The Geometry of Axisymmetric Ideal Fluid Flows with Swirl

The sectional curvature of the volume preserving diffeomorphism group of a Riemannian manifold $M$ can give information about the stability of inviscid, incompressible fluid flows on $M$. We demonstrate that the submanifold of the volumorphism group of the solid flat torus generated by axisymmetric fluid flows with swirl, denoted by $\mathcal{D}_{μ,E}(M)$, has positive sectional curvature in every section containing the field $X = u(r)\partial_θ$ iff $\partial_r(ru^2)>0$. This is in sharp contrast to the situation on $\mathcal{D}_μ(M)$, where only Killing fields $X$ have nonnegative sectional curvature in all sections containing it. We also show that this criterion guarantees the existence of conjugate points on $\mathcal{D}_{μ,E}(M)$ along the geodesic defined by $X$.

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