Wall Shear Stress Probe Research Articles

Wall-Shear-Stress-Based Conditional Sampling Analysis of Coherent Structures in a Turbulent Boundary Layer

Abstract Coherent structures are critical for controlling turbulent boundary layers due to their roles in momentum and heat transfer in the flow. Turbulent coherent structures can be detected by measuring wall shear stresses that are footprints of coherent structures. In this study, wall shear stress fluctuations were measured simultaneously in a zero pressure gradient turbulent boundary layer using two house-made wall shear stress probes aligned in the spanwise direction. The wall shear stress probe consisted of two hot-wires on the wall aligned in a V-shaped configuration for measuring streamwise and spanwise shear stresses, and their performance was validated in comparison with a direct numerical simulation result. Relationships between measured wall shear stress fluctuations and streamwise velocity fluctuations were analyzed using conditional sampling techniques. The peak detection method and the variable-interval time-averaging (VITA) method showed that quasi-streamwise vortices were inclined toward the streamwise direction. When events were simultaneously detected by the two probes, stronger fluctuations in streamwise velocity were detected, which suggests that stronger coherent structures were detected. In contrast to the former two methods, the hibernating event detection method detects events with lower wall shear stress fluctuations. The ensemble-averaged mean velocity profile of hibernating events was shifted upward compared to the law of the wall, which suggests low drag status of the coherent structures related with hibernating events. These methods suggest significant correlations between wall shear stress fluctuations and coherent structures, which could motivate flow control strategies to fully exploit these correlations.

Wall Shear Stress in a Model of 3 · 3 Rod Bundle

The paper presents results of experimental study of an upward flow in a vertical rod bundle consisting of nine rods arranged in a square. Spacing grids were used to keep the bundle geometry. The experiments were performed in single-phase and bubbly flows using an electrochemical technique. A local wall shear stress probe was mounted in the central rod. The dependence of the local wall shear stress on the distance from the grid was measured for two angular positions of the central rod. The wall shear stress fluctuations were also measured. The effect of gas phase void fraction on the flow parameters was studied.

The pulsed wire anemometer: review and further developments

The purpose of the present paper is to review work that has been done on the pulsed wire anemometer technique and also suggest further developments that could be made in its range of application. The aper discusses the three types of probes that have been used in pulsed wire anemometry: the crossed wire velocity probe, the parallel wire wall shear stress probe and the parallel wire velocity probe. The work shows that the crossed wire and the parallel wire techniques can be used to make velocity, turbulence and wall shear stress measurements in highly turbulent flows without any upper restriction on turbulence level. Comments are also made on the potential of a parallel wire probe for use in highly turbulent flows that would enable higher order velocity cross-product terms to be measured.

The flow between a rotating and a stationary disc: application to near-wall hot-wire calibration

Several experimental works in the literature have just assumed the validity of the power series solution provided by Stewartson in 1953 for the flow in the gap between a top rotating disc and a bottom stationary disc in an effort to devise a simple and compact calibration apparatus to calibrate the hot wire as a wall shear stress or velocity probe for near-wall measurements. This work seeks to verify the applicability of the series solution via the use of direct numerical simulation (DNS) of the flow field, and in doing so explicitly justify the underlying assumption of the previous experimental works. The simulated laminar flow indicates good agreement of the velocity profiles with the series solution provided the Reynolds number (, where is the angular velocity of the rotating disc and is the gap between the discs) is kept below 3.0, and the radial position is less than 0.9 disc radius away from the end-disc effect at the disc edge. Experiments carried out with a flush-mounted hot wire calibrated as a wall shear stress probe in the rotating disc apparatus (assuming the series solution) for measurement in a known laminar channel flow further attest to the applicability of the series solution in the disc gap.

The onset of turbulence in physiological pulsatile flow in a straight tube

An empirical correlation for the onset of turbulence in physiological pulsatile flow is presented. We pumped three different test fluids of kinematic viscosity 0.008–0.035 cm2/s through four straight tubes 0.4–3.0 cm in diameter. A Scotch yoke mechanism provided an oscillatory sine wave flow component of known stroke volume and frequency. We adjusted the mean flow independently until we detected signal instabilities from hot film wall shear stress probes. The critical peak Reynolds number was found to correlate with the Womersley parameter and the Strouhal number as a power law function with a root-mean-square (rms) error of 15.2%. Experimental measurements of the laminar velocity profile are compared to theoretical predictions from Poiseuille’s law and Womersley’s solution.

