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Re: Help with a race car CFD analysis
pretty complicated analysis
consider your cad geometry's accuracy vs what was tested in the wind tunnel
fluid properties...etc.
did you have a moving floor...etc.
what's your long term goal here? i might suggest starting with a simpler model like a wing shape that has a known answer and is simple enough to run and troubleshoot. once you have a good understanding of the software you can move to a more complicated problem.
there are a bunch of really good articles about this type of analysis in the solidworks KB.
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Re: Help with a race car CFD analysis
Thanks Jared for the responce. Basically my long term goal here is to develope an aero kit for the porsche 944 because I actually have one myself.
Is there a way to make the wheels rotate at a given speed (X rads/s)? And as far a moving floor how do I do that? I have a pretty good idea as to how flow simulation creates the cells, and and good understanding of my way around the program itself.
The geometry of the model is very close to what was actually tested in the wind tunnel, so the majority of the error comes from the meshing. In your experience what gives a more accutate result more fluid cells or more particle cells?
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Re: Help with a race car CFD analysis
check out boundary condition > all
always better to have fluid or solid cells but with flow's technology, partials are nearly as accurate
your limitation here is going to be computation time. i'd shoot for trends between designs than absolute accuracy. this isn't just flow simulation, this is all CFD. you'll see what i mean by trying a few smaller examples. you may also want to look at what you can do from a part to part perspective for initial design and then scale up to the full model once you've got some data.
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Re: Help with a race car CFD analysis
Hey jared sorry its been a while ive just been swamped with school.
Like you suggested i went back and checked the boundary conditions and im getting a new error which I can't figure out. "Some of the components that were disabled are lost". Currently my model is in a separate configuration where it is just the body and wheels, with everything else suppressed. Have you ever encountered anything like this?
thanks
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Re: Help with a race car CFD analysis
if you have something applied to the components you got rid of, then you'll get that message. you need to remove their selections from the BCs that are affected
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Re: Help with a race car CFD analysis
sorry but could you elaborate? I'm not sure as to what you mean
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Re: Help with a race car CFD analysis
Thats the weird part, none of the boundary conditions show signs of any error. Maybe it isn't in the boundary conditions.
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Re: Help with a race car CFD analysis
if it isn't stopping you from running the simulation it was probably a one time message that you can ignore.
the only one that might not show an issue is component control.
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Re: Help with a race car CFD analysis
Hi Jared,
I've been running NIKA/EFDPro/FLOWSIM for over ten years with excellent results, including for very demanding aero-optical distortion.
But I'm just about to try the moving ground plane case for a landing airplane for the first time. The moving real wall BC setup is asking for a "face" selection. Does this mean I have to create a solid to represent the ground? (I'm sure others would also like to know.)
Thanks,
Flyboy2160
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Re: Help with a race car CFD analysis
Well, I built a ground slab. But when I try to set its upper surface as a moving real wall, I keep getting a message that the model doesn't lie in the solid fluid interface - even if I fully enclose the slab with the domain. I also get an error message if I try to set the domain right on the upper moving surface of the slab. What's the trick? Thanks.
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Re: Help with a race car CFD analysis
did you rebuild the flow study? if it wasn't rebuilt it won't be included.
but you should just have to sink the comp domain into the block and apply the real wall
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Re: Help with a race car CFD analysis
Thank you for he reply Jared, but I got it running just minutes before you posted! LOL Have you ever seen that great, hysterical pseudo-scientific looking paper on Why You Should Procrastinate? If you want a good laugh, I'll send it along. I'll post a picture of my results here to encourage others doing cars with FLOWSIM. (I have FLOWSIM and SOLIDWORKS and ALGOR on the plane in foot high letters until I can get Sam Adams as a sponsr.....) Steve
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Re: Help with a race car CFD analysis
Hey Steve, any chance of you posting the pic of your results as you mentioned? I'm trying to do a similar analysis. Not a car or plane, but a part on a moving conveyor belt.
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Re: Help with a race car CFD analysis
flow on a conveyor belt. that could be intersting. have you already checked out the tutorial on flow over a sphere in the tech documentation?
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Re: Help with a race car CFD analysis
Steve, I didn't get an email notice that you wrote! I'll post my results in a few minutes.
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Re: Help with a race car CFD analysis
Steve, I forgot about this and just noticed your request - I didn't get an email.
This works. A summary of my experience:
-The biggest drawback is that this takes 4-10X as many fluid cells as a regular run! I don't know why. I've tried a couple of schemes to preclude this, to no avail. The pictures below show my latest scheme. The ground solid slab is exactly the same perimeter as the domain, with the upper surface of the slab at exactly the bottom of the domain. The aircraft wheels are slightly above the bottom of the domain. The domain is smaller than I normally use for such an aero analysis because the number of cells was killing me.
(Why I don't run with symmetry? A few months ago I discovered an anomaly with symmetry that has been confirmed by SW. Moderators, if you want to break this symmetry anomaly off into a separate thread, I'll give details there.)
-As shown below, you set this up by inserting a Boundary Condition, picking the surface, and making it a Real Wall with Moving as an option. This then allows you to choose the vector direction for the surface.
- You can run with the wheels below the slab top, but then you must have the domain bottom also below the slab top and below the bottom of the wheels.
-With the domain ending at the upper surface of the slab, the slab thickness doesn't matter.
-Because of the large number of cells, I set this up starting with the coarsest mesh possible and refine it with adpative meshing.
