Diffuser / under body questions

[AeroGT3]

Given that I'm really just a racer/enthusiast with an applicable project car I feel somewhat out of my element in speaking to CFD. Can you tell me where all the CFD is derived from .. meaning is it's assumptions and variables based on actual wind tunnel results .. or did CFD rewrite aero theories?

CFD is a numerical implementation of the governing equations (continuity, momentum, energy) of fluid dynamics with other equations derived from the above to handle turbulence, etc.

Is it NOT re-writing aero theory, but rather implementing it numerically. There are particle CFD codes out there that are a direct numerical simulations that can be incredibly accurate. Their downside is that they require lots of computing power.

[Carlos]

Breezyracer I was speculating, prompted by intuition. Thank you for giving some thought to my suggestions. I find this thread very stimulating, and I know many of you are University Students specializing in aero, it is my pleasure to take part. Another post, showed the underside of a formula car and there was a detailed interview posted about the Panoz
Champcar, which mentioned underbody VG generation and interesting details. It was a detailed description, and connecting it's content to the body surface strakes used on the 2 Lemans racers I mentioned ( which were featured on the Muslanne Straight site )-- it sparked my suggestions.There was also a mention of the term "blown flaps" in one of our threads and I had goggled some sites, that were also interesting and seemed like the idea could relate to underbody aero.

I'm a "non-engineer" but have followed racing since '65 ( of the last century. )

[SoftBatch]

Links??

[Carlos]

Sorry - I'm totally computer illiterate -- although MC has been trying to coach me along, on the simplest details-- which seem beyond me.

[miqi23]

As promised, two screen shots based on the simulations I did using a simplified geometry recommended by Breezy Racer.

Well the diffuser is producing about 12.2 Newtons of downforce at 5 m/sec, do the maths on how it may perform at higher speeds.

[IMG:1209:701]http://img176.imageshack.us/img176/5153 ... er1dd7.jpg[/img]

Above showing streamlines coloured by velocity magnitude.

[IMG:1209:701]http://img152.imageshack.us/img152/9783 ... er2ra7.jpg[/img]

Diffuser coloured by Pressure coefficient and cut plane coloured by velocity magnitude capturing the vortices etc.

[BreezyRacer]

Another post, showed the underside of a formula car and there was a detailed interview posted about the Panoz
Champcar, which mentioned underbody VG generation and interesting details.

Carlos, I've looked high and low to see the underside of the new Panoz Champ car as it's a unique opportunity to see design work that is not encumbered by too many rules (because it's a spec chassis in the series), though it's design does seem to be based on the previous champ car rules. Those rules include a stepped floor very similar to F1, and the allowance of tunnels. I've looked at every photo I can find of the car and so far only come up with this type of layout.

The car has a very nice diffuser, about 9-10 degrees. The tunnel exits are on the sides of the center diffuser rather than exiting into the center diffuser. The center diffuser deminsionally isn't all that different from what our friend Miqi23 detailed below. It probably is more effective than the model below because it spends most of it's time at ride heights under 2". Ride height is a big thing with diffusers.

The VGs in the Champ car are quite extreme and I have yet to find any place on the web that could accurately detail how they work via CFD for instance. The VGs are so large that they seem to have the ability to create a powerful vortex thru the tunnels creating tons of downforce. The floors in those those cars have to be very strong to take the loads. I believe the price for the floor alone is north of $40k US for the Panoz.

I would love to see any under side photos of the Panoz, especially after some track testing. Given that it's extrememly hard to find any photos of even the older champ car floors (they also use VGs in tunnels) I imagine I'll never see a Panoz undertray on the web.

[BreezyRacer]

As promised, two screen shots based on the simulations I did using a simplified geometry recommended by Breezy Racer.

Simply astounding miqi! I don't know where to start except say thank you very much! I didn't expect anything to come of it, not because I doubted you but because you offered to CFD a FSAE floor and this is clearly not that. I will be studying this for quite a while. How much computing power did this take? Just curious.

Are you intriqued by underfloors/diffusers like I seem to be? If you were inclined it would be interesting to see what happens when the diffuser goes full width instead of 15". I'm guessing the vortexes somewhat weaken given the ratio of the diffuser. Then divide up the big diffuser into a center section (15" again) and a smaller diffuser flanking each side. That would, I expect give the best results. Great work, again thanks!

[BreezyRacer]

Well the diffuser is producing about 12.2 Newtons of downforce at 5 m/sec, do the maths on how it may perform at higher speeds.

Here's what I came up with. Could very well be wrong.
Lift in lbs ---- mph
0.00 -------- 0 (duh)
2.74 -------- 11
10.96 -------- 22
24.65 -------- 33
43.83 -------- 45
68.50 -------- 56
98.64 -------- 67 (not at all bad but based on lots of assumptions!)

[AeroGT3]

As promised, two screen shots based on the simulations I did using a simplified geometry recommended by Breezy Racer.

Well the diffuser is producing about 12.2 Newtons of downforce at 5 m/sec, do the maths on how it may perform at higher speeds.

