Quote:
Originally Posted by Piwoslaw
Thank you for explaining this 
Does this statement imply that the amount of drag at the rear of the vehicle is proportional to the circumference of the area where flow detaches, and not its surface area? I mean, the way I understand it is that drag forms along the 1-dimensional line along which airflow detaches and forms eddies, while the interior of that area is not in contact with the airflow so it does not directly play a part in drag. This would mean that the vectors of the force that is trying to slow the vehicle down are all based along the perimeter of where wake turbulance begins, not in the interior. |
I apologize for not catching this when it was 'freshly minted.'
* Reduction, or elimination of pressure drag is the primarily what 'aerodynamics' is all about, according to the pros.
* Pressure drag is a function of flow separation.
* And since 'solving' automotive forebody drag hasn't been a challenge since the 1970s, we're concerned with flow separation at the rear of the vehicle.
--------------------------------------------------------------------------------------
* The separation line defines the perimeter of the wake and it's area.
* The pressure within the wake area determines the base pressure, including longitudinally-attached vortices of extremely low pressure.
* A 'large' wake of 'high' pressure is favored to a 'small' wake of 'low' pressure.
* Ignoring surface friction drag, the pressure differential between the forward stagnation point, and the wake of the vehicle, constitutes the pressure drag.
* Low pressure of the wake is communicated to the entire area of the wake, either as pure turbulence or turbulence mixed with vortex drag.
-------------------------------------------------------------------------------------
* Depending on the vehicle length, shape, and full flow attachment, pressure vectors acting 'against' the forebody motion will be virtually cancelled, as increasing pressure vectors in the aft-body also act against the body, attempting to 'squeeze the pea from the pod;' its kinetic energy limited only by the portion lost to surface friction drag.
* Pressure drag can be completely eliminated with a 'full' streamlined body. Only the small component of friction drag will remain, an inescapable consequence of air viscosity.