Model For Polygonal Hydraulic Jumps

TL;DR

This paper introduces a phenomenological model for polygonal hydraulic jumps, capturing their shape and stability by analyzing force balances and flow structure, and explains experimental observations without relying on surface tension.

Contribution

The paper develops a new nonlinear model for polygonal hydraulic jumps based on force balances, including azimuthal effects, and analyzes stability and shape formation.

Findings

Polygonal shapes are similar to experimental observations.

Existence of polygons depends on a single dimensionless parameter.

Instability occurs at low Bond numbers, with wavelength proportional to roller width.

Abstract

We propose a phenomenological model for the polygonal hydraulic jumps discovered by Ellegaard et al., based on the known flow structure for the type II hydraulic jumps with a "roller" (separation eddy) near the free surface in the jump region. The model consists of mass conservation and radial force balance between hydrostatic pressure and viscous stresses on the roller surface. In addition, we consider the azimuthal force balance, primarily between pressure and viscosity, but also including non-hydrostatic pressure contributions from surface tension in light of recent observations by Bush et al. The model can be analyzed by linearization around the circular state, resulting in a parameter relationship for nearly circular polygonal states. A truncated, but fully nonlinear version of the model can be solved analytically. This simpler model gives rise to polygonal shapes that are very similar to those observed in experiments, even though surface tension is neglected, and the condition for the existence of a polygon with N corners depends only on a single dimensionless number {\phi}. Finally, we include time-dependent terms in the model and study linear stability of the circular state. Instability occurs for suffciently small Bond number and the most unstable wave length is expected to be roughly proportional to the width of the roller as in the Rayleigh-Plateau instability.