Pulsed-wire Anemometry Research Articles

Moderately three-dimensional separated and reattaching turbulent flow

A fully three-dimensional turbulent separated flow was set up such that it had a systematic link to two-dimensional flow, as a way of investigating the more complicated nature of this flow type. The central region of the flow was fully three-dimensional, but was bounded on its sides by regions of ‘spanwise invariance’ in which the flow was invariant in the lateral direction, or very nearly so. A special case of spanwise invariance, which is statistically two-dimensional, is one in which the streamlines are also coplanar, or at least nominally so in numerous experimental studies. Another aspect of the present arrangement is that the side regions should ideally provide well-defined boundary conditions. The separation was formed downstream of a doubly swept normal flat plate, forming a ‘v’-shaped separation line, mounted on the front of a splitter plate, mounted in the centre of the wind tunnel working section. The predominantly inward flow to the central region implies a negative lateral strain rate (∂W/∂z), but all nine strain rates are non-zero. Measurements were made using pulsed-wire anemometry techniques for mean velocities, Reynolds stresses and wall shear stress. Even though the sweep angle is mild at ±10°, the effect is to increase the bubble height by over 50% in its centre to create a ‘bulge’, symmetrical about the centreline. The degree of three-dimensionality is described as moderate in that the peak inflow velocity from the side regions is less than 0.2 of the free-stream velocity, but comparable with the peak in the reverse-flow velocity. A larger sweep angle would give a larger inflow velocity. A separate study (Cao & Hancock, Eur. J. Mech. B/Fluids, vol. 23, 2004, p. 519) has shown that the bulge persists very far downstream, so that accurate physical modelling of the separated region is likely to be important in modelling the flow well downstream. An intermediate region exists between the invariant side region and the bulge, where all the stress levels are reduced, as would be expected from the effects of streamline convergence. Although overall there is a flow inward to the centre (streamline convergence), part of the overlying shear layer is subjected to diverging flow and an intensification of Reynolds stresses near the centre of the bulge.

Scaling of the near-wall layer beneath turbulent separated flow

This paper is a continuation of an earlier paper [P.E. Hancock, Velocity scales in the near-wall layer beneath reattaching turbulent separated and boundary layer flows, Eur. J. Mech. B Fluids 24 (2005) 425–438] in which it is proposed that each Reynolds stress has its own velocity scale. Two of these, u τ ′ and w τ ′ , are directly related by definition to the r.m.s. of the wall-shear-stress fluctuations ( τ x and τ z ) in the streamwise and transverse directions. They are also velocity scales for the true dissipation of the turbulent kinetic energy and the Kolmogorov velocity and length scales at the surface. From asymptotic considerations it is shown that the other two scales are related to averages involving instantaneous gradients of wall-shear-stress fluctuations. The measurements, made using pulsed-wire anemometry into the viscous sublayer, show that u τ ′ and w τ ′ are also the velocity scales for the respective streamwise and transverse fourth-order velocity moments, together with the viscous velocity scale ( ν / y ). Normalised, the fourth-order moments show an inner-layer-like behaviour independent of both position and direction, like that seen in the second-order moments [P.E. Hancock, Velocity scales in the near-wall layer beneath reattaching turbulent separated and boundary layer flows, Eur. J. Mech. B Fluids 24 (2005) 425–438]. However, not surprisingly, the third order moments exhibit an effect of mean shear, seen in the skewing of the probability distributions. Though not measured directly, the measurements imply the behaviour of the averaged products of fluctuations in wall-shear-stress and wall-pressure-gradient ( τ x ∂ p / ∂ x ¯ and τ z ∂ p / ∂ z ¯ ). Normalised, they also are independent of position and direction. Some of the results presented apply more generally to the near-wall region beneath turbulent flow.

