Khamsin Virtual Racecar Challenge 2015

Post here information about your own engineering projects, including but not limited to building your own car or designing a virtual car through CAD.
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CAEdevice
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Joined: 09 Jan 2014, 15:33
Location: Erba, Italy

Re: Khamsin Virtual Racecar Challenge 2015

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I have a question for the staff.

When will be the charts (or test track) available? Now that the third race has gone, I'm starting to think about the new car for the last three races. I would prefer to have the charts as we had last year, but the test track would be ok to start a rough development.

Would it be possible to have a small description of the "medium df" tracks (Magny Cours, Sao Paulo, Sepang), compared to the test track (faster, slower, ...)?

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LVDH
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Joined: 31 Mar 2015, 14:23

Re: Khamsin Virtual Racecar Challenge 2015

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variante wrote: 10m/s floor --> increasing floor velocity should decrease boundary layer under the floor and in general increase air velocity under there. This would increase rear downforce. However the greatest discrepancy from the official results is focused in that area...therefore i try to be "pessimist" with my own results decreasing floor velocity. Do you see why i'm doing that now?
No, I see that you are making an assumption. I guess when you think of new parts for your car you make them as well and either verify them with CFD or see a surprise and go back to the drawing board. You should do the same with your CFD setup. Try different moving floor velocities and check if your assumption is right. I think there is a good chance you will be surprised.
variante wrote:Anyway, the problem is not the result difference itself...as long as that that difference remains pretty much constant, like it has happened during the first two races. Why isn't that happening this third round??
Well you are coming to conclusions using two samples. That is better than one but simply not enough.

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machin
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Joined: 25 Nov 2008, 14:45

Re: Khamsin Virtual Racecar Challenge 2015

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CAEdevice wrote:I have a question for the staff.

When will be the charts (or test track) available? Now that the third race has gone, I'm starting to think about the new car for the last three races. I would prefer to have the charts as we had last year, but the test track would be ok to start a rough development.

Would it be possible to have a small description of the "medium df" tracks (Magny Cours, Sao Paulo, Sepang), compared to the test track (faster, slower, ...)?

Actually.. you should find the test track is updated already for the Medium Downforce test track... I did it last week but I didn't have time to post on here...

You might note that the engine power has been adjusted down by 10%: we wanted to make sure that these tracks promote a lower downforce configuration than the starting three rounds, and that was the best way of doing it. Effectively we've turned the boost down!

The trade-off between drag and downforce for those three tracks (and the test track!) is very similar, but I don't have actual figures to quote right now: I'll look later if I can.

Link to the test track is here:-

KVRC 2015 Test Track
COMPETITION CAR ENGINEERING -Home of VIRTUAL STOPWATCH

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variante
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Joined: 09 Apr 2012, 11:36
Location: Monza

Re: Khamsin Virtual Racecar Challenge 2015

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LVDH wrote:No, I see that you are making an assumption.
A logical one, though. Isn't it? An assumption based on the principles of physics.
Obviously there could be hidden variables...for example, i can't be 100% sure that the increased air velocity can compensate the detrimental increase of inwash caused by the wheels; however the negative balance scenario seems very unlikely to me...
LVDH wrote:I guess when you think of new parts for your car you make them as well and either verify them with CFD or see a surprise and go back to the drawing board. You should do the same with your CFD setup. Try different moving floor velocities and check if your assumption is right. I think there is a good chance you will be surprised.
That's true. I'll also try a bigger wind tunnel and a more refined mesh on the floor, or OCCFD directly...when i get the time...
(that's the choice i had to make initially: use my time to optimize CFD or use my time to produce a good car assuming that the CFD i was using, despite not being completely spot on, was decently reliable. I did choose the right thing...until this third round...)
LVDH wrote:Well you are coming to conclusions using two samples. That is better than one but simply not enough
That's right. But at the same time you have to consider that these are data coming from computer calculations...each one of them should be completely reliable. That's why i thought about a parameters change of OCCFD, and that's why i asked what setting of my CFD could determine such a variation.

