  Spectroscopy is a fundamental tool in science which consists in studying the
response of a system as a function of frequency. Among its many applications in
Physics, Biology, Chemistry and other fields, the possibility of identifying
objects and structures through their emission spectra is remarkable and
incredibly useful. In this paper we apply the spectroscopy idea to a
numerically simulated hydrodynamical flow, with the goal of developing a new,
non-invasive flow measurement technique. Our focus lies on an irrotational
draining vortex, which can be seen, under specific conditions, as the analogue
of a rotating black hole (historically named a dumb hole). This paper is a
development of a recent experiment that suggests that irrotational vortices and
rotating black holes share a common relaxation process, known as the ringdown
phase. We apply techniques borrowed from black hole physics to identify vortex
flows from their characteristic spectrum emitted during this ringdown phase. We
believe that this technique is a new facet of the fluid-gravity analogy and
constitutes a promising way to investigate experimentally vortex flows in
fluids and superfluids alike.
