Numerical Investigation of Flow Transition Behavior in Cold Flow Dual-Bell Rocket Nozzles

The dual-bell nozzle is an altitude-adaptive nozzle concept. It combines the advantages of a nozzle with small area ratio under sea-level conditions and a large area ratio nozzle under high-altitude conditions. Reynolds-averaged Navier-Stokes and unsteady Reynolds-averaged Navier-Stokes simulations on two-dimensional axisymmetric grids were conducted at DLR, German Aerospace Center in Lampoldshausen to investigate the transition from one mode to the other of a dual-bell nozzle model with positive pressure gradient extension. A cold flow test campaign conducted at DLR's cold flow test facility P6.2 provided validation data for the numerical approach. The present study investigates the influence of different turbulence models and feeding pressure gradients on the dual-bell flow transition behavior. Better results were achieved for the Spalart-Allmaras and Reynolds stress turbulence model. A clear impact of the feeding pressure ramp on the dual-bell transition pressure ratio and the flow separation position velocity was shown. The transition nozzle pressure ratio was predicted with an accuracy of 1%. For the hysteresis between transition and retransition nozzle pressure ratio, an accuracy of approximately 10% was reached. The calculated values of the experimental and numerical transition duration were on the same order of magnitude.