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How to find the drag on a 3d Body?

23 REPLIES 23
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Message 1 of 24
timothyatwell
2322 Views, 23 Replies

How to find the drag on a 3d Body?

Hey,

 

I'm currently designing a  solar car for my project (3d model done in solidworks) and i'm trying to analyze it. I've only been recently introduced to Autodesk Simulation, and i was told it gives the drag area and what not when analyzing it. How do i go about analyzing my model in simulation express, and analyzing it?

 

Any help is appreciated

Thanks

23 REPLIES 23
Message 2 of 24

Hi,

 

first of all it is necessary to find out which software you are actually using. Is it Autodesk Simulation CFD, showing this splash screen:

 

splash.jpg

 

If so, you are right in this forum 🙂

 

A good way to start would be the tutorial available for this kind of simulation. Please chose "Help > Examples" and select the tutorial: "External Aerodynamics: Car". If you have specific questions on your model or simulation, feel free to post them here.



Marco Müller
Product Support Specialist
Digital Simulation
Autodesk, Inc.
Message 3 of 24

How is this CFD analysis different from that in the Multiphysics package? I have the multiphysics package. 

Message 4 of 24

This post should clarify some things, let us know if this is sufficient:

 

http://forums.autodesk.com/t5/Autodesk-Simulation-CFD/Simulation-CFD-and-Multiphysics-fluid-flow-ana...

 



Marco Müller
Product Support Specialist
Digital Simulation
Autodesk, Inc.
Message 5 of 24

It tells me that it's more or less similar for what i want to get out of it.. so does that mean that MP will also give me drag coefficients based on surface roughness etc? If so, then which tutorial can i look at because, i looked throught the flow tutorial and still didn't see it. Sorry for being a nuisance.

Message 6 of 24

I will move this post to the corresponding forum, so that my colleagues may help out here.



Marco Müller
Product Support Specialist
Digital Simulation
Autodesk, Inc.
Message 7 of 24

ok, thank you

Message 8 of 24
Joey.X
in reply to: timothyatwell

Here post an pressure result image for flow through 3D object which was done in MP years agp, this is a external flow modeling by setting set the 3D object inside a box (virtual wind tunnel). To get lift/darg forces, in post-processor, select the surfaces on the interested parts, and acquire the total reaction sum for on the drag and lift force direction. 

You can search in this MP forum for my other post regarding the setup tips of external flow (virtual wind tunnel) 

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 9 of 24

thx... However, i not seeing any of your replies in the discussion on how to go about it... If i already have the car model. should i import the box and the car?

Message 10 of 24
timothyatwell
in reply to: Joey.X

This has been my method.

 

* Build Model in Solidworks

* Place model in a box in Solid works

* Start simluation (loads model into MP)

* Set to 3D and material to Air

* Attempt a mesh.

 

Are these steps so far wrong? If they are not, what are my other steps?

 

 

Message 11 of 24
Joey.X
in reply to: timothyatwell

 Yes, your method is close to mine,  and pay attention to the other steps I added in bold font.

* (CAD) Build Model in whatever CAD software

* (CAD) Place model in a box,  beware the wall distance to the model which may affect result accuracy, which is called wall effect in experiment aerodynamics if you care about the result accurancy

* (CAD) Do Boolean (subtract) operation, i.e., (Box - model) = The fluid part to be used in MP/fluid flow analysis

* Start simulation (loads model into MP and select fluid flow analysis)

* Disable solid parts and leave fluid parts interested

* Set to 3D and material to Air

* Attempt a mesh, and exclude boundary layer mesh at inlets and outlets

* Other simulation modeling setups such formulations, slover, relaxations, load curves etc.

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 12 of 24
timothyatwell
in reply to: Joey.X

Ok...So i wasn't sure what you were referring to when you said to disable solid parts... 

How does one exclude BL meshes?

Not exactly sure with regards to the setups.. I set the load curve, but the others had me puzzled for a while.

 

Anyway,

 i attached what i attempted.

 

I see the velocities and pressures, but i don't get any reactive forces, I set the surface roughness and still nothing. What am i doing wrong?

Message 13 of 24
Joey.X
in reply to: timothyatwell

Q: (Ok...So i wasn't sure what you were referring to when you said to disable solid parts... )

A: It means deactiving solid parts if they were imported from CAD.

    Do this by Tree view/Parts/Part i/right click/check on "Suppress"


Q: (How does one exclude BL meshes?)

A: Tree view/select parts/surfaces/suface i, right click and select "CAD Mesh Options"/check "Exclude from boundary Layer"

 

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 14 of 24
timothyatwell
in reply to: Joey.X

with regards to the first question, CAD did a destructive test, so there was no solid body..

 

my MP doesn't have tree view thing that you were speaking of.

 

Thank you very much for your help thus far.

 

I'm still not getting any reachtion forces what so ever. What may i be doing wrong?

Message 15 of 24
Joey.X
in reply to: timothyatwell

The default output does not have reaction force calculation, check the model settings from setup/parameters/output tab/Option for calculating reactions/(select Caculate) 

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 16 of 24
timothyatwell
in reply to: Joey.X

Thank You very much Joey. I shall be sure to mention you in my Acknowledgements. 😄 (should i just put joey. X? )

 

I've noticed that when i select the nodes, all the values come up in the text format... Does MP have an auto sum calculation for selected nodes, or do i have to do it externally?

Message 17 of 24
Joey.X
in reply to: timothyatwell

Call me Joey.X, thanks for your acknowledgements. I am glad  knowing you have progress in the work.

Here are the tips to get lift or drag force for your model in MP post-processor

- From tree view/parts/surface, suppress all wall surface properties of your enclosure box, and suppress same properties for interior surfaces, eventually leave the surfaces only of your interested model.  

- Change the result view to reaction force x, y or z direction

- Use the rectangle selection to select all surface nodes and right click to inquire result

- Select SUM in summary, it will give you the total reaction force component in the corresponding directions. 

 

Notes

(1)   Both pressure (normal direction) and shear stress(due to velocity gradient in surface normal direction and viscous fluid media)  contribute to the reaction force. 

(2) Do simple math to get lift/drag force coefficient , and be careful of  (a) reference area; (b) reference coordinates of drag/lift force coefficient , some are in velocity coordinate, some are in body coordinate, they are different if your model has attacking angle, but you can handle it by coordinate transformation.

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 18 of 24
timothyatwell
in reply to: Joey.X

ok... So i did that... i got a  reaxtive force (in x direction) of 0.8 MN.

 

This is the "net force" being applied in this direction (assuming the sum takes into account the sign).

this can't be the drag force on the system... not at 25 m/s...

 

Given the desity of the air as 1.25 kg/m^3 ...

 

Using F= 0.5 * (rho) * (ACd) * velocity^2

ACd comes out over 2000

 

The  volume of fluid is large enough that the edges not affecting the flow around the body...

 

Am i missing something? What exactly does that reactive force represent then?

 

thanks in advance

Message 19 of 24
Joey.X
in reply to: timothyatwell

check the drag force formula, it seems that the reference area was missed.

And it's suggested to db-check the consistent unit to avoid mistake.

 

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Message 20 of 24
timothyatwell
in reply to: Joey.X

(ACd) is drag area (Reference Area * Coefficient of Drag)

 

i am currently running various velocities and getting drag areas between 500-2000 going to recheck model... not sure why this is happening...

 

Any further suggestions is appreciated.

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