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CAEdevice
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Joined: 09 Jan 2014, 15:33
Location: Erba, Italy

Re: Khamsin Virtual Racecar Challenge 2015

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variante wrote:A logical one, though. Isn't it? An assumption based on the principles of physics.
Obviously there could be hidden variables...for example, i can't be 100% sure that the increased air velocity can compensate the detrimental increase of inwash caused by the wheels; however the negative balance scenario seems very unlikely to me...
I don't think that the problem is any "hidden variable": it's a strongly not linear model, so it is difficult to estimate the performance with a different solver or using some boundary conditions (es. floor velocity) to compensate other boundary condition.

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LVDH
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Joined: 31 Mar 2015, 14:23

Re: Khamsin Virtual Racecar Challenge 2015

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variante wrote: A logical one, though. Isn't it? An assumption based on the principles of physics.
Obviously there could be hidden variables...for example, i can't be 100% sure that the increased air velocity can compensate the detrimental increase of inwash caused by the wheels; however the negative balance scenario seems very unlikely to me...
No, we are doing approximated physics, thanks to the coarse meshes, a rather rough estimation of physics.
And you might not see it, but you are changing far more than one variable at a time and driving conclusions out of a far too small data set.
CAEdevice wrote: it's a strongly not linear model
You see, even the CAD guy understands.

Just try various settings esp. for the floor velocity and maybe tell us about your findings.

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variante
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Joined: 09 Apr 2012, 11:36
Location: Monza

Re: Khamsin Virtual Racecar Challenge 2015

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LVDH wrote:No, we are doing approximated physics, thanks to the coarse meshes, a rather rough estimation of physics.
And you might not see it, but you are changing far more than one variable at a time and driving conclusions out of a far too small data set.
My previous experience with Khamsin, although not very long, showed a reliable behaviour of the solver: the system reacted as one would logically expect to both boundary condition changes and cars's geometry changes.

An example concerning moving floor: I did try the difference between a stationary floor and a moving one (moving at a half the windspeed); the moving floor did imply an increase -quite a small one- of downforce in the areas where i expected that increase to happen.

Therefore, yes, an approximation of reality, but a decently good one...
That's why, and i'm repeating it once again, i'm trying to find a logical cause for the discrepancy without attributing to the CFD approximation the issue. If the approximation happens to be the actual cause, then the challenge would loose its meaning, as we should should heavily bend our cars design to what the CFD dictates rather than to the laws of physics.

Having CFD experience yourself, have you got some data about the influence of rotating wheels VS non rotating on similar cars?
Or can you give your opinion about k-epsilon/k-omega? Should they present similar outputs in the case nothing stalls?

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LVDH
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Joined: 31 Mar 2015, 14:23

Re: Khamsin Virtual Racecar Challenge 2015

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variante wrote: Having CFD experience yourself, have you got some data about the influence of rotating wheels VS non rotating on similar cars?
Sorry, nothing at hand right now. I know that right now it is developing into a trend in the automotive world (passenger cars) to look into the effects more deeply. So far my opinion was always that you can find stuff if you just look close enough. I was never too convinced about "the huge impact" the rotating or scanned wheels have.
variante wrote: Or can you give your opinion about k-epsilon/k-omega? Should they present similar outputs in the case nothing stalls?
Yes, in my opinion there is still quite a difference. Realizable-k-Epsilon compares quite well in my opinion.
And I am not sure anymore if our CFD solver has the hybrid k-Omega-SST implemented. If yes it would make quite a difference. The y+ values in the simulations are between 20 and 60 (I think in the first round around 30) around many parts. This is not good for k-Epsilon. For hybrid k-Omega-SST it should be fine.
Nonetheless, I think you should look at your floor velocity.

