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Employee
Joey.X
Posts: 259
Registered: ‎06-14-2010

Re: How to find the drag on a 3d Body?

11-08-2011 05:28 PM 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
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Contributor
timothyatwell
Posts: 14
Registered: ‎11-07-2011

Re: How to find the drag on a 3d Body?

11-08-2011 11:06 PM 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?

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Employee
Joey.X
Posts: 259
Registered: ‎06-14-2010

Re: How to find the drag on a 3d Body?

11-09-2011 05:45 AM 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
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Contributor
timothyatwell
Posts: 14
Registered: ‎11-07-2011

Re: How to find the drag on a 3d Body?

11-09-2011 11:14 AM 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?

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Employee
Joey.X
Posts: 259
Registered: ‎06-14-2010

Re: How to find the drag on a 3d Body?

11-09-2011 11:18 AM 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
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Contributor
timothyatwell
Posts: 14
Registered: ‎11-07-2011

Re: How to find the drag on a 3d Body?

11-09-2011 11:28 AM in reply to: Joey.X

Thank You very much Joey. I shall be sure to mention you in my Acknowledgements. :smileyvery-happy: (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?

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Employee
Joey.X
Posts: 259
Registered: ‎06-14-2010

Re: How to find the drag on a 3d Body?

11-09-2011 11:56 AM 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
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Contributor
timothyatwell
Posts: 14
Registered: ‎11-07-2011

Re: How to find the drag on a 3d Body?

11-09-2011 01:46 PM 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

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Employee
Joey.X
Posts: 259
Registered: ‎06-14-2010

Re: How to find the drag on a 3d Body?

11-09-2011 01:52 PM 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
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Contributor
timothyatwell
Posts: 14
Registered: ‎11-07-2011

Re: How to find the drag on a 3d Body?

11-09-2011 01:58 PM 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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