2017 front wings downforce compared to 2010

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Dipesh1995
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Re: 2017 front wings downforce compared to 2010

Post by Dipesh1995 » Mon Jul 31, 2017 10:43 pm

n_anirudh wrote:
Mon Jul 31, 2017 7:45 pm
1. Refinement regions need to extend in all directions. How did you estimate your wall y+?
2. Did you do any resolution study
3. Can you attempt it with a polyhedral mesh, should it give you better control?
4. Use ke model as a starting point for kw
1. Wall Y+ wasn't estimated, the sim was run several times to get Y+ into the desired range.
2. Resolution study was done for the complex wing, not the two-element wing.
3. The complex wing was meshed using polyhedral mesh. Due to the complexity of its geometry, I thought an unstructured mesh would be better than a structured mesh that trimmer is. Since the two-element wing isn't as complex to use, trimmer is ok. Its a good question though, Vyssion or jjn128 can probably answer whether trimmer would work well for the complex wing or not since they're FAR more experienced than I am when it comes to CFD.
4. I started both wing simulations using kw-SST. I shifted to realisable ke for the two-element wing based on Vyssion's previous suggestion.

Dipesh1995
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Re: 2017 front wings downforce compared to 2010

Post by Dipesh1995 » Tue Aug 01, 2017 11:05 am

https://photos.app.goo.gl/N7VP5ZFO7BMdWd7i1

Ok so convergence of Tke has significantly improved, my Y+ is still high around some areas. I've also lost a significant amount of downforce (3506N) but also drag (264N) so L/D has increased.

I also ran it with SST however it became very unstable , I'm guessing because SST doesn't work very well with high Y+.

jjn9128
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Re: 2017 front wings downforce compared to 2010

Post by jjn9128 » Tue Aug 01, 2017 1:05 pm

Dipesh1995 wrote:
Tue Aug 01, 2017 11:05 am
I've also lost a significant amount of downforce
This is why mesh independence is important. This simulation may be more accurate or less so than your previous simulations. Note the mesh size, the Cz, and Cx in excel then change the minimum cell size or the number of layers in your boundary layer mesh and run again, log the results and do a scatter plot of how your mesh size changes the results. Once the plot flattens you can say the result is accurate or accurate to within a known error if you use a mesh on the curve to save CPU costs/time. It'll impress your supervisor if nothing else :lol:

I would say your y+ is now a bit on the high side, I would aim for <100 on all surfaces, as Vyssion says 60-100.
Vyssion wrote:
Mon Jul 31, 2017 5:29 pm
Just thinking out loud here, we have kind of "hijacked" the thread a little bit... :lol:
Woops I assumed this was all from the original post #-o

Dipesh1995
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Re: 2017 front wings downforce compared to 2010

Post by Dipesh1995 » Tue Aug 01, 2017 6:44 pm

jjn9128 wrote:
Tue Aug 01, 2017 1:05 pm
Dipesh1995 wrote:
Tue Aug 01, 2017 11:05 am
I've also lost a significant amount of downforce
This is why mesh independence is important. This simulation may be more accurate or less so than your previous simulations. Note the mesh size, the Cz, and Cx in excel then change the minimum cell size or the number of layers in your boundary layer mesh and run again, log the results and do a scatter plot of how your mesh size changes the results. Once the plot flattens you can say the result is accurate or accurate to within a known error if you use a mesh on the curve to save CPU costs/time. It'll impress your supervisor if nothing else :lol:

I would say your y+ is now a bit on the high side, I would aim for <100 on all surfaces, as Vyssion says 60-100.
Yeah so since this wing is not meant to be used on one my cars as such, I'll do the mesh sensitivity study using a similar mesh for one of my Formula Student car front wings. I must say I really do appreciate the help you and Vyssion have given me in sorting my CFD issues.

One question I had was, since I'll be starting to design the complex wing Mk2 soon, would trimmer be suitable mesh to use over polyhedral mesh for a complex design? One of the key benefits I saw switching from polyhedral mesh to trimmer mesh was the lack of computational time required to generate the mesh.

Vyssion
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Re: 2017 front wings downforce compared to 2010

Post by Vyssion » Tue Aug 01, 2017 8:42 pm

Dipesh1995 wrote:
Tue Aug 01, 2017 11:05 am
Ok so convergence of Tke has significantly improved, my Y+ is still high around some areas. I've also lost a significant amount of downforce (3506N) but also drag (264N) so L/D has increased.

