Microbubble effect on friction drag reduction in a turbulent boundary layer

摘要

Effects of microbubbles injection on friction drag in a turbulent boundary layer are studied using large-eddy simulation. A flat-plate turbulent boundary layer with a Reynolds number Re-theta = 1430 is generated within a short distance of inflow by introducing disturbances. The parameters investigated include the gravity, the injection height, and the volume fraction of microbubbles. It is found that the mechanism for drag reduction is threefold, that is, buoyancy acting on microbubbles, the change of coherent structures, and hydrodynamic viscosity near the wall. Under different conditions, the dominant mechanism for drag reduction varies among them. With microbubbles injected above the flat plate, the primary mechanism is the lift of coherent structures from the wall and the decrease in the size and vorticity of the streamwise vortices. When microbubble volume fraction increases, the larger buoyancy acting on microbubbles leads to coherent structures shifted further away from the wall, resulting in higher drag reduction. With microbubbles injected beneath the flat plate, the dominant mechanism is a significant decrease in hydrodynamic viscosity as buoyancy pushes microbubbles gathering near the wall and forming a thin air layer. The drag reduction is higher than that of microbubbles injected above the flat plate.