Experimental investigation of the ability of yawed riblet geometries to generate near-wall spanwise flows for control applications*

Widely spaced yawed riblets, with large height-to-spacing ratios, are investigated for their promise to generate spanwise near-wall flows. Such induced flows could potentially be exploited for control or near-wall steering of turbulent boundary layers. These generated spanwise flows are small in magnitude (generally O(U_τ)) and diminish in strength rapidly with increasing wall-normal distance from the riblet crest. To meet this experimental challenge, we use a unique rotating hot-wire probe arrangement, which enables the local mean flow angle to be determined with high accuracy. DNS data from simulations on matched surfaces are used to optimise and validate this measurement approach. Having established this measurement technique, we are able to verify the increased efficacy of yawed widely spaced riblets to generate much stronger spanwise flows as compared to conventional yawed riblet geometries. The required development length over yawed surfaces to fully form these spanwise induced flows is also investigated. This is a crucial first step in understanding the minimum viable streamwise wavelengths that will permit meandering arrangements of these riblets to generate spatially oscillating spanwise near-wall flows for control purposes.