Pulsed wire velocity anemometry near walls

A new pulsed wire probe for making velocity and turbulence measurements in the near wall region of incompressible, isothermal boundary layers of all kinds is described. Results of careful calibrations of the probe response in both laminar and turbulent flows are presented, with particular emphasis on the effects of diffusion in the very near wall region. Analytic results for the motion and distortion of a heat puff in linear shear flow near a wall are developed and these are shown to validate a very simple approximate theory that accounts for the diffusional effects. It is demonstrated that correction procedures based on the theory can be successfully implemented. Examples of the use of the probe in highly turbulent, separated flows, as well as more standard boundary layers, are given and its response near the wall is contrasted with that of the corresponding (parallel wire) probe used for surface shear stress measurements.     

Characterization of near-bed coherent structures in turbulent open channel flow using synchronized high-speed video and hot-film measurements

High-speed video recordings (500 Hz) of flow visualizations in the near wall region of a turbulent open channel flow were synchronized with hot-film measurements of flow velocity and bed shear stress. Analysis of the video images provided information about the main characteristics of coherent flow structures associated with the occurrence of low-speed streak ejections near the bed. These structures consisted mainly of oscillating shear layers that were converted in the downstream direction and lifted away from the bed. A visual detection criterion was developed to obtain ensemble averaged profiles of the velocity and shear stress data during ejection events, allowing for the characterization of the associated flow field during the occurrence of coherent structures. Conditional averaging suggests that the occurrence of such coherent patterns affects mainly the turbulence structure in the wall region, and that the observed events reveal a plausible mechanism by which energy is extracted from the mean flow by large scale turbulent fluctuations, and then further transferred towards smaller eddies, while the structures lose their coherence. The intermittent nature of production and dissipation of turbulent energy becomes noticeable, taking place about 21% of the time. The results obtained also provide evidence that seems to link the structures responsible for the turbulent vertical transport of momentum, and for the maintenance of the turbulent state, with the mechanism that triggers the entrainment of sediment into suspension. Comparison of present results with other experiments conducted in different types of flows strongly confirms a universal structure of coherent events in wall bounded flows.The support of the Fluid, Hydraulic, and Paniculate System Program of the National Science Foundation (Grant CTS-9210211) and the donors of the Petroleum Research Fund of the American Chemical Society (Grant PRF 24328-G2) is gratefully acknowledged.     

Turbulence in a three‐dimensional wall‐bounded shear flow

A new turbulent flow with distinct three‐dimensional characteristics has been designed in order to study the impact of mean‐flow skewing on the turbulent coherent vortices and Reynolds‐averaged statistics. The skewing of a unidirectional plane Couette flow was achieved by means of a spanwise pressure gradient. Direct numerical simulations of the statistically steady Couette–Poiseuille flow enabled in‐depth explorations of the turbulence field in the skewed flow. The imposition of a modest spanwise gradient turned the mean flow about 8° away from the original Couette flow direction and this turning angle remained nearly the same over the entire cross section. Nevertheless, a substantial non‐alignment between the turbulent shear stress angle and the mean velocity gradient angle was observed. The structure parameter turned out to slightly exceed that in the pure Couette flow, contrary to the observations made in some other three‐dimensional shear flows. Coherent flow structures, which are known to be associated with the Reynolds shear stress in near‐wall regions, were identified by the λ₂‐criterion. Instantaneous and ensemble‐averaged vortices resembled those found in the unidirectional Couette flow. In the skewed flow, however, the vortex structures were turned to align with the local mean‐flow direction. The conventional symmetry between Case 1 and Case 2 vortices was broken due to the mean‐flow three‐dimensionality. The turning of the coherent vortices and the accompanying symmetry‐breaking gave rise to secondary and tertiary turbulent shear stress components. By averaging the already ensemble‐averaged shear stresses associated with Case 1 and Case 2 vortices in the homogeneous directions, a direct link between the educed near‐wall structures and the Reynolds‐averaged turbulent stresses was established. These observations provide evidence in support of the hypothesis that the structural model proposed for two‐dimensional turbulent boundary layers remains valid also in flows with moderate mean three‐dimensionality. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical investigation of fully developed turbulent fluid flow and heat transfer in a square duct

Three-dimensional fully developed turbulent fluid flow and heat transfer in a square duct are numerically investigated with the author's anisotropic low-Reynolds-number k-ε turbulence model. Special attenton has been given to the regions close to the wall and the corner, which are known to influence the characteristics of secondary flow a great deal. Hence, instead of the common wall function approach, the no-slip boundary condition at the wall is directly used. Velocity and temperature profiles are predicted for fully developed turbulent flows with constant wall temperature. The predicted variations of both local wall shear stress and local wall heat flux are shown to be in close agreement with available experimental data. The present paper also presents the budget of turbulent kinetic energy equation and the systematic evaluation for existing wall function forms. The commonly adopted wall function forms that are valid for two-dimensional flows are found to be inadequate for three-dimensional turbulent flows in a square duct.     

