Quote:
Originally Posted by winkosmosis
I think the paraphrasing you posted is ignoring the fact that the favorable pressure at the front of the vehicle is exerting a rearward force. That's what drag is all about-- the surrounding air exerting rearward force on the vehicle. That's why turbulence creates drag, that's why the vacuum at the back of a vehicle creates drag.
Otherwise, how do you explain the Prius's front end? Why didn't they use more traditional looking front bodywork? The article I posted shows diagrams of different nose shapes, and they quantify the effects of noses, hood angles, and windshield angles on drag.
BTW, it's not exactly momentum I was talking about... It's the fact that it takes energy to push aside fluid when you're surrounded by the fluid. The vehicle has to compress a large volume of air. Look at the smoke lines from a wind tunnel-- a lot of air has to be displaced, and of course displacing it more slowly by using gradually sloped surfaces costs less energy. Frank Lee said earlier in the thread that the air molecules are trying to get back to where they were. Well they don't remember where they were, they're just being pushed back into the area behind the vehicle by the higher pressure surrounding air.
Just like the gradual slope at the front makes displacing air cost less energy, the gradual slope of a boat tail makes replacing that air cost less energy and so it's easier for the atmosphere to push those molecules back into place.
|
* The favorable pressure gradient at the front of the vehicle helps maintain attached flow.It travels along with the vehicle.It has nothing to do with Newtonian physics.It in itself imparts no drag to the vehicle.
* drag is predominantly about pressure differentials.
* Air does not exert a rearward force on the vehicle.
* It is the pressure differential between the base pressure of the turbulent wake and the forward stagnation point which creates the drag ( I'm ignoring skin friction,we can't do anything about it ).
* The 'vacuum' behind the vehicle,acting in concert with the forward stagnation pressure creates the net retarding force.
--------------------------------------------------------------------------
* The Prius' front end is cheap to build.It's not low drag.It's not a model for low drag.It's an attempt to not pay the piper for the more expensive compound windshield used on all high performance low drag vehicles.
* Your diagrams are circa 1986,yes?
* You may track down Chuck Torner and David Holls.Chuck was Director,Advanced Engineering,General Motors Corporation.He was involved with the AERO X.David was Director,Advanced Design,General Motors Corp.,also involved with AERO X.Ask them about your nose/cowl/windshield diagrams.
--------------------------------------------------------------------------
* Below 250 mph,no compression of the air takes place.
* You are spot-on with respect to the energy required to displace air,that's where the lion's share of your net fuel dollar is going when on the highway.
* Frank Lee is correct in what he says.As the air reaches the point of max. cross-section it will attempt to reach equilibrium as it was before the vehicle came along.
* If the body allows it,the air will ramp-down off the back of the aft-body without separation with little perturbation.As the air descends the tail it is giving up velocity pressure for static pressure and will exit at a pressure close to the undisturbed air mass of the outer regions.The atmosphere really doesn't push it back together,it just marches back to equilibrium.There will be some disturbance of course,if for no other reason than entropy,as a consequence of the surface friction and viscous shear of the real gases.
* The boat-tail is the deceleration ramp which allows the air to slow,gain pressure,without separation.