Reynolds number effects on secondary flows over ridge-type surfaces

Secondary flows can be generated by imposing spatial shear stress anisotropy using spanwise heterogeneous surfaces, commonly divided into ridge-type surfaces (spanwise variation in height) and strip-type surfaces (spanwise variation in skin friction). Several studies have looked into the flow structure that determine the nature and strength of the generated secondary currents over both of these surface types. However, information on the variation of global friction as well as the nature of these secondary flows across a range of Reynolds numbers is scarce.

In this study we investigate secondary flows generated by a ridge-type surface in a L ≈ 130H long water channel facility with a rectangular cross-section W/H ≈ 8. The ridge width Wp ≈ 0.8δ, the spanwise centre-to-centre spacing S ≈ 1.6δ, and the ridge height h/δ ≈ 0.1, where δ = H/2 is the channel half-height. The channel centerline pressure gradient is measured starting from L ≈ 65H to the channel outlet. Stereoscopic PIV in a spanwise—wall-normal plane L ≈ 110H downstream of the channel inlet is used to obtain the three-component velocity field. In this talk we present results of skin-friction against bulk Reynolds number and explore the structure of the produced secondary flows over a range of bulk Reynolds numbers.