Near-wall Velocity Research Articles

Effects of velocity fluctuations on heat transfer enhancement

The three-dimensional velocity fluctuation effects on heat transfer enhancement were experimentally investigated using a wind tunnel system and cylinders placed upstream of the test section in the wind tunnel. The cylinders with different diameters were used as turbulators to generate vortical flow motions with three-dimensional velocity fluctuations. A heated plate, part of the tunnel wall, was placed far downstream of the cylinders such that it was subjected mainly to flows with velocity fluctuations but with negligible steady vortical motions. These studies included three-component velocity measurements to characterize the near-wall and cross-section velocity fields and to obtain the turbulent kinetic energy. The temperatures were measured by thermocouples on the heated plate to obtain the convection heat transfer coefficients and the Nusselt numbers. Results indicate that the heat transfer was enhanced by the velocity fluctuations, which is attributed to the modification of boundary layer velocity profiles without the modification of boundary layer thickness. The resulting normalized Nusselt number was approximately a parabolic function of a dimensionless parameter, the product of Reynolds number and normalized turbulent kinetic energy.

Near-wall LES closure based on one-dimensional turbulence modeling

A novel near-wall LES closure model is developed based on a revised form of the one-dimensional turbulence (ODT) model of Kerstein and is tested by performing LES calculations of turbulent channel flow at Reynolds numbers based on friction velocity ranging from 395 to 10,000. In contrast to previous models, which invoke Reynolds averaging, near-wall velocity fluctuations and turbulent transport are simulated down to the smallest scales, and can be compared directly to DNS data. Thus, the approach provides more than just a boundary condition. Rather, it is itself a complete (although simplified) model for the wall-normal profiles of velocity within the near-wall region. LES/ODT coupling is bi-directional and occurs both through the direct calculation of the subgrid turbulent stress by temporally and spatially filtering the ODT-resolved momentum fluxes (up-scale coupling), and through the LES-resolved pressure and velocities impacting the ODT behavior (down-scale coupling). The formulation involves finely resolved ODT lines that are embedded within each wall-adjacent LES cell – denoted the inner region. LES cells that are within approximately one LES filter width of the inner region belong to an overlap region where both ODT and LES modeling is active. All other cells are treated using a standard LES approach. Although more expensive than simpler models, the cost of the model relative to the LES portion of the simulation scales favorably with problem size, leading to computationally affordable simulations even at relatively high Reynolds numbers.

Near-wall structure of turbulent boundary layer with spanwise-wall oscillation

An investigation was carried out with an aim to better understand the mechanism of turbulent drag reduction with spanwise-wall oscillation, by carefully analyzing the experimental data of the near-wall structure of the boundary layer modified by the wall motion. It was found that the mean velocity gradient of the turbulent boundary layer was reduced close to the wall, and the logarithmic velocity profile shifted upwards by the wall oscillation. We argue that these changes in the mean velocity profile are mainly due to a negative spanwise vorticity created in the near-wall region of the boundary layer over the oscillating wall. The resultant near-wall velocity reduction seems to have weakened the near-wall turbulence activity by hampering the stretching of the quasistreamwise longitudinal vortices, leading to a reduction in skin-friction drag. Indeed, the signatures of the sweep events over the oscillating wall indicated that the duration and the strength were reduced by 78% and 64%, respectively. The reductions obtained by a 2D numerical model, incorporating the streamwise flow and associated stretching of the longitudinal vortical structure, were 47% for the sweep duration and 23% for the sweep strength, although these reductions were observed within the first cycle of wall oscillation. By phase averaging the conditionally sampled velocity data, we were able to show that the frequency of sweep events was reduced with a reduction in streamwise velocity when the negative spanwise vorticity is created by the wall oscillation in the near-wall region. It was also shown that the turbulent skin-friction reduction with spanwise-wall oscillation can be optimized with a nondimensional, spanwise wall velocity, and nearly 45% drag reduction can be obtained in the turbulent boundary layer at an optimum value of w+=15.

