Passive Trailing Edge Noise Attenuation With Porosity, Inspired By Owl Plumage
Open Access
- Author:
- Yoas, Zachary
- Graduate Program:
- Bioengineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- May 12, 2021
- Committee Members:
- Daniel Hayes, Program Head/Chair
Michael H Krane, Thesis Advisor/Co-Advisor
Adam S Nickels, Committee Member
William O Hancock, Committee Member - Keywords:
- aeroacoustics
porosity
trailing edge noise
vortex rings
owl wings - Abstract:
- The quiet flight of large owl species has been attributed to their porous plumage of their wings. Specifically, the wing porosity modifies the sound produced by the interaction of eddies in the turbulent boundary with the trailing edge of their non-compact trailing wings. Theoretical predictions have demonstrated that this porosity changes both the radiated sound levels and directivity. Moreover, the radiated sound depends on open area α and porosity diameter R, relative to acoustic wavenumber k, through the nondimensional parameter μ/k, where μ= α/R. These predictions have proved difficult to validate in wind tunnels because as porosity increases, the trailing edge noise source decreases in amplitude, relative to other sources of sound in the tunnel. The current study addresses this issue by a) reducing the problem to its fundamental element, the sound produced by the convection of a single vortical eddy past the edge of a non-compact surface, and b) by removing all other flow features by utilizing an anechoic chamber to collect measurements. This approach has been shown to be effective for validating the theoretical sound power scaling laws for a vortex ring convecting past the edge of an impermeable large flat plate. Measurements of the sound produced by this interaction were performed in the ARL Penn State anechoic chamber for a series of plates, each with a different porosity, where the control case being a rigid impermeable plate. The vortex rings, produced by a shock tube, developed from a 6mm diameter nozzle. Vortex ring motion and size were estimated from high speed Schlieren imaging of the vortex ring motion, captured at 25.1 kHz. Ring speed ranged from 39 m/s to 86 m/s, while the ring radius was 6.5 mm when the vortex ring was directly above the edge. Twelve microphones, arranged in a circle centered on the plate edge were used to measure far-field sound pressure and directivity. These measurements were used to estimate the exponent in the sound power ~Uⁿ and ~L ͫ scaling laws. Predicted changes in n, m, far-field sound directivity, and source waveforms for increasing porosity show favorable comparisons to measurements.