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Old Oct 15, 2012, 08:26 AM
polyfractal's Avatar
United States, SC, Charleston
Joined Jun 2012
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The CFD Simulation Thread

Anyone else like to play with Computational Fluid Dynamics (CFD) simulations? I've been playing around with Project Falcon, a really simple CFD package by Autodesk. It certainly is not the same caliber and accuracy of the leading modeling packages like openFOAM, but it works great for amateurs like myself.

I've been fooling around with simulations of flying swept wings and C-wings. Here are two of my recent simulations. Sorry for the video quality - my computer has a hard time recording and simulating at the same time. I'll try to get better quality videos next time

Airflow over a Flying Wing (3 min 15 sec)


Airflow simulation of a C-Wing flying wing (4 min 23 sec)


Both simulations have a 25m/s headwind (roughly 55mph). I'm going to simulate angles of attack for my next batch.
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Old Oct 15, 2012, 04:13 PM
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Have you tried to simulate the flow through a propeller?
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Old Oct 16, 2012, 07:32 AM
polyfractal's Avatar
United States, SC, Charleston
Joined Jun 2012
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Quote:
Originally Posted by TangoKilo View Post
Have you tried to simulate the flow through a propeller?
Not yet, but I'm not sure how accurate it would be. Falcon is really simple, and doesn't allow for moving parts. Everything is static. So you'd see the airflow around the propeller at a single snapshot in time, not while it is actively turning through the air.

I think for a good propeller simulation you'd have to use one of the real CFD programs (which I'm afraid I have no idea how to work )
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Old Oct 16, 2012, 01:21 PM
FS One
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United States, IL, Champaign
Joined Sep 2005
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This sounds pretty interesting. The specs say that it can read in myriad 3D file formats (you'd expect this from AutoDesk, but it's a tall order). I wonder how well it will work w/ *.obj formats which is an open format and used by a lot of graphics modelers, e.g. those modeling airplane graphics for sims.
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Old Oct 16, 2012, 02:20 PM
polyfractal's Avatar
United States, SC, Charleston
Joined Jun 2012
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Quote:
Originally Posted by MSelig View Post
This sounds pretty interesting. The specs say that it can read in myriad 3D file formats (you'd expect this from AutoDesk, but it's a tall order). I wonder how well it will work w/ *.obj formats which is an open format and used by a lot of graphics modelers, e.g. those modeling airplane graphics for sims.
I've made a few test models in Sketchup, and exported to .3ds and .obj. Both formats seem to work fine in Falcon.
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Old Oct 16, 2012, 04:32 PM
the anthropocebo effect
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Does this do viscosity, or is it only inviscid flow? Does it handle low Re laminar/turbulent transition, separation, or wake relaxation?

It seems only very expensive, dedicated CFD programs can give decent results for model scale. XFLR5 can provide pretty streamline pictures, while not really 3D and without any wake relaxation, are still probably better approximations than flow modellers that have no capability for low Re transition and separation. XFLR5 uses the very good XFOIL 2D simulation of low Re transition and separation as a basis, and gives sort of 2 1/2D results.

Kevin

Edit: attached screen capture from XFLR5. It also shows the pressure coefficients, gives lift and drag estimates over your specified AoA range, stability analysis, etc. The results are rough estimates only, but it is free.
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Old Oct 16, 2012, 05:51 PM
polyfractal's Avatar
United States, SC, Charleston
Joined Jun 2012
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Technical details about Falcon are few and far between unfortunately (probably because it's only a "tech preview"). I found this from one of the developers on the autodesk forum:

Quote:
Hey klzsailing - there are no custom inputs for fluid density and viscosity. Just air at standard conditions for now.
And taken from their validation docs:

Quote:
The solver technology that drives Falcon utilizes the following CFD techniques to simulation
the air flow over the body:
Transient, Incompressible fluid flow solver
Finite Volume Method
Full 2D and 3D Navier-Stokes fluid solution
LES turbulence model
Falcon can do pressure maps on the model itself, I just turned it off because it seems to be very computationally intensive and it slows down my computer considerably.

You seem to know a lot about CFD (definitely more than me!)...does that answer your question? Would you mind explaining a little about the "low Re laminar/turbulent transition, separation, or wake relaxation"?
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Old Oct 16, 2012, 06:42 PM
the anthropocebo effect
kcaldwel's Avatar
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It seems hard to find out very much about the details of Falcon, but it does make pretty pictures:

Autodesk Labs: Project Falcon wind tunnel analysis promo (2 min 40 sec)


Low Reynolds number flow like on an RC model wing is very difficult to accurately simulate in CFD. The flow tends to form laminar separation bubbles, and it is difficult to accurately predict where they are located and the size they will. The bubbles drastically affect the flow, and the lift and drag of a wing, and can cause separation and large drag increases. XFOIL is one of the few tools that does accurately predict the flow at low Re, but it is 2D only - it is free thanks to Dr. Drela and MIT.

It is really hard to say how good Falcon might be. It definitely incorporates viscosity, but it would be interesting to know how it handles an RC airfoil, even in 2D. The Large Eddy Simulation can be good, but again I can't find any detail on what type of LES they are using.

Until I saw some comparison to good wind tunnel results at model Re, I'd treat it as a pretty picture maker.

I keep meaning to set-up a Linux box to try OpenFoam. I'm not sure how it performs at low Re either.

Kevin
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Old Oct 16, 2012, 08:03 PM
Sink stinks
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United States, GA, Atlanta
Joined Apr 2005
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Does it do gridding automatically? I see it uses an LES turbulence model... as far as I know in order to get a fine enough grid to really use LES effectively in the boundary layer your grid would become prohibitively fine to be run on a single desktop. That's why we typically use combined RAS-LES models where RANS handles the turbulence in the boundary layer, but even in that case your grid would have to be pretty coarse to run it locally. I'd be wary of the results.

Even if you did have a large enough grid and powerful enough computer at your disposal, as Kevin said it is very difficult to simulate the flow at our Reynolds numbers, mainly because of the laminar-turbulent transition problem. Most turbulence models are designed for higher Reynolds number and just assume the boundary layer will be turbulent everywhere.

I really think the best tool for simulating the flow over finite wings, for a desktop anyway, would be something like XFLR5 or AVL. These are fully 3D in calculating lift and induced drag, but have to use a "quasi-3D" approach in modeling the profile drag, so some error is going to be introduced there. It's also understood that they will not perform well beyond the linear region of the lift curve.
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