(The purpose of this run is to determine the aerodynamic torque of the aircraft about the main landing gear tire rotation point. This torque must be greater than the countering torque from the aircraft CG trying to keep the nose down. This is a check on my hand calculations, which show that
I have sufficient elevator authority to rotate the aircraft.)
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Re: Help with a race car CFD analysis
could you post the details of the symmetry issue? i haven't seen that in my work
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Re: Help with a race car CFD analysis
I only used symmetry at home on my plane - work was full size due to asymmetry. After running symmetry for years, I decided to see how a widely used wing lift hand calculation called Schrenk's Approximation compared to CFD. I started with my usual half wing symmetry and noted a velocity anomaly at the centerline which was not there if I ran full span. SW confirmed this behavior a few months ago with the latest software (I'm running 2012.)
This anomaly resulted in only a small discrepancy in the lift that would have been dismissed by most analysts under the bromide "well, you know CFD can't get closer than 20-30%. But I had been using NIKA/EFD/FLOWSIM for15 years and seen it validated in a couple of very demanding cases with results much closer than this, so I wasn't going to let this go without asking SW.
What's even stranger is that my full span models solve much quicker than the symmetry models even though they have twice as many cells! The solutions are much better "damped." In symmetry mode, some would take forever to settle down.
The SW tech guy who looked into this said ~ "That's why we have the caveat in the manual about using symmetry only when you're sure it's OK."
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Re: Help with a race car CFD analysis
nice analysis by the way
what does a cut plot of the mesh look like?
what kind of accuracy do you think you got?
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Re: Help with a race car CFD analysis
Thank you Jared.
Here's a cut plot with the adaptive mesh. I was wondering if the moving floor would somehow create a layer of small "boundary layer" cells near it, but I don't see any.
I have no idea how close this is: This software "won" a CFD shootout at work because of it's exceptional results with separated flow (<1% off the wind tunnel results!) and with aero-optical distortion, but I have had differences with traditional hand calc checks on my plane at home. The half span longitudinal stability CFD runs were ~10% different than the hand calcs. This went down to ~5% when I began running full span - but this is an important number. I'm shooting for 5% stability. Do I have the 5% or is it 0%?
The separated flow results at high AOA at landing are still way off the hand calcs, ~20-30% This is really strange to me since it did so well at work.................??????
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Re: Help with a race car CFD analysis
I'm sorry Jared, I am not really following what you are asking?
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Re: Help with a race car CFD analysis
Have you checked component control?
Does the analysis run even though there is an error?
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Re: Help with a race car CFD analysis
Could you post the CAD model of the Porsche and some pictures of your simulation? I would like to run it myself to see if I could replicate your issues.
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Re: Help with a race car CFD analysis
Ok so i was able to fix the error i was having before, simply by creating a new flow study. So it must have just been a setup error or something. However, yesterday I tried to run my study and it simply kept on crashing once the mesh was created. Then I tried reducing the mesh all the way to 1 having only 500 000 fluid cells and it continued to crash. Its not because of lack of ram or computing power, so I cant figure whats going on now...Can you maybe shed a bit of light on this for me?
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Re: Help with a race car CFD analysis
crashing or solver failed?
version/service pack?
do other problems run? (like if you put a block in a virtual wind tunnel)
if you suppress the part in your analysis and replace it with something simple, does it run? (tells you if the study is ok and if it is a geometry problem)
if you haven't done it already, I would check these things: http://www.hawkridgesys.com/blog/flow-simulation-4-starting-flow-simulation-analysis/
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Re: Help with a race car CFD analysis
Thanks for the link it was helpful. I just did some problem solving and my flow simulation is running fine without the boundary conditions but crashes with them. So in terms of making an inlet velocity, outlet to static pressure and a rolling floor (which I know how to do) what is the best way to accomplish this? At the moment i have a thin plate at the inlet, outlet and for the floor. Is this the correct way to do it?
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Re: Help with a race car CFD analysis
It is an external analysis. The two green ends are 1mm thick and the floor is ~5mm thick.
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Re: Help with a race car CFD analysis
if you're doing an external analysis your velocity inlet and pressure openings are like portals into another universe, they aren't needed. your external conditions are what define your wind tunnel.
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Re: Help with a race car CFD analysis
So what your saying is that I dont need an inlet velocity and outlet to environment pressure when im doing this kind of analysis? Cause if you go back to my first post I set the velocity to 160 km/h without using a boundary condition and it converged at 136 km/h.
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Re: Help with a race car CFD analysis
create a model with no vehicle
external analysis
160km/h
check what the average, min and max speed is
it should be 160km/h
then introduce a blockage in the flow
i suspect those values will change because now you'll have some flow that is slower (behind), some that is faster...etc.
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Re: Help with a race car CFD analysis
Ok, so over the past few days i've been fidling with a basic flow simulation of a sphere. And I have to say thanks for the advice i now have a better understanding of the values present in my simulation. I went back to my original mesh setting (listed above) and ran the simulation and got a coefficient of drag of 0.383 which gives a 16% error from the manufacturer value of 0.33. I am going to refine the mesh a little more in order to reduce calculation time.
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Re: Help with a race car CFD analysis
This is probably long ago a moot point: Why not just use the default settings? I get very good results using them with a coarse starting mesh and 1-3 adaptive refinements.
And more cells doesn't always mean the answer is better. I usually get the same answer with a "low" number of cells as with 4X as many.
This software "won" the CFD shootout on an asymmetric bluff body (like an upside down R2D2 with a slice out of his head) where I work - at less than 1% off the (very expensive) wind tunnel results! So I would expect much better than 16% on a sphere.
With this software, I no longer accept the old "CFD can't get within 20-30%" mantra.
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