I’m a CFD hobbyist, Computation Fluid Dynamics or “Colors For Directors” can be used on propellers to analyze the flow around them, so one can get an idea about drag, lift and noise. I will be talking in this post about one of the many different method to do these kind of simulations. This method is called MFR (Multiple Frame Refernce) and can be described as such :

  • The computational domain is divided into a rotary domain and a fix domain 
  • interfaces had to be created between these two domains
  • In CFX, this method is often referred to as “frozen rotor”

The geometry: fix fluid domain, inner rotation domain

The MFR is an approximation of the instantaneous (unsteady) flow solution when the rotor is in the position defined by the (fixed) mesh.

As you can see in the picture, the domain where Navier-stocks equations are solved is  divided into a  rotaty domain, the one that looks like a hockey puck, and fix fluid domain (the bigger one).

The two domain are linked to each other  via  interfaces. 

The propeller’s mesh (sorry about the dark background) is made from tetra elements. There  is  an inflation layer of about 5, with a growth ratio of 1.5.  

Folks into  CFD, DO THE DAMN MESH DEPENDENCY TEST !!! 

Make sure your analysis  does not depends on the mesh. 

This mesh  was done using ICEM CFD.   With a total number  of nodes of around 4 millions

The mesh

Rotating Fluid: setup

So, when you get your two separate domain, you need to setup some properties.

For the fix fluid domain:

– inlet can be set as “entrainment” in CFX, “pressure-outlet”  can maybe do the job in Fluent, but you will have some negative flow value…

– the rotating flow has a velocity of 4100 RPM, this is actually the velocity of the rotating propeller.

– the rest is basic fluid setup like turbulence model and air properties. 

When you decide whether the lift and drag is correct you can impress your boss with some useless colors like this one. I have plotted  here the turbulence intensity around the propeller with the velocity gradient as color (click on the picture for bigger  version)

Pressure contour on the extrados. Turbulence intensity around the propeller