Flow resistance over heterogeneous roughness made of spanwise-alternating sandpaper strips

The present work provides novel reference data for spanwise-alternating sandpaper strips as one idealized case of a heterogeneous rough surface. Experimental data are presented and analysed in direct comparison with drag measurements of homogeneous sandpaper surfaces and numerical simulations.

Based on the homogeneous roughness data the related challenges and sensitivities for the evaluation of roughness functions from experiments and simulations are discussed. A hydraulic channel height is suggested as an alternative measure for the drag impact of rough surfaces in internal flows. For the investigated heterogeneous roughness it is found that turbulent flow does not exhibit a fully-rough flow behaviour, indicating that the assignment of an equivalent sand grain height as commonly applied for homogeneous roughness is not possible.