Deposition of a fine powder in horizontal pipelines and bends

The deposition of a very fine powder in a horizontal, lean-phase pneumatic conveying conduit containing a 90° bend has been studied experimentally. The total deposition and the deposition pattern were studied as a function of superficial gas velocity, solids loading and bend geometry: one sharp and three smooth bends of different radii of curvature were used. Most deposition was seen in the sharp bend, while in the smooth bends the deposition did not vary much with radius of curvature. The deposition decreased significantly with increasing superficial gas velocity. The experimental results were compared with CFD simulations of the flow field. The observations are consistent with the notion of deposition in regions with low near-wall gas velocity, indicating that models for deposition should be sought in considerations of the deposit stability for this type of system.

In situ calibration of hot wires close to highly heat-conducting walls

A calibration procedure has been derived that permits reliable hot-wire measurements close to walls. When hot wires are calibrated in a free flow and subsequently used for near-wall velocity measurements, erroneous velocity information results because of additional heat losses to the wall. On the other hand, laser-Doppler anemometry (LDA) measurements of local time mean velocities are very little affected by the presence of the wall and this readily suggests in situ calibration of hot wires located just behind the LDA measuring volume and at the same distance from the wall. Calibrations of this kind are described for highly heat-conducting walls and the results show good agreement with corresponding data obtained through numerical investigations. The present investigations permit a generally applicable correction curve to be suggested for hot-wire velocity measurements close to walls of high thermal conductivity.

Effect of a moving wall on a fully developed, equilibrium turbulent boundary layer

Experiments were conducted in a wind tunnel to assess the effect of a moving wall on a fully developed, equilibrium turbulent boundary layer. Pitot-static and total head probes were used in conjunction with both single- and two-component hot-wire anemometer probes to quantify the effect of wall motion on the boundary layer velocity statistics. A variable speed, seamless belt formed the wind tunnel test section wall. When stationary, the belt was found to possess a fully developed, equilibrium turbulent boundary layer in excellent agreement with archival data. With the tunnel wall moving at the free-stream speed, and at a sufficient distance above the wall, the velocity statistics in the moving-wall boundary layer were found to collapse well when scaled as a self-preserving turbulent wake. The near-wall mean velocity profile of the moving wall was found to exhibit an extended region of linearity compared to canonical turbulent boundary layer and internal flows. This can be attributed to the reduced shear resulting from wall motion and the subsequent reduction in Reynolds stress.

Correlation of Wall Vorticity and Streamwise Velocity Fluctuations in a Turbulent Boundary Layer

A simultaneous measurement of wall vorticity and near-wall streamwise velocity fluctuations has been performed using a V-type wall vorticity probe and an I-type velocity probe to investigate the relation between them. Long-time averaged space-time correlations show that the wall vorticity is highly correlated with a turbulence structure which is tilted from the wall in the streamwise direction and that there is a streamwise vortex pair near the wall. It is shown that a structure correlated with the streamwise wall vorticity is smaller than and prior to a structure correlated with the spanwise wall vorticity. Tilting angles are obtained from the phase shift between the wall vorticity and streamwise velocity fluctuations. The tilting angle of the structure correlated with the streamwise wall vorticity is larger than that of the structure correlated with the spanwise wall vorticity. The convection velocity of the near-wall streamwise velocity fluctuations obtained from the space-time correlation is in good agreement with previous results.

Surface Pressure Fluctuations Beneath Two- and Three-Dimensional Turbulent Boundary Layers

Surface pressure fluctuation measurements were made in two-dimensional turbulent boundary layers at two Reynolds numbers (Re θ = 7.3 × 10 3 and 2.34 × 104) and pressure-driven three-dimensional turbulent boundary layers at two Reynolds numbers (approach Re θ = 5.94 x 10 3 and 2.32 × 104). The collapse of spectral levels at middle and high frequencies and the effects of inner and outer boundary-layer scaling variables are shown for a wide range of Reynolds numbers (1.4 X 10 3 < 2.34 × 104) for the two-dimensional flows. Such scaling parameters do not collapse the pressure spectra beneath three-dimensional flows, which have a nearly constant, or flat, midfrequency range, and at some measurement stations and spectral levels within the flat- and high-frequency spectral ranges that significantly raise p'. Additionally, dimensional spectral levels within the flat-frequency range are independent of Reynolds number. Analysis based on the Poisson equation shows that the variation of the high-frequency spectral levels is related to the variation in near-wall mean velocity gradients and ν 2 structure due to the spanwise pressure gradient

