Numerical Study Of Interaction Between Droplet And Flexible Thin-walled Structure

Phenomena of droplet interacting with flexible thin-walled structure are very com-mon in natural and industrial systems.In this thesis,a numerical algorithm for the coupling of droplet and flexible thin-walled structure is developed,and it demonstrates the ability of the algorithm to simulate elastocapillarity phenomena.Then,we use this algorithm to simulate the problem of droplet impact on a flexible substrate.The phys-ical mechanism related to the effect of substrate flexibility on droplet impact is found.The main work and research results of this article are as follows:(1)A numerical algorithm for the interaction between droplet and flexible thin-walled structures is developed.Specifically,the conservative interface tracking equation and the incompressible Navier-Stokes equations are solved by the multiphase Lat-tice Boltzmann method(MLBM);the equation of a flexible thin-wall structure is solved by the geometric nonlinear finite element method(FEM);the no-slip bound-ary condition is imposed by the interpolated Bounce-Back method;the momentum exchange method and the diffuse-interface integration method are adopted to cal-culate the external load on the structure;the wettability boundary condition for the given contact angle is handled by the minimum weighted square method.The de-veloped algorithm has been fully verified,including the wetting of a droplet on a plane and a curved surface,the swinging of a filament under gravity,and the swing-ing of a filament or an inverted plate in a uniform flow.The results demonstrate the effectiveness of the multiphase fluid solver and the flexible solid solver.Then,the elastocapillarity phenomenon of droplet resting on a ultra-flexible membrane is simulated.It is found that the numerical results agree well with the experimental and theoretical results.It demonstrates the ability of the numerical algorithm to deal with elastocapillarity phenomena.(2)The problem of droplet impact on a flexible substrate is simulated,and the effect of substrate flexibility on droplet impact dynamics is studied.It was found that two conditions should be met to reduce the contact time and increase the remaining upward momentum after the droplet rebounds.Firstly,the Weber number should be higher than the critical Weber number to overcome the viscous and capillary forces.Secondly,the ratio of the natural frequency of the plate to that of the droplet should be approximately equal to the reciprocal of the contact time of a droplet impact a rigid surface at the same Weber number.Only in this case,the kinetic energy of the droplet will be converted into the surface energy of the droplet and the elastic energy of the plate simultaneously,and vice versa.In addition,based on the double spring model,the scaling laws of the maximum deflection of the plate and the maximum spreading diameter of the droplet are proposed,and the numerical results agree well with the theoretical predictions.Finally,the effect of flexibility on the droplet bouncing for different wettability was qualitatively investigated.It is found that the flexibility could effectively help the droplets bounce off at the smaller contact angles.