Mechanical Engineering - Research Publications
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ItemA wall-wake model for the turbulence structure of boundary layers. Part 2. Further experimental support(Cambridge University Press, 1995)In Part 1 an extension of the attached eddy hypothesis was developed and applied to equilibrium pressure gradient turbulent boundary layers. In this paper the formulation is applied to data measured by the authors from non-equilibrium layers and agreement with the extended theory is encouraging. Also power spectra of the Reynolds stresses as developed from the extended theory compare favourably with experiment. The experimental data include a check of cone-angle effects by using a flying hot wire.
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ItemA wall-wake model for the turbulence structure of boundary layers. Part 1. Extension of the attached eddy hypothesis(Cambridge University Press, 1995)The attached eddy hypothesis developed for zero pressure gradient boundary layers and for pipe flow is extended here to boundary layers with arbitary streamwise pressure gradients, both favourable and adverse. It is found that in order to obtain the correct quantitative results for all components of the Reynolds stresses, two basiv types of eddy structure geometries are required. The first type, called type-A, is interpreted to give a 'wall structure' and the second, referred to as type-B, gives a 'wake structure'. This is an analogy with the conventional mean velocity formulation of Coles where the velocity is decomposed into a law of the wall and a law of the wake.If the above mean velocity formulation is accepted, then in principle, once the eddy geometries are fixed for the two eddy types, all Reynolds stresses and associated spectra contributed from the attached eddies can be computed without any further empirical constants. This is done by using the momentum equation and certain convolution integrals developed here based on the attached eddy hypothesis. The theory is developed using data from equilibrium and quasi-equilibrium flows. In Part 2 the authors' non-equilibrium data are used.
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ItemWall turbulence closure based on classical similarity laws and the attached eddy hypothesis( 1994)A new look at the closure problem of turbulent boundary layers is taken here using recently derived analytical expressions for the shear stress distributions. These expressions are based on logarithmic law of the wall and law of the wake formulation of Coles [J. Fluid Mech. 1, 191 (1956)] with the mean continuity and the mean momentum differential and integral equations. The concept of equilibrium layers of Clauser [Adv. Mech. 4, 1 (1956)] is extended and using similar ideas as Rotta [Prog. Aeronaut. Sci. 2, 1(1962)] for self-similarity, a closure scheme is proposed for layers developing in arbitrary pressure gradients for the case where the streamwise derivative of the Coles wake factor is not too large. For a given flow case, this Coles wake condition can be tested with internal consistency checks. The mathematical framework is most suitable for incorporating Townsend’s attached eddy hypothesis as recently developed by Perry, Li, and Marusic [Phils. Trans. R. Soc. London. Ser. A 336, 67 (1991)] for closure. This gives an opportunity to incorporate coherent structure concepts into closure schemes. Possible ways of handling the difficult case where the streamwise derivative of the Coles wake factor is significant are discussed.
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ItemTowards a closure scheme for turbulent boundary layers using the attached eddy hypothesis(Royal Society Publishing, 1991)In this paper, an attempt is made to formulate a closure hypothesis for adverse pressure gradient turbulent layers using the attached eddy hypothesis of Townsend and Perry & Chong, which was developed originally for zero pressure gradient layers and parallel duct flows.To the authors’ knowledge, this work represents one of the few attempts to use coherent structure ideas in the formulation of a closure scheme. At present this closure scheme is primitive and many of the assumptions are of an arbituary nature but the analysis at least points out where the difficulties are and which areas need more work.
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ItemSimilarity law for the streamwise turbulence intensity in zero-pressure-gradient turbulent boundary layers( 1997)A similarity relationship is proposed to describe the streamwise broadband-turbulence intensity in a zero-pressure-gradient boundary layer. The formulation is applicable to the entire region of the flow beyond the viscous buffer zone and is based on the attached eddy hypothesis, the Reynolds-number-similarity hypothesis and the assumed existence of Kolmogorov eddies with a universal inertial subrange. Experimental data of the authors and those from various published works covering a large Reynolds number range are investigated in light of this formulation.
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ItemA study of the Reynolds-shear-stress spectra in zero- pressure-gradient boundary layers( 1989)Measurements of Reynolds-shear-stress spectra in smooth-wall zero-pressure-gradient turbulent boundary layers are presented for four different Karman numbers. The results are plotted using various scaling variables and found to collapse with 'inner-flow' scaling for a modest range of mid-range wavenumbers, the extent of collapse increasing for higher Karman numbers. The results are discussed in light of the attached eddy hypothesis of Townsend (1976)as extended by Perry and Chong (1982). A computation of the cross-power spectra, based upon the same theory, is also presented.
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ItemAn experimental and computational study on the orientation of attached eddies in turbulent boundary layers( 1992)The flow visualisation results of various researchers suggest that the attached eddies align themselves with the direction of the principal rate of mean strain. The principal rate of mean strain is influenced by pressure gradient. With the knowledge of this dependence and using the wall turbulence model proposed by Perry & Chong (1982) and later modified by Perry et al. (1986, 1991) it may be possible to predict the characteristics of a turbulent boundary layer flow subjected to any arbitrary pressure gradient.This work investigates quantitatively the orientation of attached eddies in a turbulent boundary layer. This is achieved by maximising the two point correlation coefficient between the signals obtained from two normal hot-wires having a streamwise offset and a predetermined normal to the wall spacing between them. The experimental results will also be compared with some computational results based on the attached eddy hypothesis.
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ItemCone angles and Reynolds stresses in an adverse pressure gradient boundary layer( 1992)A comparison of the Reynolds stresses measured with a stationary and flying hot-wire is made for flow in a turbulent boundary layer on a smooth wall with streamwise pressure gradient. A cone angle is defined, based on the p.d.f. of velocity vector angles for 90o X-wire. From these measurements it is possible to evaluate the conditions under which a stationary X-wire will give erroneous estimates of the Reynolds stresses in fields of strong turbulence intensities. The response of the Reynolds stresses to the application of an adverse pressure gradient is discussed.
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ItemA comparative study of the spectra of turbulent jets and boundary layers at high wavenumbers( 1994)This paper describes measurements of spectra taken in a coflowing jet and in a boundary layer with zero streamwise pressure gradient. The spectra are compared to examine universality of the small scales and to examine the scaling laws of Kolmogorov. Comparisons are also used to examine differences in the distribution of the Reynolds shear-stress spectra in wavenumber space between the two flows and these experimental observations are explained in terms of models for the turbulence structure which have been developed by the authors for each of these flows.
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ItemSimilarity predictions based on the attached eddy hypothesis in turbulent boundary layers( 1995)The paper presents a similarity formulation for the streamwise component of the fluctuating velocity u1 in a turbulent boundary layer based on the attached eddy model of wall turbulence being developed at the University of Melbourne by Perry and co-workers. The consequences of this formulation for increasing Reynolds numbers is tested against recent high Reynolds number data. The model is based on the assumption that there exist eddies of different length scales in a turbulent boundary layer and the probability density function (p.d.f.) of the eddy length scale distribution follows an inverse power law for eddies in the turbulent wall region. Such a distribution is necessary to obtain a log-law of the mean velocity: the spectral scaling laws provide indirect evidence for this. In this paper the results from a template matching technique will be presented which gives further support for the proposition of an inverse power law p.d.f. of attached eddy length scales.