Turbulent Shear Stress Research Articles

Non-adiabatic annular flow experiments in a subchannel geometry with spacer grid

Boiling annular flow in a subchannel geometry with functional spacers is studied using X-Ray tomography in the Dryout Tomography Experiment (DoToX) facility at ETH Zürich. The liquid film distribution on the walls and the liquid holdup within the gas core of the annular flow in the subchannel, both under the influence of the spacer vanes, are extracted from tomographic images. The subchannel cross section consists of a main subchannel in the center surrounded by four heating flow sections representing fuel rods and connected to four partial subchannels with simplified geometry. This effectively removes unnatural no-slip boundary conditions at the narrow gap of the main subchannel.Chloroform is used as a working fluid because of its low enthalpy of evaporation and a low boiling point around 60 °C close to atmospheric pressure. A liquid heating system with water at slightly above 100 °C provides the heating power for the boiling process. This ensures continuous operation of the facility even in case of a dryout scenario, avoiding structural damages by imposing a maximum system temperature. The high thermal capacity of the water allows for an almost isothermal boundary condition. Although not identical to real BWR settings, this experiment is well suited to generate data for model validation.Originally designed for tomographic imaging with both cold neutrons and X-rays, this paper focuses on results obtained by X-rays. Since the X-ray contrast between the working fluid chloroform and the structural material of the channel, aluminum, is very small, differential imaging with adaptive centering of reconstructions with and without the liquid film were required. The measurement campaign consisted of three measurements with constant chloroform mass flows at 5.00, 4.05 and 3.25 kg/min and constant heating liquid temperature of 100–104 °C, in addition to the empty channel reference measurement. The thermal power transferred to the working fluid under these conditions was about 10 kW of heating power. The resulting liquid film thicknesses (LFT) upstream of the spacer decreased from 91 to 49 µm. The effect of the spacer vanes is visualized and quantified over an axial section of 10 hydraulic diameters. Reference samples of the film thickness and the liquid holdup were taken upstream of the spacer. Dryout was nearly reached in characteristic regions of reduced film thickness due to turbulent shear stresses, while other regions show a significant local increase of the liquid film thickness. The deposition of about 30% of the liquid holdup from the gas core contributes to the increase. The results are consistent with previous adiabatic experiments with similar geometries.

Fluid flow analysis of a turbulent offset jet impinging on a wavy wall surface

The fluid flow characteristics of a turbulent offset jet impinging on a wavy wall surface has been investigated numerically. Two-dimensional Reynolds-averaged Navier–Stokes (RANS) equations are solved by the finite volume method. In the governing differential equations, the convective and diffusive terms are discretized by the power law upwind scheme and second-order central difference, respectively. The semi-implicit method for pressure linked equation algorithm is utilized to link the pressure to the velocity. The offset ratio is set to 7.0 and the Reynolds number is fixed to 15,000. The width of the jet is taken as the characteristic length. The amplitude of the wavy wall surface is varied from 0.1 to 0.7 with an interval of 0.1 and the number of cycle is fixed to 10. The results of fluid flow and turbulent characteristics of the offset jet are presented in the form of contours of streamline, velocity vector, turbulent kinetic energy, dissipation rate, pressure, and Reynolds shear stress. The variation in integral constant of momentum flux, wall shear stress, and pressure along the wall is presented and also compared. The decay in the maximum streamwise velocity in the downstream direction and jet half-width along the streamwise direction are also presented and discussed. The wavy surface introduces some remarkable features, which are not present in a normal plane wall case. These features have been discussed in detail.

Self-similar compressible turbulent boundary layers with pressure gradients. Part 1. Direct numerical simulation and assessment of Morkovin’s hypothesis