Velocity Statistics of Plane Self-Preserving Buoyant Turbulent Adiabatic Wall Plumes

Measurements of the velocity properties of plane buoyant turbulent adiabatic wall plumes (adiabatic wall plumes) are described, emphasizing conditions far from the source where self-preserving behavior is approximated. The experiments involved helium/air mixtures rising along a smooth, plane, and vertical wall. Mean and fluctuating streamwise and cross-stream velocities were measured using laser velocimetry. Self-preserving behavior was observed 92–156 source widths from the source, yielding smaller normalized plume widths and larger near-wall mean velocities than observations within the flow development region nearer to the source. Unlike earlier observations of concentration fluctuation intensities, which are unusually large due to effects of streamwise buoyant instabilities, velocity fluctuation intensities were comparable to values observed in nonbuoyant turbulent wall jets. The entrainment properties of the present flows approximated self-preserving behavior in spite of continued development of the wall boundary layer. Measurements of temporal power spectra and temporal and spatial integral scales of velocity fluctuations are also reported. [S0022-1481(00)00504-1]

Interposition vein cuff anastomosis alters wall shear stress distribution in the recipient artery.

Interposition of a vein cuff between a prosthetic infrainguinal bypass graft and a recipient infrageniculate artery can improve graft patency. There is evidence that the improved performance may be explained by a redistribution of myointimal hyperplasia (MIH) away from the critical areas at the heel and toe of the cuff-artery anastomosis. It is widely accepted that there is an association between hemodynamic forces, more specifically, low wall shear stress (WSS), and the development of MIH. The aim of this study was to determine whether the reported redistribution of MIH in the interposition vein cuff (IVC) may be explained by differences in magnitude and distribution of WSS. Design of Study and Method: Detailed flow velocity measurements were made in life-size models of conventional end-to-side (ETS) and IVC anastomoses using a two-component laser Doppler anemometer under pulsatile flow conditions. Velocity vectors were determined in the plane of symmetry of the anastomosis, and the variation of WSS was estimated from near-wall velocity measurements on the floor and upper wall of the artery. The main flow features in the ETS anastomosis were flow separation at the graft hood, strong radial velocity at the heel, and a stagnation point on the floor of the artery that moved slightly during the flow cycle. In the IVC anastomosis, a coherent vortex that occupied most of the cuff volume was present from the systolic deceleration phase to end diastole. A stagnation point on the anastomosis floor was found to oscillate by about 4 mm. Critical regions of low mean WSS (ie, below 0.5 N/m(2)) were identified. In the ETS anastomosis, they were found at the heel and along the floor. In the IVC anastomosis, low mean WSS was found only on the floor, and it was generally less extensive than in the ETS anastomosis. The vein cuff anastomosis alters the mean WSS distribution within the recipient artery and removes the area of low WSS at the heel. This may explain the redistribution of MIH away from important sites in the recipient artery.

An in vitro comparison of the hemodynamics of two inferior vena cava filters

Purpose: The effectiveness of an inferior vena cava (IVC) filter in preventing pulmonary embolism while preserving caval flow is significantly affected by its hemodynamic characteristics. Flow fields surrounding two types of IVC filters were compared to assess how the design of a filter may influence performance.Methods: The 12F Titanium Greenfield and VenaTech LGM inferior vena cava filters were studied in vitro with a noninvasive flow visualization technique, the photochromic flow visualization and measurement technique. Axial velocity profiles and wall shear stress distributions were measured. These results were compared with analytical data corresponding to the flow field in the absence of a filter to determine the relative extent of the flow disturbances.Results: The reductions in near-wall axial velocity and wall shear stress caused by the VenaTech filter were more extensive and severe than those caused by the Greenfield filter. These changes were the consequence of differences in the geometry and dimensions of the struts of the two filters. The measurements showed the flow fields to be laminar, with no evidence of turbulence in both cases.Conclusion: Two factors that have been linked to thrombogenesis, near-wall velocity and wall-shear stress, were significantly affected by the larger frontal profile area of the VenaTech filter. Although a larger area may increase clot-trapping efficiency, as shown by previous studies, the reduced near-wall velocities and wall shear stresses may increase the potential for thrombogenesis and, thus, caval occlusion. In contrast to other in vitro flow visualization studies, no turbulence was observed with either filter.

