The effect of surface skewness on the super/postcritical coefficient of drag of roughened cylinders

Surface skewness (Rsk) is a further roughness parameter in addition to maximal (Rz) and average (Ra) roughness. So far, the aerodynamics and flow transition of spherical and cylindrical bodies were tested at different Rz and constant Rsk or variable Rz and variable Rsk. The purpose of this study is to test surfaces of different Rsk and same Rz. Eight surfaces of constant relative roughness of 0.9% and variable Rsk from –5 to + 5 including infinite skew (smooth surface) were rapid prototyped and wrapped around a cylinder. The drag force of the cylinder was recorded in a wind tunnel at speeds from 20-140 kph at 10 kph increments. The larger Rsk, the “rougher” is the surface profile, i.e. the larger is the postcritical coefficient of drag, and the smaller is the critical Reynolds number.