[Nolwenn]

Hello everybody,

I try to simulate a rotor and i am very surprised by the results I obtain.
The rotor is rotating at 1.125 rad/s in a flow of 2.25m/s.

I run one case using MRF, here is the fvOptions file

FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvOptions;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

MRF1
{
    type            MRFSource;
    active          true;
    selectionMode   cellZone;
    cellZone         rotationCellZone;

    MRFSourceCoeffs
    {
        origin      (0 0 0);
        axis        (1 0 0);
        omega       -1.125;  //rad/s
    }
}

I run one case using rotatingWallBC, U is :

FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    location    "0";
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

#include        "include/initialConditions"

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform $flowVelocity;


boundaryField
{
    //- Set patchGroups for constraint patches
   //- Set patchGroups for constraint patches
    #include "${WM_PROJECT_DIR}/etc/caseDicts/setConstraintTypes"
    #include "include/lowerAndUpperPatches"
    #include "include/fixedInlet"

    outlet
    {
        type            inletOutlet;
        inletValue      uniform (0 0 0);
        value           $internalField;
    }

   "rot.*"
    { 
    type rotatingWallVelocity;
    origin (0 0 0);
    axis (1 0 0);
    omega -1.125;  // rad/s
    value uniform (0 0 0);
    }

     fixeGroup
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }

}

And one with funkySetField. funkySetBoundaryDict :

FoamFile 
{ 
    version     2.0; 
    format      ascii; 
    class       dictionary; 
    object      funkySetBoundaryDict; 
} 
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // 
 
 
velocities1 { 
    field U; 
    expressions 
    ( 
        { 
            target value; 
            patchName rot_rotor; 
            expression "-1.125*vector(0,-pos().z,pos().y)"; 
        } 
    ); 
} 
velocities2 { 
    field U; 
    expressions 
    ( 
        { 
            target value; 
            patchName rot_blade; 
            expression "-1.125*vector(0,-pos().z,pos().y)"; 
        } 
    ); 
}

For the 3 cases the BCs on the rotor are correct (see solidBC.jpg)
Mx moment is very small for rotatingWallBC and MRF
Mx moment is very high for funkySet

The velocity around the blades is very different from one case to the other (see rotatingWallBC.jpg, MRFrame.jpg and funkyBC.jpg).
By plotting streams around the rotor I see almost no rotation of the fluid with MRF and rotatingWallBC, and very important rotation with funkyBC.

If someone can help me to understand why my results are so different, it would be very appreciated !

Nolwenn

[Ruli]

Hi Nolwenn,

just a guess: You are comparing absolute and relative velocity fields.

Search the Urel function on this forum and convert absolute intro relative velocity fields an than compare again.

Best regards
Julian

[Nolwenn]

Hello Julian,

Thanks for your answer, I try to calculate Urel (not working for the moment ...) and I will check this.

Nolwenn

[Nolwenn]

Not better ...
Considering Urel or U, the velocity field obtain with MRF is totally different from the one obtain with funkyBC

[gschaider]

The sign for omega is correct?

Anyway: why would you not want to use the rotational-BC? It is built for this purpose.

[Nolwenn]

Hello Bernhard,

Thank you for your answer, the sign of omega is correct (even worst results if I change the sign !).
I tried to use rotational BC (rotatingWallBC) but I am very surprised by the results, the moment around the axis is a lot of smaller than what I expect and the flow around the blades don't seem to rotate (on the picture only the front part and the blades have a rotating BC, the cylinder and the back are fixed).

Nolwenn

[maHein]

When I do MRF-Simulations, I only use fixedValue boundary conditions since the MRF-Zone already handles the rotational velocity of the walls within the rotating zone (except for those defined as nonRotatingPatches).

[Nolwenn]

Hello maHein,

I also use fixedValue boundary conditions when I run MRF simulation. RotatingWallBC is used in another case to compare. But in both cases the resultant moment is to small.