Quote:
Originally Posted by renault_megane_dci
I have never quite understood this dead air theory.
What bothers me is air is highly compressible so the layer of air is more likely a turbulent area for me, with air coming in and out and maybe some air staying deep down.
I can only think about it like coffee cup you would try to fill with the tap wide open. The dead content of it would be quite shallow and you would end up with water all around ...
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Actually,air is considered an incompressible fluid until transonic velocities are achieved,typically 250-mph for cars.
Up to that speed,air is just like water.
If you take an open,full bottle of beer, and submerge it in a fast moving stream,with the neck of the bottle normal to the flow,the beer will just stay in the bottle,with the water skimming over the opening.You might get some resonance (Helmholtz),but that's it.
Depending on the well design,it's possible that air will pool inside portions of it and just travel along with it,with the outer active flow just skimming over it.
Just like a rear bumper cover which pools air (upside-down) under the car,as low as suspension components,with active flow skimming along the pool as flat stones skipping on a lake or pond.
It like shark skin in a macro environment.All the denticles capture dead water in between,and the 'water' touches 'water',not the shark's skin.