Experimental Study of Wall-Induced Cavitation and Acoustic Softening Leading to Choking in Aerospace Fluids

Microbubble nucleation under reduced pressure is a fundamental threat to liquid stability in aerospace fuel and coolant lines, propellant systems, and high-altitude physiological environments. This study presents an integrated in vitro investigation that experimentally quantifies how non-ideal wall features and geometric constrictions govern the onset of decompression-induced cavitation and subsequent flow instability. Using high-speed imaging in both static and dynamic flow configurations, we observed microbubble formation across surfaces of varying roughness (80-400 grit) and through engineered constrictions, including a stent-simulating metallic mesh, using water and aviation fuels (diesel, petrol, and Jet A1). Visual evidence consistently demonstrated that wall roughness and constrictions act as dominant nucleation sites, significantly lowering the effective cavitation threshold and generating dense bubble fields. The resulting increase in local void fraction was shown to cause a dramatic acoustic softening—a catastrophic reduction in the mixture speed of sound, which dropped to 10-100 m/s in extreme cases, as robustly predicted by Wood’s equation. This acoustic softening enables the bulk liquid velocity to attain the sonic condition, leading to Sanal Flow Choking. The work provides an empirical demonstration of a critical cascade: Wall Friction initiates Cavitation, which leads to Acoustic Softening, and culminates in Compressible Flow Instability. This extends classical theory by confirming that wall-driven bubble inception can induce compressible-flow dynamics even in systems traditionally considered incompressible.