Turbulent flow and heat transfer in channels with shark skin surfaces: Entropy generation and its physical significance

In order to understand the mechanisms by which drag is reduced and heat transfer is affected when a wall is covered by a shark skin like texture the local entropy generation rate was determined and compared to that for a flow over smooth walls. Based on direct numerical simulations (DNSs) details of the turbulence close to the wall could be analyzed and lead to the conclusion that basically certain turbulent structures are lifted off the wall and get rearranged, a mechanism we call lift off and alignment (LOA). The distribution of (time mean) entropy generation rates supports this interpretation, showing a reduction next to the wall and an increase further away, however, such that the overall effect leads to a drag reduction. By solving the thermal energy equation it is shown that also heat transfer is reduced thus increasing the thermal body protection.