A direct numerical simulation study of self-similar compressible flat-plate turbulent boundary layers (TBLs) with pressure gradients (PGs) has been performed for inflow Mach numbers of 0.5 and 2.0. All cases are computed with smooth PGs for both favourable and adverse PG distributions (FPG, APG) and thus are akin to experiments using a reflected-wave set-up. The equilibrium character allows for a systematic comparison between sub- and supersonic cases, enabling the isolation of pure PG effects from Mach-number effects and thus an investigation of the validity of common compressibility transformations for compressible PG TBLs. It turned out that the kinematic Rotta–Clauser parameter $\unicode[STIX]{x1D6FD}_{K}$ calculated using the incompressible form of the boundary-layer displacement thickness as length scale is the appropriate similarity parameter to compare both sub- and supersonic cases. Whereas the subsonic APG cases show trends known from incompressible flow, the interpretation of the supersonic PG cases is intricate. Both sub- and supersonic regions exist in the boundary layer, which counteract in their spatial evolution. The boundary-layer thickness $\unicode[STIX]{x1D6FF}_{99}$ and the skin-friction coefficient $c_{f}$, for instance, are therefore in a comparable range for all compressible APG cases. The evaluation of local non-dimensionalized total and turbulent shear stresses shows an almost identical behaviour for both sub- and supersonic cases characterized by similar $\unicode[STIX]{x1D6FD}_{K}$, which indicates the (approximate) validity of Morkovin’s scaling/hypothesis also for compressible PG TBLs. Likewise, the local non-dimensionalized distributions of the mean-flow pressure and the pressure fluctuations are virtually invariant to the local Mach number for same $\unicode[STIX]{x1D6FD}_{K}$-cases. In the inner layer, the van Driest transformation collapses compressible mean-flow data of the streamwise velocity component well into their nearly incompressible counterparts with the same $\unicode[STIX]{x1D6FD}_{K}$. However, noticeable differences can be observed in the wake region of the velocity profiles, depending on the strength of the PG. For both sub- and supersonic cases the recovery factor was found to be significantly decreased by APGs and increased by FPGs, but also to remain virtually constant in regions of approximated equilibrium.

Numerical Simulation of Flow over a Flat Unglazed Transpired Solar Collector (UTC) and it’s Performance Prediction

An unglazed transpired solar collector is a system that can leverage the abundant solar energy for various purposes. The solar collector is available in flat or corrugated form and is seen to be installed as an exterior layer of building facades. The cladding thus made absorbs radiation from the sun and heats up air being sucked by fan and flowing through perforations. In this research the focus has been to understand the correlation of plate temperature, exit temperature, the velocity distribution in the chamber and perforation location when air flows past an unglazed transpired solar collector (UTC). The establishment of correlations was carried out in the dataset of flow variables obtained after solving the problem using Navier-Stokes (NS) equations along with standard two-equation (k-ε) turbulence models and Shear Stress Transport (SST ) k-ω models for turbulent flow. The same problem was also solved using NS equation using laminar model. An attempt has also been made to compute Pearson’s correlation coefficient of any two variables to understand their strong and weak correlations. A linear regression analysis was done through an open source software Rstudio for a dataset produced during the computational modeling using a commercial CFD solver, Ansys® Fluent. At the end a Monte Carlo simulation has been done to predict the likelihood of using the flat UTC for drying as well as to understand the dependency of system efficiency on plate exit temperature, suction velocity and freestream temperature.

Comprehensive shear stress analysis of turbulent boundary layer profiles

Motivated by the need for accurate determination of wall shear stress from profile measurements in turbulent boundary layer flows, the total shear stress balance is analysed and reformulated using several well-established semi-empirical relations. The analysis highlights the significant effect that small pressure gradients can have on parameters deduced from data even in nominally zero pressure gradient boundary layers. Using the comprehensive shear stress balance together with the log-law equation, it is shown that friction velocity, roughness length and zero-plane displacement can be determined with only velocity and turbulent shear stress profile measurements at a single streamwise location for nominally zero pressure gradient turbulent boundary layers. Application of the proposed analysis to turbulent smooth- and rough-wall experimental data shows that the friction velocity is determined with accuracy comparable to force balances (approximately 1 %–4 %). Additionally, application to boundary layer data from previous studies provides clear evidence that the often cited discrepancy between directly measured friction velocities (e.g. using force balances) and those derived from traditional total shear stress methods is likely due to the small favourable pressure gradient imposed by a fixed cross-section facility. The proposed comprehensive shear stress analysis can account for these small pressure gradients and allows more accurate boundary layer wall shear stress or friction velocity determination using commonly available mean velocity and shear stress profile data from a single streamwise location.

