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Suppression of Large Scale Structures in a Turbulent von Karman Flow using Random Forcing

Hanna Berning
Institute of Fluid Dynamics ETH Zurich 8092 Zurich, Switzerland

Tim Grunberg
Institute of Fluid Dynamics ETH Zurich 8092 Zurich, Switzerland

Thomas Rosgen
Institute of Fluid Dynamics, ETH Zurich, CH-8092 Zurich, Switzerland


Low wavenumber instabilities in a von Karman flow cell are significantly reduced in an experiment by applying a variable forcing. By reversing the disc rotation sense pseudo-randomly or regularly, only the poloidal inward pumping remains at sufficiently high change rates. We investigate the influence of the forcing on the macroscopic scale with ultrasound Doppler measurements along a line in the transversal plane of the cell. The suppression of slow, persistent instabilities throughout the cell results in a cut-off of low wavenumber modes in the velocity spectrum and decreasing characteristic autocorrelation times. Turbulence properties are further examined in the center of the cell by temporally and spatially resolved Lagrangian ultrasound particle tracking. Turbulence features such as peak height of the compensated second order structure functions or higher order moments of the velocity fluctuations remain as expected for a fully turbulent flow. With only the poloidal base flow mode remaining, this forcing method allows to investigate turbulence phenomena in the center of the cell without superimposed effects.