nonexact gradient values
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Hi OpenFOAMers!
I've recently found a really strange behaviour on the calculation of the gradients on version 1.6ext: Starting from a uniform field, called P (partial pressure), which is uniform and equals 1, even at the boundaries, I want to evaluate the gradient of P, which is expected to be zero. However, it is not!! Details on the case and code: 1. the mesh is uniform, hence nonorthogonal corrections are not required. CheckMesh is o.k. 2. boundary conditions of P are own made based on fixedValue type but, for this simple verification case, the value is set to 1 (and can be checked when looking at P values on the boundaries) 3. two different strategies are used for the evaluation of the gradient: 3.a.In the attached figure you can find P map, dPdx_sn (from option 3.a.) and dPdx (from option 3.b.). As the mesh is orthogonal, both options yield to the same results. But the gradient is NOT zero!!!! What's wrong? Am I going crazy? any help is appreciated.... elisabet 
The gradient is (Phi_P  Phi_f)/mag(d). I'll assume d=1e4m, and you get a small error in the computation of the difference of 1e16 (that's about the precision you can get using double if I'm not mistaken). Dividing that by d leads to 1e12, about the order of errors you are seeing.
Further reading: http://en.wikipedia.org/wiki/Machine_epsilon 
Thanks akidess!
Obviously that was the point! sometimes, when one doesn't get the right solution begins to be suspicious of everything... sorry! now the bad new: I must have a mistake somewhere else in my code... let's see... elisabet 
No worries and good hunting!

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