[DanielaB]

Dear Foamers,

I am currently trying to simulate a propeller within a strout. I am trying to show what influence the holder of the strout has on the force on the surface of the strout.

Now my Problem: I implemented the propeller with the actuatordisk. I used parameters of experiments from that propeller named KT for ct and calculated the cp with the given P.
KT can also be calcualted with:
ct=T/(rho*n^2*D^4)
and
cp=P/(rho*n^3*D^5)

Now the Results aren’t the way I expected them to be. The mean velocity of the flow is 2m/s and the disk accelerates the flow to values like 900 m/s (until the calcualtion is aborting).
I know the code of the actuatordisk. But I don’t see how I calculate a when I don’t know if my values for ct and cp are right (and obviously they are not, since the calcualtion wouldn’t crash if they where).
I will attach my fvOptions.
I am sure the Problem is an error in my setup, but I am helpless to find it right now.
Thanks in advance!

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.0                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "constant";
    object      fvOptions;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

disk1
{
    type            actuationDiskSource;
    active          on;

    actuationDiskSourceCoeffs
    {
        fields      (U);

        selectionMode   cellSet;
        cellSet         actuationDisk1;
        diskDir         (1 0 0);    
        Cp              0.3273;
        Ct              0.2728;
        diskArea        0.036;
        upstreamPoint   (0 0 0);
    }
}

With kind regards,
Daniela

[DanielaB]

My suggestion is, I need different coefficients, since most explanations are for wind turbines. So normally the disk ist for taking energy out of the stream, while I want to induce energy to it. Unfortunately I don't find any explanations about this case.
I found that if I use the thrust loading coefficient I get (more) plausible results.




But I can't explain it nor can I be sure it is the right way, when I don't find any source to look the topic up.
Additionally I calculated the Thrust with the equation in the source code but don't get a result near the measured.

Constant values for momentum source for actuation disk
    \f[
        T = 2 \rho A U_{o}^2 a (1-a)
    \f]
    and
    \f[
        U_1 = (1 - a)U_{o}
    \f]
    where:
    \vartable
        A   | disk area
        U_o | upstream velocity
        a   | 1 - Cp/Ct
        U_1 | velocity at the disk
    \endvartable

Someone must have used this utility for an ship propeller before, right?

Thanks again in advance and best regards,
Daniela

[Sachin m]

Hi,

I saw your post on one of the forums related to actuator disc on ship. I am also stuck with a same problem.

What is the value of Cp and Ct to be used.
I can get the values of J,Kt,etta from propeller openwater test.

Thanks

Sachin

[Kbshariff]

Originally Posted by Sachin m

Hi,

I saw your post on one of the forums related to actuator disc on ship. I am also stuck with a same problem.

What is the value of Cp and Ct to be used.
I can get the values of J,Kt,etta from propeller openwater test.

Thanks

Sachin

Hello,

1. You can modify the definition of Ct in the actuator disc model to suit your equation.
here

2. To use the actuator disc as a propeller, the sink should be 'false' to create a production thrust force.

[Sachin m]

Could you share a sample case if you happen to have one?

[Sachin m]

Originally Posted by Kbshariff

Hello,

1. You can modify the definition of Ct in the actuator disc model to suit your equation.
here

2. To use the actuator disc as a propeller, the sink should be 'false' to create a production thrust force.

Can you guide me on ow to convert the Openwater values Kt and Kq for propeller into Cp and Ct values for OpenFoam actuator disc case.

[Kbshariff]

Originally Posted by Sachin m

Can you guide me on ow to convert the Openwater values Kt and Kq for propeller into Cp and Ct values for OpenFoam actuator disc case.

I am not working on a propeller so I don't have a file with such modification.

You have to get your hands a bit dirty to customize the equation.

here are some steps:

1. Copy the actuatorDisk folder and rename it to say myActuatorDisk.

2. Modify all the actuatorDisk keywords to myActuatorDisk.

3. Compile the new actuator disk and run it to ensure you have the same result.
NB: the previous actuator disk should remain original. so as not to mess up the code in the future.

4. If the new myActuatordisk is compiled successfully, run a test simulation to ensure you have same results. ( remember to change the keyword also in the fvOptions to myActuatordisk)

5. If all is ok, then you can go to the file here in your modified folder.

6. At first you can modify the code like this

    const scalar a =  your calculated ct value;//1.0 - Cp/Ct;
    const scalar T = 0.5*rhoRef*diskArea_*magSqr(Uref & diskDir_)*a*; this according to your equation of thrust force //2.0*rhoRef*diskArea_*magSqr(Uref & diskDir_)*a*(1 - a);

7. Save and recompile the code.

8. With this modification, the Cp and Ct defined in the fvOptions are not considered during calculation since you change the equation in the source term.

9. Ensure the sink is 'false' to apply the disc as production, not extraction of energy

10. If all is ok, then you can edit the code to control the parameters Kt and Kq directly in fvOptions.

11. Goodluck, Openfoam is hands-on.