Near-field investigation of turbulence produced by multi-scale grids

Abstract

We investigate grid turbulence behind two multi-scale grids and one conventional one, with particular emphasis on the properties of the near-field turbulence. All three grids are designed to produce the same integral scale, ℓ0, at a downstream distance of two meters (x = 2 m). We find that the turbulence is highly inhomogeneous for x < 50ℓ0, but approaches a homogeneous, fully developed state for x > 50ℓ0. The structure of the near-field turbulence for the present grids is dependent on the type of grid used, exhibiting a number of curious properties, which is consistent with earlier studies [D. Hurst and J. C. Vassilicos, “Scalings and decay of fractal-generated turbulence,” Phys. Fluids 19, 035103 (2007)]. However, the far-field is remarkably similar for all three grids. For example, the far-field energy decay rate is virtually identical for all three grids, as was shown by Krogstad and Davidson [“Freely-decaying, homogeneous turbulence generated by multiscale grids,” J. Fluid Mech. 680, 417 (2011)]. This ability of the turbulence to largely forget its initial conditions is consistent with numerical studies. There has been considerable recent interest in the possibility of an exponential decay of energy in homogeneous turbulence. We confirm a region of apparent exponential decay behind all three of our grids over a limited range of x. However, the range of x is narrow, and other functional forms fit the data equally well. Indeed, when the streamwise extent of the investigated region is increased, a classical power law fits the data even better. In any event, the region of apparent exponential decay lies in the highly inhomogeneous near-field, and so is not representative of homogeneous turbulence.