The friction coefficient of heterogeneously rough surfaces
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The friction coefficient of heterogeneously rough technical surfaces exposed to flow is of interest, but its prediction remains challenging. Existing suggestions for prediction tools are based on averaging of smooth and rough values, but reference data is needed for validation. To widen the data base, an extensive experimental campaign is carried out investigating spanwise alternating smooth and rough strips. The portion of the surface covered with sandpaper is kept constant for all configurations, whilst the strip width s is varied systematically in relation to the half channel height δ. The pressure drop in a channel flow facility is employed to obtain the global friction coefficient of the surfaces over a wide Reynolds number range. The results are presented in a Nikuradse-type diagram showing the friction coefficient as a function of Reynolds number. Differences and commonalities are figured out based on the curves resulting in the conclusion that different flow mechanisms must be present in the different Reynolds number regimes. The conference contribution will feature results from hot wire anemometry and DNS on selected cases to shed more light on the different mechanisms.