100% Bluff Body Drag Reduction Theory

[gone-ot]

Originally Posted by thatguitarguy

Beam me up Scotty...

...spoken in a heavy scottish broge: "...sorry, Captain, but the Di-Lithium Crystals have all gone flat!"

[Christ]

Phil -

In short, you're right. Everything I've read and seen about this particular function of aerodynamics places it so far outside the realm of "normal" operation that it's particularly useless to discuss in our forum.

It's still an interesting read, though, if you're into that sort of thing.


Rokeby -

Thanks for finding my thread. I forgot it was here.

[aerohead]

Originally Posted by Christ

Phil -

In short, you're right. Everything I've read and seen about this particular function of aerodynamics places it so far outside the realm of "normal" operation that it's particularly useless to discuss in our forum.

It's still an interesting read, though, if you're into that sort of thing.


Rokeby -

Thanks for finding my thread. I forgot it was here.

Oh yeah,I love those Russian rocket-propelled torpedos!

[Rokeby]

I agree that supercavitation, while interesting in its own right, is outside the
envelope for application to land-bound vehicles used on public roadways.

That said, I'm still poking around the 'net for more info on the original idea,
as it might be inelegantly put; "a plate on a pole."

Edit:
One little factoid from my reading to date: drag in water is ~1000 times that
in air. (I suspect you guys already know this, but it's news to me) This would
confirm that putting model cars in water flow tanks is good for generalized
flow visualization... and only that.

[aerohead]

Originally Posted by Rokeby

I agree that supercavitation, while interesting in its own right, is outside the
envelope for application to land-bound vehicles used on public roadways.

That said, I'm still poking around the 'net for more info on the original idea,
as it might be inelegantly put; "a plate on a pole."

Rokeby,that will be great,and greatly appreciated.
No stone should remain unturned,and discoveries in any branch of study could potentially be of benefit to us all.
We never know where the voyage may ultimately lead.Thanks!

[thatguitarguy]

I prefer "clown nose on a pole."---

But it is frustrating when people lose sight of an idea because of improper terminology or semantics. It's about the big picture. Or in this case the little picture - the one in example C in post #1. I think this is more applicable than the flat surface "blocking" for the round shape. Our cars are closer to "boxy" as in the first plate, than "round" or "spherical" as in the 2nd plate.

I am going to see if I can figure out some attachment scheme to try my mirror "blocking" idea.

[aerohead]

drag reduction requires a "single pass".No "circulation" is allowed.
The turbulence behind the lead structure is followed by pure turbulence.The circular motion of these turbulent eddies prevents their kinetic energy from EVER being converted to static pressure behind the trailing structure.It will simply be lost to viscous attrition at some point downstream converting all it's energy to useless heat.
Simply softening the edges of the leading edges of the second structure will accomplish everything the lead structure is accomplishing and WITHOUT any circulation and loss of kinetic energy,making this conserved energy available to increase the base pressure behind the second structure,reducing its profile drag to a value lower than that possible when using the lead structure.
So far,and for sub-transonic velocity,three-dimensional flow,the 2.5:1 teardrop body-of-revolution remains the lowest drag aerodynamic form to be discovered and published about.

[Otto]

Originally Posted by aerohead

........So far,and for sub-transonic velocity,three-dimensional flow,the 2.5:1 teardrop body-of-revolution remains the lowest drag aerodynamic form to be discovered and published about.

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.

[Christ]

Is there any info on the speeds at which the "pie plate on a stick" idea was successful?

I think, for all intents and purposes, this will be an exercise in futility at 60 MPH.

I'm saying this without experience in the field, because it's something I've never really gotten into too much, but I suspect that at the speeds that we're traveling, and even at 100 MPH, the object would still have to be ~60% of the vehicle's frontal area to have any notable effect. The lower the speed, the more of the vehicle's frontal area, and the closer to the vehicle, the object has to be to open a "pocket" of airflow.

What we're talking about here basically is cavitation, except without supersonic speeds being necessary.

If I had to chance a guess, I'd suggest that what's going to occur with this at our speeds, the airflow is going to begin to close behind the object, making it have to change direction more drastically to begin attached flow on the vehicle.

I guess the image that keeps flowing through my head right now is a truck "blocking" for a trailer... the cab's aero structure has to be larger than the trailer to maintain attached flow at highway speeds, so the flow tripping off the edges of the cab fairing doesn't begin to roll into the space between the structures.

[Rokeby]

I've found another paper dealing with the phenomena presented in my OP,
that is the reduction of drag on a square cylindrical prism (3D solid) "bluff
body" by placement of a separate cylinder in the up-stream airflow.

This new find has the drawing showing the hypothetical best placement of
the main body and cylinder/plate in common with the first reference. It has
different graphics though. It's biggest drawback is that other than a brief
introductory paragraph it is in Japanese.

Here it is, you can get to the document by using the PDF button in the:
Read/Search Full Text section:

CiNii- Drag Reduction of a Square Prism

OK, does anybody know which of the many web-based translators will produce
a readable Japanese to English version?
(Note: I'm not 'net/web savvy enough to extract Japanese characters out of
a PDF to input to a translator.)

Edit:
Here is a list that includes other studies which may shed light on this
phenomena, most of them are also in Japenese:

CiNii Square Prism