Full or scale Aerodynamic Wind Tunnel Testing

[Lyds]

Was just wondering who has done either part scale or full scale wind tunnel testing? If you did, how good are the results and what sort of Cd are you looking at? For those of you that haven't, have you done any CFD simulations or anything?

Cheers!

Lyds

[Lyds]

Was just wondering who has done either part scale or full scale wind tunnel testing? If you did, how good are the results and what sort of Cd are you looking at? For those of you that haven't, have you done any CFD simulations or anything?

Cheers!

Lyds

[syoung]

I messed around with some 20% scale representations (they weren't good enough to be called models!) last year as part of an individual project, which was strictly not FS-related. Our wind-tunnel's 1x1m section and 15 m/s max airflow meant that what would have been pretty bad results anyway were almost entirely useless. My Reynolds number was more than an order of magnitude out, and I couldn't do any better than that.

There is a section in a book by Katz (Race Car Aerodynamics: Designing for Speed) if you want to find out some more info about wind tunnel methods, as well as an overview of road- and CFD-testing.

This is dangerously close to reigniting the old wings debate here, but our aero has been styled by eye as we don't consider it to be as important as other things on our list this year!

[Jarrod]

http://www-personal.monash.edu.au/~fsae/frameset.htm

there is a video on this page from a TV erport of our car in the tunnel, we did around 60- 80 hours in 2002 and 2003, testing wings and also some cooling studies, (I believe RMIT may have looked at cooling as well?) and have some more planned later this year. What sort of results are you looking for? Wings and radiators are probably the only things of any interest. 2D cfd was done to determine wing profiles, some 3d work was done, but took far too long to solve. The work we are doing is back to back tests, to determine what configurations work best, with some flow vis to see what is happening. We don't have a moving floor though.

[Joel Miller]

I think FSAE cars have a Cd somewhere around 0.9-0.95. Not that it matters; aero drag has only a small influence on fuel economy cos its effect is dwarfed by the energy losses associated with frequent braking. Acceleration is affected slightly, but this doesn't matter too much as we find that the car is rarely at full throttle during an enduro. There is up to a 10th or two penalty for high downforce configuration at acceleration event, wing mass included, but you wouldn't do it in full downforce setup anyway...

I agree with Jarrod; wind tunnel studies are only really useful for wings and radiators. We found that the solving time in 3D CFD takes a few hours, or possibly a day, which is okay, but generating a good mesh is really difficult and time consuming.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>This is dangerously close to reigniting the old wings debate here <HR></BLOCKQUOTE>
Bring it on!

[Lyds]

We're looking at cooling but also the overall drag and the performance of the diffusers. We testing our 2004 car in the MIRA wind tunnel before christmas and if I remember rightly we were looking at a Cd of about 0.5. The car was designed with a styling issue in mind and not aerodynamics. We're not looking at wings...but I wont go there!! http://fsae.com/groupee_common/emoticons/icon_wink.gif CFD calcs look ok, I've looked at doing some analysis using FLUENT, although getting the boundary conditions and mesh size is highly time consuming.

Lyds

[Didier Beaudoin]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Lyds:
We're looking at cooling but also the overall drag and the performance of the diffusers. We testing our 2004 car in the MIRA wind tunnel before christmas and if I remember rightly we were looking at a Cd of about 0.5. The car was designed with a styling issue in mind and not aerodynamics. We're not looking at wings...but I wont go there!! http://fsae.com/groupee_common/emoticons/icon_wink.gif CFD calcs look ok, I've looked at doing some analysis using FLUENT, although getting the boundary conditions and mesh size is highly time consuming.

Lyds <HR></BLOCKQUOTE>

A Cd of 0.5 with open wheels is something I doubt is possible. I bet there wasn't a moving floor in the wind tunnel.

[Lyds]

Nope, no moving floor, but I think that the scale tests that we are planning on doing will have utilise a moving floor rig designed by a guy last year. So hopefully some more accurate values.

[Tudor Miron]

Didier,
While I agree that moving flour is VERY important but please explane how does it relate to opened wheels? I mean considerable values.

Thanks...
Ted

[Lyds]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Didier Beaudoin:
A Cd of 0.5 with open wheels is something I doubt is possible. I bet there wasn't a moving floor in the wind tunnel. <HR></BLOCKQUOTE>


Didier, having spoken to my project supervisor today, I think that you may be wrong on this. He said that 0.5 Cd was acceptable for an FS car as most of them are not producing large amounts of downforce that would increase this coefficient. F1 cars are about Cd 1.0 but that is considering the amounts of downforce that they are producing. Although I agree with you that moving floor would be more important, I am informed that it would only vary by about 10%. In case that you question the validity of my supervisor's information, he is very experienced in this field. In fact, he did some coastdown testing ont he Bennetton car with Schumacher in it, and has also done testing for all other formulae.

