Correlation of frictional drag and roughness length scale for transitionally and fully rough turbulent boundary layers

Abstract This study investigates zero-pressure-gradient turbulent boundary layers for several rough surfaces in the transitionally rough and fully rough flow regimes. The tested surfaces include, but are not limited to marine antifoulings as irregularly rough engineering surfaces. The boundary layer profiles were measured by using a two-dimensional Laser Doppler Velocimetry system. The coatings were applied with different procedures to simulate the effect of different application types. An attempt was made to find a new roughness length scale which provides a good correlation to represent the roughness functions for both transitionally and fully rough flow regimes. Surface roughness measurements with a laser profilometer device were carried out to determine several roughness parameters to be used in the calculation of the roughness length scale. Different roughness calculation methods, with varying low-pass filter window lengths and sampling lengths, were applied to determine their effect on the roughness parameters and hence the roughness function correlations. The paper presents a new definition for the roughness length scale and covers the details of the measurements, analyses, discussions regarding the differences between the surfaces, the effect of the roughness calculation methods and application type of the antifouling coatings.