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shrimp September 5, 2008 17:51

how to consider gravity in CFX
For a pump simulation, the roation axis is set as Z Dirn. As the medium is water, I think gravity should be considered. But when I set "Buoyancy" -> Gravity Y Dirn: -9.8 m s^-2, Gravity X,Z Dirn: 0. I got a error message shown as below. I think it is werid. Because according to the momentum equation, gravity can be added even it is not aligned with rotation axis. Am I right? and if so, how can I add gravity in steady-state?

| ERROR #001100279 has occurred in subroutine ErrAction. | | Message: This simulation involves a gravity vector which is not aligned with the rotation axis. This is valid only if the simulation type is transient, but it is currently set to steady-state. Please change the setup accordingly

Glenn Horrocks September 7, 2008 19:38

Re: how to consider gravity in CFX

If you are doing a frozen rotor multiple frames of reference simulation then obviously you cannot have the gravity vector in any direction except the rotation vector as it cannot be averaged over the rotation.

Why do you need to include gravity? Can't you do it just by putting a inlet and outlet pressure difference?

Glenn Horrocks

shrimp September 8, 2008 02:59

Re: how to consider gravity in CFX
Thanks for your reply. Yes, I do use frozen rotor multiple frames of reference. I thought frozen rotor just keeps the relative position unchange, but no idea on how this would affect the gravity vector. Well, I can not understand why this type of interface doesn't allow the variables that can not be averaged over the rotation? How can I find more information on this? Thanks again!

Glenn Horrocks September 8, 2008 18:57

Re: how to consider gravity in CFX

The various MFR options (multiple frames of reference) options are discussed in the documentation.

If you want to include gravity in an impeller simulation with gravity not along the impeller shaft you will have to run a full transient-rotor stator simulation. You would only do this if you were modelling priming a pump say, where there was lots of air and water around.

For general running of the pump bouyancy has no effect, except to create a head for the pump to push against, and that is more easily done with pressure boundary conditions.

Glenn Horrocks

shrimp September 8, 2008 20:41

Re: how to consider gravity in CFX

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