The Beauty of Active Grids and Their Infinite Possibilities of Turbulence Generation

Wind tunnel experiments are of major importance for fundamental turbulence research as well as investigation of objects in realistic reproducible conditions. Active grids are used to increase the possibilities of generating turbulent flows. There are several approaches to generate turbulent flows, as different needs require different excitation approaches and different grid designs. In this study, we focus on the needs of future wind turbine studies, which require a tailored turbulence intensity gradient. Experiments are conducted in the closed test section of the large wind tunnel in Oldenburg. A new half active grid is built, which consist of a classic active grid at the lower half and a completely empty upper half. In this way the new half active grid enables to generate inhomogeneous flows with strongly varying turbulence parameters. A reduction of the turbulence intensity with height, as present in the atmospheric boundary layer, can be generated. There are already many different methods to generate turbulent flows with active grids. However, there is still a wide range of possibilities to excite different turbulent flow properties and to further adapt the flows, either by adapting the shaft motion or by adapting the active grid design.