Aeroacoustic Characterization of Single- and Multiscale Porous Fences

An experimental characterization of the acoustic behavior and the flowfield past several wall-mounted porous fences is presented. Particle image velocimetry is conducted in the near field of the fences. It shows a spanwise periodicity (associated with the periodic geometry of the grid) of regions where well-defined wakes appear as opposed to regions with a lower mass flow rate influenced by strong recirculations. The acoustic behavior is characterized using a microphone array enabling the spatial location of noise sources to be determined. Broadband noise is observed for all grids except for the fractal square grid, where a clear peak appears. The intensity of noise spectra displays scalability with Mach number M6 (dipoles), and the preferential frequency for the fractal square case is shown to scale with the freestream velocity (suggesting a shedding mechanism). For certain grids, the mean and instantaneous spatial locations of the noise sources also present a spanwise preferential arrangement coincident with regions where the flow presents well-defined wakes. Regarding the intermittency of the unsteady sources, they present smaller temporal width, separation, and spatial extent for increasing freestream velocity. However, the ratio between these quantities seems to be constant to within 6% for different Mach numbers.