[CFD-Lover]

Hi All,

I am trying to manipulate a volume source term that was add to the simpleFoam solver to simulate the flow over a horizontal axis wind turbine. The governing equations used in the code is as follows;

The elementary lift and drag forces on the blade section are given by:
eq1.jpg

The axial and circumferential forces are;
eq2.jpg

The circumferential force can be further analyzed to the y and z directions of the Cartesian grid:
eq3.jpg

If the rotor is composed by B blades, the forces per unit surface will be;
eq4.jpg

The code was tested and validated successfully and all simulations were done in 3D. My aim is to apply the boundary condition wedge i.e. axisymmertric simulations to save some computational cost. However, the volume force terms programmed in the source code using the above equation are in 3D. I am currently having an issue on which source term should be excluded to be able to make it axisymmetric. Does anyone understand my question here? Can anyone help or suggest any document might help me to get it to work in Axisymmetric?

Thank you again for your time.

[piu58]

My 2C:

Without changing anything at the simulation you may transform your problem into an axialsymmetric *manually*, with replacing the x coordinate by
log(r/r_i). The coefficents are not correct for this geometry. You may transform the coefficients, or - may be simpler - export the velocities and forces and re-transform them to x/y. In this retransformed system all stream values are at the right place and the coefficients may calculated bu numerical integration / simple summation.

[CFD-Lover]

Hello Pilz,

Thank you for the kind clarifications and help. I don't fully understand how would I replace the x-coordinates by log(r/r_i) .

Originally Posted by piu58

Without changing anything at the simulation you may transform your problem into an axialsymmetric *manually*, with replacing the x coordinate by
log(r/r_i).

I definitely agree with you that the coefficients are not correct since the wedge boundary condition requires 5 degree extrusion.

Originally Posted by piu58

The coefficents are not correct for this geometry.

Unclear how would I transform them if they are already incorrect from the axisymmertic simulations?

Originally Posted by piu58

export the velocities and forces and re-transform them to x/y. In this retransformed system all stream values are at the right place and the coefficients may calculated bu numerical integration / simple summation.

Thanks again.

[CFD-Lover]

To be more specific, here are the results of the forces and velocity for both simulations i.e. Axisymmetric and 3D results.

Axi.tar.gz

3Dsimulation.tar.gz

Thanks in advance,

[CFD-Lover]

It seems that no one in the CFD-Online knows how the wedge BC works.

[piu58]

I look at your simulation the next few days and try to find out something. Please give me some time for that.

[piu58]

> I am currently having an issue on which source term should be excluded to be able to make it axisymmetric. Does anyone understand my question here?

I only have experiences with changing a 2D geometry in a cylindric one with the help of MakeAxialMesh. The tool needs to be added to a standard installation:

http://openfoamwiki.net/index.php/Contrib_MakeAxialMesh

The tool warps a 2d geometry in an axialsymetric one. The result is still 3d, that means 1 cell thick in radial direction (was the z direction). The wedge boundary condition has to be used at these boundaries. As far as I understand this b.c. takes into account that the planes are not parallel anymore. So "empty" cannot be used.

You don't need to use that tool. It is not too hard to create a axial symmetric geometry form scratch. Start with a wegde and add hex elements which get larger in r direction. The wedge b.c. has to be used too.

In short: The geometry remains 3d as always in openfoam.

[CFD-Lover]

Dear Pilz,

Thanks again for your time and clarifications. My issue is not that I was not able to create an axiysmmetric mesh from a 2D geometry but instead the main issue is that the three body forces introduced have to be adjusted in the code in order to perform axisymmetric simulations. I need to reduce the problem into two dimensions through the three body forces introduced earlier i.e. fx, fy, fz. I will provide more detail tomorrow so that you can understand what am trying to do.

Thanks again,...

[tareqkh]

Hi,

I think that you need to assume the forces are uniformly distributed along the circumferential direction.

Hope that helps,