Spatial resolution issues in rough wall turbulence

Abstract

The question whether spatial resolution effects should be accounted for when performing hot-wire (or particle image velocimetry) measurements in turbulent wall-bounded flows over rough surfaces in the fully rough regime is addressed and answered by exploiting an existing direct numerical simulation database of open-channel flows at Re_tau =500 over a rough surface consisting of randomly distributed roughness elements. The results show sizeable attenuation effects of the streamwise velocity fluctuation intensity, similar to smooth-wall flows with up to 70% for a wire width of around 100 viscous units. A suitable correction scheme is applied to successfully compensate for the spatial resolution effects. Both results indicate that spatial resolution effects are relevant also in fully rough flows and can be corrected a posteriori when the state-of-the-art criteria on sensor size cannot be achieved in experiments.Graphical