100% Bluff Body Drag Reduction Theory

[Bicycle Bob]

The reduced drag on the main body is due to drafting in the wake of the leading object, which is designed for near-maximum drag. The CDa, or effective frontal area would almost surely go up.

[dwestmodesto]

How about a frame mount 2" trailer hitch receiver mounted to the front of the truck. All of the 4wd's have them available for winch instalation. How about a spare tire mount out 36" forward of the bumper. It wouldn't be the 72" ideal...but might yeild some benefit. You have to put the spare tire somewhere...lots of campers mount a carrier box on the front of their truck....just forget the box.
Dave

Originally Posted by Rokeby

A large part of implementing a "leading plate" would be holding it in place. I
think it would take more than just a pole. It would probably require an A-
frame at top and bottom. The low point connection wouldn't seem to be a big
problem. But the upper point of connection with the body work would be
around the top of the windshield.

Maybe the leading plate wouldn't have to extend vertically the full height of
the car. Perhaps just part way up, say to the belt line/bottom of the
windshield.

[Edit:
There is always the possibility that turning the plate on its side, so it was
horizontal, might work too. I think it would be easier to implement at any
rate.]

Who knows? Not me for sure.



I think I meant to say that aero mods should first be made at the front.

Yes, for the typical car. (But not for a car with a boat tail.) If this worked,
perhaps there would be little need to work at the rear.

At this time this is an "idea experiment," and I'm not capable of mounting a rigorous defense of it.

[aerohead]

Originally Posted by Otto

Actually, Zedan et al at University of Houston about 30 years ago found ~3.33:1 body of revolution to be best, with point of max thickness at ~44.22% of length. Carmichael et al have published on this, saying essentially the same. Hertel, too. Fast swimming fishes such as tuna are shaped in such proportions-otherwise they don't get breakfast, but are breakfast.

But, since we cannot drive around in fish-like bodies of revolution, and have comparatively crappy vehicle shapes to deal with, how about somebody trying a pie-plate-on-a-pole mod for a motorcycle?

After all, a motorcycle is about the worst shape on the road aerodynamically, so may have the most to gain with the bike running in the wake of the pie plate. Might be handy breaking up the wake turbulence and buffeting behind trucks, too, leaving the bike rider in a relatively calm pocket of air, saving fuel into the bargain.

The 2.5:1 value comes from DVL,Germany,1945.Above this fineness ratio skin friction works against any reduction in profile drag for a net loss.
The 3.33:1 value is very close to the minimum for 2-dimensional flow around a strut.
The difference between skin friction for a 3.33:1 may be a trade-off for stability as is certainly the case for airships.
The blue-fin tuna has the lowest drag coefficient of the pelagic fishes,it's frontal area occurs at aprox. 33 % of body length as of those of the lowest drag fusiform shark with 2/3rds boat tail.
I just enjoyed a preliminary drive in a half body-of-revolution fish-like body and am not convinced that we can't drive around in them.
With respect to the motorcycle,are they exempt from the laws of fluid mechanics?
And if there was any crosswind present,would the motorcycle not "miss" the wake of the pie-plate.
And if the MC were already in the wake of a truck,would the pie-plate not also be embedded within the turbulence of the truck,with no hope of performing while "submerged" in turbulence?

[Otto]

Originally Posted by aerohead

The 2.5:1 value comes from DVL,Germany,1945.Above this fineness ratio skin friction works against any reduction in profile drag for a net loss.
The 3.33:1 value is very close to the minimum for 2-dimensional flow around a strut.
The difference between skin friction for a 3.33:1 may be a trade-off for stability as is certainly the case for airships.
The blue-fin tuna has the lowest drag coefficient of the pelagic fishes,it's frontal area occurs at aprox. 33 % of body length as of those of the lowest drag fusiform shark with 2/3rds boat tail.
I just enjoyed a preliminary drive in a half body-of-revolution fish-like body and am not convinced that we can't drive around in them.
With respect to the motorcycle,are they exempt from the laws of fluid mechanics?
And if there was any crosswind present,would the motorcycle not "miss" the wake of the pie-plate.
And if the MC were already in the wake of a truck,would the pie-plate not also be embedded within the turbulence of the truck,with no hope of performing while "submerged" in turbulence?

