# The KdV equation on the half-line: Time-periodicity and mass transport

## TL;DR

This paper investigates whether the KdV equation models the experimentally observed time-periodic wave profiles in a flume with a periodic boundary forcing, demonstrating asymptotic periodicity but revealing a subtle mass conservation issue.

## Contribution

It applies the Fokas method to analyze the KdV equation with periodic boundary conditions, showing asymptotic time-periodicity and identifying a second-order mass conservation failure.

## Key findings

- Solutions become asymptotically periodic in time at fixed points.
- Mass conservation fails at second order in wave amplitude.
- The KdV model aligns with experimental periodicity observations.

## Abstract

The work presented here emanates from questions arising from experimental observations of the propagation of surface water waves. The experiments in question featured a periodically moving wavemaker located at one end of a flume that generated unidirectional waves of relatively small amplitude and long wavelength when compared with the undisturbed depth. It was observed that the wave profile at any point down the channel very quickly became periodic in time with the same period as that of the wavemaker. One of the questions dealt with here is whether or not such a property holds for model equations for such waves. In the present discussion, this is examined in the context of the Korteweg-de Vries equation using the recently developed version of the inverse scattering theory for boundary value problems put forward by Fokas and his collaborators. It turns out that the Korteweg-de Vries equation does possess the properly that solutions at a fixed point down the channel have the property of asymptotic periodicity in time when forced periodically at the boundary. However, a more subtle issue to do with conservation of mass fails to hold at the second order in a small parameter which is the typical wave amplitude divided by the undisturbed depth.