The primary objective of this project was to investigate the silent flight of the owl and adapt its unique plumage to enable ultra-quiet propulsion of aircraft including unmanned aerial vehicles. The night owl, particularly the species Tyto Alba or barn owl, is well known for its nearly silent flight. The feathers of the night owl have three unique features that are likely responsible for its silent flight. These are 1) a stiff leading-edge ?comb?-like structure, 2) a pliable ?serrated? trailing edge, and 3) a canopy on the lifting surface of the owl wing. This project focused on numerical and experimental investigations of features 1 and 3. Numerical simulations were conducted using the a high-fidelity flow solver, coupled with an in-house acoustics code. These simulations were verified with experimental data available in the literature as well as some concurrent experiments conducted in the Stability Wind Tunnel at Virginia Tech. Demonstrations of the technology for public dissemination were developed and setup in the anechoic chamber at Iowa State on model propellers. The project also led to significant enhancements of two courses - a graduate course in aeroacoustics and an undergraduate course in turbomachinery. These included addition of new modules and term projects focusing on acoustic analysis using existing methods and a new technique developed as a part of this project.

A major focus of the work was on numerical modeling of owl-feather-inspired features. These features were simplified and idealized to understand the working mechanisms as well as to ensure that the technology could be transitioned to practice - in aircraft, wind turbines and unmanned aerial vehicles (UAVs). The leading-edge comb geometry was approximated as a sinusoidal variation in airfoil chord while maintaining a smooth variation in airfoil thickness. The canonical rod-airfoil problem was then selected to investigate leading-edge noise. A two-step numerical approach was used to predict the radiated noise ? an eddy-resolving, unsteady flow simulation is performed in the first step, and this unsteady flow field, gathered over a surface surrounding the noise sources, is then processed in the second step using an integral method to predict farfield noise. The procedure was first validated against experimental data. Since the simulated span of the geometry was intentionally smaller than the experiments to save computation time, a frequency-dependent span correction that accounts for spanwise coherence was used to correct the predicted noise spectrum. With this correction, the numerical predictions matched the data quite well. Numerical simulations of the idealized leading-edge comb geometry for the rod-airfoil problem gave a broadband noise reduction of the order of 5 decibels in the mid-to-high frequencies. The main causes of noise reduction with the owl-inspired leading-edge airfoil are:  (1) the power spectral density of unsteady lift, which is the main source of sound, is reduced with the wavy leading edge, (2) the spanwise coherence is reduced, leading to less portions of the airfoil radiating in unison, and (3) there is larger destructive interference in the coherent region. A key contribution of this work is the identification of the physical mechanisms behind the observed noise reduction.

Prior experiments at Virginia Tech had observed reduction in trailing edge noise when a porous canopy was placed in the aft section of the airfoil. These canopies were formed using 'finlets' which were constructed using i) razor-thin flat plates (called fences) mounted perpendicular to the airfoil surface, and ii) thin cylindrical rods that were lifted away from the surface by a support structure. We performed detailed, eddy-resolving simulations of such canopies to understand and explain the noise reduction mechanisms. First, a simulation of turbulent boundary layer over a baseline blade (no canopy) was validated against established measurements and prior simulations. The numerical simulations showed that the surface pressure and friction coefficients were reduced on the airfoil surface in the region between neighboring fences; however, due to the added surface area of the fences, the net skin friction drag increased slightly due to the finlets. The unsteady surface pressure in the trailing edge region of the airfoil reduced due to the fences, which lead to a reduction in farfield noise. Our detailed numerical simulations have revealed two causes for radiated noise reduction: 1) increased separation between the source and trailing edge, and 2) reduced spanwise coherence. This new understanding of how finlets reduce trailing edge noise is a key contribution to fundamental science and is critically required for designing novel noise reduction technologies using bioinspiration from the owl canopy. 

This project has resulted in improved understanding of the mechanisms responsible for the silent flight of the owl, which will guide the development of next-gen, quiet aircraft, jet engines, wind turbines, and UAVs. It has raised public awareness of aerodynamic noise and has benefitted graduate, undergraduate and K-12 students, as well as public in general through improved curricula, laboratory demonstrations, publications, and news media outlets.

Last Modified: 02/12/2023
Modified by: Anupam Sharma