Near-wall Velocity Research Articles

Instantaneous skin-friction pattern analysis using automated critical point detection on near-wall PIV data

Near-wall PIV provides qualitative and quantitative information that can be used to analyse the skin-friction pattern of the flow around a wall-mounted cube. This is addressed by a brief comparison between oil-flow visualizations and near-wall mean velocity fields. This approach is expanded to the analysis of instantaneous pseudo shear-stress patterns obtained from PIV. A post-processing algorithm based on proper orthogonal decomposition and critical point identification is devised for this purpose. It is shown to be very robust and efficient. A statistical analysis of its results enables us to automatically extract the salient features of the resulting POD filtered instantaneous velocity fields: their critical points type, distribution and connectivity. It provides valuable insights into the time-dependent behaviour of the flow. Issues associated with the interpretation of such results are discussed.

Effects of local blowing from a slot on a laminar boundary layer

The influence of local blowing on a laminar boundary layer is analyzed numerically. Steady local uniform blowing is imposed in a cross flow through a thin slot. A regime diagram is constructed by varying the local blowing amplitude for three Reynolds numbers. As the local blowing amplitude is increased, the boundary layer flow begins to separate at a critical blowing rate, and on further increase of the blowing amplitude, spanwise vortex shedding occurs behind the slot. The simulation results show that in the regime where the flow is separated, the time-mean separation bubble length increases approximately linearly with the local blowing amplitude. In the vortex shedding regime, however, the time-mean separation bubble length decreases with increasing blowing amplitude. Other flow characteristics such as shedding frequency, the reattachment length, pressure coefficient, and near-wall mean velocity profiles are also examined.

Effects of temporal flow acceleration on the detachment of microparticles from surfaces

The detachment of microparticles from surfaces subjected to various temporal accelerations of air was studied experimentally. Stainless microspheres, approximately 70 μ m in diameter, deposited as a sparse monolayer on smooth glass substrates, were subjected to temporal accelerations ranging from 0.01 to 2.0 m/s 2. Higher accelerations up to 23 m/s 2 were investigated without particles on the surface to further characterize the temporally evolving flow. Microvideographic images of the particles on the surfaces were acquired. These were used to determine the detachment fraction of the particles versus the free-stream flow velocity at different temporal flow accelerations. For the relatively slower flow accelerations, approximately 0.3 m/s 2 or less, microparticle detachment was independent of acceleration to within the experimental uncertainty. For the relatively more rapid accelerations, from approximately 0.3 m/s 2 to at least 2.0 m/s 2, the flow velocity required to detach one half of the microparticles increased with acceleration. Near-wall velocity measurements supported that rapid temporal acceleration delayed the onset of turbulence, thereby affecting the boundary layer characteristics, causing a decline of turbulent bursting in the wall-layer, and, hence, suppressing the detachment process.

STUDY OF INSTANTANEOUS AND FILTERED FLOW FIELDS NEAR THE BED OF OPEN-CHANNEL FLOW BY DNS

A direct numerical simulation (DNS) has been conducted to examine the near-wall structure of turbulence in rough-bed channel flow. It is intended to explore a better wall boundary condition in such simulation as Large-Eddy Simulation (LES) of open channel flows at high Reynolds numbers over rough bed. It is found that the instantaneous velocity profiles cannot be described by the instantaneous logarithmic-law similarity. The filtered velocity field is found to be closely related to the magnitude of the filtered temporal normal velocity near the bed. It agrees with the previous study based on experimental data and indicates that the near-wall velocity field over rough surface is related more to the sweep-ejection type modes of motion than the local and temporal values of the wall shear stress. A possible wall model using an assumed shear correlation is suggested.