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CAEdevice
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Joined: 09 Jan 2014, 15:33
Location: Erba, Italy

Re: Khamsin Virtual Racecar Challenge 2015

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I have a question for LVDH, but it's about KVRC2014 (sorry if it is slightly off topic).

I used a eps-omega solver (no other options) and I was never able to match the OpenFoam results (the floor produced half the df). All the BC where identical and the mesh was as refined as the official. Do you you think it could be dependent on the vortex law? At the end I could obtain good results only using a time dependent solution with a quite large timestep... but the KVRC was almost completed. That is why this year I choose the official framework.

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RicME85
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Joined: 09 Feb 2012, 13:11
Location: Derby

Re: Khamsin Virtual Racecar Challenge 2015

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Seems bizarre to me that you would use anything other than Khamsin in previous years and OCCFD this year seeing as the competition uses them for the official results.
Why try and imitate something when you can have the actual thing?
Khamsin was easy to use once the wind tunnel file was supplied, getting it running without it was a bit if a pain admittedly.
OCCFD is just painless, so easy to use and now that between us we have got the AWS features working its even better.

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LVDH
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Joined: 31 Mar 2015, 14:23

Re: Khamsin Virtual Racecar Challenge 2015

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CAEdevice wrote:I used a eps-omega solver
Are you sure, sounds strange? The Epsilon and the Omega both relate to turbulent decay.
CAEdevice wrote:Do you you think it could be dependent on the vortex law?
Do you mean Crocco's theorem? If yes, normal CFD software does not explicitly solve this equation. I think it can be derived from the Navier-Stokes equation. But I think it is actually limited to non-viscous flows. So unless you were not using some very exotic solver the answer is probably no.

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CAEdevice
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Joined: 09 Jan 2014, 15:33
Location: Erba, Italy

Re: Khamsin Virtual Racecar Challenge 2015

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Just received the result of the simulation for the 3rd race.
It will be a very disappointing race, but it was my fault: I had only two days to make same changes and I took the risk to modify the rear wing without testing it.
The good news is that there is an excellent matching between my simulation (OCCFD on a windows workstation, with small adjustments to keep the mesh light) and the official solver.

etsmc
etsmc
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Joined: 04 Apr 2012, 13:20

Re: Khamsin Virtual Racecar Challenge 2015

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Well happy with the improvement in the car. Gone from
round 2
Drag: 1774.47 N
Drag coefficient - Cd: 0.84
Drag area - Cd.A: 1.48 m2
Downforce: -1950.38 N
Downforce coefficient - Cl: -0.92
Downforce area - Cl.A: -1.63 m2
CoP of downforce: 1.488 m along streamwise (Y) direction from Y = 0.00 m.
KVRC Only: Corrected CoP of downforce: 1.569 m along streamwise (Y) direction from Y = 0.00 m.

To round 3
Drag: 1785.08 N
Drag coefficient - Cd: 0.86
Drag area - Cd.A: 1.49 m2
Downforce: -2523.82 N
Downforce coefficient - Cl: -1.21
Downforce area - Cl.A: -2.11 m2
CoP of downforce: 1.417 m along streamwise (Y) direction from Y = 0.00 m.
KVRC Only: Corrected CoP of downforce: 1.534 m along streamwise (Y) direction from Y = 0.00 m.

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CAEdevice
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Joined: 09 Jan 2014, 15:33
Location: Erba, Italy

Re: Khamsin Virtual Racecar Challenge 2015

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Hi Julien, Hi Chris, is there a way to use this OpenFoam Windows compilation (2.3.1) instead of OpenFoam 2.2?

https://sim-flow.com/download/openfoam-windows/

cdsavage
cdsavage
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Joined: 25 Apr 2010, 13:28

Re: Khamsin Virtual Racecar Challenge 2015

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We now have a full set of results for round 3. Here is some speed data:

Image

Image

Julien deserves special thanks this round as he's made a big effort to squeeze in the simulations while being very busy with other commitments.