Bionics

What is trailing edge noise?• Unsteady air flow (turbulent flow) and sharp edges are common in

airplanes.• These two combination create loud and unwanted noise called airfoil

trailing edge noise.• Common Noise: broadband noise (annoying) created due to the above.• Less common (equally annoying) : noise generated by vortex shredding

(laminar or turbulent).• These kinds are generally seen in wind turbines, cooling fans, rotors in

helicopter, cooling fans, from control surface of submarines etc.

Why is trailing noise created?• Noise due to stress produced by Unsteady fluid motion (UFM) .• Due to the fluctuating fluid transporting momentum in time and

space stress are generated. These stresses radiate acoustic energy (as quadrupole source).• UFM along with sharp edges create more noise.• An edge provides a scattering surface that improves the acoustic

efficiency of the turbulent flow.

How does Owl come into the picture?• How the design of its feather help reduce the noises?

Video:• Owl wings produce very little turbulencehttps://www.youtube.com/watch?v=qu8eOgf4nN0