Axial stent strut angle influences wall shear stress after stent implantation: analysis using 3D computational fluid dynamics models of stent foreshortening

IntroductionThe success of vascular stents in the restoration of blood flow is limited by restenosis. Recent data generated from computational fluid dynamics (CFD) models suggest that the vascular geometry created by an implanted stent causes local alterations in wall shear stress (WSS) that are associated with neointimal hyperplasia (NH). Foreshortening is a potential limitation of stent design that may affect stent performance and the rate of restenosis. The angle created between axially aligned stent struts and the principal direction of blood flow varies with the degree to which the stent foreshortens after implantation.MethodsIn the current investigation, we tested the hypothesis that stent foreshortening adversely influences the distribution of WSS and WSS gradients using time-dependent 3D CFD simulations of normal arteries based on canine coronary artery measurements of diameter and blood flow. WSS and WSS gradients were calculated using conventional techniques in ideal (16 mm) and progressively foreshortened (14 and 12 mm) stented computational vessels.ResultsStent foreshortening increased the intrastrut area of the luminal surface exposed to low WSS and elevated spatial WSS gradients. Progressive degrees of stent foreshortening were also associated with strut misalignment relative to the direction of blood flow as indicated by analysis of near-wall velocity vectors.ConclusionThe current results suggest that foreshortening may predispose the stented vessel to a higher risk of neointimal hyperplasia.

Probability distribution function of near-wall turbulent velocity fluctuations

By large eddy simulation (LES), turbulent databases of channel flows at different Reynolds numbers were established. Then, the probability distribution functions of the streamwise and wall-normal velocity fluctuations were obtained and compared with the corresponding normal distributions. By hypothesis test, the deviation from the normal distribution was analyzed quantitatively. The skewness and flatness factors were also calculated. And the variations of these two factors in the viscous sublayer, buffer layer and log-law layer were discussed. Still illustrated were the relations between the probability distribution functions and the burst events—sweep of high-speed fluids and ejection of low-speed fluids—in the viscous sub-layer, buffer layer and loglaw layer. Finally the variations of the probability distribution functions with Reynolds number were examined.

Effect of an external excitation on the flow structure in a circular impinging jet

The effect of an external excitation on circular impinging jet flow is studied experimentally. Velocity fields in moderate air flows (Re=10000) are investigated by hot-wire anemometry. The basic flow is excited by a small sinusoidal modulation of the nozzle exit velocity. The phase-averaging technique is used to study the behavior of vortex structures in the jet, specifically rolling-up, pairing, and interaction with the wall. The jet is found to be sensitive to excitation in the frequency range characterized by a Strouhal number, Ste=feD∕U, from 0.3 to 3. Different flow regimes are identified in the excited impinging jet: a periodic flow regime with the same frequency as the excitation, a regime with a frequency corresponding to a subharmonic of the excitation frequency, a regime alternating between these two frequencies, and two border regimes with more complicated behavior. The low-frequency excitation leads to the formation of vortices, which are larger than those occurring in an unexcited jet. Consequently, the near-wall velocity fluctuations are enhanced and the unsteady flow separation induced by vortex impingement is more pronounced. By contrast, excitation at higher frequencies, characterized by value 0.017 of a Strouhal number based on the shear layer thickness θ, Stθ=feθ∕U, leads to the roll-up of small vortices and hence to the attenuation of the near-wall velocity fluctuations. In this case, the flow separation is suppressed because the small vortices are unable to induce it. Finally, the vortex merging process is sensitive to the double-frequency excitation with subharmonic components. For example, a combined excitation at Ste=1.60 and 0.53 leads to the merging of three vortices.

Effect of near-wall turbulence enhancement on the mechanisms of particle deposition

The modification of deposition mechanisms of small particles in wall turbulence due to enhanced near-wall fluctuations is presented. The direct numerical simulation database of turbulent air flow over a water surface populated by gravity-capillary waves of small wave slope was used to mimic the enhancement in fluctuation intensity. Lagrangian tracking of particles is performed under the assumption of one-way coupling between the particles and the flow. Two sets of particles have been considered with inertial response times of 5 and 15, respectively, normalized using the friction velocity at the air–water interface and the kinematic viscosity of air. Compared to wall-bounded flow, the particle deposition rates on the interface were found to be considerably higher; specifically for the low-inertia particles, an eightfold increase was observed. The deposition rate for particles of higher inertia increased by only 60%. The correlation characterizing particle deposition rates for wall-bounded flows, where the deposition rate is proportional to the square of the particle response time, was found to be invalid for the flow with enhanced near-wall turbulence. Comparison with experimental results on particle deposition onto rough walls showed better correlation. Depositing particles were divided into free-flight and diffusional deposition populations. Since the primary effect of the interfacial waves is to increase the turbulence intensity in the near-interface region with high particle concentration, a remarkable increase in diffusional deposition is observed. As in wall-bounded flows, diffusional deposition is seen to be the dominant mechanism of deposition. The free-flight mechanism, where particles acquire velocities high enough to travel directly to the interface, remains unaffected by enhanced near-wall velocity fluctuations.

