Quote:
Originally Posted by jakobnev
I'm thinking there are two kinds of wake at the extremes: One where a bubble is formed behind the car that has the same air in it all the time, the other where new air that has been accelerated flows into the wake constantly. (In reality you would always be somewhere between these extremes)
The first kind would of course be better since air would flow around the car and then around this bubble and be left behind not much disturbed and the second kind would leave behind a trail of fast moving-energy sucking air.
This effect could be measured, for example, in a wind tunnel by turning a smoke generator on and off and observing how long the area behind the car stays smoky.
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I believe that all separated wake flow is described as random eddy turbulence of which its kinetic energy cannot be converted to useful static pressure and is therefore lost to friction heating of viscous attrition.Typically,there is very violent mixing into and out of this region.
When you describe a 'bubble' I believe you are referring to a special case which can occur at very small aft-body truncations in which the wake behaves like an attached vortex in 3-dimensions,embodying the form of a 'phantom' boat-tail of which the surrounding flow field will pass over as if it was a solid boundary.
This can only occur in about the last 20 % of the tail.