Experimental Investigation of Homogeneous Anisotropic Turbulence

Abstract

Motivated by the need to substantiate the existing literature on homogeneous turbulence with experimental data, a novel zero-mean homogeneous turbulence chamber is presented. Despite the anisotropy of the large scale velocity fluctuations, the experimental apparatus is found to well approximate homogeneous, shear-less turbulence over scales larger than the integral lengths of the flow for four separate cases at Reynolds numbers (based on the Taylor microscale) ranging between 154 and 412. This enables a detailed investigation of the turbulence statistics as obtained by 2D particle image velocimetry, which confirms the existence of inertial scaling ranges in both the second-order structure functions and energy spectra. It is found that the anisotropy of the flow persists down to the smallest scales, though its influence decreases with decreasing scale. The coherent structures, identified using a percolation analysis, are however isotropic in their geometry and generally collapse across cases using the Taylor microscale as a normalization length scale. Two types of scale interaction analyses are applied to the turbulent fields and indicate that there exists substantial coupling between scales small and large; challenging the classic assumption that a range of scales might emerge which is independent of the large scales. Employing the generalized Karman-Howarth-Monin equation in scale space, the energy cascade is found to move energy downscale across all cases, which is also confirmed using a filter space technique. The magnitude of the non-linear energy transfer in scale space is however found to be increasingly anisotropic for increasing large-scale RMS velocity ratio u'_1/u'_2. Using a conditional sampling procedure based on the activity of the small-scales, the non-linear energy transfer is found to have a strong dependence on the relative small-scale activity (as well as the presence of coherent structures), which causes enhanced downscale non-linear energy transfer or upscale non-linear energy transfer of moderate magnitude depending on the subset. In addition to showcasing accurate PIV measurements of homogeneous turbulence over a large range of scales, the results point to the complex nature of the energy cascade in the jet-array driven facility, with simultaneous upscale and downscale transfers at each instant as well as spatially concurrent interactions across all scales of the flow.