Saturday, June 25, 2011

The Beauty of Prandtl-Glauert Singularities

In the last 8 months, I've experienced more air travel than I have in the last 8 years combined.  And I have to admit, every time I'm in a pressurized aluminum tube accelerating down a one mile long runway towards rotational speed, I am still impressed / in disbelief / in at least a little bit of awe by the precise symphony of physics and fluid dynamics that are providing thrust in the engines and lift on the wings.  This compressible fluid we breathe in each day, composed almost 3/4rds worth of inert nitrogen, is being pushed and squeezed and driven to lift humans into the air, safely (in Western countries).

And since humans are so visually oriented, when the passing phenomenon of a burst of condensation over the wings of a large commercial jet airliner happens at low altitudes and in high humidity, it still strikes me as remarkable.   I know it's not "breaking the sound barrier" and it's just another observation of a Prandtl-Glauert Singularity, but it is so very cool.  I catch myself looking out the window often, when the aircraft in which I am flying is at full flaps, hoping to see this phenomenon - gotta be careful not to strain one's neck doing this, especially if the landing is experiencing turbulence or is in a Southwest 737 piloted by an former Navy airman replicating a carrier landing.

And yes, when you see this at an air show (like Paris, or San Francisco Bay) and there's a large burst cone around a fighter jet, that is often a trans-sonic phenomenon.  But a Boeing 747, or a B-52 cannot reach Mach 1 (in normal or sustained operations, at least, not intentionally, or for long) and both aircraft produce some spectacular Prandtl-Glauert singularities. 
This happens not only on the tops of the wings, but also in the air intake engine cowlings.  It's nothing to be alarmed about.  It's normal fluid dynamics in progress - visibly observable aerodynamic phenomenon that is happening all the time in the world around us, just without the humidity and a keen eye, humans don't notice it.

Now it's time for me to do some yard work, before the ambient climbs into the triple digits here in Phoenix, and before the monsoon blows all the fallen lemon tree leaves I have into my pool.

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