Cheers

Lyds

[Homemade WRX]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Although I agree with you that moving floor would be more important, I am informed that it would only vary by about 10%. In case that you question the validity of my supervisor's information, he is very experienced in this field. <HR></BLOCKQUOTE>
did he mention that with open wheel cars you not only worry about the drag but the lift created by the open wheels, so consider that as well

[Didier Beaudoin]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Lyds:
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Didier Beaudoin:
A Cd of 0.5 with open wheels is something I doubt is possible. I bet there wasn't a moving floor in the wind tunnel. <HR></BLOCKQUOTE>


Didier, having spoken to my project supervisor today, I think that you may be wrong on this. He said that 0.5 Cd was acceptable for an FS car as most of them are not producing large amounts of downforce that would increase this coefficient. F1 cars are about Cd 1.0 but that is considering the amounts of downforce that they are producing. Although I agree with you that moving floor would be more important, I am informed that it would only vary by about 10%. In case that you question the validity of my supervisor's information, he is very experienced in this field. In fact, he did some coastdown testing ont he Bennetton car with Schumacher in it, and has also done testing for all other formulae.

Cheers

Lyds <HR></BLOCKQUOTE>

Oh well, I must be the one who's wrong, but I was under the impression the wheels' revolution increased drag and lift a lot. Of course, I don't have any data to back my impression, so you'd better beleive your supervisor.

But I swear I have heard in several occasions that the Cd for a FSAE car was around 0,9, which is considerably more than 0,5.

[Didier Beaudoin]

And by the way, I think I've heard Formula one cars have Cd that's greater than 1, because of the large wings and aero bits that create a lot of induced drag.

[Joel Miller]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Didier Beaudoin:
[...] I was under the impression the wheels' revolution increased drag and lift a lot. Of course, I don't have any data to back my impression, so you'd better beleive your supervisor.
<HR></BLOCKQUOTE>
Rotating wheels do have slightly increased aerodynamic drag, but they also have slightly less lift than stationary wheels. This is fairly straightforward if you consider what is happening at the boundary layer at the top of the tyre - it is moving forward at twice the speed of the car, thus more drag and higher pressure than otherwise on top of the tyre. The moving ground is important for front wing or undertray analysis, but I think you should be able to get a good estimate of Cd without it.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Didier Beaudoin:
But I swear I have heard in several occasions that the Cd for a FSAE car was around 0,9, which is considerably more than 0,5. <HR></BLOCKQUOTE>
I agree. 0.5 is only slightly above the range of an ordinary passenger vehicle. Our cars are open cockpit, have open wheels, circular tubes and firewalls everywhere, Pretty horrid really. On the plus side they have a fairly low frontal area, and thus drag values are fairly modest even though the coefficients of drag are high.

[Joel Miller]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Joel Miller:
Our cars are open cockpit, have open wheels, circular tubes and firewalls everywhere, Pretty horrid really. <HR></BLOCKQUOTE>
Actually Lyds, I see your car has fewer circular tubes and and jutting action than most. Pretty nice really.

Try some CFD. You'll probably need to do a lot of simplification on your CAD model to get a mesh you can work with. Use the plane of symmetry to halve the cell count. The boundary conditions shouldn't be too difficult to specify, I use a moving ground and rotating tyres with roughness values, a velocity inlet, outflow and symmetry on the midline, far wall and ceiling.

If you get a chance to verify the calculations on the previous wind tunnel testing, or get to do some of your own, let us know what Cd value you get.

[Axle_Greece]

WE have basically determined that FSAE cars are really bad parachutes (kinda heavy), but there's some definite downforce to be gotten. If your wind tunnel is YOUR wind tunnel and you don't have a rolling road (moving floor generally means you can yaw the floor left or right) you can install a sucker system to kill the boundary layer on the floor. At least you get it thin enough so it doesn't come up and smack the underside of the vehicle. 10% off sounds about right without rolling road. Maybe look at jacking the car up on blocks and running the engine so the tires spin?
Doesn't fix the front tires, but it'll get you closer to real data.

Scale models don't work to effectively for this sort of thing either... your reynolds numbers skyrocket as you get smaller and smaller, so when you scale your data back up, the thing'll look like a mack truck. Or two. Lift completely goes away at small scale, so your downforce measurements on a scale model will be confusing, and less than useless since they'd be a waste of time building the model and running it in the tunnel. I would expect a Cd of .6 with open wheels and round stuff sticking out as it does, also with an open rear end (body) you're getting recirculating flow, and a radiator hanging off the side of the car can't help. To be honest though, open/closed cockpit doesn't affect a whole lot, since there's a body in there. Put on a little windscreen and you can keep recirc. down to almost nil... which is why formula helmets have spoilers on 'em... they're nice and smooth and round, and people complain when they generate more lift than their heads weigh. *chokes on chin strap - cough cough*

Something cost effective is to do CFd after you do some back of the envelope calcs for various aerofoils. Build a small wind tunnel and check out full scale, partial chord wings. length doesn't affect your wing much, just test two of different lengths, and you can eliminate the area affected by the tip slipstream (box the ends of the wing) and figure out a scaling factor for span. more complicated front wings may not be suited to this technique, since they've often got weird curves and involve polyhedral spans.
Any questions, please feel free to email me,
emote001@odu.edu

[Scott Wordley]

Hey Lyds,

Read the "Aero data" thread in the open discussion section, I gave some numbers from our testing there.