# Adding gravity in rotating domain

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 November 14, 2013, 05:21 Adding gravity in rotating domain #1 New Member   Join Date: Oct 2013 Posts: 21 Rep Power: 3 Hello, in my Oil/Air multiphase flow calculation with a rotating domain (only 1° is modeled) i want to add gravity in the radial direction z. The rotating axes is x. If I set the buoyancy model and use x=0 [m/s^2] y=0 [m/s^2] z=9.81 [m/s^2] the solver automatically counter-rotates the relative Frame gravity vector and so the vector components y and z Change with the Rotation angle. How can i avoid this? Could i use a subdomain and define a momentum source in order to model the gravity? What do i have to add there? The unit is [Nm] best regards Thomas

 November 14, 2013, 07:01 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,824 Rep Power: 85 If the domain in rotating in the X axis and gravity is along z then I cannot see how a 1° segment with periodic or symmetry conditions can model that. You are going to have to model the full 360°. But to keep gravity pointing the same direction, you could use a momentum source, or you could just make the gravity vector a function of time. They both sound like they would work.

 November 14, 2013, 07:34 #3 New Member   Join Date: Oct 2013 Posts: 21 Rep Power: 3 Hello Glenn, i think gravity is the wrong word. In my rotational domain (which could be imagined as a rotational couette flow) I get a state where oil droplets are surrounded by air, so they hover in space with no velocity. With the additional body force I want to get them out of the Domain in the radial z direction. I still tried to make the gravity (buoyancy) vector as a function of time but the calculation is not exact because my rotational speed also changes with time. So what I have to add in the momentum source term to apply a Body acceleration in z? regards

 November 14, 2013, 17:35 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,824 Rep Power: 85 I do not understand how gravity can be used to help in the strange situation you explain. But I do not need to understand, I will just answer your question If your rotation changes with time then simply link the rotation of the gravity vector to the rotation speed. Isn't that the easiest approach? I think some tutorials use source terms - not sure which ones, have a look.

 November 21, 2013, 03:49 #5 New Member   Join Date: Oct 2013 Posts: 21 Rep Power: 3 Thanks for your help! I will figure it out.

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