Evolution of a spectrally local disturbance in grid-generated, nearly isotropic turbulence

Abstract

A grid-generated ‘isotropic’ turbulent flow has been subjected to a spectrally local perturbation in the form of a high wavenumber, sinusoidal ripple in the mean velocity. It is introduced as the wake of a fine wire, low solidity screen (a ‘zither’), operating below its vortex-shedding Reynolds numbers. The perturbation appears most clearly downstream as a strongly periodic component in the transverse correlation of streamwise turbulent velocity, R11(r2). Its Fourier transform, E11(k2, a ‘one-dimensional spectrum’, shows a corresponding local ‘spike’. The downstream evolution of this perturbation has been chronicled for four different spectral locations. Their decays are approximately exponential. Measurements of the one-dimensional spectrum E11(k1) at several downstream stations show evidence of energy transfer to other wavenumber regions.The decay of the velocity correlation ripple is approximately consistent with the decrease in time of the narrow-band correlation function presented by Comte-Bellot & Corrsin (1971). It is also found that the linear perturbation response calculated by Kraichnan (1959) for a disturbance wavenumber larger than those of the incident turbulence shows fair agreement with this decay.