Numerical Simulation of Two-Dimensional Acoustic Liners with High-Speed Grazing Flow

Resonant acoustic liners are used to dissipate acoustic energy in engine ducts in the presence of grazing flow. Future applications have been proposed that will expose liners to much higher grazing flow velocities than are typically encountered. To study their characteristics at these conditions, a fully predictive liner eduction technique is developed by solving the compressible Navier-Stokes equations with accurate boundary conditions. Validation of the numerical approach is demonstrated by comparing the predicted absorption characteristics of a 3 kHz resonant liner geometry without any background flow against existing computational and experimental data. Using the validated approach, simulations at more realistic liner operating conditions are performed. Incident grazing acoustic waves of different intensity and frequency are performed at grazing flow Mach numbers up to 0.85. The liner impedance is evaluated and compared with existing semiempirical models; reasonable agreement is found in most cases. The simulation databases are then used to examine candidate quantities for recently suggested time-domain liner models. It is found that, at high grazing Mach numbers, the vortex shedding characteristics continue to be a strong function of frequency, with incident waves of lower frequency leading to stronger vortex shedding. It is also found that the aperture wall shear stress and displacement thickness are strongly coupled to the in-aperture flow details, while the mass flux through the aperture is more closely linked to the incident sound frequency.