Characterization of the flow over periodic hills with advanced measurement and evaluation techniques

Due to the complex nature of turbulence, the simulation of turbulent flows is still challenging and numerical models have to be further improved. For the validation of these numerical flow simulation methods, reliable experimental data is necessary. A typical test case is the flow over periodic hills. The numerical prediction is difficult, since flow separation and reattachment are not fixed in space and time due to the smooth geometry (Temmerman et al., 2003; Fröhlich et al., 2005). Furthermore, the separated and fully three-dimensional flow from the previous hill impinges on the next hill, which results in very complex flow features. With the increasing computer performance available, it becomes possible to examine larger Reynolds numbers with DNS and LES. Typical grid sizes are in the order of several (3-10) Kolmogorov length scales h for LES and approach h for DNS (Breuer et al., 2009). The resolution of currently available measurements is in the order of 30 h (Re = 8, 000) and above which is not sufficient to resolve the large gradients in the shear layer at the hill crest. Even more severe, the contribution of the small eddies is averaged over a region associated with the measurement resolution. Thus an important part of the turbulent energy cannot be measured and is lost for the validation of turbulence models. Since these models are supposed to simulate the contribution of these small eddies it is of inherent interest to increase the resolution in the experiment. The aim of the current measurement campaign was therefore to increases the spatial resolution in order to provide a new data set for the validation of numerical tools.