Mechanical Engineering - Research Publications
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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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ItemApplication of the wavelet transform in turbulence( 1997)Traditionally, Fourier transforms have been used to elicit the scale-based behaviour of the turbulent motion and one speaks synonymously of its wavenumber components with scales (large scales are associated with small wavenumbers and vice-versa). Although, this approach is theoretically correct, many workers have questioned its appropriateness on the grounds that a Fourier mode represents a wave like disturbance which is global in the physical domain, whereas an eddy is a disturbance with finite spatial extent. Consequently, a more appropriate scheme should involve a local decomposition of the velocity field which is more reminiscent of eddy like phenomena. In this paper we have explored the feasibility of the wavelet transform as an analyzing tool in deducing the turbulence spectrum.
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ItemApplication of the attached eddy hypothesis for the evolution of turbulent boundary layers( 1997)The wall-wake attached eddy model of Perry & Marusic [1] is incorporated in a new approach to the classic closure problem for turbulent boundary layers recently proposed by Perry, Marusic & Jones [2]. This involves using the well known mean-flow scaling laws such as Prandtl's law of the wall and the law of the wake of Coles together with the mean continuity and the mean momentum differential and integral equations. The important parameters governing the flow in the general non-equilibrium case are identified and are used for establishing a framework for closure.
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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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ItemNew evolution equations for turbulent boundary layers in arbitrary pressure gradients( 1997)A new approach at looking at the classic closure problem for turbulent boundary layers is presented. This involves using the well known mean-flow scaling laws such as Prandtl's law of the wall and Coles' law of the wake together with the mean momentum integral and differential equations. The important parameters governing the flow in the general non-equilibrium case are identified and are used to formulate the closure hypothesis. Once the mean flow field has been determined, relevant turbulence quantities can be computed using a coherent structure eddy model based on the attached eddy hypothesis.
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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¿ .