Near-wall velocity profile with adaptive shape functions for turbulent forced convection

This article applies Reichardt's velocity profile to turbulent convection analysis. In contrast to a conventional law-of-the-wall formulation with three regions, a single profile represents the entire region from the viscous sublayer to the fully turbulent region. Special shape functions are developed for this profile in a hybrid finite element/volume formulation, together with dissipation rate boundary conditions, which are consistent with the velocity profile modeling. Applications to turbulent channel flow are successfully predicted with a k− ɛ turbulence model.

Friction drag reduction achievable by near-wall turbulence manipulation at high Reynolds numbers

The Reynolds-number dependence of the drag reduction achievable by diminishing to zero the near-wall turbulent velocity fluctuations is clarified. This reduction could be obtained by a virtual active feedback control system. The formula derived suggests that large drag reduction can be attained even at high Reynolds numbers if turbulence fluctuations adjacent to the wall are completely damped. For example, 35% drag reduction rate can be obtained at Reτ=105 if the turbulence only below y+=10 vanishes. Thus, the active feedback control strategy, which has been studied mostly at low Reynolds numbers, would be much promising even in high Reynolds number flows of real applications. Results from the direct numerical simulation of turbulent channel flow at a Reynolds number of Reτ=642 are also presented to clarify the phenomena in the controlled flow.

AN ASSESSMENT OF k–ω AND v 2–f TURBULENCE MODELS FOR STRONGLY HEATED INTERNAL GAS FLOWS

Both k –ω and v 2– f turbulence models are used to model an axisymmetric, strongly heated, low-Mach-number gas flowing upward within a vertical tube in which forced convection is dominant. The heating rates are sufficiently high, so that fluid properties vary significantly in both the axial and radial directions; consequently, fully developed mean flow profiles do not evolve. Comparisons between computational results and experimental results, which exist in the literature, reveal that the v 2– f model performs quite well in predicting axial wall temperatures, and mean velocity and temperature profiles. This may be contrasted with the k –ω model results, in which the wall heat transfer rates and near-wall velocities are significantly overpredicted.

Active control of flow over a sphere for drag reduction at a subcritical Reynolds number

Active control of flow over a sphere at $). For the forcing frequencies smaller than this critical frequency, the drag is either nearly the same as, or slightly smaller than, that without forcing. The critical forcing frequency is found to be closely associated with the onset of the boundary-layer instability. It is shown from the surface-pressure measurement, surface oil-flow visualization and near-wall streamwise velocity measurement that the disturbances from the high-frequency forcing grow inside the boundary layer and delay the first separation while maintaining laminar separation, and they grow further along the separated shear layer and high momentum in the free stream is entrained toward the sphere surface, resulting in the reattachment of the flow (thus forming a separation bubble above the sphere surface) and the delay of the main separation. The reverse flow region in the wake is significantly reduced and the motion in that region also becomes weak owing to the forcing. Finally, the variation of drag by the present forcing with respect to the Reynolds number is very similar to that by dimples on the surface, but is different from that by surface roughness.

