Nonaxisymmetric flow characteristics in head-on collision of spinning droplets

Abstract

Effects of spinning motion on the bouncing and coalescence between a spinning droplet and a nonspinning droplet undergoing the head-on collision were numerically studied by using a Volumeof-Fluid method. A prominent discovery is that the spinning droplet can induce significant nonaxisymmetric flow features for the head-on collision of equal-size droplets composed of the same liquid. Specifically, a non-axisymmetric bouncing was observed, and it is caused by the conversion of the spinning angular momentum into the orbital angular momentum. This process is accompanied by the rotational kinetic energy loss due to the interaction between the rotational and radial flows of the droplets. A non-axisymmetric internal flow and a delayed separation after temporary coalescence were also observed, and they are caused by the enhanced interface oscillation and internal-flow-induced viscous dissipation. The spinning motion can also promote the mass interminglement of droplets because the locally non-uniform mass exchange occurs at the early collision stage by non-axisymmetric flow and is further stretched along the filament at later collision stages. In addition, it is found that the non-axisymmetric flow features increase with increasing the orthogonality of the initial translational motion and the spinning motion of droplets.

Spin-affected droplet coalescence and separation undergoing off-center collision

“…The present study adopts the Volume-of-Fluid (VOF) method, which has been implemented in the open source code, Gerris [20], featuring the three-dimensional octree adaptive mesh refinement, the geometrical VOF interface reconstruction, and continuum surface force with height function curvature estimation. Gerris has been demonstrated to be competent for highfidelity simulation of a wide range of multiphase flow problems [14][15][16][17][21][22][23][24].…”

“…A coarse mesh would induce "premature" coalescence of the droplets that realistically bounce off. Thus, the successful simulation of droplet coalescence and subsequent collision dynamics in previous studies [17,22] were obtained by choosing an appropriate mesh resolution near the interface. It is worthy to mention that mesh dependence is not a significant issue for the simulation of droplet coalescence followed by separation in the present study because the drainage of gas film occurs shortly upon the droplet collision at large We.…”

“…To improve computational efficiency, different mesh refinement level (N g ,N l ,N i ) is used in three physical zones, namely the gas, the droplet, and the interface zones, that divided from the entire computational domain. As a balance between computational cost and accuracy, a mesh refinement level of (4, 7, 8) was used [17] for all simulations in the present study. It takes about 200 hours of real time to run the simulation up to T=2.0 on two Intel Xeon(R) Gold-6150 processor with 72 cores (36 cores for each processor).…”

“…For collisions at sufficient large B, stretching mechanics was claimed to account for the stretching separation without large droplet deformation [3,5], by considering the stretching inertia force against the surface tension force and viscous force. Furthermore, for collisions at intermediate B and large We, a new regime named "rotational separation" were introduced [13] that located in the coalescence regime, whose the criterion is interpreted by balancing surface tension force and angular momentum [4,5] or via an evaluation of the rotational energy against surface energy, because previous experimental [3,5] and numerical [14][15][16][17] studies have demonstrated that the merged droplet can spin after off-center collisions. It is worthy to noted that in practical situations the droplet collision is more complicated than the case discussed above.…”

“…In particular, the off-center collision-induced spinning droplets can collide with each other because subsequent collisions are highly probable in practical dense sprays [18,19]. In recent study, He and Zhang [17] studied the head-on bouncing and found the spinning droplet can induce significant non-axisymmetric droplet deformation because of the conversion of the spin angular momentum into the orbital angular momentum. Further, they also studied the headon coalescence between a spinning droplet and a non-spinning droplet of equal size and found the spinning motion can promote the mass interminglement of droplets because the locally nonuniform mass exchange occurs at the early collision stage by non-axisymmetric flow and is further stretched along the filament at later collision stages.…”