CFD analysis of BRZ/FRS rear diffuser
 

:confused0068:

Related links:
http://www.ft86club.com/forums/showt...t=42605&page=9
http://www.ft86club.com/forums/showt...t=34937&page=6

So, as most of you know I have been building and tweaking my rear diffuser model for a few months now. Some experiments with success, others with failure.

I love our automotive community and would rather help foster growth and success from within, instead of looking to outside sources for work, technology and effort. So, I contacted @plucas from Hancha Group, who is not only a part of our community but also an owner and enthusiast of our platform. Hancha builds a couple parts for us and are working on more. They are in the same boat as me, they love what they do and are working to get better.

Anyhow, the moment many people have been waiting for. Hancha uses a 3D model that they are continuing to build more and more detail into. It isn't complete. This data is not 100% fact, it is meant as a function to show where and how my diffuser functions in an environmental simulation. It provide graphical representation which I can back with real world accounts that reinforce the data.

Would I love to be able to get into a wind tunnel? Yes. Would I love to run the model in UNC's engineering computers? Yes. Can I afford any of that shit? No. So if any millionaires have complaints about the testing data, feel free to paypal me some cash, because the government doesn't even want to pay me my check right now.

Without further ado.

http://www.ft86club.com/forums/pictu...pictureid=6290
http://www.ft86club.com/forums/pictu...pictureid=6289
http://www.ft86club.com/forums/pictu...pictureid=6291
http://www.ft86club.com/forums/pictu...pictureid=6292
http://www.ft86club.com/forums/pictu...pictureid=6293
http://www.ft86club.com/forums/pictu...pictureid=6294
http://www.ft86club.com/forums/pictu...pictureid=6295
http://www.ft86club.com/forums/pictu...pictureid=6296
@plucas @Gunman @FT-86 SpeedFactory @robispec

About time you posted this. Nicely done

Great stuff. I'm definitely feeling a difference at high speeds with mine.

Question: Is the lift decrease the same thing as downforce or what?

Decrease in lift = downforce.

I know we're enjoying it on track. :)

With the APR wing, the car is amazing.

Great job, Wonderbar. Worth every penny.

I promise I am working on the front end too! Just a little tough with other things going on.

I totally forgot to mention a special thanks to @Gunman. Insight, mentorship and nerding out on race car stuff is much appreciated!

For anyone tracking the car, I suspect a front splitter would help reverse the 9% rearward shift of the Cp, IF you think the aerobalance needs it.

On an FR-S which is looser from the factory, maybe the aeroshift helps balance it?

Any time @Wonderbar , a former boss once told me, "racing isn't a job, it's a lifestyle", and that applies to us in the design office too!

Great stuff!

Thats 108lbs of lift decrease. This may take some clarification from hancha, but is that a decrease from stock? If so, IIRC, thats only about 18lbs of downforce. I seem to remember that stock, there was about 90lbs of lift. I could definitely be wrong though.

I guess it depends on how you look at it. It would still be 108lbs, but if it negated the entirety of the lift over "stock" and created additional force over that, I'd still call it a win personally.

Was this modeled with a stock muffler shape?

The muffler shape or type makes no difference with the diffuser on. The diffuser encloses the entire cavity and covers the entire muffler.

I know. I am talking about the model without the diffusor. It matters whether you are comparing the diffusor to a stock car or a car with aftermarket mufflers.

Ah, gotcha. I do believe it's the stock muffler design. That's a question for. @plucas

Yes from stock. The car makes lift stock which is why it is described as a decrease in lift. The car makes downforce with Driveway Labs diffuser. Exact number won't be eact since the model is simplified. Cfd is a tool to improve designs, not something that gives absolute numbers. Even windtunnels won't give exact numbers ( i wrote about this in our blog).

Also a decrease in lift will give you more available grip just as making downforce.

Correct stock muffler design.

It's all about the trends, not the exact numbers.

Awesome to see this

Thanks for this! Just a couple questions.

1-Is this w/ underpanels or w/o? Can't tell if the model is a GT86, or FRS or if the front fascia is just the model and you're running BRZ limited numbers. I assume not w/o a spoiler.

a-Would it be possible to offer sims w/ the underpanels if they are not included?

2-Is it possible to model the OE aftermarket diffusers available from Subie just for the sake of comparison? I don't think they'll be able to match what you have but it would be a nice comparisons for consumers.

3-Would it be possible to do a few sims running the Seibon and VIS CSL trunks?

4-Same for those rear roof spoilers?

I understand if all that is too time consuming or a bit OT. All of those are an order of magnitude below the diffuser in question performance wise I'd presume. Figured it couldn't hurt to ask if you find the time and resources.

Thanks again!