I also ran it with SST however it became very unstable , I'm guessing because SST doesn't work very well with high Y+.
Yeah, so because of your cell count restrictions, we're aiming for a higher y+ with a different turbulence model - so yeah SST would become unreliable at best.
It is good that your Tke has improved - from the looks of the residual plot, there was still a downward trend in the residuals... It is probably worthwhile running for 1500-2000 iterations, at least once, so that you can get a good 300ish steps at the fully converged state. Because you are running a RANS simulation, the results that you get are kinda meant to be interpreted as something like "the average flow path (or movement etc.), that the fluid follows over an infinite time"... basically, you will miss any transient or unsteady behaviour, because you are taking the fluctuations that each cell experiences over the total number of iterations, and then averaging them to get ONE value. So a good thing to do at first, is to run the results for an extra 300-500 iterations past the point you define as them being converged, and then grab all 500 force values from those iterations and just do an average (in excel or something), and that is "technically" more accurate than taking the final value of the simulation, because at that iteration, you may be at the peak of an oscillation or something.

As a "general rule of thumb";
  • A decrease in residuals by three orders of magnitude (10^-3) indicates at least qualitative convergence. At this point, the major flow features should be established.
  • Species residuals may need to decrease to 10^-5 in order to achieve species balance.
  • Energy residuals should decrease to at least 10^-6 (for a pressure-based solver).
Where the y+ is still quite high, I would suggest decreasing the minimum size of the prism layer cells, because not only will that hopefully help with the overall y+ distribution, but it would allow for smaller cells to be used at the leading and trailing edges of each aerofoil. At the moment, the y+ in those regions is within the buffer region, and so they no longer follow the viscous sub-layer theory, nor the log-law, and so cannot be accurately predicted. The leading and trailing edges are arguably, the most important bit of an aerofoil to capture. I would set your surface wrapper cell size on the aerofoils to something like 1.0mm, and then set your minimum prism layer cell size to be around 0.5mm. I would then also suggest making the refinement boxes you have around the leading and trailing edges a teeny bit further forward and backward respectively, and then reduce the cell size within there to maybe 50% what it is now (70% if cell count is an issue after the previous suggestion). You could also try increasing the surface curvature setting a little more too. If that still doesn't fix it, you can then increase your layer count to somewhere between 8-14 pretty safely, but the higher you go trying to "fix" the y+, the more likely it is that a different setting needs to be changed first :D
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Dipesh1995 wrote:
Tue Aug 01, 2017 6:44 pm
One question I had was, since I'll be starting to design the complex wing Mk2 soon, would trimmer be suitable mesh to use over polyhedral mesh for a complex design? One of the key benefits I saw switching from polyhedral mesh to trimmer mesh was the lack of computational time required to generate the mesh.
To give you a general idea of the differences, the trimmer meshing method is more efficient for filling large volumes (i.e. the big size of the computational domain for a motorsport CFD analysis) and uses less memory per cell because it has less cell face/boundaries that it needs to keep track of throughout the simulation, however, it means that your total cell count will increase. The polyhedral mesher is more suited to complex geometries, and multi-region problems, and it converges quicker (and more reliably) than trimmer meshes usually do; however, because some of the polyhedra that the mesher creates sometimes have 20+ faces, it means that it is quite memory intensive to keep track of all those boundaries. The meshing process (depending on software) can also take longer for polyhedral as some software packages (such as ANSYS FLUENT - CFX doesn't offer polyhedral) will first mesh a fully hex-tetrahedal (cube) mesh, and then perform another meshing pass to "convert" that hex-tet into a polyhedral one. When used correctly, it can kind of allow your setup to be a little less rigorous given its affinity to complex shapes etc, however, use with caution :lol: That comment is not a get out of jail free card!!
The pictures here show you that the polyhedral looks like it has fewer cells etc, however, you can still see that the size difference between the ones at the leading and trailing edges of the aerofoils vs. the larger main fluid domain cells is still a few orders of magnitude.
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Dipesh1995 wrote:
Tue Aug 01, 2017 6:44 pm
Yeah so since this wing is not meant to be used on one my cars as such, I'll do the mesh sensitivity study using a similar mesh for one of my Formula Student car front wings. I must say I really do appreciate the help you and Vyssion have given me in sorting my CFD issues.
No worries mate - We love this stuff :lol: :D
If you can't explain it simply, then you don't understand it well enough.
The great thing about facts is that they are true, whether or not you believe them. - Neil deGrasse Tyson
Vyssion Scribd - Aerodynamics Papers
G&K

