Abstract
The influence of the orientation of ratchet-type rough surfaces on their fluid dynamic roughness effect is investigated using direct numerical simulations of turbulent channel flow at Re_{tau }=395. The ratchet length-to-height ratio is varied from ell /k=2 to 16 for a fixed ratchet height of k/delta =0.1 where delta is the mean channel half-height. The results show that both roughness function and mean flow and turbulence statistics strongly depend on the ratchet orientation. Existing empirical formulae, which estimate the roughness function Delta U^+ or the equivalent sand-grain roughness k_s based on surface-slope related parameters such as the effective slope or the Sigal-Danberg parameter, fail to accurately predict the differences between ratchet surfaces with high windward slopes and ratchet surfaces with high leeward slopes.
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