Direct numerical simulation of low Reynolds number flows in an open‐channel with sidewalls

A direct numerical simulation of low Reynolds number turbulent flows in an open‐channel with sidewalls is presented. Mean flow and turbulence structures are described and compared with both simulated and measured data available from the literature. The simulation results show that secondary flows are generated near the walls and free surface. In particular, at the upper corner of the channel, a small vortex called inner secondary flows is simulated. The results show that the inner secondary flows, counter‐rotating to outer secondary flows away from the sidewall, increase the shear velocity near the free surface. The secondary flows observed in turbulent open‐channel flows are related to the production of Reynolds shear stress. A quadrant analysis shows that sweeps and ejections are dominant in the regions where secondary flows rush in toward the wall and eject from the wall, respectively. A conditional quadrant analysis also reveals that the production of Reynolds shear stress and the secondary flow patterns are determined by the directional tendency of the dominant coherent structures. Copyright © 2009 John Wiley & Sons, Ltd.     

Vortex structure of turbulence over permeable walls

In order to understand the effect of the wall permeability on the turbulent vortex structure near porous walls, based on PIV experimental data, a probability density analysis of fluctuating velocities, statistical quadrant and quadrant-hole analyses of the Reynolds shear stress are performed. The investigated flow fields are turbulent channel flows whose bottom walls are made of porous media. The porous media used are three kinds of foamed ceramics which have almost the same porosity (∼0.8) but different permeability. From the discussions on those analyses, a conceptual scenario of the development of the vortex structure near a permeable wall is proposed for a moderate permeability Reynolds number case. It explains the reason why the near-wall long streaky structure tends to vanish near a porous wall with increasing wall permeability.     

Low Reynolds number axisymmetric turbulent boundary layer on a cylinder

In the present study, an axisymmetric turbulent boundary layer growing on a cylinder is investigated experimentally using hot wire anemometry. The combined effects of transverse curvature as well as low Reynolds number on the mean and turbulent flow quantities are studied. The measurements include the mean velocity, turbulence intensity, skewness and flatness factors in addition to wall shear stress. The results are presented separately for the near wall region and the outer region using dimensionless parameters suitable for each case. They are also compared with the results available in the open literature.The present investigation revealed that the mean velocity in near wall region is similar to other simple turbulent flows (flat plate boundary layer, pipe and channel flows); but it differs in the logarithmic and outer regions. Further, for dimensionless moments of higher orders, such as skewness and flatness factors, the main effects of the low Reynolds number and the transverse curvature are present in the near wall region as well as the outer region.     

Wall shear stress modified by bubbles in a horizontal channel flow of silicone oil in the transition region

We performed laboratory experiments on bubbly channel flows using silicone oil, which has a low surface tension and clean interface to bubbles, as a test fluid to evaluate the wall shear stress modification for different regimes of bubble migration status. The channel Reynolds numbers of the flow ranged from 1000 to 5000, covering laminar, transition and turbulent flow regimes. The bubble deformation and swarms were classified as packing, film, foam, dispersed, and stretched states based on visualization of bubbles as a bulk void fraction changed. In the dispersed and film states, the wall shear stress reduced by 9% from that in the single-phase condition; by contrast, the wall shear stress increased in the stretched, packing, and foam states. We carried out statistical analysis of the time-series of the wall shear stress in the transition and turbulent-flow regimes. Variations of the PDF of the shear stress and the higher order moments in the statistic indicated that the injection of bubbles generated pseudo-turbulence in the transition regime and suppressed drag-inducing events in the turbulent regime. Bubble images and measurements of shear stress revealed a correlated wave with a time lag, for which we discuss associated to the bubble dynamics and effective viscosity of the bubble mixture in wall proximity.     

