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Forces for airfoil test case

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Old   June 29, 2012, 08:59
Default Forces for airfoil test case
  #1
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Martin Vymazal
Join Date: Dec 2009
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Hello,

I would like to use the 'libforces.so' with simpleFoam to compute forces and force coefficients on an airfoil. I have several issues to which I didn't find answers in the documentation, so I hoped someone could be so kind and help me out.

First, the airfoil consists of multiple parts, let's say surface1, surface2, surface3. Is there a way of listing on which surfaces the forces should be computed? I tried to set the following in the controlDict file:

Code:
forces
{
   type forceCoeffs;
   functionObjectLibs ( "libforces.so" );
   ...
   patches (surface1,surface3);
   ...
}
Unfortunately, OpenFOAM doesn't like this syntax. I tried to create a list of names:
Code:
airfoil_surfaces
(
  surface1;
  surface3;
);

forces
{
   type forceCoeffs;
   functionObjectLibs ( "libforces.so" );
   ...
   patches ($airfoil_surfaces);
   ...
}
Again, OpenFOAM is not happy with this. Is there a way of specifying the surfaces on which forces should be computed? Should I repeate the whole forces{ ... } block with different patch names?

Second, I would like to get both forces AND force coefficients. Therefore I wrote:
Code:
functions
{
    forces
    {
        type                forces;
        functionObjectLibs  ( "libforces.so" );
        ...
    }

    forces
    {
        type forceCoeffs;
        functionObjectLibs ( "libforces.so" );
        ...
    }
}
OpenFOAM outputs only one file with entries such as this:
Code:
# Time  forces(pressure, viscous) moment(pressure, viscous)
500 (((5.55683 -156.714 1.05751e-18) (-0.628621 0.13499 1.34059e-19)) ((43.0962 1.52813 -82.8966) (-0.0371222 -0.172871 0.000622457)))
I don't understand what am I looking at: why are there four triples of numbers for one time step? What do they mean?

Finally, I would really like to compute the aerodynamic center of the airfoil. Is this possible with OpenFOAM?

Thank you for every suggestion how to solve this.

Best regards,

Martin Vymazal
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Old   July 2, 2012, 11:58
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Joe
Join Date: Dec 2011
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Posts: 67
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jferrari is on a distinguished road
Quote:
Originally Posted by Martin_ View Post
Hello,

I would like to use the 'libforces.so' with simpleFoam to compute forces and force coefficients on an airfoil. I have several issues to which I didn't find answers in the documentation, so I hoped someone could be so kind and help me out.

First, the airfoil consists of multiple parts, let's say surface1, surface2, surface3. Is there a way of listing on which surfaces the forces should be computed? I tried to set the following in the controlDict file:

Code:
forces
{
   type forceCoeffs;
   functionObjectLibs ( "libforces.so" );
   ...
   patches (surface1,surface3);
   ...
}
Unfortunately, OpenFOAM doesn't like this syntax. I tried to create a list of names:
Code:
airfoil_surfaces
(
  surface1;
  surface3;
);
 
forces
{
   type forceCoeffs;
   functionObjectLibs ( "libforces.so" );
   ...
   patches ($airfoil_surfaces);
   ...
}
Again, OpenFOAM is not happy with this. Is there a way of specifying the surfaces on which forces should be computed? Should I repeate the whole forces{ ... } block with different patch names?

Second, I would like to get both forces AND force coefficients. Therefore I wrote:
Code:
functions
{
    forces
    {
        type                forces;
        functionObjectLibs  ( "libforces.so" );
        ...
    }
 
    forces
    {
        type forceCoeffs;
        functionObjectLibs ( "libforces.so" );
        ...
    }
}
OpenFOAM outputs only one file with entries such as this:
Code:
# Time  forces(pressure, viscous) moment(pressure, viscous)
500 (((5.55683 -156.714 1.05751e-18) (-0.628621 0.13499 1.34059e-19)) ((43.0962 1.52813 -82.8966) (-0.0371222 -0.172871 0.000622457)))
I don't understand what am I looking at: why are there four triples of numbers for one time step? What do they mean?

Finally, I would really like to compute the aerodynamic center of the airfoil. Is this possible with OpenFOAM?

Thank you for every suggestion how to solve this.

Best regards,

Martin Vymazal


Have you tried different combinations? Such as a line for each surface? Or calling the forces function for each surface? I haven't done this before, so I'm not speaking from experience, I'm just thinking out loud.

I believe that OpenFOAM spits out 4 sets of 3 values because it is the x, y, z pressure and viscous forces and moments.

When you call the forces libraray you specify an origin and axis about which you calculate moments. If you have the moments and the axis location, you can set up a statics problem to calculate where your aerodynamic center is.
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