This thesis contains three independent parts. The first part presents a proof of existence of weak global solutions to a Vlasov-incompressible-Navier-Stokes system with variable density. This system is obtained formally from a classical Vlasov-incompressible-Navier-Stokes model with fragmentation for which only two values for the particules radii are considered: a radius r1 for non fragmented particules and a smaller radius r2 for particules created by fragmentation. The asymptotic model is obtained in the limit r2 vanishing. The second part deals with the modeling of a wave impact on a rigid wall. The purpose of our work is to study and model the escape of the gas between the liquid and the wall. In the numerical model we have replaced the liquid wave with a solid mass, and developed an ALE-VFFC code for the numerical simulation of the system. Scaling the system of equations allows us to obtain the dimensionless numbers governing the escape phenomena. The mean escape velocity is compared to the velocity in the case of incompressible gas. Finally, a parametric study with respect to the dimensionless numbers is carried out. We present in the third part the principles of the coupling between an efficient numerical method for hyperbolic systems (and non conservative equations arising in multiphase flows), namely the FVCF scheme, on the one hand; and a particle method for the Vlasov-Boltzmann equation (of PIC-DSMC type), on the other hand. Numerical results illustrating this coupling are shown for a problem involving a spray (droplets inside an underlying gas) in a pipe which is modeled by a 1D fluid-kinetic system.
