Turbulent drag reduction for rough wall boundary layers by spanwise wall oscillations*

Spanwise surface/wall oscillation has proven to be a successful active flow control technique for reducing turbulent boundary layer drag. Marusic et al.¹ recently demonstrated that this technique could yield drag reduction at high Reynolds numbers via two possible strategies: (i) the well-known inner-scale actuation strategy, targeting the viscosity dominated motions, and (ii) the novel outer-scale actuation strategy, targeting the inertia-dominated motions. However, these strategies have only been tested for smooth wall boundary layers. This study investigates the efficacy of both these strategies for transitionally and fully rough wall boundary layers. Emphasis is placed on comparing the performance of the inner-scale actuation strategy for smooth and fully rough wall boundary layers, considering the near-wall viscous cycle is severely disrupted in the latter. For this, we consider low-Reynolds-number direct numerical simulations, and high-Reynolds-number drag measurements over smooth and rough wall flows, imposed with spanwise surface oscillations at matched actuation parameters.

¹Marusic, I., et al., (2021) An energy-efficient pathway to turbulent drag reduction. Nat. Commun., 12, 5805.