Waves from an oscillating point source with a free surface in the presence of a shear current

TL;DR

This paper analytically studies the wave field generated by an oscillating point source in a shear flow with a free surface, revealing wave behaviors and resonances depending on the Froude number.

Contribution

It provides a detailed analytical solution for wave fields in shear flows with a free surface, including the effects of resonance and critical layers, which was not previously characterized.

Findings

Wave field splits into regular and critical waves below resonance

Critical wave forms a downstream street of flow structures

Resonance occurs at a specific Froude number, amplifying wave amplitude

Abstract

We investigate analytically the linearized water wave radiation problem for an oscillating submerged point source in an inviscid shear flow with a free surface. A constant depth is taken into account and the shear flow increases linearly with depth. The surface velocity relative to the source is taken to be zero, so that Doppler effects are absent. We solve the linearized Euler equations to calculate the resulting wave field as well as its far-field asymptotics. For values of the Froude number $F^2=\omega^2 D/g$ ($\omega$: oscillation frequency, $D$ submergence depth) below a resonant value $F^2_\text{res}$ the wave field splits cleanly into separate contributions from regular dispersive propagating waves and non-dispersive "critical waves" resulting from a critical layer-like street of flow structures directly downstream of the source. In the sub-resonant regime the regular waves behave like sheared ring waves while the critical layer wave forms a street of a constant width of order $D\sqrt{S/\omega}$ ($S$ is the shear flow vorticity) and is convected downstream at the fluid velocity at the depth of the source. When the Froude number approaches its resonant value, the the downstream critical and regular waves resonate, producing a train of waves of linearly increasing amplitude contained within a downstream wedge.