Boundary layer development after a region of three-dimensional separated flow

The development of the boundary layer flow down stream of a region of three-dimensional turbulent separated flow has been investigated using hot-wire anemometry and pulsed-wire anemometry for wall shear stress. The flow, generated by means of a ‘v-shaped’ separation line, was set up so as to generate a central region of fully three-dimensional flow, bounded on each side by the degenerate case of spanwise-invariant flow, which is closely related to two-dimensional co-planar flow. The flow developing in the central region has fundamentally different features from that after a region of two-dimensional separated flow. A bulge in the boundary layer, generated in the separated flow, has a strong wake-like characteristic that sits very persistently in the outer part of the layer, its shape and relative size changing little in ∼25 reattachment length scales. The Reynolds stresses in this bulge region, driven by the mean velocity gradient of the wake-like flow, are very much larger than the normal levels in the boundary layer or in the developing region downstream of a two-dimensional separation. The inner layer that lies beneath the bulge is subjected to a high level of non-shear-stress-containing motion, the normal stresses being much higher than the shear stress which exhibits a constant level in the inner layer despite much higher levels further out.

INFLUENCE OF GEOMETRY ON THE MEAN FLOW WITHIN URBAN STREET CANYONS - A COMPARISON OF WIND TUNNEL EXPERIMENTS AND NUMERICAL SIMULATIONS

A comparison between numerical simulations and wind tunnel modelling has been performed to examine the variation with streamwise aspect ratio (width/height, W/H) of the mean flow patterns in a street canyon. For this purpose a two-dimensional (2-D) cavity was subjected to a thick turbulent boundary layer flow perpendicular to its principal axis. Five different test cases, W/H = 0.3, 0.5, 0.7, 1.0 and 2.0, have been studied experimentally with flow measurements taken using pulsed-wire anemometry. The results show that the skimming flow regime, with a large vortex in the canyon, occurred for all the cases investigated. For the cavities with W/H ≤ 0.7 a weaker secondary circulation developed beneath the main vortex. The narrower the canyon, the smaller the wind speed close to the cavity ground, giving increasingly poor ventilation qualities. The corresponding numerical results were obtained with the Computational Fluid Dynamics (CFD) code CHENSI that uses the standard k-e model. The intercomparison showed good agreement in terms of the gross features of the mean flow for all the geometries examined, although some detailed differences were observed.

Active control of the turbulent flow over a swept fence

The aim of this experimental investigation has been to reduce the length of the three-dimensional separation region downstream of a swept surface-mounted fence by time-periodic blowing and suction generated by loudspeakers along a spanwise slot either upstream of or at the tip of a fence. The sweep angles were 0° and 20° and the Reynolds number Re h ≡ U N h/ ν=5330. Two flow phenomena were used for the manipulation of the shear layer and the reduction of the separation length: • the coherent structures of the separated shear layer (Kelvin–Helmholtz instability) were influenced to increase entrainment by single and bimodal forcing at the tip of the fence. Here the effective disturbance amplitude could be small (e.g., A= O(0.1 U N )); • the corner flow upstream of the fence was manipulated by the upstream actuator-slot so that a starting-type vortex roll formed at the tip of the fence, increased entrainment just downstream of the fence and reduced the separation length. Here the effective disturbance amplitude was large (e.g., A= O( U N )). Reattachment could be reduced at most by about 35% of the non-manipulated reattachment length. Measurements in the flow field were performed by LDA and DPIV and on the wall by pulsed-wire anemometry (PWA).

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.

Aerodynamics of chain link stoker mats

This paper describes experimental work on the aerodynamic features of a chain grate stoker link mat and interprets and analyses the results of flow visualisation and associated quantitative measurements. Particular attention was paid in the experimental work to local air flow characteristics by using smoke tracer techniques on an actual link mat and pulsed wire anemometry on an enlarged scale model mat. Examples of jetting flow, recirculation, wakes and eddies of long residence time have been recorded for typical chain grate combustion air flow rates. Such information represents a significant step forward from the traditional empirical development and allows the proposal of improved, commercially applicable grate link designs for the enhancement of the role of coal in a competitive fuels market where environmental legislation is ever tightening. The results are being used in a subsequent part of a project to develop a new design of stoker mat link.