Pressure and shear stress measurements at the wall in a turbulent boundary layer on a cylinder

The fluctuating wall shear stress, wall pressure, and streamwise velocity were measured simultaneously in a cylindrical boundary layer at a momentum thickness Reynolds number of Reθ=2160 and a boundary layer thickness to cylinder radius ratio of δ/a=5 using a hot wire wall shear stress probe mounted just upstream of a hearing aid microphone and a hot wire velocity probe. Variable Interval Time Averaging (VITA) event detection on streamwise velocity indicates that streamwise accelerations are associated with positive wall pressure peaks and sudden increases in wall shear stress. Likewise, positive pressure peak events are associated with streamwise accelerations and sudden increases in wall shear stress. VITA detection on wall shear stress reveals that increasing wall shear stress corresponds to streamwise accelerations and small-amplitude pressure rises, not distinct intense pressure peaks. Detection of strong adverse and favorable instantaneous pressure gradients indicates that a shear layer at y+=13 coincides with a positive peak in the wall pressure, suggesting that a positive wall pressure peak event is the key wall pressure signature associated with the burst cycle. Measurements of the cross-correlation indicate that the pressure–shear stress relationship is about two times weaker than the pressure–velocity relation and about ten times weaker than the shear stress–velocity relation. Thus, a strong relationship exists between wall pressure and streamwise velocity as well as between wall shear stress and streamwise velocity, but the relationship between wall shear stress and wall pressure is quite weak. Because of the similarity of the near-wall structure of all wall-bounded turbulent flows, regardless of transverse curvature, these conclusions should be applicable to planar boundary layers.

An in vitro study on the effect of branch points on the stability of coronary artery flow

Empirical correlations for the onset of turbulence in pulsatile flow through a straight tube and a 45° T-bifurcation are presented. We pumped three different test fluids of kinematic viscosity 0.008–0.035 cm 2 s −1 through four straight tubes 0.4–3.0 cm in diameter and three 45 degree T-bifurcations 0.45–2.2 cm in diameter. A Scotch yoke mechanism provided an oscillatory sine wave flow component of known stroke volume and frequency. To determine transition to turbulence, we adjusted the mean flow independently until we detected signal instabilities from hot film or electrochemical wall shear stress probes. The critical peak Reynolds number was found to correlate with two independent dimensionless groups: the Womersley parameter and the Strouhal number. We derived power law functions of these groups to provide an accurate and convenient method of predicting transition in both straight and bifurcating tubes. When compared to pulsatile flow through the straight tube, the presence of flow separation within the 45° T-bifurcation induced flow instabilities at lower values of the peak Reynolds number. The correlation for the 45° T-bifurcation is also a suitable model for predicting transition at coronary branch points, which we previously studied in an in vitro pulse duplicator. Flow instabilities at coronary branch points may play an important role in atherogenesis.

On the use of Irwin and Preston wall shear stress probes in turbulent incompressible flows with pressure gradients

Abstract A set of wall shear stress pressure probes, of the type developed by Irwin and Preston, was tested in a duct flow under various positive and negative pressure gradients. A hot wire wall shear stress probe with temperature compensation was employed as the reference instrument. In the present study, the parameter Δ used by Patel to quantify the severity of the pressure gradient was made to vary between −0.035 and 0.05, thus extending the range of results available in the literature. The experimental facility constructed allowed the probes to be calibrated and tested without changing of position, as well as obtaining the same value of Δ with different types of flow. It was found that both pressure probes studied had broadly the same behaviour, and that, in the case of positive pressure gradients, the measuring errors produced by the Irwin and Preston probes depended very strongly on the conditions of the flow development. The use of an integral parameter to complement Δ is suggested in order to try to predict the errors produced by the pressure probes under study.

Phase-Averaged Wall Shear Stress, Wall Pressure, and Near-Wall Velocity Field Measurements in a Whirling Annular Seal

The flow field inside a 50 percent eccentric whirling annular seal operating at a Reynolds number of 24,000 and a Taylor number of 6600 has been measured using a three-dimensional laser-Doppler anemometer system. Flush mount pressure and wall shear stress probes have been used to measure the stresses (normal and shear) along the length of the stator. The rotor was mounted eccentrically on the shaft so that the rotor orbit was circular and rotated at the same speed as the shaft (a whirl ratio of 1.0). This paper presents mean pressure, mean wall shear stress magnitude, and mean wall shear stress direction distributions along the length of the seal. Phase-averaged wall pressure and wall shear stress are presented along with phase-averaged mean velocity and turbulence kinetic energy distributions located 0.16c from the stator wall, where c is the seal clearance. The relationships between the velocity, turbulence, wall pressure, and wall shear stress are very complex and do not follow simple bulk flow predictions.

