Results of an attempt to generate a homogeneous turbulent shear flow

Abstract

An approximately homogeneous turbulent shear flow is generated in an open-return wind tunnel test-section by a plane parallel-rod grid of uniform rod diameter and non-uniform rod spacing. The grid design is based upon an analysis by Owen & Zienkiewicz (1957). Hot-wire measurements taken in this flow include mean velocities, component turbulence intensities, shear and two-point space correlations, and energy spectra. In addition, microscales, obtained both from instantaneous time derivatives of the hot-wire signal and from two-point space correlations, and integral scales, calculated both from correlations and energy spectra, are reported.Based upon these results, it is concluded that, far enough away from the grid and the test-section wall boundary layers: The turbulence intensities are maintained at uniform values by the nearly constant mean shear. The turbulent shear stress approaches an asymptotic value. Measured two-point space correlation coefficients and one-dimensional energy spectra attain self-preserving forms. When distance downstream of the grid is measured in terms of the number of ‘local’ grid rod spacings, (see discussion of microscales obtained from time derivatives), the Taylor microscale defined by the correlation coefficientR_uu(r_X, 0, 0) grows linearly with this ‘effective’ distance over most of the region measured. The limited number of integral scale determinations and experimental uncertainty allow only the statements that the magnitude of the longitudinal scale is roughly one-eighth the lateral dimension of the square test-section and tends to increase slightly with ‘effective’ distance from the grid. The lateral integral scales are approximately one-half the longitudinal scales and also increases with distance from the grid. The integral scale which characterizes the size of the eddy primarily responsible for momentum transfer is roughly one-tenth the test-section lateral dimension (measured at one point only).