[KVRC] Khamsin Virtual Racecar Challenge 2016

[machin]

The problem for me is that I was purposefully very cautious with my front end downforce for round 1, because I didn't want to lose any power....

I suggested a compromise agreement, allowing milder rules for who has had cooling issues (the strange behaviour of the solutor could be a consequence of low pressure, not a cause): it would be applied only if all the participants would accept it....

My personal view is that the rules and points for Round 1 should stay as they were. (i.e. same cooling rules, 40% power minimum reduction, etc) because they were the rules we were playing to at the time, and the rules we designed our cars around: If I'd known there would only be a 75% minimum power I would have put more downforce on the front end, because there would've been less risk of doing so.

If the rules are going to change for Round 2 they need to be agreed by all before hand. I've already made the changes to my car for round 2 and don't really plan on doing any more changes, so my preference would be for the rules to stay as they are for Round 2... unless the round 2 submission date is pushed back.

As an aside: if the minimum Power would be retrospectively changed to 75% for Round 1 then Mantium would beat my time, but wouldn't beat yours... so the proposal affects me more than it does you (I'm not suggesting you picked the 75% specifically to achieve that scenario, just pointing out that any changes now affect some people more than it affects others).

[Jolle]

If the race starts wet, I think there are a bunch of young eager drivers who never "did" Monaco in the wet. Will be fun!

[LVDH]

Hi guys,

so I have been experimenting a lot on what is going on with these strange simulations with the strange pressure values on the cooling inlets and outlets.
For this I changed the meshing settings to something close to how I configure my snappyHexMesh. The KVRC and my meshes produce similar results.
I tested different numerical settings. Again I can replicate the strange results.
So I was wondering if it is the boundary condition itself causing a strange behavior. I suspected that having the cooling surfaces touching regular walls could be an issue as the cooling surfaces have a velocity of about 3m/s applied to them while the walls are at 0. So I created an area between the two with a slip boundary condition. Again I get the same old results.
At some point I was wondering if most the CFD setup is OK and we are looking at (imposed) physics at work. Luckily I have some simulations with pretty much my car and a heat exchanger.
So we can now compare the results:



The upper picture is my car from the last race (have fun copying everything) using the cooling boundary conditions from our challenge.
The lower one is a similar car but with a heat exchanger modeled as porous media.
In both I show static and total pressure along the visualized line.
The lower one is a nice example of what the pressure should be doing along its way through the car. The static pressure is slowly increasing approaching the heat exchanger as it is slowing down. When passing it it drops a lot of pressure as energy is lost. After exiting the duct it gets subjected to the external pressure again. The total pressure is steadily decreasing with the expected sudden drop at the heat exchanger. So this is all good and the cooling air travels through the car without a problem.
On the upper picture we see different stuff however.
The static pressure at the inlet shows a huge (unrealistic?) variation approaching the cooling inlet. So now measuring the pressure difference strongly depends on where you are measuring. Looking at the pressure at the outlet you see it is between the values seen at the inlet.
Then there is the other issue that the total pressure (apparently) increases from inlet to outlet.
So this shows that we are imposing strange physics by using the boundary conditions we are using for the cooling simulation.

[CAEdevice]

Thank you very very much Mantium, I hope that your study will help Julien and Chris.

Meanwhile I am running a test to compare the physical model used now (with the BC on the inlets) and a simplified one (measuring only forces on the inlets).

For the moment, here is the car for the next race: not many new parts: mirrors, more detailed rear wheel arches, a ugly fron wing (only for Monaco specs).

[CAEdevice]

The problem for me is that I was purposefully very cautious with my front end downforce for round 1, because I didn't want to lose any power....

I suggested a compromise agreement, allowing milder rules for who has had cooling issues (the strange behaviour of the solutor could be a consequence of low pressure, not a cause): it would be applied only if all the participants would accept it....

My personal view is that the rules and points for Round 1 should stay as they were. (i.e. same cooling rules, 40% power minimum reduction, etc) because they were the rules we were playing to at the time, and the rules we designed our cars around: If I'd known there would only be a 75% minimum power I would have put more downforce on the front end, because there would've been less risk of doing so.

If the rules are going to change for Round 2 they need to be agreed by all before hand. I've already made the changes to my car for round 2 and don't really plan on doing any more changes, so my preference would be for the rules to stay as they are for Round 2... unless the round 2 submission date is pushed back.

