Investigating The Effects Of Passive Compliant Coatings On Trailing-Edge Noise

Abstract

Trailing-edge noise is a significant noise source for various rotorcraft applications such as wind turbine rotors or propellers of unmanned aerial vehicles (UAVs). With growing dependence on wind energy and increasing usage of UAVs in military and commercial applications, it is imperative to address this shortcoming and ensure their wide spread application. In this dissertation, we investigate the noise mitigation potentials of passive compliant coatings through Computational Aeroacoustics Analysis (CAA) and experimentation through wind tunnel testing. It is hypothesized that compliant coatings damp the normal component of turbulent kinetic stresses through elastic surface deformation. Turbulent drag reduction and mitigation of flow-induced noise generated at the surface are direct byproducts. In the present study, CAA was performed on a flat plate for a chord-based Reynolds number of Rec = 460,000 using the SST k-ω Improved Delayed Detached Eddy Simulation and the Ffowcs Williams and Hawkings acoustic analogy. Trailing-edge noise was accurately predicted from 750 – 7000 Hz. Next, baseline noise results were compared with the case when two coatings with different material properties are applied onto flat plate. It was observed coating-1 may increase trailing-edge noise by 10-15 dB throughout the frequency range of interest. Whereas coating-2 shifted the energy content in trailing-edge noise to a lower frequency range and reduced noise by 2-4 dB from 600-1575 Hz, thus demonstrating that choice of coating material properties plays a crucial role in its ability to mitigate trailing-edge noise. Furthermore, experiments conducted in a closed loop wind tunnel revealed that coating-2 reduced farfield noise by 3 dB from 40 – 1750 Hz with maximum reduction of 5.11 dB at 475 Hz, thus demonstrating the favorable effects of compliant coatings on trailing-edge noise.