Flow patterns of large eddies in a wake and in a boundary layer

Abstract

The turbulent velocity fluctuations at eight positions on sections of a plane wake and a boundary layer have been sampled simultaneously and recorded in digital form on magnetic tape for subsequent numerical analysis. Two configurations have been used (lines of equally-spaced sensors in planes normal to the flow, and arrays with three rows) with sensors responsive both to streamwise and cross-stream components of the fluctuations. To the extent that the Taylor approximation of ‘frozen’ flow is valid, the recorded fluctuations may be interpreted as instantaneous values at grid points in the volume swept out by the array.The records have been examined, (a) to find evidence for flow patterns with marked periodicity in one direction, and (b) to select dimensions and orientations for simple eddy flow patterns whose random superposition would lead to correlation functions with a close resemblance to those calculated from the recorded data. In the wake, clear evidence was found for periodic flow patterns that resemble the eddies of a von Kármán street, but, although the spacing of eddy centres in each group was uniform, it varied considerably from one group to another, suggesting that groups are being observed in different stages of development.Two kinds of correlation were calculated from the records, (i) simple mean values of velocity products, and (ii) mean values of the products weighted by the total intensity or Reynolds stress in the effective volume swept by the array of sensors. For both kinds, the correlations are well described by simple inclined roller-type eddies, but the correspondence is greatly improved by weighting in favour of intensity or Reynolds stress. It appears that the eddies contributing most to intensity or Reynolds stress are less variable in form than all the eddies together, and that those contributing most to Reynolds stress are significantly different in shape and in orientation from those contributing most to turbulent energy.