Khamsin Virtual Racecar Challenge 2014

[cdsavage]

When I do that I get a page telling me that it isnt complete yet. I think whatever generates the case files has stopped working.

[astracrazy]

I've e-mailed Julien now

[cdsavage]

Portal seems to be working now, thanks.

Just checking, what number is being used for density in the Cl.A and Cd.A calculation? I've seen both 1225 and 1204 mentioned.

[CAEdevice]

Portal seems to be working now, thanks.

Just checking, what number is being used for density in the Cl.A and Cd.A calculation? I've seen both 1225 and 1204 mentioned.

Are these numbers constant? They shouldn't be projected ares, different for each car?

[cdsavage]

Portal seems to be working now, thanks.

Just checking, what number is being used for density in the Cl.A and Cd.A calculation? I've seen both 1225 and 1204 mentioned.

Are these numbers constant? They shouldn't be projected ares, different for each car?

To calculate Cd and Cl you would need to know the projected area, but Cd*area and Cl*area are being used to avoid this.

[machin]

Is it worth competitors having a read of this:-

http://en.wikipedia.org/wiki/Drag_equation

????

Lift/Downforce is calculated in the same way but Cd is replaced with Cl.

In Virtual Stopwatch I use an air density (rho) of 1.225 kg/m^3 which is the figure usually used for air at 15degC at sea-level (http://en.wikipedia.org/wiki/Density_of_air). In reality air density changes based on temperature and altitude, so it wouldn't be that surprising if in real life the place where you do your testing has a different air density to where you do your racing... and it'll change from track to track depending on temperature and altitude.

Julien will have to advise on he uses for the CFD runs... but a value of 1.204 is only 1.7% different to 1.225, and will affect everyone the same; I wouldn't get too hung up on it.

[CAEdevice]

My car for Magny Cours is ready (the submissin last time is tomorrow, 27h,I have a bit more than 24h to test it, am I right?)!
I'm happy about general efficency that could improve, but downforce is still too low. I'm going to mount a monkey seat, I hope it can help.

Sorry that I could not see yet some of the car that run the first race, especially one of the three car that was faster than mine
I think that if we didn't use the forum and the official website to deal only with numbers would be boring!

[variante]

Sorry that I could not see yet some of the car that run the first race, especially one of the three car that was faster than mine
I think that if we didn't use the forum and the official website to deal only with numbers would be boring!

soo much agree with that...
I'm sorry i haven't shown much of my car yet, but i've been very busy. Hopefully i'll soon reveal the definitive version of it, the results of my CFD testings and so on
I'm also sorry that not all the cars have been revealed at all... i was waiting for the official CFD images...


Also, i'd like a greater interactivity between the competitors. It's true that we're challenging each other, but we've got much more to earn sharing our knowledge than from winning the KVRC prize...

[machin]

I know Julien and Luke are pretty snowed under sorting everything out... maybe it should be a condition of the entry for each race that all competitors must post a picture (or two) of their car on this forum in order to be eligible for a time/place in the standings? That would avoid adding additional work for Julien and Luke?

Julien or Luke can then do a quick visual check to make sure that the car submitted for the race is the same as the one posted on the forum (to stop people hiding some radical new feature from their competitors!)....

????

[variante]

Actually, i'd be more intrested in seeing how the airflows behave when they interact with some weird shape, rhater than just seeing the weird shape...

But yes, your idea would still be a step forward, since at the moment we can only get to see numbers, not the cars...

[astracrazy]

Hi Guys

Thanks for the feedback. We tried to push people towards the forum this year and encourage renders. We put links in the e-mails, allocated team colours and numbers to give a personal touch. It hasn't really worked how we would have liked. Maybe going forward this season all cars will have a picture taken when i check and we upload these?

I have to admit i've been slack too doing my proper render for the website (logo's etc). Pretty bad of me. I'll do it for this race - just been really busy.

I fully agree I would like to see more interaction too. It is a challenge, but its not f1. I think a good thing to do going forward is for all of us to me more open about asking how to improve our car if we have a problem, it might get everyone talking, sharing ideas etc.

I'd be really keen to here you opinions with the visual race. Did it add something at all? Are you encouraged to watch that first? What could be improved?

CFD images: They are missing because the first race images weren't good enough to use. We will be using images from race 2. I hope we can get some really good flow images or something, we will keep improving.

If you feel we can do anything to improve the competition please let us know as well

[CAEdevice]

I am not sure about showing CFD pictures: in the real F1 it would reveal too much.

On the contrary, I think that it would be necessary to see, for each race, the pictures of the racing cars. It is not necessary to see a rendering: pictures taken from CAD or SketchUp would be ok!

[astracrazy]

I think i'll go with every race creating a gallery with a picture of each car. I won't have time to render, but its more than we have now

[CAEdevice]

I think i'll go with every race creating a gallery with a picture of each car. I won't have time to render, but its more than we have now

It would be perfect! To obtain a uniform style, you could use the same viewing setup in SKTP for all cars.

[julien.decharentenay]

Density - Short answer: The CFD model uses a density of 1.205kg/m3 - but the value is irrelevant...

Density - Long answer: The CFD model uses a density of 1.205kg/m3, which is assumed to be constant . The CFD solver is a pressure based solver that uses an incompressible assumption (i.e constant density). The actual variable being solved is p/rho (pressure divided by density). The density of 1.205kg/m3 is used when calculating the forces from the normalised pressure p/rho:

F = Integral over surface ( rho * p/rho * dN) where dN is the surface normal.

The drag and downforce coefficient are:

Cd/CL = F / 0.5*rho*V*V*A where V is the reference velocity and A is the projected area.

Introducing the expression of F above into the drag and downforce coefficient and considering that rho is constant:
Cd/CL = rho * Integral (p/rho * dN) / 0.5*rho*V*V*A = Integral (p/rho * dN) / 0.5*V*V*A

So as mentioned above the value of rho is nearly irrelevant - from a CFD perspective - in the extraction of the drag and downforce coefficients that are used in Virtual Stopwatch...

To be 100% honest: The value of rho has some indirect relevance in the laminar viscosity induced forces - but this is order of magnitude smaller than the pressure induced forces and likely to be smaller than the turbulent viscosity forces...