Multi-scale generation of turbulence with fractal grids and an active grid

Turbulence plays an important role in our everyday life, yet it is still not well understood. Wind tunnel experiments can help to develop generalized descriptions of turbulent flows. However, creating turbulent flows with suitable characteristics for various experiments is still challenging. In this work, fractal and active grids were used to generate multi-scale turbulent flows. Using hot-wire measurements we investigated the influence of different boundary conditions, bar sizes and solidity for fractal grids. We found that the evolution of the flow generated by a fractal grid does not depend on the considered boundary conditions. An alternative to these rigid structured grids is an active grid, which allows for a dynamical generation of flow fields with comparable properties. Experiments were conducted with an active grid in which the distribution of the local solidity was actively changed. A transition between classical and fractal grid type decaying turbulence depending on the active grid excitation protocol was observed. We conclude that the distribution of the local solidity of these grids has a strong influence on the evolution of the generated turbulent flow.