Measurement of instantaneous flow reversals and velocity field in a conical diffuser

An 8° total divergence angle conical diffuser fed with a fully developed turbulent pipe flow at its inlet was experimentally investigated. Through effective use of pulsed-wire anemometry, some quantitative information about instantaneous reversals was obtained in conical diffuser flow. Profiles of streamwise mean and fluctuating velocities, skewness factor, and flatness factor are presented for the stations in the final stages of the diffuser flow, where appreciable instantaneous reversals were detected in the wall region. Results are also presented for the stations in the earlier stages of the diffuser, where smaller instantaneous backflow was present. Comparative results of conventional pressure measurements and hot-wire anemometer measurements were obtained to determine quantitatively the effects of instantaneous flow reversals on these techniques. In the regions of instantaneous backflow, the pulsed-wire technique was found most accurate for measuring the mean and fluctuating velocities. The hot-wire and pulsed-wire anemometer results were also used to obtain guidelines for hot-wire use in highly turbulent instantaneously reversing flows. The integral constraint of continuity was used at the diffuser stations to evaluate measurement techniques for obtaining mean velocity in a conical diffuser flow. The distributions of streamwise mean and fluctuating wall shear stress were measured with a pulsed-wire skin friction probe. Pulsed-wire wall shear stress probe results indicated serious errors in the values of the mean wall shear stress obtained by the conventional Preston tube in the region of appreciable instantaneous backflow. Some of the separation prediction criteria reported in the literature were evaluated for their validity using the present results of quantitative instantaneous backflow. These measurements illustrate important characteristics of instantaneous flow reversals and the velocity field in the highly turbulent axisymmetric flow developing under a severe adverse pressure gradient.

Direction-specific characteristics of a three-component electrodiffusional velocity probe

Various types of electrodiffusional wall shear stress and velocity probes have been suggested in the last years. Most attention has been paid to probes consisting of three electrodes which allow detailed information on the velocity field to be obtained. A three-segmented circular wall shear stress probe was developed in [1, 2]. A triple-split velocity probe was suggested in [3]. To study the detailed fine structure of the flow velocity miniature probes are needed to make possible the detection of high-frequency fluctuations without accounting for the probe frequency response. A small sized probe may also be of importance in measurements in gas-liquid flows with small gas bubbles. A design of a two-component liquid velocity probe which satisfies these requirements was suggested in [4-6]. This was a V-shaped velocity probe similar to an X-type cross hot wire probe [7]. The development of an analogous probe for a three-dimensional flow is presented in this paper.

Analysis of wall shear stress probes in large amplitude unsteady flows

The measurement of the mass transfer rate to flush mounted wall electrodes is a commonly used method to determine the wall shear stress. An analysis for the performance of these probes in the presence of large amplitude unsteady flows is presented. This is accomplished by using numerical solutions of the direct (the variation of the wall shear stress with time is given) and the inverse problem (the variation of mass transfer rate with time is given). For cases in which flow reversal occurs additional information about the flow direction is needed.

The identification of two distinct laminar to turbulent transition modes in pipe flows accelerated from rest

A study was undertaken to investigate transition in a pipe flow accelerated from rest. Experiments were carried out on a vertical tube under a constant head of liquid: flow was initiated by opening a solenoid valve. A wall shear stress probe used in the role of an event recorder identified two transition events, separated by the passage of a turbulent to laminar front and a period of laminar flow. Evidence suggests that the first comprises a laminar to turbulent interface arising from a ‘natural’ stable/unstable front moving up the tube as local conditions become met, while the second is consequent upon the formation of a continuous turbulent structure carried down the tube from the inlet by the bulk flow. The paper provides a formal explanation of a phenomenon which is typical of that which is observed in starting pipe flows with a disturbed inlet.

Some properties of the large structure in a fully developed turbulent duct flow

Some characteristic features of the large scale in a fully developed, two-dimensional, turbulent duct flow have been investigated with the use of a wall shear stress probe and a single hot wire, traversed across the duct. From space-time correlation measurements of wall shear stress and velocity fluctuations, the large scale structure is observed to be inclined at an angle of about 13° for y/d ≳ 0.15, where d is the duct half-width. The convection speed Uc of wall shear stress fluctuations is equal to 0.62 U0 for Rd ≳ 3.5×104, where U0 is the center-line velocity and Rd is the Reynolds number based on center-line velocity and duct half-width. The convection speed of velocity fluctuations is approximately constant over a wide range of Reynolds numbers. The integral length scale of wall shear stress fluctuations is nearly 0.38d. Joint probability density functions of velocity and wall shear stress fluctuations are presented and discussed.

Large structure in a turbulent boundary layer

Using an array of hot wires and wall shear stress probes results are presented which support the hypothesis that an organized structure exists in the boundary layer. From the correlation between wall shear stress and velocity it is shown that this structure is at an oblique angle to the free stream. The inferred angle from this correlation is found to agree remarkably with recent flow visualization photographs. Evidence is found that the large scale motion in the organized structure produces a slowly varying component in the wall shear stress and, importantly, a high frequency large amplitude fluctuation occurring near the maximum in the slowly varying wall shear. It is suggested that this latter fluctuation is associated with the bursting phenomenon. A model which accounts for these results is described. It leads to a description of the bursting process in terms of a rotational instability and while too approximate to give more than good order of magnitude estimates, it gives values of the wall streak spacing which are not inconsistent with previous measurements.