## Fluids

#### question about define speed of rotation and speed of revolution in CFX

• weij2018
Subscriber

Hello, everyone! I have got this question about simulating flow in the bearing. I define the speed of revolution in the bearing domain, and I also need to define the speed of rotation of the ball of the bearing. I insert a new coordinate frame, and its origin is set in the center of the ball. I want to set the frame as a rotating frame, but after that, I can't define the wall velocity of the ball by the new rotating frame to define a relative speed. If I set the frame as stationary, I can define the wall velocity of the ball by the new rotating frame, but the frame is stationary. And here is my question: if I set the new frame as stationary, will the revolution speed of the bearing domain be added to the ball for I want to define a relative speed of the ball; otherwise, if I set the new frame as a rotating one, how could I define the wall velocity? Thank you very much!

• weij2018
Subscriber

• DrAmine
Ansys Employee

I will do it in this way: Define your Coordiante System without frame motion. Assing the domain the new coordinate system and let it rotate along its axis of rotation. For the wall you can set as stationary and if not rotating either rotating with negative revolution or just as counter rotating.

• weij2018
Subscriber

Thank you for your kind reply! I know I should set the coordinate frame as stationary by your response. However, I'm still confused about this question: for the domain is rotating along the axis of the global z-axis, and the ball wall is rotating along the local z-axis, the two axes are not the same, so could I define two rotating axes and two rotating speed for both the domain and the wall?

• DrAmine
Ansys Employee

Will the ball belong to the rotating domain? If yes: they should use the same axis of rotation.

Best to add some pictures to understand your geometry and describe the domains you have and to which domain the ball wall belongs.

• weij2018
Subscriber

Thank you so much for your reply! As you can see, the domain consists of a cavity and three bearings. The domains of all three bearings rotate around the global axis, and the ball of each bearing is spinning around the circle of the ball itself. And my question is: if I define the domain to be rotating around the global z-axis by a speed of revolution, should I define the wall speed of the ball around the local z-axis of itself by a rotation speed? If I define it this way, will the rate of revolution of the whole domain be added to the ball? Thank you so much again for your kind help!