Pulsed-wire anemometry near walls

This experimental investigation compares two configurations of pulsed-wire probe for measurements in the very near wall region, as introduced by Castro and Dianat (1990) and Devenport et al. (1990). Diffusion of the thermal wake causes significant errors in regions where the velocity is low and the velocity gradient high. These errors, which are about the same for either configuration, are large in the viscous sub-layer. An analysis of diffusional effects is made, and a method of correction is given that applies equally in laminar and turbulent flow. It is necessary to calibrate a probe for the effects of shear in a known flow.

Point measurement of turbulence quantities in separated flows - a comparison of techniques

Accurate measurement of turbulence quantities in regions of high turbulence intensity is notoriously difficult. In the particular context of turbulent separated flows, only two `point-based' (rather than full-field) techniques are generally acknowledged to be viable: laser-Doppler anemometry (LDA) and pulsed-wire anemometry (PWA). This paper presents the first serious comparison of data obtained in the same high-intensity, separated flow using both techniques. It is shown that, with sufficient care and understanding of the instrument responses, mean velocity and turbulence stress measurements yielding similar results can generally be obtained by the two techniques, although PWA data are in some circumstances inevitably less accurate. In addition, the results are close to those obtained from standard crossed-hot-wire anemometry in regions where the latter can sensibly be used. LDA and PWA are based on very different physical principles and hence there are very different sources of error. Whilst the present results from either technique do not, on their own, necessarily imply high accuracy, taken together they therefore do suggest that the measurement errors can usually be made acceptably small in both cases. Consequently, they increase the confidence with which each technique can be used for such complex flows although there are some unavoidable drawbacks in PWA which impose additional limits on the scale of flow which can be successfully probed and make accurate assessment of the errors rather difficult.

Structure of Turbulence in an Incipient-Separating Axisymmetric Flow

The relative intensity, skewness, and flatness of fluctuating streamwise velocity along the centerline of an 8 deg included angle conical diffuser show dramatic rapid growth in the final stages of the flow under the increasing influence of growing instantaneous reversals in the wall-layer. Pulsed-wire anemometry was effectively used for the measurement of quantitative instantaneous reversals and the turbulent flow field. In the severe adverse pressure gradient of the diffuser flow, the maxima of the streamwise and transverse fluctuating velocities, Reynolds shear stress, and turbulent energy production coincide and move away from their near-wall position in the pipe, also the velocity triple products show completely opposite nature as compared to the pipe flow. These measurements reveal the strong influence of instantaneous backflow on the structure of turbulence. The present results further corroborate the ability of the “structural” turbulence model of Nagano and Tagawa (1990) to predict velocity triple products in an axisymmetric diffuser flow.

Behavior of mean and fluctuating skin friction in a conical diffuser with instantaneous flow reversals

Measurements were made of the mean and fluctuating skin friction in an 8° total divergence angle conical diffuser using modern pulsed-wire anemometry. The development of streamwise and spanwise fluctuating skin friction, skewness and flatness factors of streamwise skin friction, and probability density distribution of the wall shear stress show the rapidly changing nature and complexity of the wall layer turbulence structure in an axisymmetric severe adverse pressure gradient flow. The relative intensities of skin friction increase consistently and achieve very high values in the final stages of diffuser flow. The quantitative instantaneous backflow and the mean and fluctuating skin friction distributions were examined to determine their interrelationship. The initiation of instantaneous reversals was found to strongly influence the near-wall flow field and to coincide with the precipitous drop in skewness factor of the streamwise fluctuating wall shear stress, to approximately 10% of its initial value near the diffuser inlet. The flatness factor of the streamwise fluctuating wall shear stress also decreases in the latter stages, indicating a lesser inrush of high-velocity fluid from the central region. Comparison with conventional Preston tube data indicated the appropriateness of pulsed-wire skin friction measurements in severe adverse pressure gradient flows. In this diffuser the minimum streamwise wall shear stress near the diffuser exit is close to 3% of its inlet value, indicating that the flow approaches mean-flow separation. The present results of mean and fluctuating skin friction also differ substantially from those reported by other investigators using hot-film anemometry, both in level and shape of profiles. The differences are attributed to the rectification errors in hot-film anemometry at high turbulence intensities (near-wall turbulence level over 80%) and the appreciable instantaneous backflow (instantaneous reversals up to 30% of the time) present in the final stages of conical diffuser flow.

