A Quantitative Hydrodynamic Limit for the Vlasov-Fokker-Planck/Navier-Stokes System in the Fine Particle Regime

We justify the hydrodynamic limit of the Vlasov-Fokker-Planck equation coupled with the incompressible Navier–Stokes equation in the fine regime with a quantitative convergence rate. In this regime, the limiting dynamics is described by the inhomogeneous incompressible Navier-Stokes system for the fluid-particle mixture. In contrast with the existing relative entropy approaches, the present work extends to a pointwise convergence of the hydrodynamic limits with an explicit rate on any given time interval [0,T]. The proof is based on a Macro-Micro decomposition around the moving local Maxwellian, together with an $\epsilon$-weighted energy-dissipation structure adapted to the fine particle relaxation scales. Furthermore, we remove the classical small background restriction inherent in high-order energy methods, allowing the limiting profile to be possibly large. This is accomplished by exploiting a hidden Kelvin-Voigt type damped wave equation structure within the macroscopic velocity error system, yielding the critical extra dissipation required to close the uniform estimates.