[IMG:1209:701]http://img176.imageshack.us/img176/5153 ... er1dd7.jpg[/img]

Above showing streamlines coloured by velocity magnitude.

[IMG:1209:701]http://img152.imageshack.us/img152/9783 ... er2ra7.jpg[/img]

Diffuser coloured by Pressure coefficient and cut plane coloured by velocity magnitude capturing the vortices etc.

Do you have the case file for that? Was it inviscid? Laminar? Turbulent?

[miqi23]

Another post, showed the underside of a formula car and there was a detailed interview posted about the Panoz
Champcar, which mentioned underbody VG generation and interesting details.

Carlos, I've looked high and low to see the underside of the new Panoz Champ car as it's a unique opportunity to see design work that is not encumbered by too many rules (because it's a spec chassis in the series), though it's design does seem to be based on the previous champ car rules. Those rules include a stepped floor very similar to F1, and the allowance of tunnels. I've looked at every photo I can find of the car and so far only come up with this type of layout.

The car has a very nice diffuser, about 9-10 degrees. The tunnel exits are on the sides of the center diffuser rather than exiting into the center diffuser. The center diffuser deminsionally isn't all that different from what our friend Miqi23 detailed below. It probably is more effective than the model below because it spends most of it's time at ride heights under 2". Ride height is a big thing with diffusers.

The VGs in the Champ car are quite extreme and I have yet to find any place on the web that could accurately detail how they work via CFD for instance. The VGs are so large that they seem to have the ability to create a powerful vortex thru the tunnels creating tons of downforce. The floors in those those cars have to be very strong to take the loads. I believe the price for the floor alone is north of $40k US for the Panoz.

I would love to see any under side photos of the Panoz, especially after some track testing. Given that it's extrememly hard to find any photos of even the older champ car floors (they also use VGs in tunnels) I imagine I'll never see a Panoz undertray on the web.

Ill perform some CFD to show how the vortex generators work when used under bodies, as you said its all limited by rules. I have attached a screen shot of an F1 diffuser with streamlines showing the flow profile coloured by velocity magnitude, basically it answers your second question too where by you mentioned the use of small tunnels by a large one in the middle. You are right, they are effecient.

[IMG:1209:701]http://img316.imageshack.us/img316/47/f1diffuserpi9.jpg[/img]

If you have any recommendations on the Champcar dimenions, it would be great as a starter or simply I will stick some vortex generators on the simplified geometry shown before and compare the downforce values.

I will also increase the diffuser angle to a maximum of 15 degrees on the simplified model and show you what happens!

[Carlos]

Here is a picture that relays some of what Breezyracer and I touched on in connection with underbody VG:

http://www.mulsannescorner.com/LolaUnderbody.jpg

[BreezyRacer]

Ill perform some CFD to show how the vortex generators work when used under bodies, as you said its all limited by rules. I have attached a screen shot of an F1 diffuser with streamlines showing the flow profile coloured by velocity magnitude, basically it answers your second question too where by you mentioned the use of small tunnels by a large one in the middle. You are right, they are effecient.


If you have any recommendations on the Champcar dimenions, it would be great as a starter or simply I will stick some vortex generators on the simplified geometry shown before and compare the downforce values.

It might take me some time to find (if possible) working deminsions for the VG setup for Champ cars. The photo posted by Carlos is probably the best example I've found so far, but I have some more that aren't as detailed but do somewhat show the profile of the tunnels. In Champ car they've been doing this since something like 1991, you would think it wouldn't be the big secret that it seems to be.

And where did you find that undertray shot of the F1 car? Great stuff!!!!! That does answer a lot about the development of VGs in barge boards to run the vortex right down the 2" step and right into the diffuser inlets! A true work of art, it is! Thanks! Is that a commonly found image? I've never seen it. More replies to come ..

[Carlos]

This is 29 pages long, and it may be familiar material -- but it may have some application or interesting value:


http://www50.dt.navy.mil/reports/CC.pdf

Just an idea -- Does the "Coanda-effect circulation control to low aspect ratio wings" have any application to underbody "flow control" or wing design? I was reading a little about it and wondered if anyone else had
an idea about possible applications.

[BreezyRacer]

I will also increase the diffuser angle to a maximum of 15 degrees on the simplified model and show you what happens!

I know that angles as high as 17 degrees are pretty common in very controlled environments like F1 but angles like that are very ride height sensitive so from what I have found so far 10m degrees provides a very nice ride height range.

So what year of F1 car is that CFD based on? Again thanks for posting that. It really helps to answer a lot of questions about how the F1 guys use the stepped floor to their advantage. I bet they are producing more use-able DF with the stepped floor than they might with a flat floor. With a flat floor you can get too low and close off the airflow. With the stepped floor the diffuser inlet never gets closed off.

It's funny .. we had a guy in another part of the forum that claimed he was an F1 aero guy and he hasn't said boo, except for that claim. If I was going to believe anyone was on a team at this point I would have to say it's you. Thanks for participating.

[Carlos]

This is a proposed design for a valveless pulsejet engine. Could the diagrams of the combustion and intake phase contribute to the design of an underbody diffuser?

http://aardvark.co.nz/pjet/coanda.shtml