Active and passive flow stabilization of Mach-4 free jets: A numerical study

Free-jet wind tunnels (FJWT) are widely used for supersonic aerodynamic testing; however, their effectiveness is primarily limited by the formation of conical or diamond-shaped jet cores downstream of the nozzle exit. These flow structures arise from complex interactions between expansion waves, shock reflections, and shear-layer growth, resulting in reduced flow uniformity and a shorter effective test-section length. Although numerous mitigation strategies have been proposed, including pressure matching, geometric modification, and active flow control, their relative effectiveness and underlying physical limitations have not been fully characterized. In this work, a comprehensive numerical investigation is conducted to elucidate the dominant mechanisms governing conical free-jet formation under Mach-4 operating conditions and to assess selected passive, active, and hybrid flow-conditioning strategies within a unified computational framework. Quantitative uniformity metrics are used to enable direct comparison across control approaches under identical operating conditions. The results show that pressure-based and active control strategies can locally alter the shock structure but are highly sensitive to tuning and facility-specific constraints, thereby limiting their robustness and scalability. By contrast, passive geometric modification at the nozzle exit is shown to provide a reliable and energy-neutral means of flow stabilization. Straight cylindrical nozzle-exit extensions delay shock formation, suppress jet contraction, and displace shock–shock interactions downstream of the test section, resulting in a marked increase in the usable uniform-flow region while maintaining Mach number variations within accepted experimental tolerances. These findings provide new physical insight into the limitations of conventional free-jet control strategies and offer practical guidance for improving flow quality in supersonic FJWT.