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Helical symmetry is invariance under a one-dimensional group of rigid motions generated by a simultaneous rotation around a fixed axis and translation along the same axis. The key parameter in helical symmetry is the step or pitch, the magnitude of the translation after rotating one full turn around the symmetry axis. In this article we study the limits of three-dimensional helical viscous and inviscid incompressible flows in an infinite circular pipe, with respectively no-slip and no-penetration boundary conditions, as the step approaches infinity. We show that, as the step becomes large, the three-dimensional helical flow approaches a planar flow, which is governed by the so-called two-and-half Navier–Stokes and Euler equations, respectively.

In this article we consider weak solutions of the three-dimensional incompressible fluid flow equations with initial data admitting a one-dimensional symmetry group. We examine both the viscous and inviscid cases. For the case of viscous flows, we prove that Leray-Hopf weak solutions of the three-dimensional Navier-Stokes equations preserve initially imposed symmetry and that such symmetric flows are stable under general three-dimensional perturbations, globally in time. We work in three different contexts: two-and-a-half-dimensional, helical, and axisymmetric flows. In the inviscid case, we observe that as a consequence of recent work by De Lellis and Székelyhidi, there are genuinely three-dimensional weak solutions of the Euler equations with two-dimensional initial data. We also present two partial results where restrictions on the set of initial data and on the set of admissible solutions rule out spontaneous symmetry breaking; one is due to P.-L. Lions and the other is a consequence of our viscous stability result. © 2013 Society for Industrial and Applied Mathematics.

The global existence of weak solutions for the three-dimensional axisymmetricEuler-$\alpha$ (also known as Lagrangian-averaged Euler-$\alpha$) equations,without swirl, is established, whenever the initial unfiltered velocity $v_0$satisfies $\frac{\nabla \times v_0}{r}$ is a finite Randon measure with compactsupport. Furthermore, the global existence and uniqueness, is also establishedin this case provided $\frac{\nabla \times v_0}{r} \in L^p_c(\mathbb{R}^3)$with $p>{3/2}$. It is worth mention that no such results are known to beavailable, so far, for the three-dimensional Euler equations of idealincompressible flows.