Lattice-Boltzmann Very Large Eddy Simulation of a Multi-Orifice Acoustic Liner with Turbulent Grazing Flow

A lattice-Boltzmann Very Large Eddy simulation of a multi-orifice acoustic liner, grazed by a turbulent flow at Mach number equal to 0:3 and a planar acoustic wave with amplitude equal to 130 dB and frequency equal to 1800 Hz, is carried out. The geometry of the liner replicates the experiments carried out in the Grazing Flow Impedance Tube (GFIT) facility at NASA Langley. It is found that the impedance, obtained from numerical simulations using the Dean’s method, is a function of the orifice location. This is attributed to two phenomena: the interaction between the wake behind the upstream orifices and the downstream ones; and the interaction between the flow fields in the cavity induced by the ejected vortices. Results show that, for the investigated configuration, two vortical structures are generated in the orifice: one, formed along the downstream inner wall of the orifice, weakly penetrates into the cavity; the second, formed at the bottom downstream corner of the orifice, is ejected into the cavity up to three orifice diameters. The direction along which the latter is ejected varies with the orifice location. The ejected vortices are characterized by an annular vortex and a trailing vortex similarly to what found for synthetic jets. In the cavity, large scale vortical structures are found. On the face sheet, it is found that the turbulent wakes behind the upstream orifices increases the vertical velocity component within the orifices. These findings suggest that local measurement techniques, such as the Dean’s method, might be affected by the sampling location in realistic configurations.