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

[MTrenk]

Oh, and thanks to MetroMPG for changing the title, I think it's very appropriate and much more oriented towards problem solving.

[slowmover]

slowmover:
--> It's my opinion that many cooling techniques used on the tops of cars can do well on the bottoms of cars . . . . Or you could even just use metal mesh type holes. If you cover the bottom of your truck with a belly pan, I think you will be able to achieve the kind of cooling you need. You'll need to go with something heat resistant/something that doesn't get destroyed by heat over time. I've had lots of ideas about trucks and SUV's over the years, so maybe soon I can sit down and draw them out.


Thanks for addressing this. Yes, plenty of ideas from the world of Class 8, also, where hoods are modified to speed underhood air out (to accommodate smaller radiator openings).

Any thing you choose to add is welcomed by truck owners as they tend to be double duty (making some money, directly or indirectly, and doing personal service as well).

As to heat control on a turbocharged diesel, there are manifold blankets, turbocharger blankets and exhaust wrap to keep some heat problems within narrower confines, but airflow is the sine qua non. "Expanded metal" [mesh] has been mentioned before, but how effective it is as a bellypan material . . . ?

Oilpan4, I'm aware of water mist cooling. It has limits. At best a short duration solution (not dismissing it, just contextualizing it). SNOW PERFORMANCE has a line of products that their website describes for those interested. We used water misting on big block V8's in the 1960's and '70's for trailer towing in mountains and desert.

Further, with a grille block and/or winter front I can easily remove or modify the opening based on workload against conditions. A bellypan might be set up to do the same, but I don't think it would be as workable (one would want to leave it alone).

.

[aerohead]

Originally Posted by MTrenk

aerohead:
--> Does the flat side really limit turbulence? I think there's enough potential from such a large flat plate to end up creating MORE drag than pumpkins and exhaust bits. I'm completely serious. Flat underpans are required in Formula One to reduce top speeds. :P
--> Diffusers are good.
--> If airflow volume is constant at fixed velocity, then what good does it do to analyze it when cars have changing velocity, not only due to acceleration, but changes is wind speed?
--> Is the golf ball effect really all over the car ever? Because at 20 mph I'd bet money that the hood has laminar attached flow, and a 20 mph golf ball has turbulent attached flow. Perhaps I don't know what you are trying to say.
--> Dimpling the underside of a bellypan would in fact allow you to make a flat one, which is obviously more convenient than turning the underside of your car into an airfoil profile. Let's say that a smooth sphere and a smooth flat underbody are traveling at 40 mph along side a golf ball, and a dimpled flat underbody. Suddenly they encounter wind resistance, nothing is pushing them. We all know what happens to the sphere vs. golf ball (equal diameter). Can we imagine what would happen to the underbodies (same dimensions)?
--> Yes, creasing metal panels, dimpling, or whatever sort of bending manufacturers do is related to many things: panel strength, good looks, airflow, damping properties, etc.

Nevyn:
--> Ask Formula One, they know all about dealing with flat panels and Venturi tunnels under and behind the car, and they use them to their advantage in certain way, while doing their best to eliminate them in other ways. This deals with aerodynamics that I won't know much about until I go to graduate school in approximately 3 years. Some of this stuff is trade secret ya know?

Xist:
--> As for the drawings, I just have to find someone to let me borrow their scanner.
--> I haven't owned a car in about 8 months. I'm paying for my degree by working and playing in the university orchestra for scholarship money. I have just enough for rent and food. Cars are expensiveeeeeeee to keep legal.

freebeard:
--> I'm not really sure what's going on in that diagram... Raised ribs just increase panel strength, but also increase frontal area and probably don't do anything other than just push the air around. I'll try to talk more about sidewalls later, in conjunction with underbodies and front air dams/splitters.

slowmover:
--> It's my opinion that many cooling techniques used on the tops of cars can do well on the bottoms of cars.




Or you could even just use metal mesh type holes. If you cover the bottom of your truck with a belly pan, I think you will be able to achieve the kind of cooling you need. You'll need to go with something heat resistant/something that doesn't get destroyed by heat over time. I've had lots of ideas about trucks and SUV's over the years, so maybe soon I can sit down and draw them out.

oil pan 4:
--> Not really sure I understood anything you meant.

Tried to answer posts bottom to top!

*the panel is a physical barrier which prevents the air access to areas which would be a torture chamber to laminar flow.
*diffusers are good
*we don't need to analyze for different velocities if we're just talking mpg.
*measure the length of your car and tell me what your Reynolds number is at 20 mph.Then go back into your Fluid Mechanics text and tell me what kind if boundary layer you have.Since the golf ball dimples are strictly for transition to a turbulent boundary layer,explain how they will affect your Rn/boundary layer.
*with respect to the balls traveling side by side,if they are scaled to the size of automobiles,both balls will have turbulent boundary layers and separation at 115-degrees behind the forward stagnation points.

