In this paper, we are interested in the linear and the nonlinear Rayleigh-Taylor instability for the gravity-driven incompressible Navier-Stokes equations with Navier-slip boundary conditions around a laminar smooth density profile ρ 0 px 2 q being increasing in an infinite slab 2πLT ˆp´1, 1q (L ą 0, T is the usual 1D torus). The linear instability study of the viscous Rayleigh-Taylor model amounts to the study of the following ordinary differential equation of the vertical component of perturbed velocity on the finite interval p´1, 1q,with the boundary conditionswhere g ą 0 is the gravity constant, λ is the growth rate in time, k is the wave number transverse to the density profile and two Navier-slip coefficients ξ ˘are nonnegative constants. For each k P L ´1Zzt0u, we define a threshold of viscosity coefficient µcpk, Ξq for linear instability. So that, in the ksupercritical regime, i.e. µ ą µcpk, Ξq, we provide a spectral analysis adapting the operator method of Lafitte-Nguyễn in [12] and then prove that there are infinite solutions of (0.1)-(0.2). Based on infinitely unstable modes of the linearized problem, we consider a wide class of initial data to the nonlinear perturbation problem, extending Grenier's framework [6], to prove nonlinear Rayleigh-Taylor instability in a high regime of viscosity coefficient, namely µ ą 3 sup kPL ´1Zzt0u µcpk, Ξq.
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