I am using Lagrange approximation.
I asked about that example just to use as a reference of problem that
use second order and to change to first order.
My model is a CahnHilliard mixed linear formulation. I already have a
code running with first order approximation. (not using libmesh)
Now I am trying to do the same with libmesh. But only work for second
order approximation. When I use first order approximation work for 15x15
elements but if I use 20x20 or more elements doesn't give me the correct
solution.
So I wasn't sure if I was changing the correct place.
I will try to set the nodal value directly, thank you.
Em 20121205 15:39, Kirk, Benjamin (JSCEG311) escreveu:
> On Dec 5, 2012, at 2:54 PM, ernestol <ernestol@...> wrote:
>
>> Dear all,
>>
>> I am just starting using libmesh and trying to implement the
>> CahnHilliard equation. I already had implemented that by myself and
>> now
>> I am trying to do with libmesh to compare the results and futher use
>> adaptative mesh.
>>
>> The problem is that when I use an initial condition like a X with 1
>> on
>> the X and 1 outside the X. The way libmesh project the initial
>> condition when using second order approximation give me values under
>> 1
>> and above 1, that I don't want it.
>
>
> This is because the projection is using a leastsquares type
> approximation which can cause ringing for discontinuous data.
> Alternatively you can set the nodal value directly, or tune the
> quadrature rule used to project the function to achieve this effect 
> what type of finite element approximation are you using?
>
>> I am trying to use first order approximation so I can compare with
>> my
>> program and don't have the problem with the initial condition.
>>
>> Even with the initial condition problem, the model work with second
>> order approximation, but every problem that I tried to use didn't
>> work
>> first order approximation.
>>
>> Just a basic question so, in system_of_equation_ex2 if I want to
>> approximate u and v as first order. I just need to put first oder
>> there
>> and take off the mesh.all_second_order()?
>>
>> because when I do this doesn't work.
>
> This definitely won't work, because in example 2 we solve the
> incompressible NavierStokes equations, and there are strong limits
> on
> compatibility between the velocity and pressure approximation. In
> general, for finite element problems without this type of
> restriction,
> all you would need to do is change the ORDER when defining the
> variable.
>
> Ben
