Active grid excitation for tailored turbulence generation

Wind tunnel experiments are essential to understand flow phenomena or investigate new aerodynamic concepts. Especially for investigations of the interaction of wind turbines with the flow it is crucial to create realistic turbulence in the wind tunnel. To do so active grids are used, which allow to modulate the flow velocity and hence to reproduce the stochastics of atmospheric turbulence on a wide range of time scales. Herein a simple approach is shown to create various turbulent wind fields with different stochastic behaviours, on e.g. one-point and increment probability density functions and power spectral density. In this approach the excitation of the individual active grids shafts is defined by a Langevin time series. By varying the drift and diffusion coefficient of the Langevin time series the stochastics of the flow can be varied. In combination with a characterisation of the active grid behaviour a prediction of the flows one point and increment statistics on the computer is possible. This allows for an easy a priori tailoring of different turbulent wind fields for wind tunnel experiments.