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Global Existence and Long Time Asymptotics for Rotating Fluids in a 3D layer

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Niveau: Supérieur, Doctorat, Bac+8
Global Existence and Long-Time Asymptotics for Rotating Fluids in a 3D layer Thierry Gallay Institut Fourier (UMR CNRS 5582) Universite de Grenoble I B.P. 74 38402 Saint-Martin-d'Heres, France Violaine Roussier-Michon Institut de Mathematiques de Toulouse (UMR CNRS 5219) INSA Toulouse 135 av Rangueil 31077 Toulouse cedex 4, France December 5, 2008 Abstract The Navier-Stokes-Coriolis system is a simple model for rotating fluids, which allows to study the influence of the Coriolis force on the dynamics of three-dimensional flows. In this paper, we consider the NSC system in an infinite three-dimensional layer delimited by two horizontal planes, with periodic boundary conditions in the vertical direction. If the angular velocity parameter is sufficiently large, depending on the initial data, we prove the existence of global, infinite-energy solutions with nonzero circulation number. We also show that these solutions converge toward two-dimensional Lamb-Oseen vortices as t? ∞. 1

  • velocity ?

  • u˜ ?

  • large initial

  • velocity field

  • sufficiently large

  • nsc system

  • dimensional vector

  • navier-stokes system

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Global Existence and Long-Time Asymptotics for Rotating Fluids in a 3D layer
Thierry Gallay Institut Fourier (UMR CNRS 5582) UniversitedeGrenobleI B.P. 74 38402Saint-Martin-dHeres,France thierry.gallay@ujf-grenoble.fr
Violaine Roussier-Michon InstitutdeMathematiquesdeToulouse(UMRCNRS5219) INSA Toulouse 135 av Rangueil 31077 Toulouse cedex 4, France roussier@math.univ-toulouse.fr
December 5, 2008
Abstract
TheNavier-Stokes-Coriolissystemisasimplemodelforrotatinguids,whichallowsto study the in uence of the Coriolis force on the dynamics of three-dimensional o ws. In this paper, we consider the NSC system in an in nite three-dimensional layer delimited by two horizontal planes, with periodic boundary conditions in the vertical direction. If the angular velocityparameterissucientlylarge,dependingontheinitialdata,weprovetheexistence of global, in nite-energy solutions with nonzero circulation number. We also show that these solutions converge toward two-dimensional Lamb-Oseen vortices ast→ ∞.
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Introduction
Inrecentyearsalotofactivityhasbeendevotedtothemathematicalstudyofgeophysicalows, andinparticulartovariousmodelsofrotatinguids.Takingadvantageofthestrati catione ect due to the Coriolis force, signi can t results have been obtained which are still out of reach for the usual Navier-Stokes system, such as global existence of solutions for large initial data [1, 3] and stability of boundary layers for small viscosities [10, 15]. We refer the interested reader to therecentmonograph[4]whichcontainsageneralintroductiontogeophysicalows,anoverview of the mathematical theory, and an extensive bibliography. In this article we study the so-called Navier-Stokes-Coriolis (NSC) system in a three-dimen-sionallayerdelimitedbytwoin nitehorizontalplanes,assumingasusualthattherotationvector is constant and aligned with the vertical axis. This is a reasonably simple model for the motion of the ocean in a small geographic zone at mid-latitude, where the variation of the Coriolis force due to the curvature of Earth can be neglected. More realistic systems exist which take into accountthevariationsoftemperatureandsalinityinsidetheocean,andincludeboundarye ects modelling the in uence of coasts, the topography of the bottom, or the action of the wind at thefreesurface,see[9,17].Nevertheless,keepingonlytheCoriolisforceismeaningfulina rst approximation, because its e ect is very important on the ocean’s motion at a global scale due to the fast rotation of Earth compared to typical velocities in the ocean. Our main goal is to investigate the long-time behavior of the solutions to the NSC system fora xed,buttypicallylarge,valueoftherotationspeed.Asin[1,3]weshallusethee ect of the Coriolis force to prove global existence of solutions for large initial data, but the long-time asymptotics of those solutions turn out to be essentially two-dimensional and are therefore not a ected by the rotation. Thus we shall recover as a leading term in our expansion the Lamb-Oseen vortex which plays a similar role for the usual Navier-Stokes system in the plane R2[8] or the three-dimensional layerR2(0 To1) [19]. avoid all problems related to boundary layers,weshallalwaysassumethattheuidmotionisperiodicin the vertical direction. This hypothesishasnophysicaljusti cationandisonlyaconvenientmathematicalwaytodisregard the in uence of the boundaries. Although boundary conditions do play an important role in the problem we study and will have to be considered ultimately, in this paper we chose to focus on the motion of the uid in the bulk. We thus consider the Navier-Stokes-Coriolis system in the three-dimensional layerD=R2 T1, whereT1=R/Z points of Theis the one-dimensional torus.Dwill be denoted by (x z), wherex= (x1 x2)R2is the horizontal variable andzT1 Theis the vertical coordinate. system reads tu+ (u r)u+ e3uu rp divu= 0(1) = whereu=u(t x z)R3ehevitsty elociatnhdeludido,fp=p(t x z)Ris the pressure eld. Here and in what follows, it is understood that di eren tial operators such asror  act on all spatial variables (x z), unless otherwise indicated. System (1) di ers from the usual incompressible Navier-Stokes equations by the presence of the Coriolis term e3u, where Ris a parameter ande3= (001)t term is Thisis the unit vector in the vertical direction. due to the fact that our reference frame rotates with constant angular velocity /2 around the verticalaxis.Notethat(1)doesnotcontainanycentrifugalforce,becausethise ectcanbe included in the pressure termrpikehamenvcitocsitysithofuieasdh.Forsimplicity,tebne rescaled to 1, and the uid density has been incorporated in the de nition of the pressurep. As in the ordinary Navier-Stokes system, the role of the pressure in (1) is to enforce the incompressibility condition divu= 0. eliminate the pressure, one can apply to both sides the To Leray projectorP, which is just the orthogonal projector inL2(D)3onto the space of divergence-
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