# Rotating mesh CFD analysis using the MRF approach

We have seen in the Mixing Vessel tutorial how to setup a physical mesh rotation of the impeller in the mixing vessel. While this is the best approach to simulate rotating bodies,  it is also computationally expensive. If we are more interested in the effect of the rotation on the surrounding flow rather than the details of the interaction with the impeller (for instance we are not interested in the impeller detailed design) we can use the Multiple Reference Frame (MRF) approach.

The Multiple Reference Frame (MRF) approach applies the rotation effect as an additional source term in the momentum equations, meaning that it is not necessary to physically rotate the mesh. The MRF CFD analysis can be both transient and steady state, with the latter one being obviously computationally much less demanding than the transient one. The steady state solution would show the flow field of a fully converged solution after an infinite time, since we are interested in the time evolution of the phenomenon we will carry out a transient analysis and we will compare it with the previous mixing vessel tutorial results.

This tutorial is based on the Mixing Vessel tutorial completed using our CFD software SimWorks. We would recommend to complete that tutorial first before proceeding because this tutorial will share the setup with the original one.

## Setup the transient simulation with MRF

The fist step is to right click on the rotating mesh mixing vessel analysis and click on Duplicate.

Then just turn the Volume type to MRF zone leaving the rotational speed and all the simulation parameter as the previous mixing vessel with rotating mesh case. You can see that a Non rotating parts box has appeared, here you can select any geometry (provided that you defined a Part Group for it) inside the MRF volume which you do not want to rotate. Since in our case the only geometry inside the MRF volume is the impeller and that one does rotate with the MRF we will leave it as default:

Run the setup and mesh phases and then run the simulation. Once the simulation has completed, you can load the results in Fields to see the evolution of the passive scalar isosurfaces with progressing time steps (displayed the isosurfaces of passive scalar with selected variable passive scalar at time steps 0.2, 0.5, 1.0 and the last one 2.0).

As you can see even if the impeller is stationary its dragging effect on the surrounding flow is well captured. The final result is very similar when compared with the results obtained with a rotating mesh.

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