The structure of instantaneous reversals in highly turbulent flows

Structure of instantaneous flow reversals has been measured in a highly turbulent axisymmetric diffuser flow using pulsed-wire anemometry. In this 8° nominal included angle conical diffuser, the adverse pressure gradient (APG) is strong enough to cause appreciable instantaneous flow reversals (instantaneous backflow up to 30% of the time), but the time-averaged flow is non-separated. The results are compared with the other severe APG separating flows reported in literature. An increase in entry Reynolds number indicated a decrease in the size of near-wall instantaneous reversals region as well as a decrease in the magnitude of instantaneous backflow. Also, the region of instantaneous reversals moves slightly downstream at appreciably higher Reynolds numbers. The initiation and growth of instantaneous reversals in a conical diffuser was found to strongly influence the wall-layer and the central region. Present results also suggest that the instantaneous backflow should be considered for modelling of instantaneously-separating diffuser flows. In the final stages of a conical diffuser, the magnitudes of cross-stream pressure gradient were found to be appreciably larger than that of the longitudinal pressure gradient, indicating that accurate representation of a conical diffuser flow can not be achieved without considering V-momentum equation. A comparison of various separating flows revealed remarkable similarity of instantaneous reversals regions and distributions even in different flow configurations.

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.

Asymptotic velocity defect profile in an incipient-separating axisymmetric flow

Presence of a new asymptotic mean velocity profile (velocity defect law) was found concomitant with appreciable incipient separation (instantaneous detachment > 20 % of the time) and extremely high-turbulence intensities in the final stages of an 8-deg total divergence angle conical diffuser fed with a fully developed turbulent pipe flow. Pulsed-wire anemometry was used for the measurement of incipient flow separation, mean velocities and skin-friction distributions. The conical diffuser flow shows some attributes .of quasiequilibrium in the final onethird length. In the second-half of the diffuser the mean velocity profiles indicate a distinct point of inflection, the wall regions have less sensitivity to the initial conditions, and the flow exhibits some insensitivity toward the local boundary conditions. A comparison with the other asymptotic velocity defect profiles in Perry-Schofie ld coordinates revealed the distinctly different shape of the present asymptotic profile found in the conical diffuser. The validity of this asymptotic mean velocity profile was confirmed at two Reynolds numbers. Application of rapid distortion theory indicated coexistence of the regions of slowly and rapidly changing turbulence in the conical diffuser. Results of the present study also support the theory that turbulence is not universal and depends on the initial and boundary conditions.

Pulsed-wire anemometry on rough surfaces, with application to desert sand dunes

The use of pulsed-wire anemometry to measure surface shear stress in rough wall boundary layers is discussed. It is shown that if the size of the probe is not too dissimilar from the size and spacing of the roughness elements, reasonably accurate measurements of shear velocity are possible, provided the (active) tip of the probe is located near the zero plane displacement height. The technique is illustrated via an experiment on the flow over a 1/200 scale model of a “barchan” desert sand dune. Measurements of surface stress using the pulsed-wire probe on a modelled dune are shown to be consistent with those deduced from extrapolated crossed hot wire data and also with the implications of full-scale measurements of sand transport rates. These results, in addition to confirming the viability of pulsed wire anemometry on rough surfaces, have led to the resolution of a long-standing paradox concerning the dynamics of such dunes.

