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[Kent-Andre Mardal <question146882@xxxxxxxxxxxxxxxxxxxxx>]

Question #146882 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/146882

    Status: Open => Answered

Kent-Andre Mardal proposed the following answer:
Hi JJ,

you would solve a variational problem

VV = VectorFunctionSpace(..)
u = TrialFunction(VV)
v = TestFunction(VV)

a = dot(u,v)*dx
L = dot(grad(scalar1), v)*dx

problem = VariationalProblem(a,L)
c = problem.solve()

Kent


On 25 February 2011 19:25, JJ Jiang <question146882@xxxxxxxxxxxxxxxxxxxxx>wrote:

> New question #146882 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/146882
>
> Hi all,
>   This should be a naive question since I am about 14 days new to Dolfin at
> this point.
>    I want to calculate gradients for either a scalar or a vector field.
> Both scalar or vector fields are read from external files. The mesh below
> refers to a triangulated surface.
>
>    V = FunctionSpace(mesh, "CG", 1)
>    scalar1 = Function(V, "1.xml")
>    c = grad(scalar1)
>
>   I want to have c as a vector so that I can save that to a vtk file for
> visualization or do some further operations on that.
>   outfile = File("example.pvd")
>   outfile << c
>  Alternatively, I could convert c to 3 different scalar functions. Is there
> anyway to do such operation in Dolfin.
>
>  For a vector field v(x,y,z), the gradient operation will create a
> tensor/matrix. How can I manipulate them? For instance, I only need dv/dx,
> dv/dy and dv/dz. Thanks so much for your help.
>
> Best regards,
> --JJ
>
>
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