Decrement of spanwise vortices by a drag-reducing riblet surface

Abstract

In this paper, the spanwise vortices in a turbulent flow over drag-reducing V-shaped riblet grooves have been investigated experimentally. The height and lateral spacing of each V-shaped riblet groove are 176.8 m and 300 m, respectively. A high-resolution PIV system was employed to measure the instantaneous velocity fields of the turbulent boundary layer developed over the riblet surface under a drag-reducing condition of s(+) = 10.4. The contours of the spanwise vortices which show a locally swirling motion were derived by using a vortex-identification technique. After extracting the cores of the spanwise vortices from instantaneous PIV velocity fields, the number distributions of the vortex cores were analyzed statistically. In addition, the spatial distributions of mean velocity, turbulence intensity and Reynolds stress were also obtained. All experimental results were compared with those over a flat smooth surface. As a result, the number distributions of the small-scale spanwise vortices were significantly decreased on the riblet surface. The turbulence intensity and Reynolds stress were also reduced by the drag-reducing effect of the riblet grooves. The decrease of small-scale vortices observed in this study indicates the reduction of small-scale coherent structures in the boundary layer.