Logarithmic spirals in 2d perfect fluids

TL;DR

This paper investigates logarithmic spiral solutions to 2D incompressible Euler equations, establishing well-posedness, bifurcation, and long-term behavior, including blow-up or homogenization, for various initial conditions.

Contribution

It introduces a framework for analyzing logarithmic spiral vortex sheets, proving existence, bifurcation, and stability results, and characterizing their long-term dynamics.

Findings

Well-posedness in L^p and atomic measures

Existence and bifurcation of multi-branched spiral vortex sheets

Dichotomy in long-term behavior: blow-up or homogenization

Abstract

We study logarithmic spiraling solutions to the 2d incompressible Euler equations which solve a nonlinear transport system on $\mathbb{S}$. We show that this system is locally well-posed in $L^p, p\geq 1$ as well as for atomic measures, that is logarithmic spiral vortex sheets. In particular, we realize the dynamics of logarithmic vortex sheets as the well-defined limit of logarithmic solutions which could be smooth in the angle. Furthermore, our formulation not only allows for a simple proof of existence and bifurcation for non-symmetric multi branched logarithmic spiral vortex sheets but also provides a framework for studying asymptotic stability of self-similar dynamics. We give a complete characterization of the long time behavior of logarithmic spirals. We prove global well-posedness for bounded logarithmic spirals as well as data that admit at most logarithmic singularities. This is due to the observation that the local circulation of the vorticity around the origin is a strictly monotone quantity of time. We are then able to show a dichotomy in the long time behavior, solutions either blow up (either in finite or infinite time) or completely homogenize. In particular, bounded logarithmic spirals should converge to constant steady states. For logarithmic spiral sheets, the dichotomy is shown to be even more drastic where only finite time blow up or complete homogenization of the fluid can and does occur.