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 (

) and Reynolds number (

italic Re Re $\textit{Re}$

). 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 $\textit{ISPL} = {150}\,\textrm{dB}$

, the acoustic–hydrodynamic energy conversion is highest when

italic St less than or equals 4 italic St Subscript 0 Baseline St ≤ 4 St 0 $\textit{St} \le 4\textit{St}_0$

, corresponding to an effective Keulegan–Carpenter number (

upper K Subscript c K c ${K_c}$

) 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 $\textit{St}$

, the Stokes-layer confinement produces X-shaped near-slit modes, reducing the energy input by approximately 50 %. The influence of

italic Re Re $\textit{Re}$

depends on amplitude. Larger

italic Re Re $\textit{Re}$

corresponds to suppressed broadband fluctuations and sharpened harmonic peaks at

150 dB 150 dB ${150}\,\textrm{dB}$

. At

italic ISPL equals 120 dB ISPL = 120 dB $\textit{ISPL} = {120}\,\textrm{dB}$

, the boundary layers remain attached, vortex shedding is weak, absorption monotonically scales with viscosity and the

italic Re Re $\textit{Re}$

- and

italic St St $\textit{St}$

-dependencies become comparable. Across all conditions, more than 99 % of the VL is confined to a compact region surrounding the slit mouth. The KE–VL spectra identify regimes that enhance or suppress acoustic damping in slit geometries, providing a physically interpretable basis for acoustic-based design.