walking on a treadmill or riding a stationary bicycle while being monitored. In patients that are unable to exercise, pharmacologic stress testing is performed, either with vasodilatory agents (e.g. adenosine) or dobutamine, which is a pro-inotropic and chronotropic drug. During dobutamine infusion, the heart rate increases, but there is a negligible increase in blood pressure. The result is that the heart rate increases as a function of the dosage. Thus by monitoring brain activity while patients are undergoing dobutamine stress testing the effect of increased blood pressure on the brain is removed. It was found in the results of one patient that there are five areas of the brain which show a change in the frequency of EEG activity that is a function of dobutamine dosage. using fourier transforms it was established that this change occurred at a frequency around 12.5 hz. The regions of the brain where the increased activity were a function of heart rate occurred in the cZ-PZ, c4-P4, T5-O1, T4-T6 and c3-P3 regions. In undertaking these calculations it was realized that the stress on the ventricle could be calculated. The physicians monitor EcG of the patient to evaluate the cardiac status of the patient. Since currently the attending physician does not have data on the stress being applied to the myocardium, it would be useful to have this calculated on a real time basis. The motivation for the present preliminary study is to both investigate the feasibility of producing such information for clinicians as well as to investigate the variation between different patients as the heart rate varies. It was found that generally the surface shear stress increased with heart rate around most of the left ventricle. While the time averaged shear stress may be important for diagnosis the maximum shear stress probably is the limiting factor in terminating testing.

Endothelial Gene Expression in Regions of Defined Shear Exposure in the Porcine Iliac Arteries

The effect of hemodynamic shear stress on endothelial gene expression was investigated in the porcine iliac arteries. A novel statistical approach was applied to computational fluid dynamics simulations of the iliac artery flow field to identify three anatomical regions likely to experience high, medium, and low levels of time average shear stress magnitude. Subsequently, endothelial cell mRNA was collected from these regions in the iliac arteries of six swine and analyzed by DNA microarray. Gene set enrichment analysis demonstrated a strong tendency for genes upregulated or downregulated in one of the extreme shear environments (low or high, relative to medium) to be regulated in the same direction in the other extreme shear environment. This tendency was confirmed for specific genes by real-time quantitative PCR. Specifically, beta-catenin, c-jun, VCAM-1, and MCP-1 were all upregulated in low and high shear stress regions relative to the medium shear stress region. eNOS expression was not significantly different in any of the regions. These results are consistent with the notion that endothelial cells chronically exposed to abnormally low or high shear levels in vivo exhibit similar genetic responses. Alternative explanations of this outcome are proposed, and its implications for the role of shear stress in atherogenesis are examined.