Pathline analysis of traveling wavy blowing and suction control in turbulent pipe flow for drag reduction

Skin friction drag is much greater in turbulent flows as compared with that in laminar flows. It is well known that traveling wave control can be used to achieve a large drag reduction. In the present study, a direct numerical simulation of a turbulent pipe flow was performed to clarify the mechanism of the drag reduction caused by the traveling wave control. The flow induced by the control was evaluated using pathline analysis. Near the wall, a “closed flow” was formed, wherein the injected particles return to the wall owing to the suction flow. The random component of Reynolds shear stress was perfectly suppressed in the closed flow, which suggests that there was no turbulence. The controlled flow was categorized into four patterns, and each flow characteristic and drag reduction effect was discussed. When the closing rate is high, the drag decreases, while when the closing rate is low, i.e., when the injected particles are released into the main flow, the turbulence is maintained. If the thickness of the layer suppressing turbulence is insufficient, a significant effect in terms of the drag reduction cannot be expected. The large drag reduction owing to the traveling wave control can be attributed to the elimination of turbulence in the region near the wall.     

Simultaneous wall shear rate measurements by a three segmented electrodiffusion probe and laser-Doppler-anemometry

 In a fully developed turbulent channel flow, the instantaneous wall shear gradient was measured simultaneously by an electrodiffusion (ED) probe and by laser-Doppler anemometry (LDA). The LDA measurements were done in the viscous sublayer and the linear velocity profile was used to calculate the instantaneous shear gradient at the wall. The electrodiffusion probe requires presence of a suitable electrolyte (e.g. an iodine and potassium-iodine aqueous solution) for reduction of the species on the working electrode (cathode). This results in a concentration gradient of the ions due to the convective diffusion of the [I₃]^- to the cathode. Due to the limited molecular diffusion rate and due to the dimension of the electrodiffusion probe its frequency resolution is limited. It is shown in which limits the correction of the signal on the concentration boundary layer inertia is not necessary and the probe can be used for measuring instantaneous wall shear gradients in turbulent flows. The results are compared with those obtained by LDA and elucidate the capability of the ED probe for measuring instantaneous shear rates at low Reynolds numbers. Received: 25 October 1995/Accepted: 16 July 1996     

Direct numerical simulation of the turbulent energy balance and the shear stresses in power-law fluid flows in pipes

The results of direct numerical simulation of turbulent flows of non-Newtonian pseudoplastic fluids in a straight pipe are presented. The data on the distributions of the turbulent stress tensor components and the shear stress and turbulent kinetic energy balances are obtained for steady turbulent flows at the Reynolds numbers of 10⁴ and 2×10⁴. As distinct from Newtonian fluid flows, the viscous shear stresses turn out to be significant even far from the wall. In power-law fluid flows the mechanism of the energy transport from axial to transverse component fluctuations is suppressed. It is shown that with decrease in the fluid index the turbulent transfer of the momentum and the velocity fluctuations between the wall layer and the flow core reduces, while the turbulent energy flux toward the wall increases. The earlier-proposed models for the average viscosity and the non-Newtonian one-point correlations are in good agreement with the data of direct numerical simulation.     

Numerical study of effects of pulsatile amplitude for transitional turbulent pulsatile flow in pipes with ring‐type constrictions

The effects of pulsatile amplitude on sinusoidal transitional turbulent flows through a rigid pipe in the vicinity of a sharp‐edged mechanical ring‐type constriction have been studied numerically. Pulsatile flows were studied for transitional turbulent flow with Reynolds number (Re) of the order of 10⁴, Womersley number (Nw) of the order of 50 with a corresponding Strouhal number (St) of the order of 0.04. The pulsatile flow considered is a sinusoidal flow with dimensionless amplitudes varying from 0.0 to 1.0. Transitional laminar and turbulent flow characteristics in an alternative manner within the pulsatile flow fields were observed and studied numerically. The flow characteristics were studied through the pulsatile contours of streamlines, vorticity, shear stress and isobars. It was observed that fluid accelerations tend to suppress the development of flow disturbances. All the instantaneous maximum values of turbulent kinetic energy, turbulent viscosity, turbulent shear stress are smaller during the acceleration phase when compared with those during deceleration period. Various parametric equations within a pulsatile cycle have also been formulated through numerical experimentations with different pulsatile amplitudes. In the vicinity of constrictions, the empirical relationships were obtained for the instantaneous flow rate (Q), the pressure gradient (dp/dz), the pressure loss (P_loss), the maximum velocity (V_max), the maximum vorticity (ζ_max), the maximum wall vorticity (ζ_w,max), the maximum shear stress (τ_max) and the maximum wall shear stress (τ_w,max). Elliptic relation was observed between flow rate and pressure gradient. Quadratic relations were observed between flow rate and the pressure loss, the maximum values of shear stress, wall shear stress, turbulent kinematic energy and the turbulent viscosity. Linear relationships exist between the instantaneous flow rate and the maximum values of vorticity, wall vorticity and velocity. The time‐average axial pressure gradient and the time average pressure loss across the constriction were observed to increase linearly with the pulsatile amplitude. Copyright © 1999 John Wiley & Sons, Ltd.     

