Energy dissipation mechanisms in an acoustically driven slit

We quantify the conversion of incident acoustic energy into vortical motion and viscous dissipation for a plane wave passing through large-aspect-ratio slit geometries. We perform direct numerical simulations over a broad imposed parameter space in incident sound pressure level (ISPL), Strouhal number ( italic St St St ) and Reynolds number ( italic Re Re Re ). Spectral proper orthogonal decomposition yields energy-ranked coherent structures at each frequency, from which we construct mode-by-mode fields for spectral kinetic energy (KE) and viscous loss (VL) to examine the acoustic absorption mechanisms. At italic ISPL equals 150 dB ISPL = 150 dB ISPL = 150\,dB , the acoustic–hydrodynamic energy conversion is highest when italic St less than or equals 4 italic St Subscript 0 Baseline St ≤ 4 St 0 St ≤ 4St₀ , corresponding to an effective Keulegan–Carpenter number ( upper K Subscript c K c Kc ) larger than 40. In this regime, three-dimensional simulations show that the dominant flow response is two-dimensional, with the oscillatory shear layer near the slit corners rolling up into shedding vortices. The VL accounts for 20 %–60 % of the KE contribution. For larger italic St St St , the Stokes-layer confinement produces X-shaped near-slit modes, reducing the energy input by approximately 50 %. The influence of italic Re Re Re