Mechanism of early-stage drop deformation in shock induced flow at limited Weber numbers

Early-stage deformation of water drops under Weber numbers ranging from 2 100 to 2 700 is investigated by experimental, numerical and theoretical methods, to reveal the influences of primary flow parameters on drop deformation as well as the mechanism behind them. Images of the drop deformation with noteworthy differences under different test conditions are captured with high-speed photography technique, demonstrating that though the Weber numbers are similar, drop deformation can be largely affected by the involved primary flow parameters, such as gas velocity, gas density and drop diameter. By substituting the liquid drop with a rigid sphere body, the gas flow field is numerically simulated, and the aerodynamic forces acting on sphere surface are distilled based on which the drop deformation is theoretically computed. The results show a good agreement between the theoretical and experimental deformation trends. The early-stage deformation of the drop is found to be in coherence with the flow separation and vortex distribution characteristics of the gas flow. Evolution of the gas flow field can be divided into a transient separation developing period and a following globally steady period. The pressure distribution exerted by the gas flow and the radial acceleration induced by it exhibit large differences in the two periods. The characteristic time of the separation development relative to the drop deformation, which can be represented by the square root of gas-liquid density ratio, is found to be a dominant parameter determining the drop deformation pattern in early stage of aero-breakup. A higher gas density leads to a higher occupation of the separation developing period in the whole drop deformation process, and the drop tends to develop a single ridge on its rear surface; on the contrary, multiple ridges with similar amplitude are more likely to happen when the gas density is lower, reflecting the characteristics of the outer flow in the globally stable period.