On the dynamics of drop acceleration at the early stage of velocity relaxation in a shock wave

The early stage of velocity relaxation of a water drop in a flow behind a shock wave under conditions of drop deformation and breakup is studied experimentally. The aerodynamic drag of the drop is measured on the basis of the dynamics of motion of its leading edge, and a strong dependence of the drag on the observation time is demonstrated. These data are compared with the drag obtained on the basis of the dynamics of the center of mass of the drop. The drag of the center of mass is shown to be comparable with the drag of a solid sphere and to be substantially smaller than the value determined from the leading edge motion. A possible physical mechanism of this effect is proposed, which is based on the delay of the beginning of motion of the center of mass of the drop, owing to certain specific features of stabilization of external and internal flows at the early stage of velocity relaxation.