Force calculation in CFD Post

  • Last Post 08 November 2019
marduin posted this 03 October 2019

I was wondering if someone could let me know how CFD post calculates force.  I am simulating a rotating helical gear dipped in lubricant, and when I look at the y force calculated by CFD post the location of the coordinate system seems to have a great affect.  The global coordinate system is at (0,0,0) and my coordinate system has different numbers for y and z.  The x and z forces change a little if the coordinate system is changed, but the y force changes dramatically and almost linearly as the coordinate system changes.  The magnitude of the y force increases as the coordinate system is moved closer to the center of the gear.  I was wondering if you would have any ideas why this would be happening and if it was due to how CFD post was calculating force. 

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Steve posted this 09 October 2019

Hi marduin,

The following describes how CFD-Post calculates force, 

The force calculation is based on the coordinate frame specified. So if the coordinate frame changes, the resulting force will change too. If you want to calculate the forces on the gear, it would make sense to align the coordinate frame with the gear shaft.


rahul221sharmaa posted this 10 October 2019

mail anmelden


EDIT - I've just removed the link. Please post "full" links if you're trying to point out a resource. This also helps up catch/stop SPAM attacks. 

marduin posted this 15 October 2019


Thank you for the resource of how the force is calculated, but it doesn't completely answer my question.  All of my axis are pointing in the same direction as the global coordinate system the origin point is the only difference.  I get that the force is calculated based off of pressure and shear, but I still don't understand why changing the origin of the coordinate system changes the force in one direction so much.  Shouldn't the force on the wall be the same in the y direction no matter what the origin of the coordinate system is as long as the y direction is the same for both?  Why does the origin point of the coordinate system have such an effect on force?



Steve posted this 20 October 2019

Hi marduin,

I think we'll need to look at some screenshots to get a better idea of what could be going on. Please attach screenshots showing as much information as possible including the set up of the force calculation and the coordinate system. Also include images of the model with the global coordinate triad.


marduin posted this 08 November 2019


When using the function calculator to calculate force the only change is which coordinate system is used, as seen in the images below.


the coordinate frames were created with the settings in the images below


Assuming that the global coordinates are at (0,0,0) the global coordinate system is as follows

The only thing changing between the coordinate systems is the origin point, so I don't get why the force is changing especially for the y force as the coordinate system is changed from global to coordinate system 1.  The x and z forces also change a little but not as significantly as the y force.  As seen in the following images.