Thermal contributions to aero

[gcdugas]

Does any know how/if the teams try to simulate the thermal effects the exhaust, tyres and the radiators have upon the overall aero package in the wind tunnel? I would imagine these effects to be considerable. Hot air is much less dense and would certainly affect the performance of the rear wing. Also the exhaust plume interferes with the wing/air flow but I have never heard of any team factoring this in. I remember the F2001 had exhausts flush with the bodywork which works great in the wind tunnel but fails to take much into account. The F2002 had periscope exhausts and little radiator exits surrounding the pipes. While this is almost certainly not the first attempt to account for the exhaust plume, it represents a shift in thinking. Does any team simulate, in the tunnel, exhaust plumes with compressed air? Likewise does any team attempt to mimic thermal radiator effects perhaps using an electrical heating element? And what of the tyres? They are VERY hot and the air in their wake cannot help but be affected.

[mx_tifoso]

Isn't much of this taken into consideration when working with CFD, or whatever they use before actual production?

[bazanaius]

It's an interesting question - I'd agree that in cfd it will probably be taken into account; but is it simulated in the wind tunnel? If the model is just a shell, I'd presume not - in which case is it a source of possible non-correlation between cfd and tunnel?
Is the error just absorbed by a 'fudge factor', or is there something cleverer going on?

B

[timbo]

Interesting question indeed. I don't believe that temperature (in that range) makes HUGE difference on aero (except radiator exits, where volumes of incoming and exiting air are unequal), however, termoplasticity and geometry changes hot/cold of fins, engine cover etc. may have it's effect.

[bazanaius]

Can anyone provide some known temps for a few things in the thermal image above?
Say, tyres, the floor, maybe a helmet?
If so, we could probably calibrate the image and deduce roughly the temps of the gases leaving the cooling system/exhausts, passing over the rear wing?

[myurr]

Interesting question indeed. I don't believe that temperature (in that range) makes HUGE difference on aero (except radiator exits, where volumes of incoming and exiting air are unequal), however, termoplasticity and geometry changes hot/cold of fins, engine cover etc. may have it's effect.

Ooo that raises an interesting idea - using engine heat to change the flexibility of the cars floor such that it can pass the FIAs tests when cold, but during the race when the engine is heating the floor, it is able to flex.

[timbo]

Can anyone provide some known temps for a few things in the thermal image above?
Say, tyres, the floor, maybe a helmet?
If so, we could probably calibrate the image and deduce roughly the temps of the gases leaving the cooling system/exhausts, passing over the rear wing?

I don't have any good source other than rFactor simulator. There tyre temperatures are somewhat 110-120 oC

2myurr. Yep - that was what I meant partially. However I don't think that it is practical to make floor flex much at the operating temperature, as it would be hard to control downforce gain. But making flexing engine cover, rear wing components to steal few kphs via less drag - is possible in my mind.

[bazanaius]

I wonder what temperature the air is when it reaches the rear wing. Assuming that there will be a fair bit of mixing with external air flow between the cooling outlets and the wing, maybe it cools sufficiently to have a negligable effect?

[timbo]

I wonder what temperature the air is when it reaches the rear wing. Assuming that there will be a fair bit of mixing with external air flow between the cooling outlets and the wing, maybe it cools sufficiently to have a negligable effect?

Probably yes. The airfoils itself are cool. However the gearbox top to which the wing is attached is hot and it is probably possible that it flexes.

[gcdugas]

Can anyone provide some known temps for a few things in the thermal image above?
Say, tyres, the floor, maybe a helmet?
If so, we could probably calibrate the image and deduce roughly the temps of the gases leaving the cooling system/exhausts, passing over the rear wing?

The temp is easy to measure at the track with instruments. But it is where along the longitudinal axis these temps are generated that matters. And what about the exhaust plume turbulence? But more to the point, has anyone ever heard of a team trying to simulate these things in the tunnel using the aforementioned compressed air or whatever (for exhaust plume) and a heating element to represent the radiators. In the original post I posited that the F2002 met this challenge with the periscope exhaust and the surrounding shrouds. Its 2008 so surely this area is routinely addressed by the engineers. I just want to know how. And by the way some of you talk, CFD is magic. If it was they wouldn't need those expensive tunnels. And don't be fooled, I know they still use those "old school" smoke wands to visually see things in the tunnels too.

In Phoenix, AZ in 2003 they have to close the airport to big planes (only for an hour) when the temp hit 122F due to increased take off distances. So it affects lift/negative lift. Something like humidity is a constant and can is the same at the front and rear of the car but thermal matters are different front and rear. I didn't ask about the very considerable thermal effects of the brakes because most of that is routed to the sides out of the flow to the bodywork.

