Optimizing the performance of an active grid to generate high intensity isotropic free stream turbulence

Abstract

We present an optimized procedure for generating isotropic, homogeneous, high intensity turbulent flows behind an active grid. The double random protocol (R. E. G. Poorte, “On the motion of bubbles in active grid generated turbulent flows,” Ph.D. thesis, University of Twente, 1998 and subsequent works) used for operating the active grid [H. Makita, “Realization of a large-scale turbulence field in a small wind tunnel,” Fluid Dyn. Res. 8, 53 (1991)] is optimized based on the tip speed ratio α ≡Vtip⟨U⟩=2πΩC⟨U⟩, where Vtip is the speed of the active grid winglet at its tip, 2C is the winglet chord length, Ω is the mean winglet rotational speed, and U is the mean flow speed. A systematic assessment of the resulting turbulent flow is carried out using both hot-wire anemometry (single point and two point) and particle image velocimetry (PIV). It is shown that just using the double random protocol does not guarantee an isotropic flow, and the tip speed ratio of the active grid elements is also important to achieve isotropy. Too high a tip speed ratio, however, results in decreased turbulence intensities. Optimal values of the tip speed ratio, given the trade-off between isotropy and turbulence intensity, are given for the double random protocol. Furthermore, our measurements clearly show that conclusions about the structure and isotropy of active-grid turbulence cannot be based on single-point measurements alone; at the least, two-point or multi-point PIV measurements are essential to evaluate the performance of active grids. This is supported by observing the behavior of longitudinal and transverse correlation functions along the arbitrary directions and the convergence of proper orthogonal decomposition reconstructions to the true value. Interestingly, a simpler test based on the ratio of longitudinal to transverse turbulence intensities appears more robust than the one based on single-point correlations.