Effects of riblets on transitional characteristics of a boundary layer

Abstract

Experimental study has been carried out using a single hot-wire to investigate the effects of riblets on the transition characteristics of a naturally developing low-speed boundary layer. The detailed measurements were carried out across the boundary layer at several streamwise locations for the smooth and riblet surfaces. The semicircular riblets have height h = 1.0 mm and spacing s = 2.2 mm and corresponds to 8 and 18 respectively when normalized with wall variables corresponding to freestream test velocity of 2.8 m/s. The results indicate that the presence of riblets hastens the onset of transition. However, the development of turbulence is slower and the length of the transition is slightly increased in case of riblet flow. At the same turbulence development stage, there are no significant differences in the normal distribution of turbulence intermittency factor over the two surfaces. The frequency of occurrence of turbulence spots is decreased over the riblets in the early stage of transition, and turbulence spot is found to maintain itself for a longer duration over a riblet surface.