The use of scalar transport probes to measure wall shear stress in a flow with imposed oscillations

Recent studies of the influence of imposed oscillations on a turbulent flow have focused on the need for accurate measurements of the phase-averaged wall shear stress. Flush mounted wall transfer probes offer the opportunity of obtaining such results. However, considerable error can arise in interpreting these measurements (in particular, the phase of the wall stress relative to the phase of the imposed oscillations) if the time response of the scalar boundary-layer over the probe is not taken into account. This paper presents a method to correct measurements for this effect and applies it to the case of an oscillating turbulent flow.     

Dynamic response of micro-pillar sensors measuring fluctuating wall-shear-stress

We present in this paper test results of flexible micro-pillars and pillar arrays for wall shear stress measurements in flows with fluctuating wall shear stress such as unsteady separated flows or turbulent flows. Previous papers reported on the sensing principle and fabrication process. Static calibrations have shown this sensor to have a maximum nonlinearity of 1% over two orders of wall-shear-stress. For measurements in flows with fluctuating wall shear stress the dynamic response has been experimentally verified in an oscillating pipe flow and compared to a calculated response based on Stokes’ and Oseen’s solution for unsteady flow around a cylinder. The results demonstrate good agreement under the given boundary conditions of cylindrical micro-pillars and the limit of viscous Stokes-flow around the pillar. Depending on the fluid and pillar geometry, different response curves result ranging from a flat low-pass filtered response to a strong resonant behavior. Two different methods are developed to detect the frequency content and the directional wall shear stress information from image processing of large sensor films with arrays of micro-pillars of different geometry. Design rules are given to achieve the optimal conditions with respect to signal-to-noise ratio, sensitivity and bandwidth for measurements in turbulent flows.     

Turbulent separated reattached flow in a two-dimensional curved-wall diffuser

A turbulent separation-reattachment flow in a two-dimensional asymmetrical curved-wall diffuser is studied by a two-dimensional laser doppler velocimeter. The turbulent boundary layer separates on the lower curved wall under strong pressure gradient and then reattaches on a parallel channel. At the inlet of the diffuser, Reynolds number based on the diffuser height is 1.2×10⁵ and the velocity is 25.2m/s. The results of experiments are presented and analyzed in new defined streamline-aligned coordinates. The experiment shows that after Transitory Detachment Reynolds shear stress is negative in the near-wall backflow region. Their characteristics are approximately the same as in simple turbulent shear layers near the maximum Reynolds shear stress. A scale is formed using the maximum Reynolds shear stresses. It is found that a Reynolds shear stress similarity exists from separation to reattachment and the Schofield-Perry velocity law exists in the forward shear flow. Both profiles are used in the experimental work that leads to the design of a new eddy-viscosity model. The length scale is taken from that developed by Schofield and Perry. The composite velocity scale is formed by the maximum Reynolds shear stress and the Schofield-Perry velocity scale as well as the edge velocity of the boundary layer. The results of these experiments are presented in this paper.     

Two-component hot-film probe for measurements of wall shear stress

A two-component, hot-film probe has been developed, to measure the wall shear stress as a vector quantity in time-dependent flows. The probe has been applied for the measurements of the bottom shear stress in a flow generated by combined waves and current in a water basin where the magnitude and the direction of the bottom shear stress varied periodically with respect to time. The probe has also been used to measure the bottom shear stress around a vertical cylinder exposed to water waves generated in a wave flume.     

