Effect of high skewness and kurtosis on turbulent channel flow over irregular rough walls

The skewness of the roughness height distribution is one key topographical parameters that govern effects on wall-bounded turbulence. In this paper mathematical bounds for realisable values and kurtosis are discussed in context irregular multi-scale rough surfaces, which representative typical forms engineering roughness. properties a set surfaces fully covered by features with very high positive negative investigated using direct numerical simulations turbulent channel flow at Re?=395. While an increase function observed moderate line empirical predictions previous results moderately skewed saturates extreme skewness. Overall, effect found to be more sensitive over range compared range. Surface pressure statistics show pits or peaks do not dominate surrounding (‘background’ roughness) retain significant influence. This because, while emerge as approaches values, they tend sparse decreasing their overall impact flow.