Dipesh1995
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Re: 2017 front wings downforce compared to 2010

Post by Dipesh1995 » Tue Aug 08, 2017 2:05 pm

Ok so a quick update:

I managed to get complex wing spec 1 working without changing its design, it was a combination of mesh and turbulence model that was causing the problem. The CFD images of it I sent was based on a simulation where Tke refused to converge well. Having since changed the mesh to something based on the mesh used for the 2-element wing, changed the turbulence model to re. k epsilon and ran a mesh sensitibity study for it, Tke converged well and the edge vortex formed somewhat well. Downforce also increased by 195 N to 3629 N whilst drag also increased by 38N to 745N.

https://goo.gl/photos/RsTNkv3MqxFqpFEd7

I still went ahead and designed a spec 2 wing. The main differences are the "box" dimensions of the wing (its 2 cm longer and 1.8 cm taller; span is the same as before), the size and shape of the vortex tunnel and addition of a 7th element .

Having ran a mesh sensitivity study with 9 refinements, I managed the generate a mesh that is good compromise between cell count (~3.6 million cells, Y+ between 20 and 80) and result variations compared to a Y+ 0.28-5 mesh (Drag varies by 3N and Downforce by 5N). With a Y+ of between 60-100, variation of the results compared to a Y+ 0.28-5 mesh becomes too big.

The flow field around the wing has improved although its not perfect but its not too bad for the first iteration of spec 2. The edge vortex works well and downforce has improved by ~ 400 N to 4019 N and drag has increased by ~ 50 N to 797 N (The drag is overestimated due to the base wake of the nose cone which would not be present in reality) . I reckon there's still more in it tbh with a couple of changes focusing primarily on the second element which isn't quite the ideal shape.

I planning to sort out the remaining the issues with spec 2 before moving on to a ride height sensitivity study, I'm hoping to reduce it by 10mm to 40mm but I'll see what happens to the edge vortex. After that, I'll hoping to put rotating wheel behind it and see what happens but I'm thinking maybe a couple of weeks ahead right now.

Btw, the pressure of -23kPa is a bit optimistic caused by a few erroneous cells which I can't sort out even with a Y+ of 0.28-5 mesh. I also couldn't find the setting in the vector scene to make the wall stress in the x direction rather than magnitude.

jjn9128
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Re: 2017 front wings downforce compared to 2010

Post by jjn9128 » Tue Aug 08, 2017 8:18 pm

Dipesh1995 wrote:
Tue Aug 08, 2017 2:05 pm
I managed to get complex wing spec 1 working without changing its design, it was a combination of mesh and turbulence model that was causing the problem. The CFD images of it I sent was based on a simulation where Tke refused to converge well. Having since changed the mesh to something based on the mesh used for the 2-element wing, changed the turbulence model to re. k epsilon and ran a mesh sensitibity study for it, Tke converged well and the edge vortex formed somewhat well. Downforce also increased by 195 N to 3629 N whilst drag also increased by 38N to 745N.
Good stuff. Downforce and drag increased but changing the mesh was no longer affecting results?

Dipesh1995 wrote:
Tue Aug 08, 2017 2:05 pm
The flow field around the wing has improved although its not perfect but its not too bad for the first iteration of spec 2. The edge vortex works well and downforce has improved by ~ 400 N to 4019 N and drag has increased by ~ 50 N to 797 N (The drag is overestimated due to the base wake of the nose cone which would not be present in reality).
You say drag is overestimated because of the nose box wake - are you including the nose forces in your solution for the wing downforce?

When I said about a nose/front end model I meant more along the lines of this setup.
ImageImage
This should close the nose box wake - it needs to be this long so you don't get any adverse/unrepresentative pressure effects on your front wing. At the moment you'll have a big low pressure separated region which will be sucking on your front wing. You can make it more realistic by adding suspension.