A universal velocity profile for smooth wall pipe flow

The most important unanswered questions in turbulence regard the nature of turbulent flow in the limit of infinite Reynolds number. The Princeton superpipe (PSP) data comprise 26 velocity profiles that cover three orders of magnitude in the Reynolds number from $Re=19\,639$, to $Re=20\,088\,000$ based on pipe radius and pipe centreline velocity. In this paper classical mixing length theory is combined with a new mixing length model of the turbulent shear stress to solve the streamwise momentum equation and the solution is used to approximate the PSP velocity profiles. The model velocity profile is uniformly valid from the wall to the pipe centreline and comprises five free parameters that are selected through a minimization process to provide an accurate approximation to each of the 26 profiles. The model profile is grounded in the momentum equation and allows the velocity derivative, Reynolds shear stress and turbulent kinetic energy production to be studied. The results support the conclusion that logarithmic velocity behaviour near the wall is not present in the data below a pipe Reynolds number somewhere between $Re=59\,872$, and $Re=87\,150$. Above $Re=87\,150$, the data show a very clear, nearly logarithmic, region. But even at the highest Reynolds numbers there is still a weak algebraic dependence of the intermediate portion of the velocity profile on both the near-wall and outer flow length scales. One of the five parameters in the model profile is equivalent to the well-known Kármán constant, $k$. The parameter $k$ increases almost monotonically from $k=0.4034$ at $Re=87\,150$ to $k=0.4190$ at $Re=20\,088\,000$, with an average value, $k=0.4092$. The variation of the remaining four model parameters is relatively small and, with all five parameters fixed at average values, the model profile reproduces the entire velocity data set and the wall friction reasonably well. With optimal values of the parameters used for each model profile, the fit to the PSP survey data is very good. Transforming the model velocity profile using the group, $u/u_{0}\rightarrow ku/u_{0}$, $y^{+}\rightarrow ky^{+}$ and $R_{\unicode[STIX]{x1D70F}}\rightarrow kR_{\unicode[STIX]{x1D70F}}$ where $R_{\unicode[STIX]{x1D70F}}$ is the friction Reynolds number, leads to a reduced expression for the velocity profile. When the reduced profile is cast in outer variables, the physical velocity profile is expressed in terms of $\ln (y/\unicode[STIX]{x1D6FF})$ and a new shape function $\unicode[STIX]{x1D719}(y/\unicode[STIX]{x1D6FF})$. In the limit of infinite Reynolds number, the velocity profile asymptotes to plug flow with a vanishingly thin viscous wall layer and a continuous derivative everywhere. The shape function evaluated at the pipe centreline is used to produce a new friction law with an additive constant that depends on the Kármán constant and a wall damping length scale.

Experimental Study of the Effect of Displacement of Vanes Submerged at Channel Width on Distribution of Velocity and Shear Stress in a 180 Degree Bend

Shear stress is a parameter of high significance. Through knowledge of this parameter, assessment of scour or sedimentations at different points of bed is made viable. Therefore, this paper investigated alterations in shear stress along the bend, specifically around a bridge pier, under the influence of applying submerged vanes at the upstream side of the bridge pier. With the aim of modeling submerged vanes, vanes of Plexiglas with a thickness of 20% of the pier diameter, a length of 1.5 times the pier diameter, and submergence ratio of 75% were utilized. The vanes were installed at a distance equal to 5 times the pier diameter from the pier center at a distance of 40 to 60% of the channel width from the inner bank at the upstream side of the bridge pier. Acoustic-Doppler Velocity velocimeter device was utilized for measuring three-dimensional velocity components. The experiments were conducted in a 1-meter-wide flume with a degree of curvature of 180. The results of the study suggested that upon reaching the bend apex, the maximum flow turbulence rate occurred in a transverse direction in the case of installing submerged vanes at a distance of 40% of the channel width from the inner bank towards the inner wall; while in the case of installing submerged vanes at a distance of 60% of the channel width from the inner bank, it occurred towards the outer wall, and it could be observed that the maximum longitudinal and vertical components of turbulence rate increased by 16 and 5.5% respectively upon increase in the distance of submerged vanes from the inner bank. Furthermore, the values of and turbulence shear stresses at the outer bank in the case of installing the vanes at a distance of 40% of the channel width from the inner bank were smaller than those in the case of installing the submerged vanes at a distance of 60% of the channel width from the inner bank.

