Absorption of planar waves in a draining bathtub

We present an analysis of the absorption of acoustic waves by a black hole analogue in (2 + 1) dimensions generated by a fluid flow in a draining bathtub. We show that the low-frequency absorption length is equal to the acoustic hole circumference and that the high-frequency absorption length is 4 times the ergoregion radius. For intermediate values of the wave frequency, we compute the absorption length numerically and show that our results are in excellent agreement with the low-and high-frequency limits. We analyze the occurrence of superradiance, manifested as negative partial absorption lengths for corotating modes at low frequencies.

Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)

Fundacao de Amparo a Pesquisa do Estado do Para (FAPESPA)

Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)

Engineering and Physical Sciences Research Council (EPSRC)[EP/G049092/1]