Hallo Roy,
Maybe, I didn't explain my question clearly. I need
the grad_displacemet at each node point but not at
quadrature point. In this case, I think the only thing
to do is like in MeshFunction::gradient(),
go over each element and
node_fe_value.reinit(elem,node);
Node is each node of the element.
thanx
pan
 Roy Stogner <roystgnr@...> wrote:
> On Mon, 23 Apr 2007, li pan wrote:
>
> > I have found it. MeshFunction::gradient(). Is it
> > right?
>
> Not if you want to perform these calculations
> efficiently. If your
> displacement and velocity spaces are expressed in
> the same basis (and
> probably even if they aren't) the most efficient
> thing to do is to
> manually sum the local degrees of freedom times
> basis gradients at
> each quadrature point. See the construction of
> grad_u, grad_v, etc.
> in example 13.
>
> You're *already* going to "go over all elements,
> nodes again" when
> you're assembling the displacement equation matrix
> and right hand
> side. You might as well calculate the grad(u)
> contributions at the
> same time.
> 
> Roy
>
> >  li pan <li76pan@...> wrote:
> >
> >> Hallo,
> >> I'm calculating a kind of NavierStokes equation.
> >> After one "solve", I get the velocity vector.
> Then
> >> I'll evaluate displacement. Here, I have to
> >> transform
> >> the velocity vector from Euler coordinates to
> >> Lagrange
> >> coordinates, because of convective derivative:
> >> vel: velocity at time t
> >> u: displacement at time t
> >> I : identity matrix
> >> vel(t) = d_u/d_t + grad(u,t)*vel(t).
> >> I need to do something like:
> >> d_u/d_t = (I+grad(u,t))vel(t)
> >>
> >> My question is, how to do it effectively? I don't
> >> want
> >> to go over all elements, nodes again, to
> calculate
> >> the
> >> deformation gradient.
> >> Do you have good idea ?
> >>
> >> pan
> >>
> >>
> >>
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