Songs over water

09/22/2016
 
Air flowing over an airfoil and cavity behaves differently when a spoiler is present.
Air flowing over an airfoil and cavity behaves differently when a spoiler is present.
If you’ve ever blown over the top of an open water bottle and heard the bottle make a humming noise, you have produced what’s called an Aeolian tone.  Aeolian tones are sounds made by wind passing over or through objects.  This phenomenon is one of the studies of aeroacoustics, which is a branch of acoustics that studies flow-generated noise. 
 
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.