Flow dynamics in the vicinity of a gravel embedded vertical retaining wall: conditions corresponding to the initial stages of local erosion

The present study examines the turbulent flow characteristics in the vicinity of the leading edge of a vertical retaining wall structure embedded in a gravel bed. Conditions before and during the initial stages of local scour development were considered. To that end, three laboratory experiments were carried out: one with fixed boundaries as a reference case, a second one with an erodible bed, but immobile channel bank, and a third one characterized by an erodible channel bed and bank. Instantaneous and time-averaged flow features, and turbulence statistics associated with the three-component velocity fields, obtained with a stereoscopic particle image velocimetry system, were analyzed. Water surface examination determined that channel bank erosion alters flow behavior upstream of the retaining wall. This change is manifested through more frequent formation of eddies near the top corner of the junction between the upstream face of the protrusion and the channel bank. Time-averaged and instantaneous velocity vector fields demonstrate the presence of two counter-rotating vortices in the experiments with the fixed channel bank, one located near the junction of retaining wall and the channel bed and one away from it generated as a result of a strong downflow. The latter motion represents a section of a horseshoe vortex. During the channel bank erosion, however, these vortices are not as significant and persistent. The results also indicate that the coherency and magnitude of the streamwise vorticity field diminish as a result of scour. However, it was confirmed that local turbulent kinetic energy and shear stresses increase in the presence of scour due to enhanced local flow separation.

Observation of Rapid Seabed Erosion Near Closure Depth During a Storm Period at Hujeong Beach, South Korea

AbstractWe report extraordinary seabed erosion that occurred at a depth of ~8.3 m at Hujeong Beach, South Korea. The seabed was eroded for 0.7 m during a ~2.3‐day period under storm conditions. The maximum significant wave height was ~3.8 m, which was unexpected considering the traditional closure depth concept. Four storm events were recorded within 36 day of the experimental period, but the severe erosion only occurred during the first storm. The erosion started when the nearbed flow velocity rapidly increased to reach its maximum at 0.27 m/s, and it continued until this velocity remained at its maximum. Additional factors responsible for the unanticipated erosion process were the current and wave skewness. The erosion was facilitated when the current direction was actively changing and the magnitude of positive skewness increased during the storm period. These factors are likely to have intensified the nearbed flow turbulence and bed shear stress.

A Meshless WCSPH Boundary Treatment for Open-Channel Flow over Small-Scale Rough Bed

A weakly compressible smoothed particle hydrodynamics (WCSPH) method was developed to model open-channel flow over rough bed. An improved boundary treatment is proposed to quantitatively characterize bed roughness based on the ghost boundary particles (GBPs). In this model, the velocities of GBPs are explicitly calculated by using evolutionary polynomial regression with a multiobjective genetic algorithm. The simulation results show that the proposed boundary treatment can well reflect the influence of wall roughness on the vertical flow structure. A fully developed open channel is established, and its flume length, processing time, and turbulent model are discussed. The mixed-length-based subparticle scale (SPS) turbulence model is adopted to simulate uniform flow in open channel, and this model is compared with the Smagorinsky-based one. For the modified WCSPH model, the results show that the calculated vertical velocity and turbulent shear stress distribution are in good agreement with experimental data and fit better than the calculations obtained from the traditional Smagorinsky-based model.

Direct numerical simulation of conical shock wave–turbulent boundary layer interaction

Direct numerical simulation of the Navier–Stokes equations is carried out to investigate the interaction of a conical shock wave with a turbulent boundary layer developing over a flat plate at free-stream Mach number $M_{\infty }=2.05$ and Reynolds number $Re_{\unicode[STIX]{x1D703}}\approx 630$, based on the upstream boundary layer momentum thickness. The shock is generated by a circular cone with half opening angle $\unicode[STIX]{x1D703}_{c}=25^{\circ }$. As found in experiments, the wall pressure exhibits a distinctive N-wave signature, with a sharp peak right past the precursor shock generated at the cone apex, followed by an extended zone with favourable pressure gradient, and terminated by the trailing shock associated with recompression in the wake of the cone. The boundary layer behaviour is strongly affected by the imposed pressure gradient. Streaks are suppressed in adverse pressure gradient (APG) zones, but re-form rapidly in downstream favourable pressure gradient (FPG) zones. Three-dimensional mean flow separation is only observed in the first APG region associated with the formation of a horseshoe vortex, whereas the second APG region features an incipient detachment state, with scattered spots of instantaneous reversed flow. As found in canonical geometrically two-dimensional wedge-generated shock–boundary layer interactions, different amplification of the turbulent stress components is observed through the interacting shock system, with approach to an isotropic state in APG regions, and to a two-component anisotropic state in FPG. The general adequacy of the Boussinesq hypothesis is found to predict the spatial organization of the turbulent shear stresses, although different eddy viscosities should be used for each component, as in tensor eddy-viscosity models, or in full Reynolds stress closures.

