Experimental assessment of nozzle flow unsteadiness through optical techniques

This study presents a comparative experimental investigation of two key flow unsteadiness phenomena in an overexpanded thrust-optimized parabolic (TOP) rocket nozzle: the transition from Free Shock Separation (FSS) to Restricted Shock Separation (RSS) and the onset of the End Effect Regime (EER). Using a combination of high-speed Schlieren imaging, oil film visualization, and synchronized wall pressure measurements, the study explores the unsteady dynamics of flow separation and reattachment at varying Nozzle Pressure Ratios (NPR). Higher NPR visualization of cap shock is also showed and presented. A new truncated TOP nozzle was designed and tested in a cold-flow nozzle test rig at Delft University of Technology. The truncation influences flow behavior, leading to the onset of EER at lower NPR values (∼26) compared to a non-truncated TOP nozzle. This experimental setup allowed for a detailed analysis of flow unsteadiness associated with EER and the FSS-RSS transition. Pressure measurements confirm that the FSS-RSS transition occurs at an NPR of approximately 22, with a sudden reattachment of the FSS- induced shock to the nozzle wall. This results in strong pressure fluctuations, asymmetry-driven side loads, and the characteristic hysteresis effects between startup and shutdown conditions. At higher NPR, the End Effect Regime (EER) manifests as a global unsteady pulsation of the most downstream separation bubble, driven by external flow interactions at the nozzle lip and intermittent flow oscillations, but without hysteresis. As the reattachment point progresses toward the nozzle exit, the flow exhibits increasing unsteadiness. Comparative CFD simulations using RANS-based models successfully capture key flow structures and show strong coherence with experimental pressure data and internal nozzle flow behavior. These findings provide valuable insights into shock-induced unsteadiness in Thrust Optimized nozzles, contributing to the design optimization of high-thrust propulsion systems, where understanding and mitigating flow separation and side loads are critical for structural integrity and performance.