We'll just have to disagree on optimum fineness ratios. My 3.33:1 comments based on Zedan et al, Bruce Carmichael, Heinrich Hertel, and Dr. Hoerner's books, all in my library, based on tests of optimum fuselage pod shapes, etc., i.e., bodies of revolution, not air foils. Also measured proportions of shark, tuna, etc.. per Hertel. That's my story and I'm sticking to it.

Motorcycle pie plate efficacy depends on speed and crosswind gradient--the faster the less effect from a given crosswind. You knew that.

The pie plate on a stick trick might work pretty good on a motorcycle, since the basic essentially concave shape of an unfaired motorcycle is so bad to start with. Easy to test. Actually, those crude windscreens on unfaired Harleys reportedly help somewhat, and have some of the aspect of pieplate on a stick, just not so far out in front. A serrated edge on the pieplate might also help--makes little vortices that may counter-rotate each other and dampen turbulence. At least, that's part of the theory of serrated trailing edges on new jet engine nacelles, said to be quieter than conventional.

We won't know until we try it. Can the motorcycle mod guy be talked into this? A simple coast-town test might tell the tale.

[aerohead]

Originally Posted by Otto

We'll just have to disagree on optimum fineness ratios. My 3.33:1 comments based on Zedan et al, Bruce Carmichael, Heinrich Hertel, and Dr. Hoerner's books, all in my library, based on tests of optimum fuselage pod shapes, etc., i.e., bodies of revolution, not air foils. Also measured proportions of shark, tuna, etc.. per Hertel. That's my story and I'm sticking to it.

Motorcycle pie plate efficacy depends on speed and crosswind gradient--the faster the less effect from a given crosswind. You knew that.

The pie plate on a stick trick might work pretty good on a motorcycle, since the basic essentially concave shape of an unfaired motorcycle is so bad to start with. Easy to test. Actually, those crude windscreens on unfaired Harleys reportedly help somewhat, and have some of the aspect of pieplate on a stick, just not so far out in front. A serrated edge on the pieplate might also help--makes little vortices that may counter-rotate each other and dampen turbulence. At least, that's part of the theory of serrated trailing edges on new jet engine nacelles, said to be quieter than conventional.

We won't know until we try it. Can the motorcycle mod guy be talked into this? A simple coast-town test might tell the tale.

Otto,I'm okay with disagreement,I certainly do not "know it all" and I mean no disrespect to you nor messengers Zedan,Carmichael,Hertal et al.
And I'll digress and share what I've run across and we can see if I've missed the boat on some fundamental.
I believe that Hoerner along with every Fluids text cites Prandtl's work which includes Airship bodies.
My data reflect that for length/diameter ratios of 2.5:1-2.8:1,Cds of 0.04 are realized for a frontal area drag coefficient.
An airship with 5:1 scores an overall Cd 0.05 and the higher value is attributed to greater skin friction overwhelming the diminishing form drag.
The airship Los Angeles of even greater L/D has Cd0.071.
The data clearly appear to demonstrate a trend toward increasing drag as L/D increases beyond 2.8:1.
In Hucho's text in Fig 4.117,at the top of the table appears "Body of revolution optimized for low drag" with a sub-0.04 Cd.
This data is from reference 4.85.
Reference 4.85 is Hucho's contribution to Gino Sovran's(GM Research Labs) "General Discussion........................." which cites reference 4.3.
Reference 4.3 is 'Prospects for numerical simulation of bluff-body aerodynamics,' in Sovran's "Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles",1978.
Which would mean that the sub-0.04 form is generated from CFD.
From the Journal of Fluid Engineering,Sept 1980,Vol 102,they admit that modeling of 3D lifting bodies surface singularity methods to calculate potential flow are flawed by incompleteness of ability to model near wake flow,something MIT finally achieved with the Navier Stokes Equation in 2009.
The Journal says" Although one can learn something of the fundamentals of bluff body flow by studying isolated axisymmetric bodies this family of shapes has only limited relevance to the land vehicle problem."
My question about Zedan's form,would be :
(1) Is it real and was it developed in a windtunnel at critical Reynolds number with turbulent boundary layer?
(2) Mair's boat tail research arrived at a maximum tail angle of 22-degrees to prevent flow separation ( and Mair's own research does not include skin friction of the body ).The sub-0.04 shape in Hucho's book has the max cross-section occur at 42% of length and ends with an included angle of 41-degrees.Does Zedan's form have a tail with as radical an architecture?
(3) Is the performance of Zedan's aftbody sensitive to,and completely dependent upon the exacting architecture of the forebody?
(4) If so,would the utility of Zedan's form be of only limited value and basically represent an academic curiosity?
I'd like to see the form if you could post it.
I've tried to follow Bonneville,the HPV competition at Battle Mountain,Man-powered Subs at David Taylor Tow Basin,EAA,NASA,etc.,so far I just keep seeing the same old fluids stuff.

