swak4Foam for calculating velocity transformations
Hi,
I am trying to calculate a new transformed velocity field as a post processing step according to the following expression: dU/dt = c*dUz/dx, where c is a constant. I am trying it using swak4Foam, in which I had a couple of questions,  how to add the dUz/dx term? I can add the div(U) term, but how to ensure that I get dUz/dx alone?  for the dU/dt term, is it possible to use an explicit formulation like, U=Udelta(T)*c*<term for dUz/dx>? I went through the slides on swak4Foam (No C++, please. We’re users!, Bernhard F.W. Gschaider, June 2011), and found multiple ways to obtain such expressions.  using libswakFunctionObject and expressionField kind formulations (slide number 61/176) Ustar { functionObjectLibs ("libswakFunctionObjects.so"); type expressionField; outputControl timeStep; fieldName Ustar; expression "U = U  c*div(U)*deltaT()"; autowrite true; }  using storedValues and patchExpression (slide 131/176) Which of the above will be apt for this formulation? Lastly, I was trying to compile swak4Foam for OpenFoam v. 2.0.1, and I get the following errors (related to swakcodedFunctionObject) while compiling: swakCodedFunctionObject.C:299: error: no matching function for call to ‘Foam::swakCodedFunctionObject::updateLibrary(Foam ::word&)’ /home/OpenFOAM/OpenFOAM2.0.1/src//postProcessing/functionObjects/utilities/lnInclude/codedFunctionObject.H:111: note: candidates are: void Foam::codedFunctionObject::updateLibrary() const make: *** [Make/linux64GccDPOpt/swakCodedFunctionObject.o] Error 1 I removed the last parameter to the =codedFunctionObject=constructor in =swakCodedFunctionObject.C= as mentioned in the README, but still continue to get this error. Due to this, I am unable to use the expressionField kind of formulation as well. Rest of the functions are compiled fine, the problem is only with the codedFunctionObject. Looking forward to some directions. :) Thanks, Santosh 
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What speaks against "grad(U.z).x" ? Quote:
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"oldU=storedU;" Quote:
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thanks for the reply Bernhard, really appreciated. :)
So, if I understand correct, 1. If U is stored in the previous time step, then I can use the expressionField, with the following expression, Code:
expression "oldTime(U)  c*grad(U.z).x*deltaT()"; Code:
variables ( I will try upgrading my version of OpenFoam, and hope it works. :) Thanks again. Santosh 
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Concerning your variables: I think you'll want to exchange the last two assignments (overwrite oldU AFTER it was used. But I still don't quite understand what exactly you're trying to calculate here 
I am trying to calculate a transformation which will lead to a new steady velocity field. The constant "c" in my expression is the velocity of structures I observe in the flow. After this transformation, I will be able to obtain a velocity field which does not change with time.

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