Effect of the Riblet in the Transitional Flow Region

In this study, direct numerical simulations are performed to reveal the effect of the riblet on the transition location and the viscous drag in the transitional flow region. It is found that due to the tendency of the riblet to be more affected by the partial derivatives of instantaneous pressure in the spanwise direction, the turbulent intensity in the spanwise direction increases more upstream than on the smooth surface, resulting in promoting turbulent transition via TS instability a little. It is also found that the riblet increases the viscous drag in the transitional flow region because the coherent structure of vortices is more likely to be formed by TS waves on the riblet surface compared to the smooth surface in that region.