Dipesh1995
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Re: 2017 front wings downforce compared to 2010

Post by Dipesh1995 » Tue Aug 08, 2017 9:59 pm

jjn9128 wrote:
Tue Aug 08, 2017 8:18 pm
Dipesh1995 wrote:
Tue Aug 08, 2017 2:05 pm
I managed to get complex wing spec 1 working without changing its design, it was a combination of mesh and turbulence model that was causing the problem. The CFD images of it I sent was based on a simulation where Tke refused to converge well. Having since changed the mesh to something based on the mesh used for the 2-element wing, changed the turbulence model to re. k epsilon and ran a mesh sensitibity study for it, Tke converged well and the edge vortex formed somewhat well. Downforce also increased by 195 N to 3629 N whilst drag also increased by 38N to 745N.
Good stuff. Downforce and drag increased but changing the mesh was no longer affecting results?

Dipesh1995 wrote:
Tue Aug 08, 2017 2:05 pm
The flow field around the wing has improved although its not perfect but its not too bad for the first iteration of spec 2. The edge vortex works well and downforce has improved by ~ 400 N to 4019 N and drag has increased by ~ 50 N to 797 N (The drag is overestimated due to the base wake of the nose cone which would not be present in reality).
You say drag is overestimated because of the nose box wake - are you including the nose forces in your solution for the wing downforce?

When I said about a nose/front end model I meant more along the lines of this setup.
http://www3.imperial.ac.uk/pls/portalli ... 721717.JPG http://www3.imperial.ac.uk/pls/portalli ... 721716.JPG
This should close the nose box wake - it needs to be this long so you don't get any adverse/unrepresentative pressure effects on your front wing. At the moment you'll have a big low pressure separated region which will be sucking on your front wing. You can make it more realistic by adding suspension.
jjn9128 wrote:
Tue Aug 08, 2017 8:18 pm
Good stuff. Downforce and drag increased but changing the mesh was no longer affecting results?
Yep so the mesh is insensitive so the results have negligible change when the mesh is further refined.

jjn9128 wrote:
Tue Aug 08, 2017 8:18 pm
You say drag is overestimated because of the nose box wake - are you including the nose forces in your solution for the wing downforce?
Yes so the forces I have stated for spec 1 & 2 include the nose forces, I should have stated nose-wing assembly. If I just consider the wing (spec 2) and ignore nose forces, the wing's downforce and drag is 3833 N and 731 N respectively.

jjn9128 wrote:
Tue Aug 08, 2017 8:18 pm
When I said about a nose/front end model I meant more along the lines of this setup.
http://www3.imperial.ac.uk/pls/portalli ... 721717.JPG http://www3.imperial.ac.uk/pls/portalli ... 721716.JPG
This should close the nose box wake - it needs to be this long so you don't get any adverse/unrepresentative pressure effects on your front wing. At the moment you'll have a big low pressure separated region which will be sucking on your front wing. You can make it more realistic by adding suspension.
Ok thanks for the images, I'll do a similar set up when I get round to simulating the wing with a rotating wheel.

jjn9128
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Re: 2017 front wings downforce compared to 2010

Post by jjn9128 » Wed Aug 09, 2017 1:55 pm

Okay forgive me, you did say in your post there was a variance between meshes, but I wasn't sure if the results had reached a constant value, good to know they have.

One thing I would say is to be mindful of how you plot your results. There's the old 'joke' that CFD stands for ''colourful for directors'' - saying all that it's good for is pretty marketing pictures but not much else. This is the academic in me talking but your plots should be designed to give value.

A 2-d Cp distribution of wing A vs wing B can be more helpful than a pair of surface plots to show where the changes have affected the result. If your new wing is the same except for a different upper flap, can you plot a deltaCp on the surface (blanking out the upper flaps), again to show where improvements are made. When looking at your wings wake, is a transverse plane of Cpo contours or velocity vectors more useful than streamlines?

Cpo = total or stagnation pressure coefficient in case your nomenclature is different.

Dipesh1995
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Re: 2017 front wings downforce compared to 2010

Post by Dipesh1995 » Thu Aug 10, 2017 8:33 am

jjn9128 wrote:
Wed Aug 09, 2017 1:55 pm
A 2-d Cp distribution of wing A vs wing B can be more helpful than a pair of surface plots to show where the changes have affected the result. If your new wing is the same except for a different upper flap, can you plot a deltaCp on the surface (blanking out the upper flaps), again to show where improvements are made. When looking at your wings wake, is a transverse plane of Cpo contours or velocity vectors more useful than streamlines?

