CFD Online Discussion Forums - [swak4Foam] different values between swakExpression & faceSource areaAverage

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different values between swakExpression & faceSource areaAverage

Hello,

I'm trying to calculate an area-weighted averaging of a field (gamma) in a arbitrary plane.

First at all I did it with faceSource, cuttingPlane and the operation areaAverage:

PHP Code:


		
			
 plan0_average
     {
         type            faceSource;
         functionObjectLibs ("libfieldFunctionObjects.so");

         enabled         true;
         outputControl   timeStep; //outputTime timeStep
         outputInterval  200;

         // Output to log&file (true) or to file only
         log             true;

         // Output field values as well
         valueOutput     false;  //true;

         // Type of source: patch/faceZone/sampledSurface
         source          sampledSurface;

         sampledSurfaceDict
         {
                 type cuttingPlane;
                 planeType           pointAndNormal;
                 pointAndNormalDict
                 {
                         basePoint       (0 0 0);
                         normalVector    (1 0 0);
                 }
                 source cells; // sample cells or boundaryFaces
                 interpolate true;
         }

         // Operation: areaAverage/sum/weightedAverage ...
         operation       areaAverage;

         fields          (gamma);
     }

It seemed to work. But I also do other post-processing calculations by using the swakExpression library, and the output text file look better for me than the faceSource's one. So I decided to also calculate the area average by using a swakExpression instead of faceSource.

I'm using this code:

PHP Code:


		
			
 Test_plan0_average
     {
         type            swakExpression;

         outputControlMode   timestep; //outputTime timestep
         outputInterval  200;

         expression "gamma*area()/sum(area())"; // momentum Flux [N]

         valueType surface;
         surfaceName plan01;
         surface
             {
                 type plane;
                 basePoint   (0 0 0);
                 normalVector     (1 0 0);
                 interpolate     true;
             }

         // Operation: sumMag/min/max/weightedAverage/average/sum
         accumulations       (sum);
verbose true;
     }

For me the result should be the same than previously but it is not! Here is a comparison:

http://img32.imageshack.us/img32/9551/v0rn.jpg

Someone can explain me what is the problem with my swakExpression ? Or maybe the problem comes from faceSource ??

Best regards,
Fabien.

Quote:

Originally Posted by Paebin (Post 469172)

Hello,

I'm trying to calculate an area-weighted averaging of a field (gamma) in a arbitrary plane.

First at all I did it with faceSource, cuttingPlane and the operation areaAverage:

PHP Code:


		
			
 plan0_average

     {

         type            faceSource;

         functionObjectLibs ("libfieldFunctionObjects.so");



         enabled         true;

         outputControl   timeStep; //outputTime timeStep

         outputInterval  200;



         // Output to log&file (true) or to file only

         log             true;



         // Output field values as well

         valueOutput     false;  //true;



         // Type of source: patch/faceZone/sampledSurface

         source          sampledSurface;



         sampledSurfaceDict

         {

                 type cuttingPlane;

                 planeType           pointAndNormal;

                 pointAndNormalDict

                 {

                         basePoint       (0 0 0);

                         normalVector    (1 0 0);

                 }

                 source cells; // sample cells or boundaryFaces

                 interpolate true;

         }



         // Operation: areaAverage/sum/weightedAverage ...

         operation       areaAverage;



         fields          (gamma);

     }

PHP Code:


		
			
 Test_plan0_average

     {

         type            swakExpression;



         outputControlMode   timestep; //outputTime timestep

         outputInterval  200;



         expression "gamma*area()/sum(area())"; // momentum Flux [N]



         valueType surface;

         surfaceName plan01;

         surface

             {

                 type plane;

                 basePoint   (0 0 0);

                 normalVector     (1 0 0);

                 interpolate     true;

             }



         // Operation: sumMag/min/max/weightedAverage/average/sum

         accumulations       (sum);

verbose true;

     }

The swak-Expression looks OK with me.

This sounds completely stupid but could you please try the same surface-type (cuttingPlane) with swak (I've seen differences in the results in the past with plane vs cuttingPlane). Also try "area()" and see if the total area of the plane is correct (you should know that from your geometry).

If the problem persists: which swak-version? Which OF-version?

Thanks for your help gschaider!

Quote:

Originally Posted by gschaider (Post 469177)

This sounds completely stupid but could you please try the same surface-type (cuttingPlane) with swak (I've seen differences in the results in the past with plane vs cuttingPlane).

I tried with my swakExpression but with a cuttingPlane as you said, and the result is different from the other two!

http://img13.imageshack.us/img13/9515/1awl.jpg

Quote:

Originally Posted by gschaider (Post 469177)

Also try "area()" and see if the total area of the plane is correct (you should know that from your geometry).

the total area of the three methods is the same, and it corresponds to reality. :)

Quote:

Originally Posted by gschaider (Post 469177)

If the problem persists: which swak-version? Which OF-version?

I'm using openFoam 2.2.2 and swak4Foam-OF22X, ubuntu

Best regards,
Fabien

I think the problem of the configuration swak+cuttingPlane comes from the interpolate function. All the previous results used "interpolate true;". I changed it to "interpolate false;", then faceSource and swak with cuttingPlane give me (almost) the same result:

http://imageshack.us/a/img542/2724/oeei.jpg

I tried also to used "plane" with faceSource and it gives the same results that "plane" with swak. So the main difference is between plane and cutting plane. Also, there is a difference on the face number: 31 for plane and much more for cuttingPlane (129). In reality I have 16 cells. So I think the plane's average result should be the correct answer.

Someone can explain me what is the difference between cuttingPlane and Plane ??

best regards,
Fabien.

Quote:

Originally Posted by Paebin (Post 469286)

Not sure. I'd have to look at the source myself

Ok thanks.

So plane with faceSource or swak gives the same results when I don't use interpolation. If I active interpolation, I get a different result than previously (why not!), but the problem is that the result is different between faceSource and swak...

I don't know where is the truth! :(

Quote:

Originally Posted by Paebin (Post 469738)

To further complicate things: have you tried faceSource with plane (I've only seen cuttingPlane).

One possibility to check the "truth" is to calculate an expression whose value depends on the surface but whose value you already know. For instance the sum of "area()" (that should be the total area of the plane) - that would be independent of the interpolation.

Quote:

Originally Posted by Paebin (Post 469738)

I came across the same problem. Swak and faceSource give the same value if interpolate is set to false. However this value is different for cuttingPlane and plane. In my case the value given by cuttingPlane is closer to Paraview, so I assume that it's the correct one. Anyone has faced the same problem or knows more?