URANS Study of Air-Layer Drag Reduction in a High- Reynolds-Number Flat-Plate Turbulent Boundary Layer

The air layer formation in a high Reynolds-number flat plate turbulent boundary layer is simulated using a two-phase sharp interface Cartesian grid solver. The interface is tracked by a coupled level set and volume-of-fluid method (CLSVOF) and turbulence is modeled by a Spalart-Allmaras (SA) turbulence model with a wall function (WF) approach. The air layer along the entire test plate is successfully achieved and the drag reduction is approximately 100%, which agrees very well with the experimental findings. With reduced air flow rate, bubble drag reduction (BDR) is also observed; the computational results also qualitatively match the experiments. The transitional region from BDR to ALDR is also observed in the present simulation. However, the critical air flow rate to form the ALDR is lower in the simulations than in the experiments. Several possible reasons are likely accounting for the low critical air flow rate in the simulations, such as SA-WF turbulence model, three-dimensional instability and surface tension effects. The critical air flow rate does not change much with grid refinement.