Two Fluid Modeling of Microbubble Turbulent Drag Reduction

An unstructured 3D multiphase CFD method has been adapted and applied for the modeling of high Reynolds number external flows with microbubble drag reduction (MDR). An ensemble averaged multi-field two-fluid baseline differential model is employed. Interfacial dynamics models are incorporated to account for drag, lift, virtual mass and dispersion. Wall kinematic constraints, porous-wall shear apportionment, coalescence, breakup and attendant turbulence attenuation are also accounted for. The results of several high Reynolds number applications are presented, including quasi-1D analysis of an equilibrium bubbly boundary layer, 2D analysis of flat plate flow across a range of gas injection flow rates, and 3D analysis of a notional high lift hydrofoil with MDR. For the flat plate analyses, quantitative comparisons are made with available experimental skin friction measurements, and qualitative comparisons are made with available volume fraction profile measurements. Though some accuracy shortcomings remain, the generally good agreement observed serves to validate the appropriateness of two-fluid modeling in these flows, while elucidating areas where modeling improvements can be made. It is observed that the extraction of turbulent kinetic energy from the liquid phase by the action of bubble breakup can be a significant source of skin friction reduction. Also, the role of mixture density in the boundary layer on wall shear stress is discussed in the context of the homogenous mixture and two-fluid simulations presented.