High Re_λTurbulence in a von Kármán device

We investigate turbulent flow in a von Kármán apparatus driven by two independently rotating D = 0.5 m rotors inside a 0.8 m-high octagonal tank. Planar particle image velocimetry measurements are performed in the mid-vertical plane for rotation rate, f = [0.7-3.1] rps, and rotation rate ratios Γ_Ω = Ω_b/Ω_t = [0,−1], where Ω = 2pf. We use two modes of operation - the base mode (BM), wherein the rotors have the same frequency (f_t = f_b and Γ_Ω = −1), and the differential mode (DM) wherein they have different frequencies such that (f_t > f_b). Multiple runs, each consisting of over 50,000 images, are acquired in each mode for better convergence. The BM is characterized by the symmetric recirculation regions separated by a stagnation zone formed near the center of the device. In the DM, the flow symmetry is broken, and the stagnation zone location depends on the ratio f_b/f_t. As Γ_Ω tends from 0 to -1, the flow transitions from an asymmetric jet-dominated regime to a symmetric state seen in BM. The variation of rms velocities u_rms and v_rms, along the horizontal and vertical central lines is in line with the mean flow conditions. In the central region the ratio u_rms/v_rms that characterizes anisotropy is about 1.5 for BM and is in the range 0.8-1.8 for DM depending on Γ_Ω. The Reynolds stress 〈uv〉 plotted for the DM clearly shows a formation of shear layer at the boundary of the jet-plume observed with lower values of Γ_Ω. We compute the characteristic Reynolds number based on Taylor microscale, Re_λ = ((20/3) Re_L)^0.5 ~ 900-2000, taking Re_L = k^0.5D/ν. The device provides suitable conditions for evaluation of high-Re turbulence dynamics.