How to generate turbulence with highest Reynolds numbers in the wind tunnel

In order to study objects like buildings, vehicles or wind turbines under the influence of wind fluctuations, the generation of laboratory flows that resemble atmospheric turbulence is of prime importance. This is where active grids come into play, allowing to excite the wind tunnel flow in a user-defined way. With a blockage induced flow design, it is possible to recreate atmospheric flows through their time series or to create single coherent structures such as gusts defined by industrial standards. In addition, it is possible to generate turbulence with large integral length scales through a random driving that follows a stochastic process. Velocity fluctuations with correlation lengths and thus integral scales much larger than the transverse dimensions of the wind tunnel can be generated. By combining active grid excitation with fan speed modulation, it is additionally possible to generate a flow characterized by an inertial range of four decades and an integral Reynolds number of 2×10⁷. By a newly developed active grid it is furthermore possible to vary the turbulent properties over height to mimic height dependencies found in the atmosphere and also to generate a turbulent non-turbulent interface.