That makes your lift decrease sound very good. I would like to see the results of this same simulation, but model the car with no diffusor and a typical aftermarket muffler or muffler delete shape. This would do two things for you:

1. Validate your model. Evidence suggests the car makes more rear lift with a muffler delete. If your model shows this then you have reason to trust your model. (This is huge)

2. Show even greater performance improvements. If those numbers are compared to the stock muffler, they will be even better when compared to a muffler delete or typical non-factory muffler shape. Additionally, most people buying this are not running stock exhaust.

Same questions here. Also interested in spoiler differences and maybe even with the gurney flap.

So this diffuser works just fine with muffler deletes then?

This update just turned the diffuser from a want to a must have! :)

Well, the answer to all that costs time and money. The diffuser analysis came out of my pocket, I do not have the ability or the need to spend the additional money on all these other things. Hancha is easy to to work with and easy to contact on here, Facebook and other means if you want to commission work from them.

My intent was to show a completely stock car and how just the addition of the diffuser can improve the dynamics of the cars capabilities.

Every project has to have a start and end point with it's left and right limits. There is so much that I want to do but it is outside the scope of my intent for the moment.

The model isn't 100% detailed, so it would be closer to what the car is like with underbody panels. The way I understand it, Panels on the bottom would actually show a decreased effectiveness of the diffuser because the air flowing across the bottom is already exiting in a cleaner manner. In theory is the bottom of the model was more detailed and the air more turbulent when it got the the diffuser; the model would show more positive effect coming from the diffuser.

In an ideal world, the front of your diffuser would link up (and bolt to) the JDM underpanels.

I currently have the underpanels and am interested in this rear diffuser for my car. May be interesting to see how the two can be blended together.

Sidenote: do you have pictures of the car on a lift, of the front part of the diffuser near the rear subframe?

-alex

Ideally you'd want to bridge the gap between the JDM under panels, and the diffuser. Suspension arms may hamper that.

Awesome! Great data.

i dont understand anything in the pictures, but i want one haha

My streak of applying functional mods held!!! [until I put that MaxFlow LOL-Off Valve on my car]

I plan on building my own complete system of underbody panels and body aero. If I can come up with a solution, the entire bottom of the car will be flat.

Thanks guys! If you'd like one for your shop car, contact me. :)

So, I have brainstormed a mod for the diffuser I'll be trying to fab and test over the next few weeks.

I have an idea too :D

Ferrari 599xx diffuser

http://media.caranddriver.com/images...-s-520x318.jpg

With air evacuation fans to literally suck the air out from under the car and blow it straight out the back where the tail lights once were.

http://media.caranddriver.com/images...-s-520x318.jpg

#becauseracecar :lol:

But really though, I'm eager to hear what the DPL diffuser mod will be.

That shit is crazy! Yeah, nothing like that... Lol

Interesting idea. I'm curious how much the vacuum effect messes with the diffuser. The Chapparall 2J skirted the sides, and rear, and all the air was sucked out via snowmobile engines.
http://antholonet.com/EngineersCars/...aparral2J.html
http://antholonet.com/EngineersCars/...ral2J/Fans.jpg

I was just having a talk with one of our more experienced guys at the shop about ground contact side skirts, and what works best.

Hey guys,

I was directed to this thread by Yo from ft86speedfactory after a conversation I had with him about my experience thus far with the driveway labs diffuser. I actually have not been on the forum much recently because I have been so busy with the fr-s and several other projects. I didnt know anything about this thread prior to talking to Yo and its quite crazy how spot on my description to him of my experience is compared to the simulation data. For someone who is very sensitive to vehicle character changes, you will notice this difussers effects on the car especially at high speed. The rear is more stable and eliminates that same feeling you get when you pass a semi truck with wind variations. The vacuum on the rear side can be felt as well and was one of the main things I described to Yo as noticeable. Without the diffuser the lift on the rear end was always obnoxious to me. I run the Greddy EVO 3 exhaust so there is a rather massive cavity in the back that is now completely covered.

My difusser is sitting about 1.5" lower than intended to be mounted because I built aluminum stand-offs to fit the EVO 3 mufflers and clear my APR rear spats without modification so mine is slightly different front intended install but purely to accommodate my specific exhaust and spats.

So far I am super happy with the diffuser and the effects it has had on my personal track car. APR GT-250 here I come.

Great to hear!

Would it be too much to ask for you to send me a couple snaps of your setup? Either pm or email, driveway.labs@gmail.com

Dropping the diffuser that much, is the leading edge catching air, taking it over the top of the diffuser?

In theory as long as the lead edge is above the 7 degree line off the bottom of the rear diff, should be fine. It's why I asked for photos

Also interested in seeing pics. Got tracking info on my diffuser!