# Navier-Stokes equations in riemannian geometry

Hello,

The Navier-Stokes equations can be written on a riemannian manifold: $$\dot{u}+\nabla_u u+ \Delta u=(df)^*$$ $$d^* u=0$$ where $\nabla$ is the Levi-Civita connection, $u$ is a vector fields, $\Delta$ is the laplacian, $df$ is the differential of $f$, $(df)^*$ is the dual of $df$ by the metric, $d^*u$ is the divergence of $u$.

The problem is due to Antoine Balan.

Do you have references ?

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Have you looked at the work of Marsden and Weinstein? –  Deane Yang Sep 25 '11 at 14:05
Moreover, googling "Navier-Stokes Riemannian manifold" produces a lot of hits. –  Deane Yang Sep 25 '11 at 19:27
Do you think it would be possible to extend the results of Arnol'd to the Navier-Stokes equation ? –  user18921 Oct 31 '11 at 20:26
For instance, considering the Navier-Stokes equation as a small perturbation of the Euler equations, just as it was done by Cruzeiro et al., but on a stochastic point of view –  user18921 Oct 31 '11 at 20:34
From the point of view of extending the results from flat space to compact manifolds, there is no difficulty at all: arxiv.org/abs/0901.4412 –  timur Apr 3 '12 at 22:55

The answer and comments about Arnold and Marsden papers are a little off side. They concern the equation of inviscid fluids, called Euler equation. This differs from Navier-Stokes by the highest-order derivatives $\Delta u$. This changes completely the functional analysis background. Also, Euler equation has a geometrical interpretation (geodesics on the group of measure-preserving diffeomorphisms), whereas Navier-Stokes has not.

I am not aware of references for Navier-Stokes on manifolds. However, I don't think that this is a real problem. What has been important so far for Navier-Stokes is the space dimension and the embedding theorems we have between functional spaces like Sobolev, Besov and others. For instance, the Cauchy problem must be globally well-posed on every compact surface, and locally well-posed on $3$-manifolds.

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You are missing a $\dot u$ in your equation! We want a dynamic vector field. The sign of your $\nabla_u u$ and $\Delta u$ are usually taken to be opposite, as with the sign of your $df^*$ and $\nabla_u u$. See p. 63 of Arnol'd-Khesin's book `Topological Methods in Fluid Mechanics'. Arnol'd and Khesin definitely knew how to do this. Khesin is still alive!

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Dear Richard Montgomery, I would add a link to the text "Topological Methods in Hydrodynamics" by Arnol'd and Khesin. books.google.com/… –  Giuseppe Tortorella Sep 26 '11 at 5:24

You could look at the paper: Groups of Diffeomorphisms and the motion of an incompressible fluid, by Ebin and Marsden.

About two centuries after Euler, in 1966 Arnold gave a geometric reformulation of the classical equations for an imcompressible fluid in terms of the geodesic spray of left invariant metric on an infinite dimensional Lie Group.

Ebin and Marsden promptly employed this reformulation to obtain existence and uniqueness results for these equations on compact oriented riemannian manifolds.

This circle of ideas is one of the first important application of infinite dimensional manifolds as remarked by Stephen Smale.

By the way, should not the equation contain the time derivative of the unknown $u$?

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For what it's worth, the Navier-Stokes equation on manifolds is also mentioned in this recent paper http://arxiv.org/pdf/1107.2698, see (1.16) there, in connection with another flow for vector fields that the authors define.

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