On the Combined Influence of Wall Slip and External Forcing in Turbulent Drag Reduction

Turbulent drag reduction remains a critical challenge in wall-bounded flows despite numerical attempts with various control methods. In this study, we use direct numerical simulations to examine a composite drag-reduction strategy that combines an external forcing with wall slip in turbulent channel flow. While an external forcing alone can reduce drag by up to 26% under optimal forcing parameters such as its amplitude and frequency, it can also lead to drag enhancement at lower frequencies. Wall slip, in contrast, provides consistent drag reduction across the tested conditions. When applied together, the two methods appear to act cooperatively, yielding drag reduction up to 50%—surpassing that of the individual approaches. To further analyze the combined effect, the Reynolds shear stress is analyzed to assess changes across the inner and outer flow regions. Results show that the external forcing primarily alters the outer region, while the wall slip modifies the near-wall turbulence, leading to a redistribution of momentum transport. The composite configuration not only enhances drag reduction but also eliminates the drag enhancement observed at low forcing frequencies in the external-forcing-only case. These findings highlight the potential of combining control strategies that target different flow regions to achieve better performance in turbulent drag reduction.