2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/124585We study ``draining bathtub'' as an acoustic analogue of a three-dimensional rotating black hole. Rotating fluid near the sonic horizon necessarily gives rise to the superradiant modes, which are partially responsible for the thermodynamic quantities in this rotating vortex-like hole. Using the recently suggested thin-layer method overcoming some difficulties from the well-known brick-wall method, we explicitly calculate the free energy of the system by treating the superradiance carefully and obtain the desirable entropy formula.13 pages, 1 figure. Change in authors. The improved version to be published in J. Kor. Phys. SocGeneral Relativity and Quantum Cosmologytext