[bazanaius]

CFD isn't magic, but it does allow you to simulate things in a virtual world. The key word is Simulate. Wind tunnels are still imperative, to ensure correlation between cfd and the real world, and to visualise what is going on etc. at the same time wind tunnels are not truly representative of the test track, and so these are required too.

CFD, tunnels, testing - they're all just tools used to help work out what's going on. It's the application of these tools that determines how well a team performs. It's sensible to use them all if they're available.

[bazanaius]

I was also meaning we could get the temps off the pic to help us with our approximations, because I don't have access to temp sensors at the track - if anyone does you can save me some matlabbing!

[vyselegend]

Maybe body heat can help to build some lower pressure zone (if that is wanted somehow) as air density is lower in heat. I commonly thought that the need on the upper body is more for some pressure to have downforce (or limit overall lift), so I don't see why this would help.

But I remember the period when the teams experienced exhaust in the diffuser to improve air extraction, sure it had a lot to do with the fact it was flowing, but maybe the temperature had it's impact too. I must admit I don't remember what was the conclusion of this.

That makes me ask something. What would a technical team prefers about the amount of air that flows over the rear suspentions and the rear body under the rear wing? Imagining they dispose of enough heat at some point near the engine to have a perceptible effect on the air flowing through the chassis, would they find a benefit in there, like helping air to dispers faster, or lowering the amount/proportions of the wing-generated vortexes in a bid to reduce overall drag, etc?..

[Ciro Pabón]

I know that air temperature is controlled inside a wind tunnel, because the air heats and cools when compressed or expanded, and that is what happens on the "entrance impulsor" of the tunnel. I also know that this has been taken to extremes: Audi has a wind tunnel where they produce snow and rain to check for its influence. http://www.designtaxi.com/news.jsp?id=1 ... &year=2008

I also find a couple of references to the study of heat sinks and heat flows in wind tunnels, one of them referring specifically to racing cars. These are the links (sorry, only abstracts, full articles cost money):

http://smartech.gatech.edu/handle/1853/17124 (nozzles for wind tunnels when you use heat sinks, for "global" temperature control of air)

http://www.ghtthx.com/WindTunnel.aspx (same thing: a heat exchanger design for global temperature control, some pictures)

Heat reservoir and pump to control air temperature


They can have a more complex control of temperature: this abstract shows you can build a thermally stratified flow (with wind shears induced by the changes in temperature you get depending on the height).

http://www.springerlink.com/content/n18m3097g652j433/

Now, what about testing heat producing parts in a model? The only references I got were these:

http://news.thomasnet.com/fullstory/463126 (small wind tunnel to test computer boards, by Advanced Thermal solutions)

Really small wind tunnel, it has some plastic windows to check for smoke trails


http://ieeexplore.ieee.org/xpl/freeabs_ ... mber=18078 (another abstract about designing heat sinks for computers)

http://speedarticles.com/auto_racing_article-4.html (small article on wind tunnels used for radiator design)

Now, a third class of references: CFD is used to estimate heat flows. I quote:

"CFD is used by nearly every Formula One racing team to optimize the aerodynamics of vehicle bodies and design other parts where fluid flow or heat transfer is critical." http://www.flomerics.com/casestudies/de ... hp?id=1300

These colours indicate pressure or speed, not temperature, but the model takes in account heat sinks


It seems you can, beside temperature, take in account the exhaust pressure. I found this reference:

"At some stage, the boundary conditions of the problem have to be imposed... Other similarly treated parts of the model might be...heat sources with temperatures (for the radiator faces), and pressure boundaries (for the duct exhaust)." http://autospeed.com/cms/A_109131/article.html

Finally, I found a couple of references to Shuttle tiles (the ones that kept at bay the heat of reentry) being used in NASCAR and, of course, being tested beforehand in a wind tunnel.

http://findarticles.com/p/articles/mi_q ... i_n8879904

Can you believe this? 3M makes paper that can stop the flames, to be used in NASCAR vehicles!


I imagine you could use a suit made of paper to offset the weight of a cockpit surrounded with tiles... (actually, I think that the shuttle tiles are quite light for its heat blocking ability: putting weight into space can cost you a kidney).

[PlatinumZealot]

It should be simple to implement a hot surface into a CFD model. Even though the results may not be true to life, if there is trend it can be seen.

but all in all my input is that at the high speeds where the aerodynamic surfaces on the car start to operate, the mass flow of air is so high over these surfaces, that a moving car will not be as hot as that image posted of the car in the pits and also because of the low thermal conductivity, this huge mass flow of air will barely change temperature.

( i may be wrong )