How to create analogue black hole or white fountain horizons and LASER cavities in experimental free surface hydrodynamics?

TL;DR

This paper experimentally validates the hydrodynamic laws governing transcritical free surface flows that emulate black hole and white fountain horizons, and explores potential black hole laser instabilities in water channels.

Contribution

It provides the first experimental validation of analogue horizons in free surface hydrodynamics and proposes a study of black hole laser effects using controlled obstacle configurations.

Findings

Validation of hydrodynamic laws for transcritical flows

Identification of regimes suitable for black hole laser experiments

Phase diagram for obstacle configurations and flow conditions

Abstract

Transcritical flows in free surface hydrodynamics emulate black hole horizons and their timereversed versions known as white fountains. Both analogue horizons have been shown to emit Hawking radiation, the amplification of waves via scattering at the horizon. Here we report on an experimental validation of the hydrodynamic laws that govern transcritical flows, for the first time in a free surface water channel using an analogue space-time geometry controlled by a bottom obstacle. A prospective study, both experimental and numerical, with a second obstacle downstream of a first one is presented to test in the near-future the analogous black hole laser instability, namely the super-amplification of Hawking radiation by successive bounces on a pair of black and white horizons within cavities which allow the presence of negative energy modes necessary for the amplification process. Candidate hydrodynamic regimes are discussed thanks to a phase diagram based on the scaled relative heights of both obstacles and the ratio of flow to wave speed in the upstream region.