Transient thermal structure, turbulence, and heat transfer in a reattaching slot jet flow

The role of turbulent fluctuations on mean heat transfer coefficient in a reattaching slot jet flow is studied experimentally. Convective heat transfer rate and near-wall fluid flow are examined in the recirculation, reattachment, and post-reattachment regions for two nozzle-to-surface spacings of 0.25 and 0.75 times the width of the nozzle bottom plate. In the reattachment region, results indicate a strong correspondence between variances of near-wall velocity fluctuation and peak heat transfer rate for both spacings. Thermal structures that vary in the spanwise direction are identified in the recirculation region from low-frequency transient infrared thermographs of the heated surface. While these thermal structures are confined to regions in the vicinity of nozzle bottom plate for the low nozzle spacing, they span the entire recirculation region at larger spacings. Thermal streaks are observed past reattachment for the larger nozzle spacing, suggesting a periodic breakup and re-formation of the jet curtain. The scaling of heat transfer distribution is affected by the flow structure in the geometrically non-similar area of the recirculating flow beneath the nozzle. A correlation for peak Nusselt number is presented.

Effects of Strong Irregular Roughness on the Turbulent Boundary Layer

Some recent studies with irregular roughness suggest that the Nikuradse [Nikuradse, J., NACA TM 1292, National Advisory Committee on Aeronautics (1933)] equivalent sand-grain roughness measure gives inconsistent results of the flow characteristics. In situations where the roughness is very strong to stifle or diminish the viscous effects the viscous scaling laws alone will not be very meaningful. The present study aims to find an alternative scaling parameter for such cases. Here, the measured mean and turbulent velocity profiles on a nonuniform roughness surface, simulating a gas turbine blade roughness, are presented. A nonzero wall normal pressure gradient is caused which is believed to contribute to the velocity deficit in the near-wall rough boundary layer velocity profile. The surface pressure variation is also directly influenced by the local roughness. The normal turbulent stresses are increased on the rough surface, the vertical component more than the longitudinal component. A pressure gradient velocity scale (similar to that proposed for adverse pressure gradient boundary layer modeling by Durbin and Belcher [Durbin, P.A. and Belcher, S.E., J. Fluid Mech. 238 (1992), 699-722] is defined to capture the pressure effects induced by such roughness on the inner layer properties.

An experimental study of fluid mechanics and heat transfer in an impinging slot jet flow

An experimental study of flow field, surface pressure, and heat transfer rates of a submerged, turbulent, slot jet impinging normally on a flat plate is presented. Two nozzle-to-surface spacings of 3.5 and 0.5 nozzle exit hydraulic diameters, which correspond to transitional and potential-core jet impingement, respectively, are considered. Fluid mechanical data include measurements of mean flow field and variance of normal and cross velocity fluctuations, mean surface pressure, and RMS surface pressure fluctuations along the nozzle minor axis. Local heat transfer coefficients are calculated from detailed surface temperature measurements. The heat transfer data follow a trend similar to previous studies, exhibiting high heat transfer rates in the impingement region for transitional jet impingement, and a non-monotonic decay in heat transfer coefficient for potential-core jet impingement. The fluid flow results indicate that past impingement, locations of high streamwise fluctuating velocity variance occur in the wall jet flow for both nozzle spacings. The RMS surface pressure fluctuation profile exhibits a maximum at the impingement line for the transitional jet impingement, and corresponds well with the rise in near-wall velocity fluctuation variance in the free jet prior to impingement. For potential-core jet impingement, the streamwise location of peak RMS pressure fluctuations corresponds to the highly correlated turbulence in the outer region of the wall-bounded flow. Further, there is a good correspondence between the locations of secondary peak in heat transfer and near-wall streamwise fluctuating velocity variance. The occurrence of the outer peak prior to the near-wall peak in streamwise velocity variance for the potential-core jet impingement suggests that the interaction between correlated motion in the outer region and near-wall turbulence causes the rise in heat transfer coefficient towards a secondary peak.

A priori testing of wall models for separated flows

Databases from numerical simulation of wall-bounded flows with closed reverse flow regions were examined in order to derive models, which relate the mean wall shear stress to flow quantities in a certain distance from the wall. The approach makes use of the particular shape of the profiles of turbulent shear stress which balances the streamwise pressure gradient outside the viscosity-affected near-wall region. One model version requires as input the streamwise pressure gradient and a velocity scale UW which results from linear extrapolation of the mean flow to the wall. Its application is restricted to the part of the reverse flow region with nonvanishing adverse pressure gradient. A more general applicable skin-friction law is derived from the observation that near-wall mean velocity profiles approximately collapse if they are scaled in a defect form. This scaling holds up to approximately half a bubble length downstream of attachment. Numerical values for model coefficients vary slightly from flow to flow and seem to depend on the Reynolds number.