Cpo = total or stagnation pressure coefficient in case your nomenclature is different.
Ok, thanks for the advice. I've actually now deleted spec 1 (since I won't be developing that wing anymore) so I can't really show the difference between the wings in terms of pressure coefficients however, when I make significant changes to spec 2, I will do so.

Vyssion
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Re: 2017 front wings downforce compared to 2010

Post by Vyssion » Fri Aug 11, 2017 8:54 am

jjn9128 wrote:
Wed Aug 09, 2017 1:55 pm
One thing I would say is to be mindful of how you plot your results. There's the old 'joke' that CFD stands for ''colourful for directors'' - saying all that it's good for is pretty marketing pictures but not much else. This is the academic in me talking but your plots should be designed to give value.
YES!!!! :lol:
Dipesh1995 wrote:
Thu Aug 10, 2017 8:33 am
... I've actually now deleted spec 1 (since I won't be developing that wing anymore) ...
:o
Don't ever do that mate!! Save a copy, compress it and archive it somewhere... I learnt very quickly when I deleted some things cause my "new CFD setup was better" only to find out that I needed to revert to the old setup a week or two later... #-o Thankfully, the team I'm at now has a "no deleting" policy; everything is archived... EVERYTHING.

Have you had a chance now to put a rotating wheel in behind the wing?
Dipesh1995 wrote:
Tue Aug 08, 2017 9:59 pm
jjn9128 wrote:
Tue Aug 08, 2017 8:18 pm
You say drag is overestimated because of the nose box wake - are you including the nose forces in your solution for the wing downforce?
Yes so the forces I have stated for spec 1 & 2 include the nose forces, I should have stated nose-wing assembly. If I just consider the wing (spec 2) and ignore nose forces, the wing's downforce and drag is 3833 N and 731 N respectively.
I think you missed what jjn9128 was saying; Because your nose is cut off quite close to the front wing, there is a large separation region right behind the square back of the nose. This will generate a relatively large sucking force which will inflluence the flow around the front wing just be "being there" in the first place. It may also be what is pulling your edge vortex inwards towards the vehicle centreline. You can't just "not select" that piece of the CAD to measure your forces from; you need to redo the sim with it extended much further downstream so the "influence" of the nose is reduced towards zero.

Great job that you have got it working so far :D
If you can't explain it simply, then you don't understand it well enough.
The great thing about facts is that they are true, whether or not you believe them. - Neil deGrasse Tyson
Vyssion Scribd - Aerodynamics Papers
G&K

Dipesh1995
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Re: 2017 front wings downforce compared to 2010

Post by Dipesh1995 » Fri Aug 11, 2017 6:55 pm

Vyssion wrote:
Fri Aug 11, 2017 8:54 am
Don't ever do that mate!! Save a copy, compress it and archive it somewhere... I learnt very quickly when I deleted some things cause my "new CFD setup was better" only to find out that I needed to revert to the old setup a week or two later... #-o Thankfully, the team I'm at now has a "no deleting" policy; everything is archived... EVERYTHING.
Yeah, I've saved the spec 2 wing as I'll hopefully do a performance comparison around the areas of the wing that have changed in spec 2.1 if it works as expected. I'll save all my wing designs from now on until I'm confident that I won't have to revert to a previous spec.
Vyssion wrote:
Fri Aug 11, 2017 8:54 am
Have you had a chance now to put a rotating wheel in behind the wing?
Nope not yet. I'll hopefully be able to do that after I do the ride height sensitivity study. At the moment, I'm just working on getting spec 2.1 ready. Like I said, I reckon there's still more performance to unlock with this wing.

jjn9128
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Re: 2017 front wings downforce compared to 2010

Post by jjn9128 » Sat Aug 12, 2017 10:58 am

Dipesh1995 wrote:
Fri Aug 11, 2017 6:55 pm
Nope not yet. I'll hopefully be able to do that after I do the ride height sensitivity study. At the moment, I'm just working on getting spec 2.1 ready. Like I said, I reckon there's still more performance to unlock with this wing.
You may find yourself unlocking that performance in isolation then losing it with the presence of the wheel. The wing and wheel have opposite circulation, so the wheel will work to cancel the circulation of the wing and vice versa, considering your wings span this will be especially prevalent towards the edge vortex you're working so hard to develop. This is also very true with a ride height study, different ride heights will change the way the wing/wheel system interacts, with positive or negative results. Likewise the gap between the trailing edge of your wing and the front point of your tyre - in F1 this is fixed by the legality boxes not optimal performance.