Pulsed-wire anemometry

Pulsed-wire anemometry was first developed as a tool for making velocity and turbulence measurements in the late 1960s. It has been continuously refined and its potential exploited by a number of groups who have obtained reliable data in situations where the use of other techniques would have been extremely difficult. The technique is particularly useful in flows of high turbulence intensity and has therefore been used to greatest effect in separated flows. Although its range of applicability is much more restricted than that of laser-Doppler anemometry, it is an order of magnitude cheaper, and it does not require seeding of the flow, with all the attendant uncertainties. First the physical basis of pulsed-wire anemometry is briefly described, and the major sources of experimental error are outlined for cases in which the technique is used both remote from and close to walls. Progress in the design of probes, which have been improved in a number of ways since the early days to reduce errors, is also outlined. The author then reviews the kinds of measurements that have been successfully made and the consequent improvements in the understanding of the physics of complex flows. Examples are drawn from a wide range of work, including some unusual applications like measurements in very low velocity gas mixtures. The paper closes with a summary of the limitations of the technique and an overall assessment of the likely potential for its use in future turbulence research.

Turbulent Flow Around a Bluff Rectangular Plate. Part I: Experimental Investigation

Measurements are reported for the separted reattaching flow around a long rectangular plate placed at zero incidence in a low-turbulence stream. This laboratory configuration, chosen for its geometric simplicity, exhibits all of the important features of two-dimensional flow separation with reattachment. Conventional hot-wire anemometry, pulsed-wire anemometry and pulsed-wire surface shear stress probes were used to measure the mean and fluctuating flow field at a Reynolds number, based on plate thickness, of 5 × 104. The separated shear layer appears to behave like a conventional mixing layer over the first half of the separation bubble, where it exhibits an approximately constant growth rate and a linear variation of characteristic frequencies and integral timescales. The characteristics of the shear layer in the second half of the bubble are radically altered by the unsteady reattachment process. Much higher turbulent intensities and lower growth rates are encountered there, and, in agreement with other reattaching flow studies, a low frequency motion can be detected.

Spatial correlation measurement in turbulent flows using pulsed wire anemometry

Details of a new technique in pulsed wire anemometry, developed to allow measurement of two-point spatial velocity correlations in highly turbulent flows, are described. An outline of the interface devices necessary for linking two anemometers to a microcomputer is given and examples of the use of the technique are presented. Firstly, measurements of spatial correlations with and without time delay in the near wake of a nominally two-dimensional cylinder normal to a uniform stream confirmed the viability of the technique. Secondly, measurements in the highly turbulent, separated region behind a normal flat plate fitted with a downstream splitter plate are presented as a demonstration of the effectiveness of the technique in such ‘difficult’ flows. We believe that these are the first direct measurement of spatial correlation functions within a separated flow.

The pulsed-wire anemometer

The principles and practice of pulsed-wire anemometry are reviewed. Flow velocity is deduced from the time taken for the thermal wake of a thin wire, heated by a short pulse of current, to reach a sensor wire operating as a resistance thermometer. The advantage over the hot-wire anemometer is that reversed flows can be measured by adding a second sensor wire on the “upstream” side of the pulsed wire: the main advantages over the laser Doppler anemometer are cheapness and simplicity of use. The pulsed-wire anemometer can now be regarded as a cost-effective instrument for measurements in turbulent separated flows.

Measurements in separating boundary layers

In this paper we describe the application of pulsed-wire anemometry to the study of a two-dimensional separated flow generated by imposing a suitably adverse pressure gradient on a flat plate turbulent boundary layer. Accurate measurements of both mean velocity right down to the wall and skin friction have been made throughout the region approaching separation and within the separated region itself. The presentation includes a brief description of the development of the specially designed velocity probe, which can be traversed through the wall and enables measurements to be made throughout the viscous dominated wall region. The data obtained within the separated boundary layer are compared briefly with the results of previous studies; in particular, the similarities and differences between shear layers bounding highly turbulent separated regions and the classic plane mixing layer are briefly discussed.