Quote:
Originally Posted by winkosmosis
You'll have to explain that because I don't see how it's possible. 
Air is flowing around the stagnant bubble, applying force to it, and so the bubble itself is high pressure. How can it impart no pressure? You're saying you get something for nothing.
The whole car is surrounded by a boundary layer so does that mean there's no drag? The body of the vehicle is traveling with it. Does that mean the body imparts no force to the uniframe?
Are you familiar with the pitot tube? It's the device that aircraft use to measure airspeed. Airflow increases the pressure inside the tube, the higher the speed the higher the pressure. The air inside the pitot tube is a "bubble" moving along with the plane, and it imparts pressure, otherwise the device wouldn't work. |
* The bubble is higher pressure because it is at rest.It has no dynamic pressure.
* If the ventilation system is closed,the bubble simply travels along as if it were a wart on the body.The onset flow will simply skip over it as if it were solid.
* If the vent is open,a small volume of air from the free stream will travel into the evacuating bubble.
* The bubble imparts no pressure as it is part of the vehicle,just as the air inside your mouth,nose, or ears.It just goes along wherever you go.
* Only the portion of the vehicle with attached flow has a boundary layer against it.The rest will be rolling up into the backflow caused by separation and blended into the turbulence.
* With respect to form drag,the viscous shear during momentum transfer into the boundary layer from the 'inviscid' flow outside is where the surface drag entropy enters the equation.
* The pressure drag,what streamlining deals with doesn't even exist with full streamlining( complete boat-tailing ).Anything short of this and you have separation.The base pressure of the wake is taken at the point of separation.The further back you move the separation,the closer the wake pressure is to the forward stagnation pressure.With the full boat-tail there is no separation,no wake,only skin friction which we can do nothing about.
* I know what a Pitot Tube is.I used to make my living with them.I own only two now.The tube is actually two concentric tubes.The outer tube is cross-drilled to communicate static pressure.The inner communicates ram pressure.The two tubes communicate their pressure either to both sides of a liquid column in a manometer,or to a magnehelic gauge( airspeed indicator).
The 'velocity-pressure' which is indicated can be mathematically converted to reveal air velocity when corrected to local barometric pressure.
* You are correct that the air inside the Pitot tube moves along with the aircraft,as it is a closed system sensor.