Check out theses and papers from Pegrum, van den Berg, Heyder-Bruckner, Diasinos, and Kellar to see how the wing cannot be considered without a wheel. I know Pegrum is now very senior at McLaren, Diasinos worked for Toyota when they were in F1, and the Kellar paper included Pearse who I think is now head of aero performance at Mercedes so they're top sources. I would also try to use a more representative tyre profile, check out Sprot's SAE paper which has scans with coordinates of the contact patch and upper deformation of a F1 tyre on a 13" rim.

Dipesh1995
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Re: 2017 front wings downforce compared to 2010

Post by Dipesh1995 » Sat Aug 12, 2017 6:51 pm

jjn9128 wrote:
Sat Aug 12, 2017 10:58 am
Dipesh1995 wrote:
Fri Aug 11, 2017 6:55 pm
Nope not yet. I'll hopefully be able to do that after I do the ride height sensitivity study. At the moment, I'm just working on getting spec 2.1 ready. Like I said, I reckon there's still more performance to unlock with this wing.
You may find yourself unlocking that performance in isolation then losing it with the presence of the wheel. The wing and wheel have opposite circulation, so the wheel will work to cancel the circulation of the wing and vice versa, considering your wings span this will be especially prevalent towards the edge vortex you're working so hard to develop. This is also very true with a ride height study, different ride heights will change the way the wing/wheel system interacts, with positive or negative results. Likewise the gap between the trailing edge of your wing and the front point of your tyre - in F1 this is fixed by the legality boxes not optimal performance.

Check out theses and papers from Pegrum, van den Berg, Heyder-Bruckner, Diasinos, and Kellar to see how the wing cannot be considered without a wheel. I know Pegrum is now very senior at McLaren, Diasinos worked for Toyota when they were in F1, and the Kellar paper included Pearse who I think is now head of aero performance at Mercedes so they're top sources. I would also try to use a more representative tyre profile, check out Sprot's SAE paper which has scans with coordinates of the contact patch and upper deformation of a F1 tyre on a 13" rim.
Ok so I'll first run the wing in isolation to make sure there's no fundamental issues with it after which I'll run it with a rotating wheel in a set up identical to what Pegrum used. One question I have is that a rotating wheel is fundamentally an unsteady problem however would I be able to get away with running with a steady solver like CD-adapco have done for their FSAE car even though they're running with rotating wheels?
@ 6.00 mins approx and 10.55 mins : https://www.youtube.com/watch?v=f1UL4o9rdJs

If not, would a implicit solver be better than a explicit solver for this problem? I'm leaning towards implicit as I can run with larger time steps and tends to be more stable.

jjn9128
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Re: 2017 front wings downforce compared to 2010

Post by jjn9128 » Sat Aug 12, 2017 7:45 pm

Dipesh1995 wrote:
Sat Aug 12, 2017 6:51 pm
Ok so I'll first run the wing in isolation to make sure there's no fundamental issues with it after which I'll run it with a rotating wheel in a set up identical to what Pegrum used. One question I have is that a rotating wheel is fundamentally an unsteady problem however would I be able to get away with running with a steady solver like CD-adapco have done for their FSAE car even though they're running with rotating wheels?
@ 6.00 mins approx and 10.55 mins : https://www.youtube.com/watch?v=f1UL4o9rdJs

If not, would a implicit solver be better than a explicit solver for this problem? I'm leaning towards implicit as I can run with larger time steps and tends to be more stable.
I would say a steady solver is fine, Vyssion may disagree. To run a properly transient case would need to run without a symmetry plane, doubling your cell count. My philosophy with CFD (certainly at university level) is to run what your machine is capable of running, if your cell count isn't as high as you'd like or your solver is a compromise, if you can critically explain the results then thats fine. Ultimately CFD is just a simulation, there are studies from Chinese facilities running tens of billions of cells and the accuracy (to a wind tunnel simulation) was not really any better, drag was better predicted but lift was worse or whatever.