Universal power law for the energy spectrum of breaking Riemann waves

TL;DR

This paper establishes a universal power law decay in the energy spectrum of one-dimensional breaking Riemann waves, valid for various nonlinear wave speeds and observed numerically even with dissipation or dispersion.

Contribution

It provides a theoretical justification and numerical evidence for a universal power law in the energy spectrum of breaking Riemann waves across different wave equations.

Findings

Energy spectrum decays as k^{-8/3} at breaking time.

Universal power law observed in viscous Burgers and Korteweg-de Vries equations.

Spectrum decay persists over time with small dissipation or dispersion.

Abstract

The universal power law for the spectrum of one-dimensional breaking Riemann waves is justified for the simple wave equation. The spectrum of spatial amplitudes at the breaking time $t = t_b$ has an asymptotic decay of $k^{-4/3}$, with corresponding energy spectrum decaying as $k^{-8/3}$. This spectrum is formed by the singularity of the form $(x-x_b)^{1/3}$ in the wave shape at the breaking time. This result remains valid for arbitrary nonlinear wave speed. In addition, we demonstrate numerically that the universal power law is observed for long time in the range of small wave numbers if small dissipation or dispersion is accounted in the viscous Burgers or Korteweg-de Vries equations.