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1. Noise generation of graded poroelastic edges from vortex rings

   Chen, Huansheng (January 2024, Lehigh University)

   The sound generated by an acoustic source near a semi-infinite edge with uniform parameters is studied theoretically. The acoustic emission of a vortex ring passing near a semi-infinite porous or elastic edge with uniform properties is formulated as a vortex sound problem and is solved using a Green’s function approach. The time-dependent pressure signal and its directivity in the acoustic far field are determined analytically for rigid porous edges as a function of a single dimensionless porosity parameter. At large values of this dimensionless parameter, the radiated acoustic power scales on the vortex ring speed U and the nearest distance between the edge and the vortex ring L as U^6L^−5, in contrast to the U^5L^−4 scaling recovered in the impermeable edge limit for small porosity values. These analytical findings agree well with the results of a companion experimental campaign conducted at the Applied Research Laboratories (ARL) at Penn State University. A related theoretical analysis of the sound scattered by uniform, impermeable elastic edges admits analytical results in a specific asymptotic limit, in which the acoustic power scales on U^7L^−6. In complement to the analysis of vortex ring sound from edges, the acoustic scattering of a turbulent eddy near a finite edge with a graded porosity distribution is determined numerically and is validated against analytical acoustic directivity predictions from the vortex-edge model problem for a semi-infinite edge in the appropriate high frequency limit. The cardioid and dipolar acoustic directivity obtained in the vortex ring configuration for low and high dimensionless porosity parameter values, respectively, are recovered by the numerical approach. An imposed linear porosity distribution demonstrates no substantial difference in the acoustic directivity relative to the uniformly porous cases at high porosity parameter values, where the local porosity parameter value at the edge determines the scattered acoustic field. However, more modulated behavior of the acoustic directivity is found at a relatively low frequency for the case of a finite edge with small graded porosity distribution. 

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2. Experimental Study of Trailing-Edge Bluntness Noise Reduction by Porous Plates

   https://doi.org/10.2514/1.j064045  

   Kershner, John R; Jaworski, Justin W; Geyer, Thomas F (August 2024, AIAA Journal)

   The acoustic and aerodynamic fields of blunt porous plates are examined experimentally in an effort to mitigate trailing-edge bluntness noise. The plates are characterized by a single dimensionless porosity parameter identified in previous works that controls the influence of porosity on the sound field. Hot-wire anemometry interrogates the velocity field to connect turbulence details of specific regions to flow noise directivity and beamforming source maps. Porous plates are demonstrated to reduce the bluntness-induced noise by up to 17 dB and progressively suppress broadband low-frequency noise as the value of the porosity parameter increases. However, an increase in this parameter also increases the high-frequency noise created by the pores themselves. The same highly perforated plate characterized by a large value of the porosity parameter reduces the bluntness-induced vortex shedding that is present in the wake of the impermeable plate. Lastly, pore shape and positional alignment are shown to have a complex effect on the acoustic field. Among the porosity designs considered, plates with circular pores are most effective for low-frequency noise reductions but generate high-frequency noise. No meaningful difference is found between the acoustic spectra from plates of the same open-area fraction with pores aligned along or staggered about the flow direction. 

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3. Acoustic and flow measurements of porous plate designs for aerodynamic noise mitigation

   Kershner, John R (August 2024, Lehigh University)

