This study investigated how embedding microphones in different cavity geometries reduce the measured turbulent boundary layer pressure fluctuations at the microphones. The cavity geometries were systematically varied using a design of experiments (DOE) methodology. This approach tested different cavity depths, diameters, chamfers, and opening sizes as well as the effect of a fine mesh covering. The resulting wind-tunnel test data was analyzed using a generalized additive statistical model (GAM). This approach quantified the relative effect of these parameters on the response variables of interest while accounting for non-linear frequency dependence. This experimental investigation showed that a mesh reduces the boundary layer noise by 8 dB. It was also shown that reducing the cavity area from the wall to the base of the microphone reduces the measured boundary layer spectral energy. Additionally, the model quantified the complex interactions between the mesh and area as well as the change in area.

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.

Acoustic liners are widely used as noise suppression devices, for example in aircraft engines. The effectiveness of the liners is measured through the impedance. In the present study, using a lattice-Boltzmann solver, the response of two liner geometries to grazing acoustic waves is examined. The two geometries have porosity equal to 0.99% and 6.89%, respectively. Impedance is computed using the traditional in-situ method. The results from the simulation are validated against previous experimental data, DNS data and predictions from semi-empirical models. Results show agreement with these reference data, allowing to use the computational setup for further analysis with a realistic liner configuration in the presence of a grazing flow.