Quantitative Link between Secondary Nucleation and Mixing Hydrodynamics in Batch Cooling Crystallization: A New Approach in Process Development

Precise control of a crystallization process requires a clear understanding of nucleation kinetics. Interpreting secondary nucleation threshold (SNT) of a given crystallization process under optimized hydrodynamic conditions can highlight the impact of critical process variables on product particle size distribution (PSD). In this work, a new approach is proposed in industrial crystallization process development in which a quantitative link was developed between batch cooling crystallization and fluid turbulent shear stress (TSS). The experiments involve solution crystallization of paracetamol in isopropanol. A novel method was adopted in which a large single seed crystal of paracetamol was held stationary in an agitating reactor. Using particle imaging velocimetry, a constant TSS was obtained across different scales which resulted in a constant SNT because of crystal nuclei breeding. This led to better control over the crystallization process which eventually resulted in a consistent PSD, independent of ...

Sensitivity of turbulent flow around a 3-D building array to urban boundary-layer stability

The mean flow and turbulence within a regular building array are analysed for a wide range of bulk Richardson numbers, Rb, using large-eddy simulation. Time-averaged streamlines in the vertical plane reveal that skimming flow persists for stable and neutral conditions; for unstable conditions, however, updrafts dominate and the counter-rotating vortex pairs in the horizontal plane are displaced towards the leeward wall. The turbulent kinetic energy, magnitude of the turbulent shear stress and mean streamwise flow speed show a sharp transition at Rb=0. Quadrant analysis, in particular the fractional stress ratio, indicates that small fluctuations are generated most effectively by neutral flow and large ones by unstable flow. Identical-twin experiments confirm that the effects of stratification are large (pointwise errors in the relative kinetic energy are up to a factor of 10 larger for unstable flow). Knowledge of the flow-regime transition at Rb=0 is relevant to urban modelling and parameterisation.

Statistical analysis of vortical structures in turbulent boundary layer over directional grooved surface pattern with spanwise heterogeneity

We examine the turbulent boundary layers developing over convergent-divergent riblets (C-D riblets) with three different heights (h+ = 8, 14, and 20) at Reθ = 723 using particle image velocimetry. It is observed that although a logarithmic region presents in the velocity profiles over the converging and diverging line, Townsend’s outer-layer similarity hypothesis is invalid. Compared to the smooth-wall case, C-D riblets with a height of 2.4% of the smooth-wall boundary layer thickness can cause a significant increase in the turbulence production activities over the converging region, as evidenced by a more than 50% increase in the turbulent shear stress and in the population of prograde and retrograde spanwise vortices. In contrast, the impact of riblets on the diverging region is much smaller. The slope of vortex packets becomes steeper, and they are more streamwise stretched in the outer layer over the diverging region, whereas their shape and orientation is less affected over the converging region. Furthermore, the number of uniform momentum zones across the boundary layer increases over the converging region, causing a reduction in the thickness of uniform momentum zones in the outer part of the boundary layer. Overall, while an increased riblet height affects a large portion of the boundary layer away from the wall over the converging region, the impact on the diverging region is largely confined within the near-wall region. Such distinct differences in the response of the boundary layer over the diverging and converging region are attributed to the opposite local secondary flow motion induced by C-D riblets.

