This propeller sliding mesh tutorial runs the unsteady Phase 2 and compares its forces to those of Phase 1.

Start with tutorial no. 1

Phase 1 makes the propeller rotate in the equations. Propeller sliding mesh makes it rotate for real : at each time step, the mesh block containing the rotor pivots by a small angle, and slides against the fixed mesh surrounding it. This is Phase 2 of Heliciel PRO CFD/BEM.

The price is known in advance, and the interface states it plainly : the checkbox that controls Phase 2 carries the note « Phase2 ≈ 10 × Phase1 ». This tutorial shows what you buy for that price.

• Prerequisites : a completed Phase 1, the one from tutorial no. 4 ;
• Duration : variable — that is precisely the point ;
• What you get : forces that vary over time, and a wake that is no longer frozen.

• Tutorial outline:

1. Taking back control of the sequence
2. What happens between the two phases
3. Launching Phase 2
4. Comparing the forces from both phases
5. What a revolution costs
6. When Phase 2 is worth its price

1: Taking back control of the sequence

By default, Heliciel chains everything : meshing, Phase 1, then Phase 2. For this tutorial, we want to see the seam. Parameters tab, Progress management frame : uncheck Run unsteady Phase2 after Phase1.

The calculation will therefore stop at the end of Phase 1, and you will launch Phase 2 yourself. This is also the setup to adopt when a machine is shared, or when you want to examine the steady results before committing hours of computation.

Two special cases to know : in a closed duct, Phase 2 is very costly and Heliciel automatically unchecks the chaining while warning you ; and if the mesh has the flaws that make a sliding interface diverge, the warning arrives before launch, not after three hours.

Progress management frame: the two checkboxes that decide the automatic chaining mesh → Phase 1 → Phase 2

2: What happens between the two phases

The transition is not a simple restart. Between Phase 1 and Phase 2, Heliciel runs a precise sequence :

1. the converged fields from Phase 1 — velocity, pressure, turbulence — are copied over as Phase 2's initial conditions. Phase 2 therefore does not start from fluid at rest, it starts from an already-established flow ;
2. the mesh is split along the cylinder that bounded the MRF zone. Where there was only an internal boundary, you now get two facing surfaces ;
3. these two surfaces are declared as a non-conformal coupling : they do not have matching faces, they do not need to, and the solver will interpolate fluxes from one to the other at every time step.

It is this interface that allows the rotation : the inner block rotates, the outer block stays fixed, and the coupling ensures flow continuity between the two.

The sliding interface after the coupling pipeline: two facing surfaces, one rotates, the other does not

3: Launching Phase 2

With the end-of-Phase-1 marker present, the CFD calculation button now launches Phase 2. The algorithm changes : it switches from steady SIMPLEC to transient PIMPLE, with no under-relaxation, using an upwind convection scheme for stability.

The simulated duration is counted in rotor revolutions. In free-stream flow or open duct, Phase 2 simulates one full revolution by default. In a closed duct, where each revolution costs much more, the duration is graded by the preset.

The time step is not fixed : a regulator adjusts it based on the observed stability, stepping it up in stages when the calculation is holding well. What you see scrolling in the Logs are these steps.

4: Comparing the forces from both phases

Here is the point of the tutorial. Results tab, switch the selector between MRF (phase 1) and Sliding mesh (phase 2), with Surface pressure checked in both cases.

Three differences jump out on an ordinary propeller :

• the wake : in Phase 1 it is smooth and steady ; in Phase 2 it carries the helical trace of each blade's passage ;
• the pressure at the blade tip, where the tip vortex takes on a structure the frozen calculation could not form ;
• the forces, which are no longer a single number but a signal. They oscillate at the blade-passing frequency, and it is the average over the last revolution that makes the result — not the instantaneous value.

Check Propeller wake in the Show results: frame to explicitly display the vortex sheet.

Phase 2 result, seen from the side : the turbulent trace of the blades and hub, in vertical cross-section. This is the structure a frozen-rotation calculation does not form.

5: What a revolution costs

The cost of a Phase 2 is dominated by two factors : mesh fineness and the number of revolutions simulated. The propeller preset catalogue grades them together, and displays a relative cost of ×1 for the Express level up to ×117 for the Reference level.

Two rules of thumb : if Phase 2 diverges early, step up one notch of preset — a finer mesh gives a better sliding interface. If it is stable but too slow, step down one notch.

6: When Phase 2 is worth its price

A decision table, so as not to pay ten times the price without reason :

Tutorial no. 16 covers precisely the static-thrust case, where this decision matters most.

Forces in Phase 2 : a signal that oscillates at the blade-passing frequency, not a number

The full set of twenty CFD tutorials

« Previous: The MRF zone and Phase 1  |  Next: Comparing BEM and CFD »

1. First CFD propeller simulation
2. Reading the OpenFOAM case folder
3. Choosing the mesh preset
4. The MRF zone and Phase 1
5. Phase 2 with sliding mesh (you are here)
6. Comparing BEM and CFD
7. Reading the convergence of a run
8. Turbulence models
9. Boundary layer and y+
10. Reading a pressure map
11. Where the forces come from
12. Checking an airfoil polar
13. Marine propeller and cavitation
14. Wind turbine and tidal turbine
15. Fan in a closed duct
16. Aircraft propeller, cruise and static thrust
17. The optimisation loop
18. Calibrating on a reference
19. From the hull to the propeller
20. The deliverable calculation file

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