The Effect of a Micro-Perforated Cavity Floor on the Propagation of Tunnel-Cavity Modes in a Low-Speed Duct Flow

Abstract

Experimental and numerical studies are presented that show how transverse Tunnel-Cavity (TC) modes are nearly trapped and attenuated by a non-symmetric shallow cavity whose base wall is micro-perforated and that undergoes a low-speed flow regime at Mach 0.09. Three effects are pointed out. First, wall-pressure measurements and 2D Lattice-Boltzmann (LB) simulations confirmed the potential of micro-perforated cavity floors to attenuate the transverse cut-on TC modes that are cut-off in the tunnel section and excited by the shear layer, albeit overestimated in the 2D simulations. LB simulations show that the maximum velocity fluctuations occur within, but also at the inlet/outlet of the micro-perforations through vortex shedding. The latter mechanism appears to play a more important role in the attenuation than visco-thermal dissipation, given a Shear number comprised between 2.9 and 5.1 up to 1 kHz. Second, elasto-acoustic coupling was observed between the transverse TC resonances and the flexural modes of the thin micro-perforated panel, that causes a downshift of the first TC-Panel resonance frequencies with respect to the uncoupled TC resonances. Third, an optimisation study has been carried out on the micro-perforated panel (MPP) parameters for the MPP input impedance to achieve maximum axial decay rate of the least attenuated TC mode at its resonance frequency. Although this mode was already attenuated by the nominal MPP, simulations showed that replacing the cavity floor by an optimized MPP provides further broadband reduction in the region of the plane wave least attenuated mode below the first TC cut-on frequency. However, this effect is limited to the spatial extent where the axial mean flow velocity takes the value assumed in the optimisation process.