Computational Analysis of High-Altitude Test Facilities

During ground testing campaigns of liquid rocket engines designed for altitude operations, high-altitude test facilities are used to avoid strongly overexpanded nozzle flow, which can cause shock waves and flow separation, complicating thrust measurement and thermo-structural integrity assessments. These systems simulate high-altitude conditions by creating a vacuum-like environment around the nozzle exit, generally employing a second-throat ejector-diffuser to manage supersonic recompression and flow separation stablization in a secondary throat. The present study assesses the capability of a computational model based on Reynolds-Averaged Navier-Stokes (RANS) equations to simulate various second-throat ejector-diffuser setups and compares results with experimental data. The findings demonstrate that the model predicts reasonably well steady-state flow behavior of these facilities, representing a useful and lightweight tool to guide design optimization. Moreover, unsteady simulations offer insights into vacuum generation dynamics, focusing on the complex unsteadiness of recirculation bubbles and oblique shock waves characterizing the system startup.