Quote:
Originally Posted by kach22i
If I'm thinking correctly you are asking about the section of the car/truck body which is typically flat across the rear.
In this case there is no flow of air.
No higher pressure or lower pressure, only a lack of pressure caused by the vehicle body moving though the air.
In part, the rolling vortexes of turbulence at the rear corners, end/top of vehicles are permitted to form because of this lack of pressure.
Had there been pressure back there, air flow could not go crazy and would be forced to conform to a flow, much as it does against the skin of the vehicle body. The body of the vehicle exerts an equal pressure, if it does not, it will flex like fabric does.
|
*The absence of 'body' at the vehicles rear triggers flow separation.
*As the air separates,counterflow initiates the formation of eddies,the boundary layer lifts away,and full-blown turbulence takes form.
*The base pressure of the turbulent wake is governed by the pressure at the separation point and this pressure is communicated throughout the entire wake region.
*This base pressure is what streamlining attempts to reduce or completely eliminate.Which in turn reduces or eliminates the pressure drag,the major constituent of profile drag.
--------------------------------------------------------------------------
The kinetic energy of the turbulence cannot ever be converted to useful pressure.It is lost forever to heat,as viscous attrition,over hundreds of feet behind the car,will eventually wear down the turbulence,eventually re-calming the air as if nothing ever happened.
It's a perfect example of the 2nd Law of Thermodynamics' entropy.
*Once you've broken Humpty Dumpty,you can never put him back together again.
