Rear diffuser

[gsracer]

Ok, i was bored today and threw something together. Obviously this will not be the finished product, it will be a mold for a 3/16 fiberglass version i plan on making. I know the wood is warped, i can straighten it when i make the mold, please do not give me the obvious stuff, ive done this before.


What im looking for is proof of concept. Basiclly if this is a good design. Or if any improvements could be made, or if its a waste of time and i should scrap the whole idea.

It goes all they way to the base of the diff and all the way to the lower control arms and will be held on with dzus fasteners.

Remember this is rough, i threw this together in an hour lol

input/thoughts/comments

pics

[bejota180sx]

i've always wanted to do something like this on my fastback, never really tackled it...
nice work, from how it looks, the end result will look nice when finished

are you gonna make it completly out of FRP or have a MDF(or whatever) base?

[gsracer]

it will be complete fiberglass, roughly 3/16 finished, i might use a foam car and laminate that but i cant find anyone who sells it locally.

[atutt]

Oh snap... When ever I see a thread by gsracer I know it's gonna be full of win!

I can't wait to see where this goes and the final result!

[Def]

Maybe you're planning on it, but the front needs a really rounded edge that goes up most of the way to the chassis to blow flow on the top of the diffuser.

I'd also extend that center ramp towards the front more to reduce the angle. Hard to tell, but it looks a little steep. I would maybe make it a bit wider if possible, but it looks like you were trying to avoid the exhaust as much as possible.

[wings_s13]

your idea is a sound one but if u can woulp prob match the front with the back, or even do a full front to back kinda thing. do u have a s13?? bc i do and would love to see some thing made for that as well? Any ways it sooks sick as hell, and for an hr hell like to see what u can do in a day lol good luck man keep it up

[Def]

Responding here instead of PM.

The conservative divergance angle of a diffuser is 7 degrees, this ensures that you don't get separation/stalling even with no favorable pressure gradient. Since we have a low pressure area(back bumper) right at the diffuser exit, we can get away with a bit more angle. So you're typically talking about 10-12 degrees as a conservative angle. If you really push it, you might be able to get away with slightly more than this, like 15-18 degrees, but the chances of getting separation is going to be very high. Any more than this and it's almost certain you'll get separation, it's just too much change in direction too quickly so the air won't follow the roof of the diffuser.

About making sure there isn't air on top of the diffuser - best thing to do is run a flat floor in front of the diffuser. Second best is to have a rounded ramp to keep the flow from going above the diffuser right before the throat(right behind the rear diff where it turns up). If you have lots of flow on top of the diffuser, it ends up just creating tons of drag as the diffuser angles up and it is fighting to get out around the rear bumper. In effect it makes the "parachute effect" of the rear bumper much more pronounced, so you end up with probably a net increase in drag, vs. a net decrease in drag which would usually happen with a diffuser.

The sides of the diffuser roof look like they diverge pretty rapidly, here you really want about 7-8 degrees max, especially with a steep roof angle.

The wider you make the throat, the more downforce the diffuser will make, so it's best to make it as wide as possible. The exhaust flow will not impede the flow out of the diffuser if the exit is close to your exhaust, as it will actually want to entrain more air and help the diffuser's efficiency. It's like a water hose squirting into a pool, right at the sides of the water hose exit you can feel the water being "sucked into" the stream. Remember, the faster air is moving, the lower its relative static pressure is. As you move air away from a source, the dynamic pressure component will make the pressure drop. The fast moving exhaust stream will be lower pressure than the diffuser area due to both of these effects.

Hope that helps a bit. I've already got a round design of a diffuser in mind, but I am waiting a bit to do some other things before I do aero work.

[gsracer]

Ok so i need to make the throat wider and lessen the angle the throat say down to 15 degrees, as for the flat portion on either side of the throat. It may not look like this in the pictures but its pretty much parallel to the floor with a rise in the back of maybe 4 degrees.

Now two more questions. The throat of the diffuser is smaller towards the diff and wider towards the rear, is there an optimum angle for this, and lastly im totally lost on the rouded edge towards the diff, do you mean rounded horizontally or rounded verticlly (ie a lip at the entrance).

[Def]

Originally Posted by gsracer

Ok so i need to make the throat wider and lessen the angle the throat say down to 15 degrees, as for the flat portion on either side of the throat. It may not look like this in the pictures but its pretty much parallel to the floor with a rise in the back of maybe 4 degrees.

Now two more questions. The throat of the diffuser is smaller towards the diff and wider towards the rear, is there an optimum angle for this, and lastly im totally lost on the rouded edge towards the diff, do you mean rounded horizontally or rounded verticlly (ie a lip at the entrance).

The pieces of either side would probably do more if they were about 6-7 degrees upward angle, but you might run out of space to do that. Might as well have them act like freestream wings.

The sides of the diffuser ramp need to be about 5-7 degrees angled from the centerline.

The rounded edge is going vertical to keep from trapping air flowing up around the diff and subframe. If you just want to build a flat portion ahead of the diffuser with some evacuation ducts it'll keep that from being as much of a problem.

[Ghostdrifter]

Def, If he uses strakes to induce turbulent flow wouldn't he be able to keep a divergence angle of 15 deg or greater w/o getting flow separation?

[Def]

Originally Posted by Ghostdrifter

Def, If he uses strakes to induce turbulent flow wouldn't he be able to keep a divergence angle of 15 deg or greater w/o getting flow separation?

Strakes help form small vorticies in the sharp corners where they meet the diffuser floor, which does help pull air into the throat, but it doesn't really affect the separation angle much. It's more an efficiency booster, as you still have the same pressure gradient from the throat to the exit of the diffuser which will dictate how much you can turn the flow with the diffuser ramp.

[bejota180sx]

i have a question that might be useful here when somebody searches about rear diffusers... Hope it doesn't bother the OP

is it more efficient to use under the bumper (like gsracer has done...) or cut off the lower portion of the bumper to fit the diffuser?

[Ghostdrifter]

I thought the strakes turned the laminar flow of air into turbulent flow, which is more resistant to flow separation. Is that correct?

[Def]

No, laminar or turbulent flow is largely a function of object size and airspeed velocity.

You can generate vorticies to "energize" the flow and help it prevent separation or otherwise influence the flow, but it has nothing to do with turbulence.