Freely decaying, homogeneous turbulence generated by multi-scale grids

Abstract

We investigate wind-tunnel turbulence generated by both conventional and multi-scale grids. Measurements were made in a tunnel which has a large test section, so that possible side wall effects are very small and the length ensures that the turbulence has time to settle down to a homogeneous shear-free state. The conventional and multi-scale grids were all designed to produce turbulence with the same integral scale, so that a direct comparison could be made between the different flows. Our primary finding is that the behaviour of the turbulence behind our multi-scale grids is virtually identical to that behind the equivalent conventional grid. In particular, all flows exhibit a power-law decay of energy, u2 ~ t−n, where n is very close to the classical Saffman exponent of n = 6/5. Moreover, all spectra exhibit classical Kolmogorov scaling, with the spectra collapsing on the integral scales at small k, and on the Kolmogorov microscales at large k. Our results are at odds with some other experiments performed on similar multi-scale grids, where significantly higher energy decay exponents and turbulence levels have been reported.