About MRF Zones

The model assumes that the MRF zone has a constant rotational speed and the non-wall boundaries are surfaces of revolution. An MRF zone is used to define cell zones that belong to a rotating frame of reference, and you can specify both the axis of rotation and the angular velocity of the zone. Solving a rotating problem using MRF is analogous to freezing the motion of the rotor and observing the instantaneous flow field within the MRF zone; this model is therefore often referred to as the "frozen rotor approach."

Although the MRF model is an approximation, it provides a simple and reliable solution to simulate rotating machinery without using moving mesh and transient simulation. It is useful in turbo-machinery applications, including mixing tanks, pumps, rotor-stator simulations, and simulations involving an impeller.

The MRF model has the following limitations:

• The interface separating a MRF zone and a stationary zone must not have any component of the frame velocity normal to it. They should be circular with the center of the circle on the axis of rotation of the MRF zone.
• The MRF model is a steady-state approximation.
• The speed of rotation cannot be changed.
• Some weak interaction exists between the MRF zone and the stationary zone.

Rotating MRF zones differ from rotating walls in the following ways:

• If the wall boundary is the surface of revolution, then you can choose either an MRF or rotating wall to solve the problem.
• The simulation of a mixing tank can illustrate how you might implement either option. If the mixing blades exist in the MRF zone and have components of their surface normals along the tangential direction. If you declare these blades as rotating walls, you would need to implement a moving mesh as well, which can allow only limited deformation and will converge more slowly than the MRF model approach.