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

Abstract

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.

On the induction period of drop breakup behind shock wave

“…A single drop of water d = 2-3 mm crossed the laser synchronization beam on the way to the channel axis and triggered the shock tube. The duration of the quasi-stationary flow behind the shock wave front is 500-700 microseconds [9,10].…”

“…1). The gas parameters behind shock front were determined as a function of the Mach number of the shock wave Ms = Vs / c in the theory of shock waves, where Vs -is the front velocity, c -is the sound velocity ahead of the front [8][9][10][11]. The velocity of the SW front was measured in two ways: by moving the shock wave front on two adjacent frames of shadow images (frames 1 and 2 in Fig.…”

“…The most effective testing of numerical technology is modeling in the calculation of the conditions realized in shock wave experiments and comparing the results with the maximum number of parameters. These can be integral quantitative parameters, such as the dynamics of acceleration and deformation of a drop [9,10], the period of induction of destruction [11]. This may include structural features that indicate the flow patterns of the droplet, the type of instability of its surface, and the deformation scenario, the physical mechanism of destruction, and the nature of the liquid erosion [11].…”

Coaxial gas-liquid jet: Dispersion and dynamics

“…The known velocity profile of the liquid phase makes it possible to determine the droplet acceleration by the gas flow as a measure of the aerodynamic force for a known mass (size), and in terms of acceleration, in turn, the droplet size can be estimated under the assumption of sphericity. Indeed, from the equation of motion of the droplet upon a sudden entry into the flow, the acceleration a depends on the droplet diameter d as follows [9][10][11]:…”

“…Generally speaking, C x of droplets due to deformation and erosion can differ substantially from the C D of a solid sphere [9][10][11], which has been well studied for various regimes in terms of the Re number [11]. Here, according to [9,10], we assume C x~2 .3 C D~ 1 -1.5 from subsonic to supersonic velocities. Figure 3 shows the results of measuring the fluid velocity (LDA) and the average droplet size ("Malvern Spraytec") on the axis of a two-phase jet, depending on the distance from the cut of the nozzle x for different flow regimes.…”