OpenFoam MRF Torque calculations and assumptions
 

Hi folks,

I know this topic has been thoroughly discussed in several posts but I have not seem a definite answer but rather conflicting information.

I would like to calculate the Power Number for a multiple level impeller.
I use simpleFoam with two MRF zones one per impeller level.
This is straightforward as seen in several threads.

I use the following (typical) code in controlDict in duplicated

functions (
forces 1
{ type forces;
functionObjectLibs ("libforces.so");
outputControl timeStep;
outputInterval 1;
patches (impeller bottom);
rhoName rhoInf;
rhoInf 1000;
CofR (-0.35 -0.55 2.07);
} );

1. My first assumption is the following: my understanding is that CofR are the coordinates of the center of the impeller (i.e., the intersection of the impeller with the shaft)?

After convergence we get the following data (dummy data for example purposes):

forces output:
forces(pressure,viscous)((-0.1 -0.2 -0.3),(-0.1 -0.2 -0.3),(-0.1 -0.2 -0.3))
moment(pressure,viscous)((-0.1 -0.2 -0.3),(-0.1 -0.2 -0.3),(-0.1 -0.2 -0.3))


My second assumption is this: to get the torque (assuming Z axis as center of rotation) we sum the red values in the above result, correct?

Assuming my assumptions are correct, I simulated my case with 1M+ nodes and got a convergence without issues.

However, when I calculate the power number for both levels individually the bottom impeller value differs from the value supplied by the vendors

The vendor states ~0.5 Power Number for the bottom impeller and I'm getting around 0.27. :confused:

I double checked everything but no luck on understanding the difference.

My BC for the simulation are:
- symmetry for the fluid surface
- wall for reactor walls and baffles
- movingWall with -6 rad/s for the shaft and impeller levels (individually)
- MRF with - 6 rad/s

Can anyone provide some insight?

Thanks.

Ruonin

Dare to bump!