This propeller pressure visualisation tutorial teaches how to set cut planes and iso-surfaces, and how to interpret what they show.

Start with tutorial no. 1

Propeller pressure visualisation is what a CFD simulation delivers most visibly — and the trickiest ground, because a beautiful image proves nothing. This tutorial first teaches how to display, then how to read, then how to doubt.

• Prerequisites: a completed calculation, Phase 1 is enough;
• Duration: half an hour, all hands-on;
• What you get out of it: spotting on a map the three or four spots where a blade's performance is decided.

• Tutorial outline:

1. Displaying the surface pressure
2. The four spots to look at
3. Setting cut planes in the fluid
4. The iso-surfaces
5. What the colour scale tells you
6. Three reasons to doubt a beautiful image

1: Displaying the surface pressure

Results tab, Show results: frame, Surface pressure checkbox. The geometry takes on colour.

If both phases exist, the MRF (phase 1) / Sliding mesh (phase 2) selector chooses the field set displayed. Get into the habit of knowing which one you are looking at: a Phase 1 map is a frozen mean state, a Phase 2 map is one instant of a living flow.

Orient the view with the Views menu: Front, Back, Left, Right, Perspective, and Recenter to regain your bearings. With the mouse: left button to rotate, wheel to zoom, right button to pan.

The pressure map on the blade: high pressure on one side, low pressure on the other, and everything else reads between the two

2: The four spots to look at

A pressure map is always read at the same spots, whatever the machine:

1. The leading edge. This is where the gradient is most violent: the fluid stops at the stagnation point, then accelerates sharply as it rounds the nose. A poorly resolved gradient here distorts everything downstream;
2. The suction side, at the first third. The low-pressure trough is generally at its deepest there: it is the section's lift engine, and on a marine propeller, the spot where cavitation first appears;
3. The blade tip. The tip vortex bleeds off the pressure there: lift collapses over the last few centimetres, and this loss is a reality that BEM handles only through a correction;
4. The blade root and hub. An often-neglected zone, often responsible for parasitic drag: the blade-to-hub fillet, sharp angles, local separation.

Rotate the blade to compare the pressure side and the suction side on the same scale. It is the difference between the two faces that makes the force, not the value on just one.

The leading edge close up: the stagnation point and the acceleration that follows, where the mesh must be fine

3: Setting cut planes in the fluid

The surface says nothing about what happens inside the fluid. Cuts frame on the Results tab:

• choose the orientation: Vertical, Horizontal, Normal to the flow, or Cylindrical;
• choose the content: Pressures, Axial velocities, Turbulence, Tang. velocities or Radial velocities;
• activate from one to four cuts with Cut 1 to Cut 4, to compare several positions at once.

The Cylindrical cut is worth knowing: it unrolls a cylinder coaxial with the rotation axis, giving the blade section at a given radius, in its flow. It is the view closest to what BEM handles — an airfoil, an apparent angle of attack — and therefore the one that allows the most direct comparison between the two methods.

For a custom position: check Custom cut position, set the position, then Refresh cut.

A vertical cut through the domain, in perspective: the quantity shown here is turbulent viscosity, whose dynamics make the wake far more readable than pressure.

4: The iso-surfaces

A cut is a slice: it shows only one plane. An iso-surface is a volume: it is the locus of every point where a quantity equals a given value.

Iso-surfaces frame: choose the quantity — Pressures, Axial velocities, Tang. velocities, Radial velocities or Turbulence — and activate up to four volumes, each at its own value.

This is the tool that makes visible what a cut only hints at: the helical sheet of the wake, the core of the tip vortices, the contraction of the streamtube downstream of a propulsive propeller or its expansion upstream of a capturing rotor.

5: What the colour scale tells you

A map without its scale cannot be read. Two points to watch:

The bounds. The same propeller looks very different depending on whether the scale is fitted to the field's extremes or fixed. To compare two calculations, both sides need the same scale — otherwise you are comparing two colour palettes.

The zero. An incompressible solver works with a relative pressure: only the differences have meaning, not the level. For a liquid, Heliciel reconstructs the absolute pressure by adding atmospheric pressure and the water column corresponding to the project's immersion depth — which lets the scale be graduated in absolute pressure and lets it place the fluid's vapour-pressure marker. That is the subject of tutorial no. 13.

Finally, know that this pressure reference is a uniform scalar, taken at the propeller's axis: a blade tip in the top position and the same tip in the bottom position see the same reference pressure. For a deeply immersed thruster, the approximation has no consequence; for a thruster close to the surface, it underestimates cavitation in the top position.

6: Three reasons to doubt a beautiful image

A polished visualisation inspires a confidence it does not always deserve. Before drawing a conclusion from it:

• Was the mesh sufficient? A map produced at a refinement level where the blade is described by only two or three cells through its thickness is smooth and convincing — and has no quantitative value;
• Had the calculation converged? A field written when stopped on an iteration limit displays exactly like a converged field;
• Are you looking at an instant or an average? In Phase 2, each instant differs from the next. An instantaneous map is legitimate, provided you say so.

The toolbar's Capture button saves the view; the File > Create PDF report entry assembles a document. In both cases, attach the preset used and the convergence state: that is what turns an image into a result. Tutorial no. 20 covers the deliverable report.

The full Results tab: cuts on the left, iso-surfaces on the right, and the scene obeying

The collection of twenty CFD tutorials

« Previous: Boundary layer and y+  |  Next: Where the forces come from »

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
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 (you are here)
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 complements the Heliciel design tutorials, which cover the BEM side: blade design, choosing the operating regime, performance curves. CFD comes after them, to verify and to see.