Nonlinear convective term UFL file

On 06/27/11 20:56, Charles Cook wrote:
> New question #162950 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/162950
>
> Hello,
>
> I am working on solving a nonlinear convective diffusion (Burger's) equation by linearizing the PDE (following suite of the fenics-book tutorial chapter).
>
> For the convective term,
>
> u*grad(u)
>
> after linearizing with u^(k+1) = u^k+du with du small so that Higher order terms may be neglected, multiplying by a test function, and integrating over d_omega, I have a term
>
> int(uk*grad(du) v, d_omega)
>
> My best guess for my bilinear term is
>
> uk*grad(du)*v*dx
>
> however, I receive indexing errors (assuming this term is of the wrong dimension).
>
> My first question is obvious, how did I go wrong,

Unfortunately, it is not obvious for us to tell you unless you provide
us with

(i) A more complete sample code (including definition of uk, du, and v)
(ii) The precise error message you are getting from running (i)

> and the second, is there a tutorial/guide for writing UFL files?
>

Have you tried taking a look at the UFL chapter (Chapter 17) in the
FEniCS book? In addition, there are the UFL and the FFC demos.

--
Marie

> Thank you!
>

(i) My apologies, my entire UFL file

element = FiniteElement("Lagrange", interval, 1)

du = TrialFunction(element)
uk = TrialFunction(element)
v = TestFunction(element)
f = Coefficient(element)
g = Coefficient(element)
nu = Coefficient(element)

#Nonlinear
a = nu*inner(grad(du), grad(v))*dx \
 + uk*grad(du)*v*dx

L = f*v*dx + g*v*ds

Note that it works sans uk on the second line defining a.

(ii)

The error message:

*** FFC: Unable to extract all indices.

Thank you, I am going through those chapters now.

Thank you very much! That of course did the trick :-)

My updated UFL file (which has all terms so as to not mislead someone)

element = FiniteElement("Lagrange", interval, 1)

# perturbation in u, du
du = TrialFunction(element)
v = TestFunction(element)

# diffusion coefficient
nu = Coefficient(element)

# previous solution of u
uk = Coefficient(element)

#Nonlinear
a = nu*inner(grad(du), grad(v))*dx \
 + uk*grad(du)*v*dx \
 + du*grad(uk)*v*dx \
 - nu*grad(du)*v*ds

L = -uk*grad(uk)*v*dx \
 + nu*grad(uk)*v*ds \
 - nu*inner(grad(uk),grad(v))*dx

Thanks Marie Rognes, that solved my question.