LOAD OSCILLATIONS CAUSED BY UNSTART OF HYPERSONIC WIND TUNNELS AND ENGINES

Large-amplitude load oscillations were observed during the tests of a hypersonic engine model in a freejet-type wind tunnel. To clarify the cause of the oscillations and their characteristics, oscillating wall pressures and loads on a drag model and engine models were investigate d. Flowfield was observed by shadowgraph to determine the cause of the large starting loads. Power spectral density functions and probabilit y functions of wall pressures and loads were derived by the fast Fouri er transform. The amplitude of the unsteady frontal pressure was corre lated with the dynamic pressure. The magnitude of the starting load wa s related to the drag coefficient of the models, and the expected maxi mum peak loads of a large-scale ramjet engine test facility were evalu ated. Engine unstart loads were also simulated by means of secondary f low injection into a small-scale model of a ramjet engine. With these methods, characteristics of engine unstart loads and the possibility o f sensing engine unstart in its early phase were studied. Engine unsta rt could be sensed with pressure measurement around the engine throat before it became severe. Furthermore, engine unstart loads associated with scramjet engine combustion were related to the drag coefficient o f the engine.