Friction velocity and virtual origin estimates for mean velocity profiles above smooth and triangular riblet surfaces

Abstract

It is well known that riblet surfaces influence only the region close to the wall (Hooshmand et al. 1983). The appropriate velocity scaling for this region is the friction velocity (u _*). Thus it is important to determine u _* very accurately. For situations where no drag balance is available various methods have been developed to estimate u _* from the mean streamwise velocity profile. Generally, the logarithmic friction law (Tennekes and Lumley, 1972) is used, better known as the log-law or the law of the wall method. However, most of the methods can only be used for surfaces where the y = 0 position is exactly known, i.e. for smooth surfaces. Choi (1989) developed a method for riblet surfaces to determine from the mean streamwise velocity profile both the friction velocity and the virtual origin of the riblets into the turbulent boundary layer. In that same year, Bechert and Bartenwerfer (1989) published a theoretical model, developed for riblet surfaces to calculate the protrusion height.