An experimental study on riblet-induced spanwise vortices in turbulent boundary layers

Coherent structure is closely related to the high frictional resistance in wall turbulence.Summarizing the influence rule of vertical micro-grooves on spanwise vortex is helpful for understanding the drag reduction mechanism by riblet surface.15998 instantaneous velocity fields over both riblet (s⁺=2h⁺=16.3) and smooth surfaces in turbulent boundary layers (TBL) at a Reynolds number of Re_τ=190 were acquired by using time-resolved particle image velocimetry (TRPIV) in a water tunnel.The swirling strength λ_ci was used to reveal spanwise vortices, and we extracted the cores of spanwise vortices by finding swirling strength's local extremum in whole field.Subsequently, the proportion of the spanwise vortex, the average swirl strength, the average diameter of the vortex and the proportion of the each scale vortex were analyzed statistically in flow field over riblet surface.All experimental results were compared with those over a smooth surface.It is found that (1) the number of prograde spanwise vortices is decreased by riblet in near wall region, while retrograde spanwise vortices'number is increased.(2) The average strength of the spanwise vortex is decreased, whether prograde vortex or retrograde vortex.(3) The proportion of small scale prograde vortices and mesoscale retrograde vortices is increased in near wall region, while mesoscale prograde vortices and large scale retrograde vortices'proportion is decreased.(4) The difference in stream-normal scale of the prograde spanwise vortex becomes smaller in inner boundary, but the retrograde spanwise vortex's scale difference without change.(5) The proportion of small scale prograde vortices is decreased in log-law region, while the proportion of large scale prograde vortices is increased.Impact on the proportion of retrograde vortices is exactly opposite of the prograde vortex.