Investigation of turbulence production and dissipation due to isotropic and anisotropic roughness components on real surfaces

Roughness generally consists of structures that are either oriented anisotropic in directions tangential to the surface or isotropic, or a superposition of both components. Interactions between the roughness elements exert a significant influence on the fluid mechanical losses. Cost-effective maintenance of the functionality of the surfaces of aerodynamically relevant components such as blades requires the quantitative prediction of the influence on the flow, which can be achieved through Reynolds-Averaged-Navier-Stokes Simulations (RANS). An established roughness parameter used to model the influence on the flow is the equivalent sand grain roughness ks. By contrast, the research presented here employs Direct Numerical Simulations (DNS) with Immersed Boundary Method (IBM) of channel flows over anisotropic, isotropic, and superimposed surfaces in order to investigate the aerodynamic losses, for example, due to turbulent production and dissipation. The simulation results show that the equivalent sand grain roughness does not correctly predict flow losses from anisotropic and superimposed surfaces, because in reality, the “angle of attack” with respect to the anisotropic structures changes the turbulence due to altered turbulent production and dissipation. A non-linear relationship between the flow resistance and this angle of attack is a result of local changes in pressure gradients.