Simulation and Analysis of Droplet Aerobreakup in a Ballistic Wave System

The interaction of droplets with the transient pressure load caused by a conical shock has so far received limited attention. This study follows prior experimental work at Sandia National Laboratories, where a conical shock wave was generated by firing a bullet near a column of water or methanol droplets. In the present work, the bullet passage is modeled by the ballistic, or N-wave, flowfield, which mimics the far-field flow of the bullet. A sharpinterface simulation (via the moment-of-fluid method) of droplet deformation and incipient breakup is then carried out. The match of simulated trajectory and droplet deformation with the measurements indicates that the N-wave model captures the experiment postshock decrease of the flow Weber number. Simulation analysis confirms a previous observation that, in the bullet experiment, the time of incipient breakup is longer than what is predicted by existing correlations. Moreover, droplet flattening predicted by the Taylor Analogy Breakup model with the coefficient C₂ = 2∕3 consistently underestimates the observed diameter growth in both experiment and simulation. The difference with respect to a gradually accelerating droplet is attributed to the baroclinic vorticity that is deposited by the shock within a thin boundary layer at the droplet surface.