Global Friction of Uniform vs. Heterogeneous Sandpaper Roughness

Skin-friction measurements of streamwise aligned sandpaper strips are presented in Nikuradse-type di-agrams obtained in a dedicated experimental facility.
These novel results shed light onto the global friction drag of heterogeneous rough surfaces. They indicate the drag-increasing presence of secondary motions known
to exist over spanwise heterogeneous roughness. The Reynolds number dependency of the skin-friction coef-ficient appears to be a superposition of the smooth wall and homogeneous rough wall behavior. The results sug-gest that the present heterogeneous rough surface does not reach a fully rough behavior in which the friction co-efficient becomes independent of Reynolds number.