Effects of plume impingements of clustered nozzles on the surface skin friction

Skin-friction values on the surfaces of clustered nozzles were measured using a pointwise local skin-friction sensor. This sensor has a floating head that can detect the local shear force using the flow passing over the head surface. Coldflow wind-tunnel experiments were carried out with clustered nozzle models. The test models had three or four clustered cell nozzles, or a nonclustered two-dimensional nozzle with a designed Mach number of 3.5, followed by a straight planar section. The experimental conditions were chosen according to the cell nozzle jet expansions. Hence, some conditions involved an oblique shock wave impinging on the bottom surface and others did not. The measured local skin-friction coefficients were compared to those predicted by using an existing prediction model. The results showed that the local skin-friction value significantly increased when the cell nozzle expansion was under the overexpansion condition, whereas the measured values were identical to those predicted by the existing model under the underexpansion condition. The physics behind this observation showed that the expansion waves hardly affected the skin friction, but the oblique shock wave and crossing neighboring jet boundaries could increase the skin friction.