An in vitro comparison of the hemodynamics of two inferior vena cava filters

The effectiveness of an inferior vena cava (IVC) filter in preventing pulmonary embolism while preserving caval flow is significantly affected by its hemodynamic characteristics. Flow fields surrounding two types of IVC filters were compared to assess how the design of a filter may influence performance. The 12F Titanium Greenfield and VenaTech LGM inferior vena cava filters were studied in vitro with a noninvasive flow visualization technique, the photochromic flow visualization and measurement technique. Axial velocity profiles and wall shear stress distributions were measured. These results were compared with analytical data corresponding to the flow field in the absence of a filter to determine the relative extent of the flow disturbances. The reductions in near-wall axial velocity and wall shear stress caused by the VenaTech filter were more extensive and severe than those caused by the Greenfield filter. These changes were the consequence of differences in the geometry and dimensions of the struts of the two filters. The measurements showed the flow fields to be laminar, with no evidence of turbulence in both cases. Two factors that have been linked to thrombogenesis, near-wall velocity and wall-shear stress, were significantly affected by the larger frontal profile area of the VenaTech filter. Although a larger area may increase clot-trapping efficiency, as shown by previous studies, the reduced near-wall velocities and wall shear stresses may increase the potential for thrombogenesis and, thus, caval occlusion. In contrast to other in vitro flow visualization studies, no turbulence was observed with either filter.

The effect of celiac and renal artery outflows on near-wall velocities in the porcine iliac arteries.

The effects of the outflow of aortic blood through the celiac and renal arteries on the flow field in the external iliac arteries were studied under steady and physiologically realistic pulsatile flow conditions. Laser Doppler velocimetry (LDV) measurements were made close to the medial, lateral, ventral, and dorsal walls of the external iliac branches of a clear, flow-through replica of a porcine aorta and its daughter vessels. The outflow from each branch of the replica was controlled so that the infrarenal aortic flow rate and the flow partition at the aortic trifurcation were the same for all experiments. LDV measurements were made with flow exiting through both the renal and celiac artery ostia, only the celiac ostium, and neither ostium. The steady flow results indicate that while the outflow through the renal arteries did not have a significant effect on near wall shear rate in the external iliac arteries, the flow through the celiac artery did. However, in pulsatile flow, three indices of near wall velocity in the iliac arteries were unaffected by celiac artery outflow, while a fourth showed a small effect that can be attributed to differences in minimum velocity. These results indicate that reliable simulations of blood flow in the external iliac arteries can be carried out without including the renal and celiac vessels, provided that the correct infrarenal flow wave is used. They also demonstrate that the flow field downstream of a region, such as a branch, that strongly alters the flow, can be nearly independent of the velocity field entering the region.

The effect of graft caliber upon wall shear within in vivo distal vascular anastomoses.

