Mechanical Engineering - Research Publications
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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.
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ItemSimilarity laws and attached eddy shapes in turbulent boundary layers( 1995)This 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 a -1 power law for eddies in the turbulent wall region. Such a distribution is necessary to obtain the log-law of the mean velocity: the spectral scaling laws provide indirect evidence of this. In this paper the results from a template matching technique will be presented which gives further support for the proposition of a -1 power law p.d.f. of attached eddy length scales.Using space time correlation coefficients further details can be obtained regarding eddy shape. The simple II-shaped representative eddy needs to be modified to give reasonable correlation coefficients.
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ItemA wall-wake model for turbulent boundary layers with pressure gradients( 1995)The attached eddy hypothesis is considered here for boundary layers with arbitrary streamwise pressure gradients. It is found that in order to obtain the correct quantitative results for all components of the Reynolds stresses, two basic 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 in 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.
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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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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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ItemThe effect of aspect ratio and divergence on the turbulence structure of boundary layers( 1995)The effect of the aspect ratio of a turbulent boundary layer on the mean flow, broadband turbulence intensities and Reynolds shear stress has been studied. The aspect ratio (AR) is defined as the boundary layer thickness divided by the boundary layer width, i.e. the effective wind tunnel width. Measurements have been taken in a nominally zero pressure gradient layer at a single station for three different aspect ratio settings, AR=1/4, AR=1/7, and AR=1/13. The measurements show that the turbulent quantities were unaffected when the aspect ratio was increased from AR=1/13 to AR=1/7. However at AR=1/4 there appears to be a slight increase in the broadband turbulence intensities and Reynolds shear stress.