[oil pan 4]

Originally Posted by MTrenk

aerohead:
oil pan 4:
--> Not really sure I understood

Sorry about that.
GM inadvertently placed an air dam under the the middle of the engine bay.
This air dam is very effective at directing air flow up into the engine bay.
It appears that air trying to come through the radiator and the air directed up into the engine bay by this air dam are at odds with each other.

I need to put more air over the radiator, since I do tow and it tends to be over 100'F outside fairly often.

[Frank Lee]

Don't know what makes the underside any different from the rest of the car as the boundary layer thickens over the car's entirety as it goes rearward.

[freebeard]

freebeard:
--> I'm not really sure what's going on in that diagram...

You know, I'm not either. It represents my thinking in the mid-1990s, as an <i>homage</i> to Jim Hall's Chapparal 2J Chaparral Cars - Wikipedia, the free encyclopedia It used 45 hp to create 1+ Gs of downforce exhausting into the wake. My thought was to exhaust through the top of the car to create an invisible 'spoiler'. I eventually decided it would be a bad idea because if the dynamic suction breaks it could end up like this:


The skirt was supposed to have zero clearance and wear in like a new set of tires. Acting passively, without any fans, if the area of the gap (viewed from the front) is less than the exhaust opening, wouldn't it effect a negative pressure underneath? And if the internal airflow sucked engine cooling air, it might represent energy savings.

Later, I inclined toward eliminating all that and make the backlight into a hatch over a wire basket that would hold 2 or 3 bags of groceries. 'Cause then I could name it 'Grocery Gitter'.

I'll try to talk more about sidewalls later, in conjunction with underbodies and front air dams/splitters.

Looking forward to that.

[MTrenk]

freebeard:
--> This is why almost all racing series have height restrictions on exhaust exits, and have banned skirts that touch the ground.
--> Nowadays, I am studying the effects of placing exhaust above the diffuser on my team's car, trying to pull air under the car to get the diffuser to 'up to speed' faster, since underbodies can become effective at as low as 20 mph under normal conditions (our average speed in the Formula SAE competition is around 30-40 mph, so it's vital we make as much use of these things as possible).
--> It wouldn't be negative pressure, just less pressure (probably).

Frank Lee:
--> The underbody is very different from the rest of the car. Yes, the further you go along any surface, the thicker the boundary layer (because of air molecules getting 'stuck' on the surface molecules, resulting in a zero velocity at the surface), but if you look at a flat panel boundary layer vs. an airfoil boundary layer, I think you'll get an idea of why it matters so much how things are curved.

aerohead:
--> Dimpling a surface changes up Reynold's numbers. Size matters, but so does everything else.

[freebeard]

freebeard:
--> Nowadays, I am studying the effects of placing exhaust above the diffuser on my team's car, trying to pull air under the car to get the diffuser to 'up to speed' faster, since underbodies can become effective at as low as 20 mph under normal conditions (our average speed in the Formula SAE competition is around 30-40 mph, so it's vital we make as much use of these things as possible).
--> It wouldn't be negative pressure, just less pressure (probably).

--> Who knows where, but I'm convinced I saved a magazine article about a Porsche racing special that had something about as big as a large muffler behind each rear wheel that had an exhaust tip in the middle of a venturi. Sort of like a high-bypass turbo-jet. Free engine cooling.

I'll have to admit that my conception of a 'diffuser' would be a ramp-bed truck bed with multiple tailfins, upside down. So when you say, "exhaust above the diffuser", I picture it exhausting at the termination of the ramp. How far wrong is that?

--> So, less turbulence, right?

aerohead:
--> Dimpling a surface changes up Reynold's numbers. Size matters, but so does everything else.

In another life I had a consuming interest in low-cost, energy efficient and aerodynamic housing. It occurs to me that it the geodesic dome has a frequency that is independent of the physical size. There should be a change in Reynolds number over the range between the icosahedron and might-as-well-be sphere.

I don't know that anyone has published these numbers. This would be relevant when I get to my plan to Template a class-A motorhome.

[freebeard]

--> This is why almost all racing series have height restrictions on exhaust exits, and have banned skirts that touch the ground.

Maybe a contrary approach?


If under the front a 4' hovercraft skirt you have, fail-safe into the ground you will.

You could steer with rockets!

[AndrzejM]

I'm just thinking... What about flat bellypan with small bumps instead of dimples? Any thoughts? Because in my opinion it's much simpler to glue some bumps to the flat surface of the belly pan than to make dimples in coroplast for example.

Edit: In addition, I have those bumps on the inside wall of Berta's rearview mirrors. Don't know the exact meanning of those, for now. I've heard that cut the noise of the air passing the mirror, but I'm not sure that's the only reason for that.