Experiments on Droplet Breakup Under Unsteady Accelerations*

At hypersonic velocities, atmospheric droplets pose a significant threat to vehicles in the air, thus restricting the use of hypersonic aircraft. Droplets interacting with hypersonic vehicles can erode the aircraft's exterior leading to performance deterioration. In order to develop high-performance hypersonic vehicles, it is essential to understand how droplets break apart upon interaction with hypersonic bow shocks. While the majority of research efforts to date have focused on droplet breakup under constant acceleration, droplets interacting with a hypersonic vehicle experience a complex system of shock and expansion waves that result in an unsteady acceleration history. The effects of droplet breakup under unsteady acceleration remain poorly understood. To address this gap in understanding, a new shock tube facility has been designed to produce a variable acceleration shock wave. The acceleration history is estimated with 1D gas dynamics and verified experimentally utilizing low-density high-speed PIV. Preliminary experiments are presented which show the deformation and onset of breakup for millimeter-sized droplets. The leading shock wave generates an initial Weber number greater than 1E3, which rapidly decreases by an order of magnitude due to coupled expansion waves. The deformation and acceleration of the droplet is imaged by high-speed shadowgraphy fully resolving the morphology and position of the droplet up to breakup. The deformation rate and acceleration are compared to extant deformation and drag models while surface morphology is compared with predictions from hydrodynamic breakup models.