Quote:
Originally Posted by 3-Wheeler
Hi Aerohead,
I'm not sure that I completely agree with what you said about "pressure" being the only factor in air movement.
Air molecules *do* have mass and because of that also have inertia.
Now the air molecule's mass is very low, but it's still there.
Regarding moving an object through the air, the biggest factor is pressure itself and the fact that the molecules "communicate" with each other at only the speed of sound, and not faster like electrons do in electrical circuits.
Air moving close to the speed of sound becomes a major issue because as you know, the "communication link" between molecules is almost nil due to the high speed. The air molecules can not communicate their localized pressure to surrounding molecules, because they are all moving at the speed of sound, and this is as fast as the pressure pulse in the air can move.
Obviously your Streamlining Template is designed to give the air time to move back into it's originally undisturbed space before our vehicle passes through, via the localized air pressure at the time. And this is partly due to particle mass, local pressure, and the speed of sound, all contributing to the general movement of air back to it's original state.
Jim.
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Jim,my pressure argument was directed towards winkosmosis' analogy of a structure going around a right-angle curve or around a smooth curve in the context of Varn's windshield.
I have wind tunnel photos of Varn's car.There is no separation anywhere in the fore-body.The radii chosen by VW are adequate for completely attached flow.I believe this has been the situation since the decade after the 1st Arab Oil Embargo.
Looking through Hucho's book,Hucho says ( paraphrasing):
* pressure drag is the largest component in the aerodynamic drag.Its minimization is the true objective of motor vehicle aerodynamics.
* on the drag problem of a body ....the front has only minor influence....the main drag originates from the rear.......... it is not important to find a proper front........ but it is very important to design a rear body surface which brings the divided streamlines smoothly together.Optimum shapes are 'streamlined' bodies having a very slender rear part.
* lower drag can only be achieved by extending the length of the vehicle's(sic) body.
* the drag coefficient for... cars may be plotted against vehicle length... (I)f the evaluation is limited to vehicles that are developed for the lowest possible drag coefficient,this expected trend is in fact confirmed.
* the fineness ratio of the ( Cd 0.15 ) ARVW is L/H=5.53...... this corresponds to an effective fineness ratio in free air of 2.27.This approaches the drag minimum recognizable in Fig.4.119.
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* Momentum does not appear in the index of the book.It is mentioned in remarks about body camber.
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* Inertia is only mentioned tangentially in Chap.13,dealing with CFD codes and the Euler equations,Navier-Stokes equations,Equation of continuity,Mach number/Reynolds number,Bernouli Theorem,and Laplace-Potential Equation
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Hucho basically blames all our problems on pressure-related effects.
We get to cheat on the front as the favorable pressure gradients there tend to hold the boundary layer against the body.
-------------------------------------------------------------------------- Our challenge is in the contour of the aft-body where no favorable pressure gradient exists,outer-flow velocity has already peaked,pressure is beginning to increase and ideal inviscid flow theory dictates that portions of the boundary layer must have their velocity retarded at locations where their velocity is already zero,triggering the flow separation which begins the turbulent wake with its low base pressure which make up the elephant in the living room we call pressure drag.
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With respect to the Template,it is based on the streamline body Hucho recommends.It has the very slender rear part.It brings the streamlines back together.It extends the body.It is separation-free.
In free air the Template has Cd 0.04 ( from Hucho's table of Cds )
In ground reflection,the Template produces a body of Cd 0.08( after Ludwig Prandtl)
Adding conventional wheels pushes it to Cd 0.13.( after Paul Jaray/Wolfgang Klemperer.
By 'integrating the wheels into the body'(Hucho) the Cd can fall below Cd 0.09 ( GM's original Sunraycer with wheel fairings )NUNA is at Cd 0.077.
Your Varna should be around Cd 0.11 according to the IHPV folks at Battle Mountain.
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The aft-body,if not very much like the Template,WILL trigger flow separation and initiate the formation of the tubulent wake.Exactly what Hucho says is paramount to avoid.
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With the exception of Jeep CJ and Hummer H-1 type vehicles,I can think of very few late model vehicles which do not provide the minimums for well attached fore-body flow.