Aerodynamics discussion

[ayau]

According to Bernoulli's principle, lowering the car will also increase air speeds under the car.

http://en.wikipedia.org/wiki/Ground_effect_(cars)

If the car is lower, then that means there's less air traveling underneath the car. Less air = less pressure = less lift/more downforce?

[EarlQHan]

My partner and I have been doing some CFD testing on the FR-S. Using the model we have, we estimate the CL to be .179, but our model has a smooth underbody. I don't expect the BRZ to have a different coefficient. A little more info is on our blog here: http://hanchagroup.wordpress.com/201...s-cfd/#more-27

[Dimman]

Originally Posted by EarlQHan

My partner and I have been doing some CFD testing on the FR-S. Using the model we have, we estimate the CL to be .179, but our model has a smooth underbody. I don't expect the BRZ to have a different coefficient. A little more info is on our blog here: http://hanchagroup.wordpress.com/201...s-cfd/#more-27

Awesome.

[ayau]

Originally Posted by EarlQHan

My partner and I have been doing some CFD testing on the FR-S. Using the model we have, we estimate the CL to be .179, but our model has a smooth underbody. I don't expect the BRZ to have a different coefficient. A little more info is on our blog here: http://hanchagroup.wordpress.com/201...s-cfd/#more-27

What are you using for your model? Is it a computer generated model or a physical model?

[Hanakuso]

Originally Posted by industrial

Does anyone have more pictures of the TRD Griffon underbody work? I'd like to try and replicate their work since it's probably the most extensively tested stuff for our cars so far.

I hope there is more clear pics as well since the only thing I know what shows it all is in the YouTube video.

I'm planning only covering the cat + res area. I don't feel comfortable with the diff only getting air from the bottom like the TRD Griffon, whereas our stock get airflow all around the diff

[serialk11r]

Originally Posted by Dimman

@serialk11r Don't know if you've seen this thread. Thought you might be interested.

Hello there. Mentions aren't showing up for me.

Ecomodder has really good information on streamlining that more people in this world should go read. I actually think it's the one subforum completely untainted by people spouting crap, though ecomodder as a whole has a lot less BS floating around than just about any other forum out there.

[serialk11r]

Originally Posted by ayau

According to Bernoulli's principle, lowering the car will also increase air speeds under the car.

http://en.wikipedia.org/wiki/Ground_effect_(cars)

If the car is lower, then that means there's less air traveling underneath the car. Less air = less pressure = less lift/more downforce?

Gets complicated. Say you have a straw. The straw is really narrow, but does that mean air actually wants to go through it and suck the walls of the straw in? Not unless you have enough of a pressure difference to make flow happen.

If you want to force air under the car, it's really easy. Just channel the air below the car. I am fairly sure the only job of the diffuser is to slow the air down as it leaves the rear and recover most of the energy that went into accelerating the air as it got under the car. Of course, the air "rubs" the ground and slows down and loses energy, so there's a cost to downforce produced in this manner, but it's not nearly as bad as the high angle of attack wings.

Originally Posted by ayau

This makes sense now when you say that most cars will generate lift. The shape of a car is similar to the shape of an airplane wing. The air above the car has to travel a longer distance than the air below the car. This pressure difference generates lift.

No, if you put an airplane wing to the ground it'll get sucked to the ground. The ground is a tricky thing.

The reason cars generate positive lift is because typically cars have a giant low pressure zone behind them (called the "wake") and a far-from-streamlined shape, so that this low pressure zone influences (and sometimes extends directly over) regions of air sitting over the top of the car. The bottom sides of a car are usually very "dirty" and so not much air tends to flow through and you don't get much negative lift from the bottom side of the car.

I guess another way to put it is that the usual car has close to zero attention paid to aerodynamics. Softened edges and tire spats have been around for a while, but they're only starting to actually put effort into aero for everyday cars.

[serialk11r]

Originally Posted by ayau

I still wonder if the OEM "diffuser" is actually decreasing lift, or if it's just there for aesthetics. For a sub 30k car, I have a feeling it's the latter.

What most people recognize as the "diffuser" isn't actually doing most of the diffuser-work. What's happening is that basically the muffler body combined with the "diffuser" is likely approximating the correct shape. Obviously, having exposed suspension, and a muffler with pipes sticking out in a few places, it's not an ideal situation.

The reason the diffuser isn't high is because there's not enough air going under the car. If you switch out the rear bumper and muffler and put on a typical aftermarket diffuser for aesthetics (yes, even the ones that are angled 10 degrees because someone thought that would always work), the diffuser will now sit in the wake and be more useless than the stock setup.

Originally Posted by EarlQHan

My partner and I have been doing some CFD testing on the FR-S. Using the model we have, we estimate the CL to be .179, but our model has a smooth underbody. I don't expect the BRZ to have a different coefficient. A little more info is on our blog here: http://hanchagroup.wordpress.com/201...s-cfd/#more-27

Thanks for doing the testing. Your C_d is lower than the manufacturer claim most likely because you essentially have a sealed front grill (radiator causes drag), smooth bottom, and non rotating wheels.

There's no need to tuft test the car as this is essentially the usual fastback. Lesson number 1 about streamlining is it's all about the rear, not the nose. Pointy nose is (slightly) worse than blunt nose at subsonic speed, just look at a Boeing 747.