Flow structures and heat transfer characteristics in fan flows with and without delta-wing vortex generators

Effects of an external delta-wing vortex generator on the flow and heat transfer characteristics in fan flows and uniform flows were experimentally investigated and compared. A heated plate, installed on the bottom wall of a duct, was used as the heat transfer surface. Three-component mean and fluctuating velocity measurements were conducted using a laser Doppler velocimetry to characterize the flow structures and to obtain the near-wall flow parameters, including the axial mean velocity, axial vorticity and turbulent kinetic energy. Temperatures on the heat transfer surface were measured using thermocouples to obtain the Nusselt numbers. Results show that the external delta-wing vortex generator in fan flows has little overall effect on the near-wall averaged axial mean velocity and axial vorticity, but increases the turbulent kinetic energy, in the investigated X/ D ranges. The increase in the turbulent kinetic energy by the delta-wing has little effect on heat transfer in the inherently vortical fan flows. Consequently, the delta-wing vortex generator in fan flows has little effect on the heat transfer augmentation.

Analytical and numerical evaluation of two-fluid model solutions for laminar fully developed bubbly two-phase flows

An analysis of the two-fluid model in the case of vertical fully developed laminar bubbly flows is conducted. Firstly the phase distribution in the central region of the pipe (where wall effects vanish) is considered. From the model equations an intrinsic length scale L is deduced such that the scaled system reduces to a single equation without parameters. With the aid of this equation some generic properties of the solutions of the model for pipes with diameter greater than about 20 L (the usual case, since L is of the order of the bubble radius) are found. We prove that in all physically meaningful solutions an (almost) exact compensation of the applied pressure gradient with the hydrostatic force ρ eff g occurs (with ρ eff the effective density and g the gravity). This compensation implies flat void fraction and velocity profiles in the central region not affected by the wall, even when no turbulence effects are accounted for. We then turn to consider the complete problem with a numerical approach, with the effect of the wall dealt via wall forces. The previous mathematical results are confirmed and the near-wall phase distributions and velocity profiles are found. With the numerical code it is also possible to investigate the regime in which the pressure gradient is greater than the weight of the pure liquid, in which case a region of strictly zero void fraction develops surrounding the axis of the pipe (in upward flow of bubbles). Finally, the same code is used to study the effect of reducing the gravity. As g decreases, so does the relative velocity between the phases, making the lift force increasingly dominant. This produces, in upward bubbly flows, narrower and sharper void fraction peaks that also appear closer to the wall.

Pitot probe corrections in fully developed turbulent pipe flow

Mean flow measurements taken in fully developed turbulent pipe flow over awide Reynolds number range are used to evaluate current methods ofcorrecting Pitot probe data. Based on this evaluation, a new form for thedisplacement correction is proposed which appears to be more accurate over awider range of conditions than those currently available. The difficulty ofobtaining the true near-wall velocity profile near the wall is explored.

Heat Transfer In Turbulent Boundary Layers Subjected to Free-Stream Turbulence—Part II: Analysis and Correlation

A new heat transfer correlation for turbulent boundary layers subjected to free-stream turbulence was developed. The new correlation estimates dimensionless heat transfer coefficients without the use of conventional boundary-layer thickness measures and the associated Reynolds numbers. Using only free-stream parameters (mean velocity, turbulence intensity and length scale), the new correlation collected many authors’ elevated-turbulence, flat-plate Stanton number data to within ±11%. The level of scatter around the new correlation compared well to previous correlations that require additional flow information as input parameters. For a common subset of data, scatter using earlier correlation methods ranged from 5–10%; scatter around the new correlation varied from 6–9% over the same data subset. A length-scale dependence was identified in a Stanton number previously defined using a near-wall streamwise velocity fluctuation, St′. A new near-wall Stanton number was introduced; this parameter was regarded as a constant in a two-region boundary layer model on which the new correlation is based.