CFD Tutorial no. 4 — Rotating MRF mesh propeller

The invisible cylinder, made visible

This rotating MRF mesh propeller tutorial shows the rotating zone, explains its disappearance after meshing, and runs a Phase 1.

Rotating MRF mesh propeller: tutorial for the steady-state Phase 1

CFD Tutorial no. 4 of 20. The actions described here are those of the HELICIEL CFD Tool window, opened from Heliciel: they assume the PRO CFD/BEM extension is active. The preset values quoted are those of the catalogue supplied with the software; the numerical results depend on your geometry and your operating point, and are to be read in the software.

How do you turn a propeller without turning the mesh? The answer has a name: rotating MRF mesh propeller, for Multiple Reference Frame. This is the method behind Heliciel PRO CFD/BEM's Phase 1, and it is what gives a result in a few minutes where physical rotation demands ten times more.

The principle comes down to one sentence: a cylinder is defined around the rotor, the flow is solved inside it in a rotating frame, and the two inertial forces that this frame brings out are added — Coriolis and centrifugal. The mesh itself does not move an inch.

Prerequisites: a propeller designed in Heliciel, a CFD case created;
Duration: half an hour, most of it spent observing;
Objective: stop enduring Phase 1 as a black box.

Tutorial outline:

Displaying the MRF zone before meshing
Why the cylinder disappears after meshing
What the rotating frame adds to the equations
Running Phase 1
What the MRF does not capture
After Phase 1

1: Displaying the MRF zone before meshing

On a rotor case that has just been created, open the toolbar's Show menu and tick mrf cylinder. A cylinder appears around the propeller, coaxial with the rotation axis.

Its radius is not arbitrary. For a free propeller, it envelopes the rotor with a margin of about 10% of the radius. For a ducted or in-duct propeller, this margin drops to 1%: the clearance between the blade tip and the wall is tight, and the cylinder must slide into it without biting into the fixed wall.

Also tick 3D model and control volume to place the three scales: the blade, the rotating cylinder, and the full calculation domain. These are three nested objects, and many meshing difficulties come from misjudging their proportions.

MRF cylinder displayed around a three-blade propeller with the domain's control volume
Show > mrf cylinder: the rotating zone, the propeller it envelopes, and the domain that contains them

2: Why the cylinder disappears after meshing

Mesh the case, then reopen the Show menu. The MRF cylinder can no longer be displayed the same way, and this is normal — the question comes up often enough to deserve an explanation.

Before meshing, the cylinder is a geometric intention: an instruction given to the mesher. After meshing, it has become a zone of cells inside the mesh, designated by a name, with no surface of its own to display. The solver, for its part, knows perfectly well where it is: it is the set of cells it will treat in a rotating frame.

There is one case where a true interface appears, with its own faces: when Phase 2 is prepared. The coupling pipeline then cuts the mesh along the cylinder and creates two facing boundaries, which will slide against each other. But this only happens after Phase 1, and tutorial no. 5 shows it in detail.

3: What the rotating frame adds to the equations

In the MRF zone, the equations are not written in the laboratory frame but in a frame rotating at the propeller's angular velocity. This change of viewpoint brings out two additional terms:

the Coriolis force, proportional to the cross product of the rotation velocity and the fluid's relative velocity;
the centrifugal force, which pushes the fluid towards the outside of the cylinder.

At the boundary between the rotating zone and the fixed zone, velocities are converted from one frame to the other. This is where the approximation lies: the blades' angular position is frozen. The propeller turns in the equations, but it always occupies the same position in space.

The turbulence model chosen for a rotor is k-ω SST, selected automatically: it handles adverse pressure gradients and boundary layer separation better than others, two phenomena that are constant on a rotating blade.

Parameters tab with the mesh preset list and the solver preset list
The Parameters tab: two preset lists — the mesh, the solver — and the estimate that goes with them. That's all you're asked for.

4: Running Phase 1

CFD calculation button, or automatically at the end of meshing if the Run steady-state Phase 1 at the end of meshing box is ticked.

Phase 1 runs in steady-state SIMPLEC. Forces on the blades typically stabilise within a few hundred iterations, which represents three to about fifteen minutes depending on mesh fineness. The convergence monitor reads the forces exerted on the blades alone — excluding the hub — and continuously compares them, including against the BEM reference of the same project: this is a useful safeguard, because a calculation that drifts sharply from its BEM almost always signals a meshing or boundary-condition problem, not a physical discovery.

One detail is settled here without you: whether the hub and the duct wall are slip or no-slip. The rule is physical, not a matter of taste — slip hub in a closed duct, no-slip hub on a free rotor — and Heliciel applies it based on the case type. It is not offered to you, because a mistake on this point silently distorts the torque, without making anything fail.

Velocity field around the blades from a Phase 1 MRF run with flow lines
Phase 1 result: the field around the blades, obtained without ever moving a single cell

5: What the MRF does not capture

The rotating MRF mesh propeller is an approximation, and it is worth knowing of what:

the interaction of a blade with the wake of the previous one is not faithfully reproduced, since the angular positions are frozen;
time fluctuations of the forces do not exist: the MRF returns an average value, not an oscillation. A vibration or acoustic analysis therefore cannot settle for it;
at zero advance speed, in hover or static thrust, the blade-tip vortices stay trapped around the rotor disk instead of being carried away. The MRF correctly computes the average induced velocity, but the gap on the forces widens.

A simple rule, expressed with the advance ratio J = V / (n·D): above J = 0.5, Phase 1 is largely sufficient. Below that, and a fortiori at V = 0, Phase 2 becomes recommended.

6: After Phase 1

At the end of Phase 1, Heliciel copies the converged fields into the initial-conditions folder, runs the coupling pipeline that prepares the sliding interface, then stops. The marker file constant/hybridPhase1.done appears in the case folder, and the Phase 2 button takes over.

This is not an interruption: it is a decision point, deliberately placed there. Your Phase 1 results are usable as they are — thrust, torque, coefficients — and you choose, with full knowledge, whether the analysis is worth the investment of Phase 2.

In the Results tab, the MRF (phase 1) / Sliding mesh (phase 2) selector switches the display between the two sets of fields as soon as both exist. This is the comparison tool of the next tutorial.

Results tab with the MRF phase 1 and Sliding mesh phase 2 selector
Results tab: the selector that switches the display between Phase 1 fields and Phase 2 fields

The collection of twenty CFD tutorials

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This series accompanies the Heliciel design tutorials, which cover the BEM side: blade design, rotation speed selection, performance curves. CFD comes after them, to verify and to see.