A Moving Reference Frame (MRF) is a relatively simple, robust, and efficient steady-state, Computational Fluid Dynamics (CFD) modeling technique to simulate rotating machinery. For example, the rotors on a quadcopter can be modeled with MRFs.

MRF CFD Simulation of a Quadcopter in FlightShows streamlines colored by velocity magnitude

An MRF assumes that an assigned volume has a constant speed of rotation and the non-wall boundaries are surfaces of revolution (e.g., cylindrical, spherical, conical). In the case of the quadcopter example the volumes between the rotor blades are designated as MRFs, assigned rotational speeds, and embedded within a multi-volume flow domain.

MRF is equivalent to running a rotational simulation and then observing the results at the instant equivalent to the position of the rotor within the MRF. MRF assumes a weak interaction between the MRF volume and the surrounding stationary volumes.

MRF CFD Simulation of Ducted FanShows velocity vectors within MRF volumes

An alternative to MRF is the moving/sliding mesh technique, which can deal with strong interactions between the moving volume and the surrounding stationary volumes. However, in practice the moving mesh technique often has robustness problems due its dependence on intersection calculations between the stationary and rotating volumes. Also the moving mesh technique is a transient (unsteady) simulation procedure which typically results in extremely long runtimes. Given the significant drawbacks of the moving mesh technique, the MRF is the preferred approach within its limitations.

Notes

• MRF is also known as the 'frozen rotor approach'
• The quadcopter (symmetrical half model) geometry was created in Caedium Professional. The CFD simulation was performed using the incompressible, steady-state RANS solver, with multiple MRF, and the k-omega SST turbulence model