High-Speed Imaging of Droplet Impact on a Hypervelocity Projectile

The effects of liquid and solid impacts on aerospace bodies are of interest to hypersonics research. Collisions with precipitation or suspended dust particles can degrade aerospace materials, reducing transparency and other desirable properties. The bulk mechanical properties of a material are not always useful for estimating the effects of these interactions due to the small length scales involved, on the order of the material's own granularity or composite microstructure. At the same time, the interaction with the shock structure around a vehicle can disrupt a droplet, potentially altering the deleterious effects of impact. Since the sphericity and mass of an impacting drop both impact the degree of damage it does to relevant materials in a high-speed collision, it is important to characterize the exact breakup behavior of droplets in a multiphase flow system around an aerospace structure. Our approach is to use the Southwest Research Institute (SwRI) Light Gas Gun (LGG) facility to accelerate relevant materials to high velocities, introduce some prescribed form of precipitation, and study the multiphase flow interactions using advanced diagnostic techniques, namely direct shadowgraphy. A 5 MFps camera was used to image the interaction of a projectile at 2.4 km/s velocity with a stream of liquid water droplets in a low-pressure nitrogen medium. Two test shots were carried out in early 2022 to validate the structural integrity of the test projectiles and to test the imaging apparatus in conjunction with the SwRI LGG and liquid dropper mechanism. These preliminary results show that edge-stripping behavior, liquid jetting from the droplet-projectile impact point, and other multiphase hypersonic flow phenomena are present in the experiment. The liquid dropper mechanism, though successfully tested in the laboratory prior to installation at the LGG facility, proved less cooperative in the field setting, and is undergoing further refinement to assure consistent droplet quality in future experiments. The velocity of liquid jetting from collisions with the alumina projectile was measured and found to be between 1 and 2 times the collision velocity. Drop flattening was measured for small drops processed by an oblique shock.