and it's a diffuser design that has the lowered center section at about 10 degs vs the two sides which are closer to double that

 

It looks to me like the cell size for the simulation might not be small enough to pick up the thin walls of the diffuser. I can't see how that air could flow across the back like that.

 

Maybe, but you'd think someone making a model that good would know to use automated mesh control to set a minimum wall thickness and pick those up. Or just use a dense enough mesh if whatever they were using didnt support that.

 

These days anyone can get their hands on expert simulation tools. The issues are often with a couple of settings that can easily get skipped or set wrong. I've done a lot of fluid sims and one setting tweak, especially cell size can drastically change the result.

Still that flow looks very wrong for some reason. If the air was really flowing past the verticals then you would surely see very high pressure zones under each. Maybe there is a large gap to the ground where the air can cross but the whole point of those fins is to prevent that happening. If the sim is correct why have the fins at all?

 

What I was pointing as is that, thats a pretty nice solid model. Its just surprising someone is able to make something that nice on the model side but then not do something fairly simple like that, or even fail to catch it when just looking at the results. It really does look like the sims are ignoring that the strakes even exist.

Theres some other oddities, like how the flow separated on the inside of the tires without significant turbulence. And there's also quite a few surface errors as well which certainly arent helping. Looking closer I dont think this is a solid model, I think someone scanned this in.

 

It looks like its either been meshed from point cloud data acquired from a laser scanner or decimated from a higher order 3D surface. Often the fluid sims need a closed volume so maybe they had a surface mesh and "shrink wrapped" a closed volume around it.

It looks ok but I know from my sims you sometimes need to bulk up the thin walls up to bigger than your cell size otherwise with the size of the cells let the air through like they are permeable.

 

This is interesting, I have been thinking about a diffuser lately. However, the max l/d is for a shape that I have not seen. It is not what is shown in photos here, as far as I can see.
Bearing in mind the data reservations above, should I be pencilling a concave, or a convex, shape?

 

Saw the Silver-Car at the MSX yesterday. Was surprised to see these roof rails feeding the rear wing:



The car has a DTM type flat bottom/rear diffuser and fiberglass skirts in addition to the wing. One of the tech guys said they saw significant gains with those roof fins during their wind tunnel testing, and they made it on to the final version of the production car.

That thing weighs 900 lbs and has 200HP...

 

I wounder if that's why the Koenigsegg One:1 uses some along those lines. But on a track car, I bet the visibility would be defiantly affected.

 

One the K1, those double as a swan neck wing mount. In the Silver-Car, the wing mount is separate. These are only for guiding airflow.

That car is rear engine and has a firewall behind the driver, so visibility is moot anyways in this case. It also doesn't extend all the way down the rear body work, it was about 4" deep after the immediate roof line. I understand it can act as a safety feature if sliding sideways, but I wonder if extending them to the leading edge of the wing provides benefits similar to spill plates on a splitter.

 

Looking at this picture, the roof fins make sense. The roof has a nice gentle slope to "put the air back together" and feed air to the wing, but I'm sure the air gets super dirty once it drops over the edge onto the rear hatch window. The roof fins should help keep the air from the sides of the car from spilling in. Seems like just raising the wing up into cleaner air would have been more effective, though.

 

many racing bodies don't allow the wing to be higher then the roof.

 

OGRacing, what wing are you running 200 or 300?

 

i got the 200. i am about to raise it to the roof line.

 

So I took some pics at 12 Hours of Sebring this past Saturday

C7R rear bumper



This I thought was a little weird, a plug hanging with a yellow extension cord



C7R rear wing of course



Ferrari Front fender



Viper's large front wheel well vents



Aston Martin #007 Front wheel well vents





Aston Martin #007 Rear Diffuser



Falken Tire 911 Porsche Rear Diffuser



Ferrari 458 Front Splitter



Ferrari 458 Rear Duct



Ferrari 458 Rear Wing with Step



Ferrari 458 Front Hood



Delta Wing's wing and about 1".5 -2" gurney flap right below the rear wing



Delta Wing Underbody channels and no those are not my feet



Delta Wing Front



Delta Wing Front Splitter

 

Mazda Skyactive Diesel Dive Planes



Mazda Skyactive Diesel Side



Prototype Corvette powered Rear Diffuser

 

Awesome pictures. Interesting step in the wing. I would think that the purpose there is to slow down the air and create a higher pressure differential between the top and bottom of the wing.

 

That is a pretty serious gurney on the Ferrari.

 

it's interesting to see that the ferrari and DW almost don't want the high pressure air in the center of the splitter. the DW is flush with the front of the bumper, and the ferrari have big gaps. wonder why?

 

The DW doesnt appear to want the drag, and the ferrari seems more concerned about getting as much air to its underbody aero as possible.