   An experimental study investigates parametrically the effects of porosity on the acoustic and aerodynamic fields about lifting- and non-lifting surfaces at two separate aeroacoustic facilities using microphone arrays and hot-wire anemometry. A single dimensionless porosity parameter characterizes the flow noise generated by a turbulent boundary layer and informs the design of the porous edge test specimens, including perforated flat plates and flat-plate extensions with a blunt or sharp trailing edge. The strong tonal peak due to vortex shedding from blunt trailing-edges diminishes in magnitude as the porosity parameter increases, and high-porosity plates eliminate this tone from the acoustic spectra. Single-microphone measurements indicate further that the porous plates examined can reduce low-frequency noise and increase high-frequency excess noise levels by up to 10 dB. DAMAS beamforming of the porous plates with sharpened edges reveal similar results on the acoustic spectra and identify that the principal effect of edge porosity on the acoustic source regions is a reduction in low-frequency noise and an increase in high-frequency noise across the entire plate. Noise generated by porous edges in the low-frequency range by the trailing- and leading-edge regions can be reduced by up to 20 dB, and porous edges increase high-frequency noise by up to 20 dB. Plates with the same dimensionless porosity perform similarly, where plates with circular holes perform slightly better (2 dB) than their counterparts with square holes at reducing low-frequency noise the most and increasing high-frequency noise the least in wind tunnel testing. Hot-wire anemometry of the flow field about blunt porous trailing edges reveals a downward shift of the bluntness-induced vortex-shedding peak in the spectra of turbulent velocity fluctuations, which are not seen in the acoustic spectra. In addition, flow field measurements for both the blunt-edged and sharp-edge plates indicate significant increases in turbulence intensity at the plate surface which are believed to be caused by the presence of holes and related to the increase in noise seen at high frequencies. The wing of a remote-controlled glider is modified with porous plates near the trailing edge to demonstrate reductions in surface pressure level fluctuations on a flying vehicle at the owl scale. Measurements of these fluctuations on the wing and fuselage indicate the capacity of porous plates to modestly reduce surface pressure levels in select frequency ranges and settings of aerial vehicles. 

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4. Exploring the Feasibility of a Thermoacoustic Metastructure for Energy Harvesting and Noise Mitigation

   https://doi.org/10.1121/2.0001670  

   Biswas, Samarjith; Manimala, James M. (December 2022, Proceedings of meetings on acoustics)

   The thermoacoustic effect provides a means to convert acoustic energy to heat and vice versa without the need for moving parts. This could enable the realization of mechanically-robust, noise mitigating energy harvesters via the development of thermoacoustic metastructures using additive and hybrid fabrication processes and materials. The mechanical, thermal and geometric properties of the porous stack that forms a set of acoustic waveguides in thermoacoustic metastructures are key to their performance. In this proof-ofconcept study, firstly, various ceramic and polymeric stack designs are evaluated using a custom-built thermoacoustic test rig. Influence of stack parameters such as material, length, location, porosity and pore geometry are correlated to simulations using DeltaEC, a software tool based on Rott’s linear approximation. Preliminary results also show a reduction in sound pressure level of around 5.28 dB across the thermoacoustic metastructure at resonance (117.5 Hz). An acousto-thermo-electric transduction scheme is employed to harvest useable electrical power using the best performing stack. Steady-state peak voltage generated was 33 mV for a temperature difference of about 30 degree Celsius across the stack at resonance. Further investigations are underway to establish structure-performance relationships by extracting scaling laws for power-to-volume ratio and frequency-thermal gradient dependencies. 

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5. Polar vortex crystals: Emergence and structure

   https://doi.org/10.1073/pnas.2120486119  

   Siegelman, Lia; Young, William R.; Ingersoll, Andrew P. (April 2022, Proceedings of the National Academy of Sciences)

   Vortex crystals are quasiregular arrays of like-signed vortices in solid-body rotation embedded within a uniform background of weaker vorticity. Vortex crystals are observed at the poles of Jupiter and in laboratory experiments with magnetized electron plasmas in axisymmetric geometries. We show that vortex crystals form from the free evolution of randomly excited two-dimensional turbulence on an idealized polar cap. Once formed, the crystals are long lived and survive until the end of the simulations (300 crystal-rotation periods). We identify a fundamental length scale, L γ = ( U / γ ) 1 / 3 , characterizing the size of the crystal in terms of the mean-square velocity U of the fluid and the polar parameter γ = f p / a p 2 , with f p the Coriolis parameter at the pole and a p the polar radius of the planet. 

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