Generating spanwise flow without penalties

Spanwise wall oscillations can reduce drag, but generating these spanwise flows typically comes with penalties, such as increased drag or energy input. Here, we show that yawed riblets can generate spanwise flows near the wall not only without penalties, but also with drag reduction. Toward understanding this phenomenon, we conduct direct numerical simulations and laboratory experiments on turbulent flows over riblets at various yaw angles. Two riblet geometries are compared: conventional riblets with a spacing-to-height ratio of s/k = 2 (s⁺ ≈ 12 –16), showing drag reduction that degrades with increasing yaw angle θ > 0°, and wide riblets (s/k ≈ 9.25 and k⁺ ≈ 8), showing drag reduction that improves with increasing yaw angle up to θ ≈ 15°. Thanks to inertial effects, wide riblets generate spanwise flow up to two to three times stronger than conventional riblets. A decomposition of the mean velocity shift △U⁺ shows that drag change in wide riblets arises not only from effective slip but also from modifications to turbulence, unlike the slip-dominated behavior of conventional riblets. These results highlight novel drag-reduction and flow-steering mechanisms and the potential of wide riblets as passive flow-control surfaces.