Mechanical Engineering - Research Publications
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ItemSurface shear stress fluctuations in the atmospheric surface layer( 2006)A lightweight, high frequency response, floating element sensor was used to measure wall shear stress fluctuations in an atmospheric surface layer. The sensor uses a laser position measurement system to track the motion of the floating element. The measurements were taken as part of an internationally coordinated experimental program designed to make extensive spatial and temporal measurements of velocity, temperature and wall shear stress of the surface layer. Velocity measurements were made with both a 27m high vertical array and a 100m wide horizontal array of sonic anemometers; 18 anemometers in total were employed. Cross-correlations of shear stress and streamwise velocity fluctuations were analysed in an attempt to identify structure angles in the flow. The results were shown to compare favourably with experimental data from controlled, laboratory turbulent boundary layer measurements at three orders of magnitude lower Reynolds number.
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ItemStudy of the near-wall-turbulent region of the high-Reynolds-number boundary layer using an atmospheric flow(CAMBRIDGE UNIV PRESS, 2006-02-10)
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ItemInvestigation of large-scale coherence in a turbulent boundary layer using two-point correlations(Cambridge University Press, 2005)Stereoscopic particle image velocimetry (PIV) measurements are made in streamwise–spanwise and inclined cross-stream planes (inclined at 45◦ and 135◦ to the principal flow direction) of a turbulent boundary layer at moderate Reynolds number (Reτ ∼ 1100). Two-point spatial velocity correlations computed using the PIV data reveal results that are consistent with an earlier study in which packets of hairpin vortices were identified by a feature-detection algorithm in the log region, but not in the outerwake region. Both streamwise–streamwise (Ruu) and streamwise–wall-normal (Ruw)correlations are significant for streamwise displacements of more than 1500 wallunits. Zero crossing data for the streamwise fluctuating component u reveal that streamwise strips between zero crossings of 1500 wall units or longer occur morefrequently for negative u than positive u, suggesting that long streamwise correlations in Ruu are dominated by slower streamwise structures. Additional analysis of Rwwcorrelations suggests that the long streamwise slow-moving regions contain discrete zones of strong upwash over extended streamwise distances, as might occur withinpackets of angled hairpin vortices. At a wall-normal location outside of the log region (z/δ =0.5), the correlations are shorter in the streamwise direction and broader in the spanwise direction. Correlations in the inclined cross-stream plane data revealgood agreement with the streamwise–spanwise plane. Ruu in the 45◦ plane is more elongated along the in-plane wall-normal direction than in the 135◦ plane, which isconsistent with the presence of hairpin packets with a low-speed region lifting away from the wall.
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ItemStrategies for the visualization of multiple 2D vector fields(IEEE COMPUTER SOC, 2006)
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ItemSimultaneous orthogonal-plane particle image velocimetry measurements in a turbulent boundary layer(Cambridge University Press, 2006)Stereoscopic particle image velocimetry (PIV) measurements were taken simultaneously in streamwise–spanwise and streamwise–wall-normal planes in a zero pressure-gradient turbulent boundary layer over a flat plate. Polarization techniques were employed to allow PIV to be taken in both planes simultaneously. Image preprocessing techniques were used to improve the quality of data near the line of intersection of the planes. Linear stochastic estimation was performed on these data, revealing the streamwise, spanwise, and wall-normal extent of swirl events primarily near the top of the log region of the boundary layer. Swirl events with rotation consistent with the mean vorticity are found to have a large footprint inthe lower limit of the log region whereas swirls with opposite-signed vorticity are found to have little influence lower in the boundary layer. These long-time-averaged statistics contain features that are consistent with the hairpin packet model (or its kinematic equivalent). This model also seems to provide a reasonable description of many instantaneous events involving large-scale coherence, where long regions of streamwise momentum deficit are surrounded by vortex cores.
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ItemExperimental investigation of vortex properties in a turbulent boundary layer(American Institute of Physics, 2006)Dual-plane particle image velocimetry experiments were performed in a turbulent boundary layer with Ret =1160 to obtain all components of the velocity gradient tensor. Wall-normal locations in the logarithmic and wake region were examined. The availability of the complete gradient tensor facilitates improved identification of vortex cores and determination of their orientation and size. Inclination angles of vortex cores were computed using statistical tools such as two-point correlations and joint probability density functions. Also, a vortex identification technique was employed to identify individual cores and to compute inclination angles directly from instantaneous fields. The results reveal broad distributions of inclination angles at both locations. The results are consistent with the presence of many hairpin vortices which are most frequently inclined downstream at an angle of 45 degrees with the wall. According to the probability density functions, a relatively small percentage of cores are inclined upstream. The number density of forward leaning cores decreases from the logarithmic to the outer region while the number density of backward-leaning cores remains relatively constant. These trends, together with the correlation statistics, suggest that the backward-leaning cores are part of smaller, weaker structures that have been distorted and convected by larger, predominantly forward-leaning eddies associated with the local shear.
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ItemTurbulence intensity similarity formulations for wall-bounded flows(CIMNE - International Center for Numical Methods in Engineering, 2004)
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ItemObservations on high Reynolds number turbulent boundary layer measurements(The University of Sydney, 2004)
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ItemSpanwise periodicity and the existence of very large scale coherence in turbulent boundary layers(Begellhouse, 2005-12-01)
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ItemDominant spanwise Fourier modes, and the existence of very large scale coherence in turbulent boundary layers( 2004)Multiple plane stereo PIV results and data from a rake often hot-wire probes are used to investigate the largest scalestructures in a zero-pressure-gradient turbulent boundary layer.Instantaneous vector fields from stereo PIV in spanwise-streamwiseplanes reveal long low- and high-speed regions,with a length that often exceeds the viewing window (> 2d).Also evident is a remarkable degree of spanwise organisation,that manifests as a persistent spanwise stripiness in the u componentof the PIV vector field. Almost all trace of such spanwiseorganisation is lost in the mean statistics, presumably dueto the multitude of scales naturally present in wall-bounded turbulence.This can be overcome by ‘de-jittering’ the instantaneousvector fields. By sorting the data according to dominantspanwise fourier modes, and then applying simple statisticaltools to the sorted subsets, we are able to extract a clear viewof spanwise organisation. Results are confirmed in the variousPIV data-sets. Since the PIV fails to adequately capture the fullstreamwise extent of the low-speed regions, a rake of hot-wireprobes is also employed to capture a continuous view of thespanwise coherence. It is found that the low-speed regions arein fact extremely persistent in the streamwise direction, oftenexceeding 20 d in length. The fact that these long features meanderappreciably in the spanwise direction will limit the overallstreamwise length-scale as witnessed by a single probe or singlepoint statistic. For instance, premultiplied one-dimensionalspectra of the streamwise velocity (kxFuu) at this z/d show apeak contribution for characteristic lengthscales of 5-7d.