Wall shear has been widely implicated as a contributing factor in the development of intimal hyperplasia in the anastomoses of chronic arterial bypass grafts. Earlier studies have been restricted to either: (1) in vitro or computer simulation models detailing the complex hemodynamics within an anastomosis without corresponding biological responses, or (2) in vivo models that document biological effects with only approximate wall shear information. Recently, a specially designed pulse ultrasonic Doppler wall shear rate (PUDWSR) measuring device has made it possible to obtain three near-wall velocity measurements nonintrusively within 1.05 mm of the vessel luminal surface from which wall shear rates (WSRs) were derived. It was the purpose of this study to evaluate the effect of graft caliber, a surgically controllable variable, upon local hemodynamics, which, in turn, play an important role in the eventual development of anastomotic hyperplasia. Tapered (4-7 mm I.D.) 6-cm-long grafts were implanted bilaterally in an end-to-side fashion with 30 deg proximal and distal anastomoses to bypass occluded common carotid arteries of 16 canines. The bypass grafts were randomly paired in contralateral vessels and placed such that the graft-to-artery diameter ratio, DR, at the distal anastomosis was either 1.0 or 1.5. For all grafts, the average Re was 432 +/- 112 and the average Womersley parameter, alpha, was 3.59 +/- 0.39 based on artery diameter. There was a sharp skewing of flow toward the artery floor with the development of a stagnation point whose position varied with time (up to two artery diameters) and DR (generally more downstream for DR = 1.0). Mean WSRs along the artery floor for DR = 1.0 and 1.5 were found to range sharply from moderate to high retrograde values (589 s-1 and 1558 s-1, respectively) upstream to high antegrade values (2704 s-1 and 2302 s-1, respectively) immediately downstream of the stagnation point. Although there were no overall differences in mean and peak WSRs between groups, there were significant differences (p < 0.05) in oscillatory WSRs as well as in the absolute normalized mean and peak WSRs between groups. There were also significant differences (p < 0.05) in mean and peak WSRs with respect to axial position along the artery floor for both DR cases. In conclusion, WSR varies widely (1558 s-1 retrograde to 2704 s-1 antegrade) within end-to-side distal graft anastomoses, particularly along the artery floor, and may play a role in the development of intimal hyperplasia through local alteration of mass transport and mechano-signal transduction within the endothelium.

Drag Reduction on a Polymer Gel Surface.

We investigated the hydrogel of poly (vinyl alcohol) (PVA) which possesses elastic behavior similar to the blood vessel. In this study, drag reduction on the polymer hydrogel tube surface was experimentally investigated at steady flow condition. A transparent PVA hydrogel tube was prepared from the solution of PVA, dimethyl sulfoxide (DMSO) and water by repetitive cooling method and the surface of the hydrogel tube was modified with poly (acrylic-acid) (PAA). Monodisperse polystyrene particles (7.21 μm in diameter) were used as a tracer of fluid flow and the flow was observed by CCD camera under a microscope. The wall shear stress (WSS) was determined from near-wall velocity gradient obtained by particle-tracking (PT) method and drag reduction was calculated from the WSS. Microscopic investigations have shown that the drag reduction on the hydrogel surface depends on the chemical nature of the hydrogel surface, which was varied with changing pH of flow liquid. Maximum drag reduction was obtained with water (flow), when the PAA of modified surface is in the dissociated condition.

Heat transfer augmentation for air jet impinged on a rough surface

An experimental investigation of heat transfer from a round air jet impinging normally from below onto a heated square plate was performed. The objective of the investigation was to study the effect of roughness on both the heat transfer and the fluid flow characteristics. Smooth and rough plates were, therefore, used in the course of the experiments. The heat transfer data were collected for four jet Reynolds numbers, ranging from 6500 to 19 000. The Reynolds numbers are based on the jet-exit velocity ( U e) and the nozzle-exit diameter ( D), Re e= U e D/ ν. The nozzle-to-plate distance ranged from 0.05 to 15 nozzle-exit diameter to cover both the potential core and the far regions of the jet flow. The roughness was composed of cubes of 1 mm dimension distributed uniformly along the plate. The local and average Nusselt number values for the rough plate showed an increase ranging from 8.9% to 28% over those for the smooth plate. Roughness was found to have a strong effect on the flow characteristics; it affected the mean velocity as well as the turbulence intensity of the flow. The mean velocity profiles for the smooth case at radial distances of r/ D=1 and r/ D=2.5 showed steeper near-wall velocity gradients compared with the profiles of the rough case, where r is the radial distance measured from the plate center along the plate centerline. In addition, roughness caused an increase in the turbulence intensity of the flow.

Vector analysis of the hemodynamics of atherogenesis in the human thoracic aorta using MR velocity mapping.