The "problems" with the rear end are as you can see the 2 giant blue vortices (this is because while the rear window is sloping downward at a nice gentle rate, the car's body isn't getting narrower laterally, so the air that gets pushed to the side of the car is still travelling fast while the air going over the top of the car is slowing down), and the green zone over the trunk (separated flow, read the aero streamlining template thread on ecomodder for more info).

The factory spoiler helps with these slightly by essentially raising the effective height of the trunk lid and bringing it closer to where it "should" be for that length of car. Lowering the car has the effect of blocking off more of the tire area "visible" to the flow, and it also affects the "fineness ratio" (in a positive way, as in reducing drag) which I understand to be a sort of height vs. length thing, I don't really understand it but I think the effect lowering has on this is because of the ground.

[Shankenstein]

Agreed. We have an opportunity to keep this forum untainted and really make something special happen.

EarlQ, the world is a better place because of your work. If you'd be willing to distribute your solid model, I (and others) would be glad to improve it and run a heap of simulations. I'm a MechE that primarily works with Pro/E, and so-so with SolidWorks and Inventor. Minimal CFD, but some magnetic and stress FEA work. If it's validation, we can do that... but I imagine the primary benefit will be identifying "hot spots" and testing solutions for both Eco and Track purposes. Alot of smart folks on here with similar interests.

Serial, you mentioned that the classic fastback shape and its implications. Which are your expectations for the FR-S? Boxy sedans will benefit from vortex generators + well placed spoiler (use turbulence to minimize the volume of "dead air" behind the car). Porsche-style cars are pretty eco-friendly out of the box, and tend to benefit from grille blocking and diffuser work. Once the car is "eco" it becomes a great platform for downforce, such as in the Hennessy Venom GT:

[Dimman]

Originally Posted by serialk11r

Gets complicated. Say you have a straw. The straw is really narrow, but does that mean air actually wants to go through it and suck the walls of the straw in? Not unless you have enough of a pressure difference to make flow happen.

If you want to force air under the car, it's really easy. Just channel the air below the car. I am fairly sure the only job of the diffuser is to slow the air down as it leaves the rear and recover most of the energy that went into accelerating the air as it got under the car. Of course, the air "rubs" the ground and slows down and loses energy, so there's a cost to downforce produced in this manner, but it's not nearly as bad as the high angle of attack wings.



No, if you put an airplane wing to the ground it'll get sucked to the ground. The ground is a tricky thing.

The reason cars generate positive lift is because typically cars have a giant low pressure zone behind them (called the "wake") and a far-from-streamlined shape, so that this low pressure zone influences (and sometimes extends directly over) regions of air sitting over the top of the car. The bottom sides of a car are usually very "dirty" and so not much air tends to flow through and you don't get much negative lift from the bottom side of the car.

I guess another way to put it is that the usual car has close to zero attention paid to aerodynamics. Softened edges and tire spats have been around for a while, but they're only starting to actually put effort into aero for everyday cars.

This has been demonstrated quite dramatically a few times on LeMans' Mulsanne straight. At least Porsche and Mercedes (don't know if others) got their closed roof prototypes airborne there when the speed and bumpy surface pitched their noses up just a tiny bit too much. The Porsche does an actual flip. Mercedes a bit more.

[ame="www.youtube.com/watch?v=rQbgSe9S54I"]Mercedes CLR-GTR Lemans Flip - YouTube[/ame]


So even the high downforce cars are pretty much wings if you take them away from the ground.

[dasklein]

This is a great thread! I'm excited to follow it!

What are you guys using for your CFD sim? I'm definitely interested in seeing the results.

One thing to note about CFD: its not actual "real life" data. Its one of the most ground breaking additions to the design and testing process, but its still a simulation. It's a big misconception that its 100% (or even 99+%) accurate to real conditions. Its all about the software, the 3D model, and how the programmer/aerodynamicist sets everything up.

In the end you still have to get it on the track or in a "real" wind tunnel...

[robispec]

some real time testing on venting the hood
don't have that fancy F1 paint but at streets of willow there were PLENTY of people kicking up dust when they went Agricultural


YES any air going out the top is air not going out the bottom so less lift up front...

[serialk11r]

Originally Posted by Dimman

This has been demonstrated quite dramatically a few times on LeMans' Mulsanne straight. At least Porsche and Mercedes (don't know if others) got their closed roof prototypes airborne there when the speed and bumpy surface pitched their noses up just a tiny bit too much. The Porsche does an actual flip. Mercedes a bit more.

So even the high downforce cars are pretty much wings if you take them away from the ground.

I think the reason those cars went airborne so easily was because they have some kind of stupid mandate on the underbody, something like it has to be flat all the way to the diffuser? I seem to recall these LeMans cars having some giant flat splitter, and they're obviously very low to the ground, so a small angle of attack, and you can run into a lot of trouble.

I would guess that F1 cars don't go airborne since 1. giant wing in front 2. their high noses have a smooth curve channeling air to the underbody.

[johnnie]

Great work guys!! Unfortunately I don't have your skills or knowledge, so I'm just watching and trying to learn something.

Keep it up.