Experimental study of influence of twin-jet interaction on over-under combined nozzle performance

Combined cycle propulsion shows great potential for wide speed and altitude range flights. However, the jets of different types of engines may interact and form complex flow structures. Given that the specific effects of jets interaction are closely related to combined nozzle configuration and working conditions, this paper analyzes the experiments of a simplified over-under combined nozzle and attempts to summarize the principles of the influence of twin-jet interaction on over-under combined nozzle performance. Firstly, twin-jet interaction directly changes the combined nozzle gross force via changing flow structure and parameters distribution inside the control volume. For example, the merging of supersonic jets forms wave systems, and the injection effect changes local pressure and the position of Free Shock Separation (FSS). Secondly, twin-jet interaction changes the force counting system. Assuming each flow-path as an isolated control volume leads to a mismatch of local pressure and reference ambient pressure. Thus, the combined nozzle should be considered as a whole control volume. Thirdly, twin-jet interaction may couple with jet-external interaction. On the one hand, jet-external interaction forms additional wave systems. On the other hand, the original expansion state of the nozzle during independent operation alters the performance baseline. Three practical experiments are conducted to verify these principles. These principles indicate that the influence of twin-jet interaction should be carefully considered in combined nozzle design. Utilizing the beneficial effect of twin-jet interaction can increase both the thrust and the lift, making the overall performance of the combined nozzle superior to the sum of two individual nozzles. However, twin-jet interaction may also decrease wall pressure, or alter the FSS position and flow direction of the over-expanded jet, resulting in a decrease in overall performance. These potential adverse effects should be avoided.