Control of laminar flow separation over a backward-facing rounded ramp with C-D riblets – The effects of riblet height, spacing and yaw angle

•As riblet spacing increases, C-D riblets firstly behave like d-type and then like k-type roughness.•By enhancing momentum transfer, C-D riblet can lead to reduction of separation zone.•The large-scale roll motion is the dominant contributor of momentum transfer.•Maximum strength of secondary flow and reduction of separation zone is obtained at s/h = 4. In the work presented in this paper, a section of convergent-divergent (C-D) riblets is applied upstream of a backward-facing rounded ramp in a fully developed laminar channel flow at a Reynolds number based on the channel height of 400. Numerical simulations are undertaken to examine the effects of riblet geometry and yaw angle on the strength of secondary flow produced by the riblets and the extent of flow separation zone. It is found that, in comparison with the baseline case with no riblets, flow separation is delayed and the reattachment occurs earlier leading to a smaller separation zone around the diverging line. The opposite phenomenon occurs around the converging line. Our results also show that for riblets with a given height a maximum strength of the secondary flow motion upstream of the ramp and a maximum reduction in the spanwise-averaged length of separation are obtained at s/h=4. At a given s/h=4, as the riblet height increases, the strength of the secondary flow motion generated by C-D riblets increases and a minimum riblet height of 3.75% of the channel height is required to produce a net reduction in the spanwise-averaged length of separation. For riblets with a given height and spacing, as yaw angle increases both the strength of secondary flow and net reduction of separation zone exhibit a parabolic trend and both of them peak at γ=45°. Overall, with the riblet setting tested here the benefit of suppressing the laminar separation bubble has not resulted in a net reduction in the total pressure losses compared to that in the baseline case. Nevertheless, an examination of the pressure losses from three different sections along the channel separately has produced some insights about the loss mechanisms and pointed to the possible ways by which the pressure losses could be reduced.