[aerohead]

Originally Posted by winkosmosis

When neither gets aerodynamic treatment like radiused edges, I think it makes sense that the conventional would be more aerodynamic than the cabover. The flat front COE might have a bubble, but the conventional has a smaller bubble, then another bubble forming a ramp between the hood and the windshield.

I believe Renault's COE holds the record for low drag in a tractor-trailer.With boat tail,it's Cd falls below 0.30.

[Christ]

Originally Posted by aerohead

I've tried to follow Bonneville,the HPV competition at Battle Mountain,Man-powered Subs at David Taylor Tow Basin,EAA,NASA,etc.,so far I just keep seeing the same old fluids stuff.

I've tried to follow a few different things as well, and maybe it's because I'm a relative child in the field of study compared to you, but I'm still fascinated by even the "same old fluids stuff" as you refer to it.

I still get excited when I see fluid math, even if it has no bearing on things that occur in my life. (Like super cavitation)

[Bicycle Bob]

Could all this discussion of fineness ratios be resolved by differentiating between the most efficient way to move a given cross-section through the air (with no crosswind) vs the most efficient way to move a given Volume through the air? Airfoils, of course, are most concerned with moving a given surface area.

[aerohead]

Originally Posted by Bicycle Bob

Could all this discussion of fineness ratios be resolved by differentiating between the most efficient way to move a given cross-section through the air (with no crosswind) vs the most efficient way to move a given Volume through the air? Airfoils, of course, are most concerned with moving a given surface area.

The car guys always defer to 'drag-factor',or,CdA.And Cds are always in the context of frontal projected area.
Since most modders aren't going to chop the roof off their car or do extreme alterations to frontal area,we've kinda defaulted to drag coefficient as the thing of primary interest.
Jaray got the whole 'Streamlined body of revolution in ground reflection' going back in 1922.
Jaray,Kamm,Lay,and so many others ended up using the body of revolution as their starting point,it seems like the convention with road vehicles.
Hucho used it at Volkswagen and he's the one who cited the L/D 2.5:1 Streamline form,or elliptical form as the 'minimum' for drag.
He chastised Mercedes_Benz for not honoring it with R&D of their C-111 III.
I've been looking for anything which demonstrated the lowest Cd with minimum structure and so far,for something in ground-effect,which cannot benefit from the jet pumping action of a body in free-flight,I've never come across anything "shorter" than L/D 2.5,which would translate to a vehicle with Length,5X it's height.
Al's closing gotta go,will pickup tomorrow.

[Christ]

Originally Posted by aerohead

The car guys always defer to 'drag-factor',or,CdA.And Cds are always in the context of frontal projected area.
Since most modders aren't going to chop the roof off their car or do extreme alterations to frontal area,we've kinda defaulted to drag coefficient as the thing of primary interest.
Jaray got the whole 'Streamlined body of revolution in ground reflection' going back in 1922.
Jaray,Kamm,Lay,and so many others ended up using the body of revolution as their starting point,it seems like the convention with road vehicles.
Hucho used it at Volkswagen and he's the one who cited the L/D 2.5:1 Streamline form,or elliptical form as the 'minimum' for drag.
He chastised Mercedes_Benz for not honoring it with R&D of their C-111 III.
I've been looking for anything which demonstrated the lowest Cd with minimum structure and so far,for something in ground-effect,which cannot benefit from the jet pumping action of a body in free-flight,I've never come across anything "shorter" than L/D 2.5,which would translate to a vehicle with Length,5X it's height.
Al's closing gotta go,will pickup tomorrow.

I was just pondering this point earlier today and part of last night... I wonder though, if the ideal really should be 5:1 in ground effect, or if ground effect changes some part of the design?

In other words, if a body of revolution of L/D 2.5:1 were placed in ground effect without being halved, would the 2.5:1 fineness ratio still be most effective, or is that the most effective ratio for an airship-type vehicle?