DRAG REDUCTION BY GAS INJECTION THROUGH DISCRETE ORIFICES UNDERNEATH A FLAT PLATE

Active gas injection through discrete orifices in a flat plate modifies the near-wall flow and can reduce hydrodynamic drag. This study combines experiments and numerical simulations to investigate the drag reduction characteristics at a fixed free-stream velocity and varying gas flow rates. Experiments are conducted in a cavitation water tunnel, with drag measured using a six-component balance. Numerical simulations couple the volume-of-fluid (VOF) method with large-eddy simulation (LES). Both experimental and numerical results show that, over the range of gas flow rates investigated, increasing the gas flow rate expands the cavity-covered area and promotes the coalescence of discrete gas strips into a continuous gas layer. The skin-friction coefficient is lower than that without gas injection in all cases considered, but does not decrease monotonically with increasing gas flow rate, exhibiting a slight increase at Q = 20 L/min. This behaviour is closely associated with cavity morphology and the extent of coalescence. Cavity-covered regions exhibit low wall shear stress, thereby reducing skin friction. In contrast, cavity expansion constricts the liquid passages between adjacent cavities, increasing local velocity gradients and wall shear stress and thus contributing to drag. Before complete cavity coalescence, the net drag reduction is therefore governed by the competition between reduced skin friction beneath the cavities and increased skin friction in the intervening liquid gaps.