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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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.
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ItemThree dimensional structure characterization and visualization in a turbulent boundary layer(CIMNE - International Center for Numical Methods in Engineering, 2004)
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ItemAnalysis of vortex packets and Reynolds stress in a turbulent boundary layer( 2003)Sets of stereo PIV measurements were obtained instreamwise-spanwise planes of a turbulent boundary layerwith Re¿ = 1060. Two-point spatial correlations obtainedfrom fields in the logarithmic region revealed that bothstreamwise-streamwise (Ruu) and streamwise-wall-normal(Ruw) correlations were significant for streamwise displacementsof more than 1500 wall units. Zero crossing datafor the streamwise fluctuating component u revealed thatstreamwise strips between zero crossings of 1500 wall units orlonger occurred more frequently for negative u than positiveu. This result suggested that the long streamwise correlationsin Ruu are dominated by slower streamwise structures.Additional analysis of Rww correlations suggestedthat the long slow-moving streamwise structures containdiscrete zones of strong upwash over extended streamwisedistances as might occur within packets of angled hairpinvortices. At a wall-normal location outside of the log region(z=± = 0:5), the correlations were shorter in the streamwisedirection and broader in the spanwise direction. Allof the correlation results are consistent with earlier studies(Ganapathisubramani et al, 2003) in which a feature detectionalgorithm identified packets of hairpins in the log regionbut not in the outer region.
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ItemEvidence of the -1-law in a high Reynolds number turbulent boundary layer( 2004)Dimensional analysis leads to a prediction of a -1-power-lawfor the streamwise velocity spectrum in a turbulent boundarylayer. This law can be derived from overlap arguments or fromphysical arguments based on the attached eddy hypothesis ofTownsend (1976). Some recent experiments have questionedthe existence of this power-law region in wall-bounded ows.In this paper experimental spectra are presented which supportthe existence of the -1-law in a high Reynolds number boundarylayer, measured in the large boundary layer facility in theWalterBasset laboratory at the University of Melbourne. The paperpresents the experimental results and discusses the theoreticaland experimental issues involved in examining the existence ofthe -1-law and reasons why it has proved so elusive.
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ItemDual-plane PIV investigation of structural features in a turbulent boundary layer( 2004)Simultaneous dual-plane PIV experiments were performed instreamwise-spanwise planes in the log region of a turbulentboundary layer at a moderate Reynolds number (Ret » 1100).The acquired datasets were used to resolve all 9 velocity gradientsfrom which the complete vorticity vector and other invariantquantities like 3-D swirl strength were computed. Thesederived quantities were used to analyze and interpret the structuralcharacteristics and features of the boundary layer. Resultsof the vorticity vector and the 2-D swirl strength from the twoneighbouring planes indicate the existence of hairpin shapedvortices inclined downstream along the streamwise direction.These vortices envelop low speed zones and generate Reynoldsshear stress that enhances turbulence production. Plots of full3D swirl strength indicate the existence of additional vorticalstructures in the middle of the low speed zones that may representheads of smaller eddies intersecting the measurementplane. This concept is in accordance with the hierarchy of structuresize in a hairpin packet proposed by Adrian et al.[2]. Computationof inclination angles of individual eddies using the vorticityvector suggests that most cores are inclined at 25± to thestreamwise-spanwise plane with a resulting projected eddy inclinationof 32±.
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ItemInclined cross-stream stereo PIV measurements in turbulent boundary layers.(CIMNE, 2004)By arranging the laser light-sheet and image plane of a stereo PIV system ininclined spanwise/wall-normal planes (inclined at both 45± and 135± to the x-axis) we have obtained a unique quantitative view of the turbulent boundarylayer in planes aligned both with and against the principle vorticity axis of aproposed hairpin model. These experiments have been repeated across a range ofReynolds numbers (Re¿ ¼ 800 ¡ 3050). In-plane swirl results indicate the presenceof inclined eddies, arranged about low-speed regions (with circumstantialevidence suggesting that these occasionally group into packet-like formations).Two-point correlations show that outer scaling is the correct way to quantifythe characteristic spanwise lengthscale across the range of Re¿ .