As an aside: if the minimum Power would be retrospectively changed to 75% for Round 1 then Mantium would beat my time, but wouldn't beat yours... so the proposal affects me more than it does you (I'm not suggesting you picked the 75% specifically to achieve that scenario, just pointing out that any changes now affect some people more than it affects others).

I proposed 75% without any aim except trying to find a compromise. 60% or 80% would be the same for me.

[machin]

I'm happy to accept any rule change as long as there is sufficient time to change our cars and the new rules are not retrospectively applied.

[CAEdevice]

I'm happy to accept any rule change as long as there is sufficient time to change our cars and the new rules are not retrospectively applied.

Ok, but consider that bc we are using now have been showed to be as "not physical" (see Mantium post and my previous ones about cooling suction effect).

Considering completely valid and point giving race 1 is ok for me. Canceling it would be the same as a retrospective rule change.

[HP-Racing]

Hi guys,

so I have been experimenting a lot on...

Hi!

Thank you for your interesting study! As someone who comes from structural analysis and has little CFD knowledge I would like to know how the boundary conditions could be changed to achieve a more realistic model, or do you think that this can only be done with the porous heat exchanger?

Alex

[LVDH]

I think there is something in OF called fixedJump. Maybe that could work, but I have never tested it.
Porous media is the way to go.

[CAEdevice]

I think there is something in OF called fixedJump. Maybe that could work, but I have never tested it.
Porous media is the way to go.

I agree, but it is not working yet with OCCFD. It will be ready in a week? Cars will be ready? Mine is designed around opt1, I don't think I can change it in a few days. After race 2 I will have to leave for a month or more, I was planning to use the same car for race 3.

What will be decided for the next race? A simplified option (no bc, only measure of the force resultant on inlets/outles) could be a good temporary solution.

I think we actually need some words from the staff.

[LVDH]

I have played around with some of the exotic boundary conditions of OF and have not found anything that will help us.
A simple fix to stay with the current regulations (we have to switch to porous media) would be to remove the possibility of rotating the cooling surfaces. This would at least prevent participants from using them to create excessive downforce. It will still affect drag however, but for the high downforce races this is not relevant.

[CAEdevice]

I have played around with some of the exotic boundary conditions of OF and have not found anything that will help us.
A simple fix to stay with the current regulations (we have to switch to porous media) would be to remove the possibility of rotating the cooling surfaces. This would at least prevent participants from using them to create excessive downforce. It will still affect drag however, but for the high downforce races this is not relevant.

I don't agree. Downforce can be obtained by suction effect without rotating the inlets. On the contrary, rotated inlets are not always used to gain downforce.

Downforce increases when a suction effects is used to extract air from then front diffuser.

From my point of view, implementing the porous media is the only complete solution, but in the short term a good compromise would be to suppress the bc on inlets/outlets and measuring the resultant force over them. It is quite easy to set a reasonable value.

Edit: for the sake of completeness, my 11800N test car (that has the odd inlet behaviour ) has no rotated inlets. On the contrary the inlets of the car I used in Race 01 (no problems over the inlets and no suction effect) have been rotated.

[CAEdevice]

I have the suspect to have misunderstood Mantium proposal: if the removal of the rotation is referred to opt2, in order to have porous media availabe, I would completely agree. The only problem is that Race 2 is very near.

[graham.reeds]

In French.

Nice design.

http://m.france3-regions.francetvinfo.f ... 05203.html

[cdsavage]

Sorry about the lack of input from us. Julien has had to wipe his PC and he won't be ready for round 2 for a bit longer - we'll need to postpone round 2 by at least a week, probably a bit more. I'll announce the new date as soon as possible.

For round 1, if we were going to award full points, we would need to find a fix, and then re-simulate every entry. I don't think this is likely to happen in any reasonable amount of time, so we're probably going to need to stick with the numbers we already have. The only decision is how to award points, I would lean towards awarding no more than 50% points, but if there's a clear majority opinion on what we should do, I have no problem following that.

The porous option is unlikely to be ready for round 2. Getting rid of the boundary conditions, while still measuring the pressure integrals, is a good possibility for the upcoming rounds if we can verify that it improves things. I'd also like to make a change to the submission process to make the inlet/outlet geometry less of an issue - this would probably involve asking for body.stl to be a single, solid body, and then producing the inlet and outlet surfaces a different way.