Our study was designed to assess the applicability of MR velocity mapping for vector analysis of the hemodynamics of atherogenesis. MR velocity mapping was used to measure axial and nonaxial elements and the length of the wall shear rate (a spatial gradient of near-wall flow velocity parallel to the vessel wall) vector at 16 time points per cardiac cycle at eight anatomic locations of the thoracic aorta in six healthy subjects. An oscillatory shear index (a ratio of blood flow volume in the recessive direction divided by the sum of blood flow volume in both dominant and recessive directions) was introduced for analysis of the degree of oscillation. The time-averaged length, axial element, and nonaxial element of the wall shear rate vector were 118+/-53 sec(-1), 106+/-55 sec(-1), and 33+/-23 sec(-1), respectively. The oscillatory shear index in the axial direction was 0.06+/-0.10 and that in the nonaxial direction was 0.07+/-0.13. At the inner wall of the distal portion of the aortic arch, the length of the wall shear rate was smallest (74+/-32 sec(-1)) and oscillation in the axial direction was largest (0.16+/-0.19). Vector analysis of the wall shear rate in the thoracic aorta was successfully done with MR velocity mapping in humans. MR velocity mapping can noninvasively evaluate the hemodynamics of atherogenesis induced by the complicated blood flow.

Mean-flow scaling of turbulent pipe flow

Measurements of the mean velocity profile and pressure drop were performed in a fully developed, smooth pipe flow for Reynolds numbers from 31×103 to 35×106. Analysis of the mean velocity profiles indicates two overlap regions: a power law for 60&lt;y+&lt;500 or y+&lt;0.15R+, the outer limit depending on whether the Kármán number R+ is greater or less than 9×103; and a log law for 600&lt;y+&lt;0.07R+. The log law is only evident if the Reynolds number is greater than approximately 400×103 (R+&gt;9×103). Von Kármán's constant was shown to be 0.436 which is consistent with the friction factor data and the mean velocity profiles for 600&lt;y+&lt;0.07R+, and the additive constant was shown to be 6.15 when the log law is expressed in inner scaling variables.A new theory is developed to explain the scaling in both overlap regions. This theory requires a velocity scale for the outer region such that the ratio of the outer velocity scale to the inner velocity scale (the friction velocity) is a function of Reynolds number at low Reynolds numbers, and approaches a constant value at high Reynolds numbers. A reasonable candidate for the outer velocity scale is the velocity deficit in the pipe, UCL−Ū, which is a true outer velocity scale, in contrast to the friction velocity which is a velocity scale associated with the near-wall region which is ‘impressed’ on the outer region. The proposed velocity scale was used to normalize the velocity profiles in the outer region and was found to give significantly better agreement between different Reynolds numbers than the friction velocity.The friction factor data at high Reynolds numbers were found to be significantly larger (&gt;5%) than those predicted by Prandtl's relation. A new friction factor relation is proposed which is within ±1.2% of the data for Reynolds numbers between 10×103 and 35×106, and includes a term to account for the near-wall velocity profile.

Methods to Set Up and Investigate Low Reynolds Number, Fully Developed Turbulent Plane Channel Flows

The tripping of fully developed turbulent plane channel flow was studied at low Reynolds number, yielding unique flow properties independent of the initial conditions. The LDA measuring technique was used to obtain reliable mean velocities, rms values of turbulent velocity fluctuations and skewness and flatness factors over the entire cross-section with emphasis on the near-wall region. The experimental results were compared with the data obtained from direct numerical simulations available in the literature. The analysis of the data indicates the important role of the upstream conditions on the flow development. It is shown that the fully developed turbulent state at low Reynolds number can be reached only by significant tripping of the flow at the inlet of the channel. Effects related to the finite size of the LDA measuring control volume and an inaccuracy in the estimation of the wall shear stress from near-wall velocity measurements are discussed in detail since these can yield systematic discrepancies between the measured and simulated results.

Dynamic response of a hot-wire anemometer. Part I: A marginally elevated hot-wire probe for near-wall velocity measurements

Experiments were carried out to generate a known near-wall fluctuating flow field for the purpose of quantifying the dynamic response of a marginally elevated hot-wire probe. It is found that the dynamic response of the hot wire is dependent on both the wire's height (h) above the wall substrate and the convective velocity at the wire's location. Both larger values of and smaller values of h improve the dynamic response of a near-wall hot wire. Further experiments were also performed to investigate the effect of the thermal conductivity of the wall substrate on the response characteristics of the hot wire. A thermally more conducting material for the wall yields a better response hot-wire probe for near-wall velocity measurements.