CFD - Computational Fluid Dynamics, Motorsport, Formula 1

Here are our CFD links and discussions about aerodynamics, suspension, driver safety and tyres. Please stick to F1 on this forum.
Wayne Kerr
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Joined: Tue Dec 20, 2016 5:19 am

Re: CFD - Computational Fluid Dynamics, Motorsport, Formula 1

Post by Wayne Kerr » Tue Dec 20, 2016 5:27 am

timhuang77 wrote::D Hi all,

I'm on an FSAE team trying to start Aero simulations for our car this year. I run Star-CCM+ v11 and are getting more and more familiar with CFD analysis. My question being: Where is the line drawn between 2D and 3D simulations? I understand that 3D is better at simulating viscous effects and estimating downforce and drag numbers. Currently, most of the vehicle simulations are done in 3D and ideally some would be done in 2D (as it is magnitudes faster computationally speaking). Our airfoil selection is done in 2D, but I would like to do more to speed up and simplify the process. Thanks
Have you watched the tutorial on steve portal for fsae? I'd start there if you haven't.

For your application, I probably wouldn't spend too much time on 2D. Depending on your computational powers, narrow it down to 2 / 3 shape choices and then start your 3D from there. I'd probably ask myself the question "how can I justify this in 2D to the judges" if you're ever caught debating 2D vs 3D

Vyssion
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Joined: Sun Jun 10, 2012 1:40 pm

Re: CFD - Computational Fluid Dynamics, Motorsport, Formula 1

Post by Vyssion » Fri Jan 06, 2017 1:44 pm

timhuang77 wrote::D Hi all,

I'm on an FSAE team trying to start Aero simulations for our car this year. I run Star-CCM+ v11 and are getting more and more familiar with CFD analysis. My question being: Where is the line drawn between 2D and 3D simulations? I understand that 3D is better at simulating viscous effects and estimating downforce and drag numbers. Currently, most of the vehicle simulations are done in 3D and ideally some would be done in 2D (as it is magnitudes faster computationally speaking). Our airfoil selection is done in 2D, but I would like to do more to speed up and simplify the process. Thanks
2D is good for initial aerofoil designing and to some extent, rough diffuser angle'shape testing. Beyond that, you cant really do much else with it as there are so many 3D effects (i.e. vortices) which you simply can't ignore when designing a vehicle. If you are serious about simplifying the process a bit, then what I would do is break the car symmetrically along its longitudinal axis to half solver time, and then if that still isn't enough, look at designing your front wing or rear wing in isolation as a smaller 3D simulation. Once you're happy with that, plug it all into the same model and simulate it all together and see what happens and adjust the design accordingly. but I would be careful with 2D things and the "numbers" you get out of it - I would only work in deltas; that is, "this design has a higher number than this one, therefore it is better". You can't really say that this 2D aerofoil gives me 1234.56789 Newtons of force where as the old one was only 1234.0000 Newtons, therefore its better.
If you can't explain it simply, then you don't understand it well enough.
- Albert Einstein


The great thing about facts is that they are true, whether or not you believe them.
- Neil deGrasse Tyson


Vyssion Scribd - Aerodynamics Papers

keroro.90
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Joined: Mon Jul 01, 2013 8:32 pm

Re: CFD - Computational Fluid Dynamics, Motorsport, Formula 1

Post by keroro.90 » Tue May 02, 2017 11:34 am

timhuang77 wrote:
Fri Dec 02, 2016 1:54 am
:D Hi all,

I'm on an FSAE team trying to start Aero simulations for our car this year. I run Star-CCM+ v11 and are getting more and more familiar with CFD analysis. My question being: Where is the line drawn between 2D and 3D simulations? I understand that 3D is better at simulating viscous effects and estimating downforce and drag numbers. Currently, most of the vehicle simulations are done in 3D and ideally some would be done in 2D (as it is magnitudes faster computationally speaking). Our airfoil selection is done in 2D, but I would like to do more to speed up and simplify the process. Thanks
I think depends strongly on what type of solver you're going to use...If you are going to use a very accurate approach with or without closure model like a DNS or a LES you will find some big difference....while if you use a RANS, the differences will be less...

Xwang
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Joined: Sun Dec 02, 2012 10:12 am

Re: CFD - Computational Fluid Dynamics, Motorsport, Formula 1

Post by Xwang » Sat Apr 14, 2018 11:21 am

I have a question for you.
Is there any limit on the scale used when doing CFD?
I know that only up to 60% model can be used in wind galleries studies, is the same for CFD or are they 100% dimension?
Is it easier to validate CFD vs wind gallery if the gallery environment is used (60% scale, gallery walls and so on) for CFD?
I've read that Force India Chief Designer has said that their CFD and gallery data correlate well, but both are different from track data.

Vyssion
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Re: CFD - Computational Fluid Dynamics, Motorsport, Formula 1

Post by Vyssion » Sat Apr 14, 2018 6:34 pm

Xwang wrote:
Sat Apr 14, 2018 11:21 am
Is there any limit on the scale used when doing CFD?
I know that only up to 60% model can be used in wind galleries studies, is the same for CFD or are they 100% dimension?
No, the limit that teams have is related to the number of TeraFLOPS they have available to use each week.

In computing, floating point operations per second (FLOPS, flops or flop/s) is a measure of computer performance for algorithms which require floating-point calculations. For these sorts of calculations, such as CFD, it is a more accurate measure than measuring instructions per second.

If you assume that each "cell" of a mesh in CFD is one FLOP (it isnt... obviously), then if your mesh has 100 milllion elements, you have 100 MFLOPS each time you solve. Since cell size is dependant on geometry and flow feature sizes, whether you are at 100% or 60%, you will have to scale your cell size down in order to maintain resolution of the geometric and aerodynamic flow features. So you would probably still end up with 100 MFLOPS regardless. (again, just as an example!!)

Xwang wrote:
Sat Apr 14, 2018 11:21 am
Is it easier to validate CFD vs wind gallery if the gallery environment is used (60% scale, gallery walls and so on) for CFD?
Typically, the order of preference with regards to obtaining data and aerodynamic performance is:
Track Testing > Wind Tunnel > CFD
The scale of a model which is different from 100% scale becomes harder in general. This is because of similarity parameters. Reynolds Number is the most famous of these, however, it is not the only one: Mach Number is another one.

In a nutshell, there are two physical qualities about air which need similarity factors to be equatable: The air's viscosity and compressibility.

Reynolds Number controls the airs viscosity (or "stickiness") and is the ratio of inertial forces to viscous forces.
Mach Number relates to the air's compressibility (or "springiness") and is the ratio of the air's velocity to the speed of sound. Usually this isn't too much of a problem due to air's ability to be essentially assumed as incompressible below about 0.3 Mach.
Xwang wrote:
Sat Apr 14, 2018 11:21 am
I've read that Force India Chief Designer has said that their CFD and gallery data correlate well, but both are different from track data.
Just be wary - "correlate" doesn't necessarily infer "equal"... Track data will always be superior to almost any wind tunnel or CFD method.
If you can't explain it simply, then you don't understand it well enough.
- Albert Einstein


The great thing about facts is that they are true, whether or not you believe them.
- Neil deGrasse Tyson


Vyssion Scribd - Aerodynamics Papers

Xwang
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Joined: Sun Dec 02, 2012 10:12 am

Re: CFD - Computational Fluid Dynamics, Motorsport, Formula 1

Post by Xwang » Sat Apr 14, 2018 6:50 pm

Thanks