Experimental and Numerical Investigation of a Supersonic Convergent-Divergent Nozzle

Experiments and large eddy simulations of a conical convergent divergent nozzle, similar to the nozzles employed on high-performance tactical jets, have been carried out. The nozzle has been studied at a slightly overexpanded condition with a nozzle pressure ratio of 4.0 (a design nozzle pressure ratio 4.1). The primary nozzle is surrounded by a secondary jet operated at three different conditions: Mach 0.1, 0.3, and 0.8. This secondary jet provides a rough forward-flight simulation over a limited axial range. It has been found that the shock pattern in the jet plume is directly influenced by the secondary jet Mach number due to the outer geometric shape of the nozzle. As the flow follows the boat-tail, it locally increases the pressure, making the nozzle operate in a more overexpanded condition. Large eddy simulations of a primary jet with forward flight fully simulated are in very good agreement with the experiments regarding both flow and acoustics. A numerical investigation of screech mode character for the case with the lowest secondary flow Mach number has been made. This shows that the screech mode is mainly built up by two counter-rotating modes with a tangential mode number of I. In the numerical simulations, additional flow-adaptive dissipation has been added in order to stabilize the solution around the shocks. The effect on the flow and acoustics is found to be very small if a shock sensor combined with a damping threshold is used.