Near-wall investigation of backward-facing step flows

The electrodiffusion technique has been used to investigate reattaching and recirculating flows behind a backward-facing step. The instantaneous wall shear rate vectors were determined using the current signals provided by a three-segment electrodiffusion probe. The near-wall extents of two counter-rotating recirculation zones located behind the step were determined under turbulent flow conditions in a water channel. The near-wall flow inside these recirculation zones was found to be very unsteady, with strong low-frequency fluctuations. The streamwise profiles of the wall shear stress were measured at several values of the Reynolds number and a high level of skin friction was obtained in the reverse-flow region. The strong dependence of the peak value of skin friction on the Reynolds number confirms the viscous-dominated character of the reverse flow appearing inside the recirculation zone. Received: 22 May 2000/Accepted: 28 March 2001     

An improved near‐wall modeling for large‐eddy simulation using immersed boundary methods

An improved near‐wall modeling for large‐eddy simulation using the immersed boundary method is proposed. It is shown in this study that the existing near‐wall modeling for the immersed boundary (IB) methods that imposes the velocity boundary condition at the IB node is not sufficient to enforce a correct wall shear stress at the IB node. A new method that imposes a shear stress condition through the modification of the subgrid scale‐eddy viscosity at the IB node is proposed. In this method, the subgrid eddy viscosity at the IB node is modified such that the viscous flux at the face adjacent to the IB node correctly approximates the total shear stress. The method is applied to simulate the fully developed turbulent flows in a plane channel and a circular pipe. It is demonstrated that the new method improves the prediction of the mean velocity and turbulence stresses in comparison with the existing wall modeling based solely on the velocity boundary condition. Copyright © 2015 John Wiley & Sons, Ltd.     

Fence probe measurement of flow reversal in separating turbulent boundary layers

A small fence probe was evaluated for measurements in the time-dependent flow reversal region of the transition from boundary layer to separated flow. For moderate and high Reynolds numbers, the fence probe is demonstrated to be a usable tool for the measurement of the reverse flow associated with separation. Although the present probe pressure transducer system was limited to approximately 200?Hz, pulses of positive and negative shear stress were readily detected. At or near the location of zero surface shear stress, the measurements were limited by the signal-to-noise ratio. For the separated flow investigated, a marked reduction in the pressure gradient occurred when the fence probe indicated approximately 20?% reversal for the higher Reynolds numbers. The reversal increased to 24?% for the lower Reynolds numbers. The measurements indicate that flow reversal alone may not be adequate to identify the degree of separation. Upstream of turbulent boundary layer (intermittent) separation, the duration of the reversed shear stress was found to be very short (0.002?C0.007?s), suggesting a local, small-scale, impulse-type separation. At and beyond the location of intermittent separation, the shear stress reversal duration was an order of magnitude longer. Estimates of the maximum and minimum surface shear stress in the separation region were also obtained with the fence probe.     

Localised dynamics of laminar pulsatile flow in a rectangular channel

The exploitation of flow pulsation in low-Reynolds number micro/minichannel flows is a potentially useful technique for enhancing cooling of high power photonics and electronics devices. Although the mechanical and thermal problems are inextricably linked, decoupling of the local instantaneous parameters provides insight into underlying mechanisms. The current study performs complementary experimental and analytical analyses to verify novel representations of the pulsating channel flow solutions, which conveniently decompose hydrodynamic parameters into amplitude and phase values relative to a prescribed flow rate, for sinusoidally-pulsating flows of Womersley numbers 1.4 ≤ Wo ≤ 7.0 and a fixed ratio of oscillating flow rate amplitude to steady flow rate equal to 0.9. To the best of the authors’ knowledge, the velocity measurements – taken using particle image velocimetry – constitute the first experimental verification of theory over two dimensions of a rectangular channel. Furthermore, the wall shear stress measurements add to the very limited number of studies that exist for any vessel geometry. The amplification of the modulation component of wall shear stress relative to a steady flow (with flow rate equal to the amplitude of the oscillating flow rate) is an important thermal indicator that may be coupled with future heat transfer measurements. The positive half-cycle time- and space-averaged value is found to increase with frequency owing to growing phase delays and higher amplitudes in the near-wall region of the velocity profiles. Furthermore, the local time-dependent amplification varies depending on the regime of unsteadiness: (i) For quasi-steady flows, the local values are similar during acceleration and deceleration though amplification is greater near the corners over the interval 0 – 0.5π. (ii) At intermediate frequencies, local behaviour begins to differ during accelerating and decelerating periods and the interval of greater wall shear stress near the corners lengthens. (iii) Plug-like flows experience universally high amplifications, with wall shear stress greater near the corners for the majority of the positive half-cycle. The overall fluid mechanical performance of pulsating flow, measured by the ratio of bulk mean wall shear stress and pressure gradient amplifications, is found to reduce from an initial value of 0.97 at Wo = 1.4 to 0.28 at Wo = 7.0, demonstrating the increasing work input required to overcome inertia.