Three‐Dimensional Flow Above River Bedforms: Insights From Numerical Modeling of a Natural Dune Field (Río Paraná, Argentina)

AbstractBedforms are ubiquitous features in rivers and shallow seas, as mobile sediment is transported by flowing water. The mutual interaction of hydrodynamics and bedform has been widely studied in the laboratory over two‐dimensional bedforms having an angle‐of‐repose (30°) lee side and a relatively simple shape. However, the influence of bedform natural morphology and three‐dimensionality on the flow is still poorly constrained. The present work looks at how a natural three‐dimensional (3‐D) bedform field influences flow properties through high‐resolution numerical modeling. A 3‐D numerical model is set up with Delft3D and verified against lab experiments of idealized 3‐D bedforms. The model is used to simulate water velocities, turbulence, water levels, and bed shear stress above a natural bedform field from the Río Paraná (Argentina). The presence and size of the flow separation zone and turbulent wake depend on the presence and properties of the slip face (defined here as the portion of the lee side with angles >15°) and not on those of the crest. When present, the flow separation and wake lengths are, for the tested settings, respectively, around 5 and 13 times the slip face height. A slip face orientation of 25° or more compared to the flow increases cross‐stream flow and suppresses flow reversal over the slip face. To understand and predict flow and bedform properties, the slip face rather than the crest position should be identified and analyzed.

Influence of turbulent shear stresses on the numerical blood damage prediction in a ventricular assist device.

The blood damage prediction in rotary blood pumps is an important procedure to evaluate the hemocompatibility of such systems. Blood damage is caused by shear stresses to the blood cells and their exposure times. The total impact of an equivalent shear stress can only be taken into account when turbulent stresses are included in the blood damage prediction. The aim of this article was to analyze the influence of the turbulent stresses on the damage prediction in a rotary blood pump's flow. Therefore, the flow in a research blood pump was computed using large eddy simulations. A highly turbulence-resolving setup was used in order to directly resolve most of the computed stresses. The simulations were performed at the design point and an operation point with lower flow rate. Blood damage was predicted using three damage models (volumetric analysis of exceeded stress thresholds, hemolysis transport equation, and hemolysis approximation via volume integral) and two shear stress definitions (with and without turbulent stresses). For both simulations, turbulent stresses are the dominant stresses away from the walls. Here, they act in a range between 9 and 50 Pa. Nonetheless, the mean stresses in the proximity of the walls reach levels, which are one order of magnitude higher. Due to this, the turbulent stresses have a small impact on the results of the hemolysis prediction. Yet, turbulent stresses should be included in the damage prediction, since they belong to the total equivalent stress definition and could impact the damage on proteins or platelets.

Topology-based characterization of compressibility effects in mixing layers

Direct numerical simulations of high-speed mixing layers are used to characterize the effects of compressibility on the basis of local streamline topology and vortical structure. Temporal simulations of the mixing layers are performed using a finite volume gas-kinetic scheme for convective Mach numbers ranging from $M_{c}=0.2$ to $M_{c}=1.2$. The focus of the study is on the transient development and the main objectives are to (i) investigate and characterize the turbulence suppression mechanism conditioned upon local streamline topology; and (ii) examine changes in the vortex vector field – distribution, magnitude and orientation – as a function of Mach number. We first reaffirm that kinetic energy suppression with increasing Mach number is due to a decrease in pressure–strain redistribution. Then, we examine the suppression mechanism conditioned upon topology and vortex structure. Conditional statistics indicate that (i) at a given Mach number, shear-dominated topologies generally exhibit more effective pressure–strain redistribution than vortical topologies; and (ii) for a given topology, the level of pressure–strain correlation mostly decreases with increasing Mach number. At each topology, with increasing Mach number, there is a corresponding decrease in turbulent shear stress and production leading to reduced kinetic energy. Further, as $M_{c}$ increases, the proportion of vortex-dominated regions in the flow increases, leading to further reduction in the turbulent kinetic energy of the flow. Then, the orientation of vortical structures and direction of fluid rotation are examined using the vortex vector approach of Tian et al. (J. Fluid Mech., vol. 849, 2018, pp. 312–339). At higher $M_{c}$, the vortex vectors tend to be more aligned in the streamwise direction in contrast to low $M_{c}$ wherein larger angles with streamwise direction are preferred. The connection between vortex orientation and kinetic energy production is also investigated. The findings lead to improved insight into turbulence suppression dynamics in high Mach number turbulent flows.

