The Underbody (flat belly pan is suboptimal - explanation & solutions offered)

[aerohead]

Originally Posted by MTrenk

I don't believe that bumps would work. Dimples create low pressure zones that keep the boundary layer attached for as long as possible. Bumps won't really create that effect.

aerohead:
I know about the Reynold's numbers and blah blah blah.
--> A flat panel will have a short laminar attached boundary layer, then an attached turbulent layer, then lose any attached flow.
--> A dimpled (flat) panel will keep the attached turbulent flow, reducing drag compared to the flat panel.
Sure things are different based on how big your object is vs. how large your dimples are and stuff, but in no way can you say that a flat panel will be the same as a dimpled panel.

Still the best way is to not have a flat panel; an airfoil shaped bottom is ideal.

*the laminar boundary layer is about 25mm long with a passenger car.
*then it transitions to turbulent boundary layer.
*the flow remains attached,with growing boundary layer thickness until it encounters the end of the car,where,in the absence of a diffuser, it separates,joining the wake turbulence.
*with the addition of a slow or up to 11-degree 'fast' diffuser,the air regains pressure,separating at a lower velocity,increasing base pressure,reducing pressure drag.
*Bumps,nubs,and an assortment of vortex-generators could actually perform better than dimples.The only reason that they're not used on golf balls,is that they'd be deformed or destroyed every time they were struck by the club head.
*any discontinuity in the aerodynamic pathway increases drag.
*Aerodynamically,a smooth panel would be preferred.
*Other considerations might compete.
*the most simple airfoils (Clark Y) have a smooth flat bottom

[aerohead]

Originally Posted by MTrenk

The Toyota is underwhelming, but it sure as hell made the business people happy I'm sure. That's the only way that car would make it to market. Toyota probably came up with a version of that car with a .26 Cd, and it was rejected by the marketing department because the business people know the market better than the engineers.

I am a student, but that doesn't really matter that much. I read all sorts of stuff, and I use my brain when I read it. I've read all sorts of vehicle dynamics books, and I don't agree with everything everyone says. There are some books out there that are considered 'definitive' in their respective fields, but they are matters of reference, not engineering bibles. I prefer to try to figure things out on my own, it's the best preface to a journey to the library, I've found. You people need to quit it with this Hucho talk, because a lot of the conclusions you guys have written down on these forums aren't really worth anything, IMO.

At the last Supermileage SAE competition, the highest score was 1475 mpg. Designed, budgeted, and built by 20 year olds. Most of those cars are under 10 grand. Nuff said.

Yeah,you wouldn't want to pay any attention to a P.hD. mechanical engineer who ran a world-class full-scale wind tunnel and invented aerodynamic detail optimization.
At 20-3 mph burn-and-coast,I'm certain that all that SAE technology will really dovetail into the world of real road-going automobiles.

[Frank Lee]

Trenk's last post was 9/20/12.

[freebeard]

I'm hungry for posts to comment in, too!

[Vman455]

Originally Posted by Frank Lee

Trenk's last post was 9/20/12.

And, according to his LinkedIn profile, he switched his major from engineering to software development shortly after starting this thread.

I guess he just had too much on his......plate.

[Hersbird]

Originally Posted by Vman455

And, according to his LinkedIn profile, he switched his major from engineering to software development shortly after starting this thread.

I guess he just had too much on his......plate.

No he just singlehandedly mastered the art of aerodynamics and moved on to artificial intelligence.

[aerohead]

Originally Posted by freebeard

I'm hungry for posts to comment in, too!

I felt that his comments,while well-meant,were significantly incorrect enough that they begged rebuttal.
It was a Yogi Berra moment,in that it wasn't what he didn't know,it was what he thought he knew that just wasn't so.
If he'd actually understood boundary layer theory,he would have known that all his comments were germane only to sub-critical Reynolds number flow,which has no bearing whatsoever with road vehicle aerodynamics.

[Xist]

Originally Posted by aerohead

It was a Yogi Berra moment,in that it wasn't what he didn't know,it was what he thought he knew that just wasn't so.
If he'd actually understood boundary layer theory,he would have known that all his comments were germane only to sub-critical Reynolds number flow,which has no bearing whatsoever with road vehicle aerodynamics.

I tried to tell him that, but I do not know what any of those words mean...

I have always kind of wanted to redo the Mythbusters Golf Ball Dimple experiment. People say installing a secondary fuel tank and weighing it is scientific, but one mile of A-B testing does not convince anyone here.

Except, I do not think it would persuade anyone. People here would ask why I wasted my time and money. Everyone else would say I obviously did it wrong because those guys are obviously smarter than me.

[freebeard]

I felt that his comments,while well-meant,were significantly incorrect enough that they begged rebuttal.

You are thinking of the lurkers.

[aerohead]

Originally Posted by Xist

I tried to tell him that, but I do not know what any of those words mean...

I have always kind of wanted to redo the Mythbusters Golf Ball Dimple experiment. People say installing a secondary fuel tank and weighing it is scientific, but one mile of A-B testing does not convince anyone here.

Except, I do not think it would persuade anyone. People here would ask why I wasted my time and money. Everyone else would say I obviously did it wrong because those guys are obviously smarter than me.

Some investigators do exactly that.It allows us to use mass (a constant) instead of volume (which can vary according to temp.).
Aircraft go by mass of fuel,as they can experience,say, 105 F,to -68 F within a matter of minutes after climb out.
When the Three Musketeers tested the Chrysler Airflow aerodynamic mule,they used mass,which allowed quick easier BSFC evaluations.
UC,Davis use mass during MPG testing for SAE Supermileage events.
It's good science!