Songs over water
Something striking about aeroacoustics is that to date, no complete scientific theory of noise generation by aerodynamic flow exists. The study employs an arrangement of the Navier-Stokes equations, which describe the motion of viscous fluids.
The various noises airplanes make during different parts of the flight, including take-off and landing, are all examples of aeroacoustics. Some propellers are designed specifically to reduce any noise produced. For example, Hartzell makes a scimitar blade that’s designed to maximize cruise performance while reducing noise. The tip design is what allows for noise reduction, while the blades’ blended airfoil design aims for maximum low-speed thrust.
One study performed by CRAFT Tech looked at controlling aircraft cavity acoustics. In the image, you can see the airflow over a flat airfoil (baseline case) compared to airflow over an airfoil with a rod spoiler (passive control case). With the flat airfoil, a large amount of flow spills into the cavity, whereas with the spoiler the flow is broken up and directed upward so that only a small portion hits the cavity.
From an aeroacoustics standpoint: What sort of noise is produced in each of these cases? How do these affect the aerodynamics of the plane?
Next time it’s a windy day and you hear the wind whistling over the treetops or shrieking as it passes by your window, remember that you are witness to a natural display of aeroacoustics.