Turbulent flow topology in supersonic boundary layer with wall heat transfer

Direct numerical simulations (DNS) are performed for the supersonic boundary layers (SBLs) with a free-stream Mach number M∞=2.2. Different cases including the adiabatic and the isothermal (cooled and heated) walls are investigated. The laminar boundary layer is excited by means of a blowing and suction strip with single-frequency and multiple spanwise wave-numbers. The incoming laminar flow is strongly perturbed with a perturbation intensity of 2.4% of the free-stream velocity to obtain the turbulent boundary layer. In the fully developed turbulent regions, the joint probability density function (JPDF) distribution and the covariance integrands’ analyses of different parameters are performed to find out the contribution of various physical mechanisms towards different transfer processes. The results reveal that behavior of the turbulent shear stress is similar to its incompressible counterpart and the wall-temperature impacts are dominant in the buffer layer region (at y+= 10). The inclination angles of coherent structures show variations arising from the wall-temperature in both the buffer-layer and the log region. The covariance integrands’ analyses of different components of the heat flux reveal the dominance of a different transfer process in case of the cooled wall, and as a result of this difference, the cooled wall acts as a heat sink.

Evaluation of transient effects in the pressure-time method

The pressure-time is a method for measuring the flow rate in closed conduits and is typically used in hydropower applications. The scope of the present paper is to examine the physics of flow during a pressure-time measurement using experimental data and two-dimensional numerical simulations. The Unsteady Reynolds-averaged Navier–Stokes (URANS) equations coupled with the low-Re k-ω SST turbulence model are employed for the simulations. The contributions of inertia, pressure gradient, viscous and turbulent shear stress terms are investigated in the flow during a pressure-time measurement. It is shown that away from the wall and in the initial moments of time, the turbulent shear stress is comparable with the pressure gradient. With increasing time, the contribution of the inertia term becomes progressively significant and comparable with the pressure gradient and turbulent shear stress terms. Close to the wall, both viscous and turbulent shear stresses are the dominant terms which are diminished by increasing the time.

Peripheral artery endothelial function responses to altered shear stress patterns in humans.

What is the central question of this study? What is the effect of altered shear stress pattern, with or without concurrent neurohumoral and metabolic activation, on the acute endothelial function response assessed via brachial artery flow-mediated dilatation? What is the main finding and its importance? Despite generating distinctive shear stress patterns (i.e. increases in anterograde only, anterograde only with neurohumoral and metabolic activation, and both anterograde and retrograde), similar acute improvements were observed in the brachial artery flow-mediated dilatation response in all conditions, indicating that anterograde and/or turbulent shear stress might be the essential element to induce acute increases in endothelial function. Endothelial function is influenced by both the direction and the magnitude of shear stress. Acute improvements in endothelial function have mostly been attributed to increased anterograde shear, whereas results from many interventional models in humans suggest that enhancing shear stress in an oscillatory manner (anterograde and retrograde) might be optimal. Here, we determined the acute brachial artery shear stress (SS) and flow-mediated dilatation (FMD) responses to three shear-altering interventions [passive heat stress (HEAT), mechanical forearm compression (CUFF) and handgrip exercise (HGEX)] and examined the relationship between changes in oscillatory shear index (OSI) and changes in FMD. During separate visits, 10 young healthy men (22±3years old) underwent 10min of HEAT, CUFF or HGEX in their left forearm. Anterograde and retrograde SS, Reynolds number, OSI and FMD were assessed at rest and during/after each intervention. Anterograde SS increased during all interventions in a stepwise manner (P<0.05 between interventions), with the change in HGEX (∆37.7±12.2dyncm-2 , P<0.05)>CUFF (∆25.1±11.9dyncm-2 , P<0.05)>HEAT (∆14.5±7.9dyncm-2 , P<0.05). Retrograde SS increased during CUFF (∆-19.6±4.3dyncm-2 , P<0.05). Anterograde blood flow was turbulent (i.e. Reynolds number≥|2000|) during all interventions (P<0.05). The relative FMD improved after all interventions (P=0.01), and there was no relationship between ∆OSI and ∆FMD. We elicited changes in SS profiles including increased anterograde SS (HEAT and HGEX) and both increased anterograde and retrograde SS (CUFF); regardless of the SS pattern, FMD improved to the same extent. These findings suggest that the presence of anterograde and/or turbulent SS might be the key to optimizing endothelial function in acute assessment protocols.