# Euler flows and singular geometric structures

## TL;DR

This paper extends classical geometric fiber bundle constructions to manifolds with boundary using $b$-calculus, and applies these ideas to analyze stationary Euler flows, revealing new structures and correspondences in fluid dynamics.

## Contribution

It introduces a $b$-calculus approach to manifolds with boundary and applies it to Euler flows, establishing new geometric structures and generalizing Arnold's theorem.

## Key findings

- New proof of Arnold's structure theorem for Euler flows.
- Identification of $b$-symplectic structures on singular sets.
- Establishment of a correspondence between $b$-Beltrami fields and contact structures.

## Abstract

Tichler proved that a manifold admitting a smooth closed one-form fibers over a circle. More generally a manifold admitting $k$ independent closed one-forms fibers over a torus $T^k$. In this article we explain a version of this construction for manifolds with boundary using the techniques of $b$-calculus. We explore new applications of this idea to Fluid Dynamics and more concretely in the study of stationary solutions of the Euler equations. In the study of Euler flows on manifolds, two dichotomic situations appear. For the first one, in which the Bernoulli function is not constant, we provide a new proof of Arnold's structure theorem and describe $b$-symplectic structures on some of the singular sets of the Bernoulli function. When the Bernoulli function is constant, a correspondence between contact structures with singularities and what we call $b$-Beltrami fields is established, thus mimicking the classical correspondence between Beltrami fields and contact structures. These results provide a new technique to analyze the geometry of steady fluid flows on non-compact manifolds with cylindrical ends.