Turbulent flows within a cyclone and behind an ahmed vehicle model

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Turbulent flows within a cyclone and behind an ahmed vehicle model

 

Author: Wang, Xiaowen
Title: Turbulent flows within a cyclone and behind an ahmed vehicle model
Degree: Ph.D.
Year: 2011
Subject: Turbulence.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xiii, 123 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2462570
URI: http://theses.lib.polyu.edu.hk/handle/200/6312
Abstract: This thesis is an experimental study of turbulent flows, consisting of two parts: (1) An investigation of the highly three-dimensional flow structure around a simplified vehicle model. The lasting high fuel cost has inspired resurgence in drag reduction research for vehicles, which calls for a thorough understanding of the vehicle wake. Flow around the Ahmed vehicle model has received considerable attention in the literature. Nevertheless, our knowledge of this flow is rather incomplete. This investigation aims to revisit turbulent flow structure behind this model. Two rear slant angles, i.e., α = 25° and 35°, of the model were examined, representing two distinct flow regimes. The Reynolds number was 5.26×10⁴ based on the model height (H) and incident flow velocity. Using particle image velocimetry (PIV), flow was measured with and without a gap (g/H = 0.174) between the vehicle underside and ground in three orthogonal planes, viz. the x-z, x-y and y-z planes, where x, y and z are the coordinates along longitudinal, transverse and spanwise directions, respectively. The measured velocity and vorticity data reconfirm the well documented major characteristics of the mean flow structure. However, both instantaneous and time-averaged PIV data unveil a number of important aspects of the flow structure, which have not been captured previously due to a variety of reasons. As such, a considerably modified flow structure model is proposed. Time-averaged velocities, vorticity components, and the Reynolds normal and shear stresses are presented and discussed, along with their dependence on g/H in the two distinct flow regimes. (2) A detailed study of the extraordinarily complicated turbulence structure in a cylinder-on-cone cyclone. The turbulent flow within a cylinder-on-cone cyclone is highly three-dimensional and complicated; our knowledge of this flow has yet to be improved. Turbulent flow structure in a cylinder-on-cone cyclone was experimentally investigated, which is characterized by a reverse flow in the core region. Two separate efforts have been made. Firstly, measurements were performed at a fixed swirl number of 3 with the Reynolds number varying from 37,100 to 74,200, based on the inlet velocity and the cyclone body diameter. The flow field in planes normal to and through the cyclone axis was measured in detail using a two-component laser Doppler anemometry (LDA) and PIV. Two dominant frequencies of vortical structures were identified based on LDA-measured tangential and axial velocity spectra. One of them agreed quite well with previous reports, but the other was observed for the first time. One explanation was proposed. Secondly, special attention was turned to the swirl number effect on the flow. The swirl number examined was from 2.4 to 5.3, with the Reynolds number fixed at 7.4 x 10⁴. Three regions of the flow have been identified after careful analysis of the data, which are referred to as the core, the outer and the wall-affected regions, respectively; each is distinct from another in terms of the vorticity concentration, frequency of quasi-periodical coherent structure, the probability density function, mean and variance of velocities. It has been found that the flow, including its Strouhal numbers and radial distributions of the mean and fluctuating velocities, depends considerably on the swirl number.

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