Concentration versus absorption for the Vlasov-Navier-Stokes system on bounded domains

We study the large time behavior of small data solutions to the Vlasov-Navier-Stokes system set on $Ω\times \mathbb{R}^3$, for a smooth bounded domain $Ω$ of $\mathbb{R}^3$, with homogeneous Dirichlet boundary condition for the fluid and absorption boundary condition for the kinetic phase. We prove that the fluid velocity homogenizes to $0$ while the distribution function concentrates towards a Dirac mass in velocity centered at $0$, with an exponential rate. The proof, which follows the methods introduced in [Han-Kwan - Moussa - Moyano, arXiv:1902.03864v2], requires a careful analysis of the boundary effects. We also exhibit examples of classes of initial data leading to a variety of asymptotic behaviors for the kinetic density, from total absorption to no absorption at all.

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