Shock-Driven Droplet Breakup Under Unsteady Acceleration

Liquid droplets in the atmosphere pose a significant threat to aircraft traveling at hypersonic velocities. Droplet impacts at these speeds can destroy the surface of the aircraft, resulting in excessive heating and drag. The bow shock in front of the aircraft rapidly accelerates the droplets before they collide with the vehicle, rapidly deforming and breaking them apart into a cloud of child drops. In contrast to the widely-studied constant-acceleration (constant Weber number) breakup regimes, droplets in a hypersonic bow shock experience a rapidly changing acceleration (changing Weber number) during the breakup process due to the complex system of shock and expansion waves found in the bow shock. As a result, it is essential to study unsteady acceleration droplet breakup to further develop next-generation aircraft. Shock-tube experiments were conducted at the Fluid Mixing at Extreme Conditions Lab (FMECL) to study variable acceleration shock-droplet interactions. Millimeter-scale droplets are broken apart by a shock-expansion wave system with an initial Mach number of 1.5. Deformation and breakup behavior is captured using high-speed Laser-Induced-Fluorescence (LIF) imaging to extract breakup time, breakup mode, and deformation rate. In order to accurately quantify the acceleration history of the droplets, high-speed low-density Particle-Imaging-Velocimetry (PIV) was performed to verify the acceleration history present inside of the shock tube. The results are compared to existing analytical models.