Re: [Libmesh-users] action of Lagrange multipliers

[Tobias M. <tob...@un...>]

Ah! ok, sure. I actually saw this before but than completely forgot 
about it.

Thank you for the explanation; now, it follows my expectations :-)


On 21.11.2017 14:40, David Knezevic wrote:
>
> On Tue, Nov 21, 2017 at 7:10 AM, Tobias Moehle 
> <tob...@un... <mailto:tob...@un...>> 
> wrote:
>
>     Dear all,
>
>     I have some problem with Lagrange multipliers: Maybe I
>     misinterpret my results, but somehow they don't have the effect I
>     expected.
>
>     Looking at the solution of systems_of_equations_ex3, (using
>     paraview) I see the (real part of) pressure r_p to be 0 at time
>     step 0 and than it jumps to  ~10, where it stays for the rest of
>     the simulation. But isn't it actually enforced to stay at 0 "forever"?
>
>
>
> This is beacuse the following lines:
>
>   // We can set the mean of the pressure by setting Falpha.  Typically
>   // a value of zero is chosen, but the value should be arbitrary.
> navier_stokes_system.rhs->add(navier_stokes_system.rhs->size()-1, 10.);
>
> So in this case we set the mean pressure to be 10. We do this just for 
> illustration purposes, to show that the Lagrange multiplier can be 
> used to set the mean pressure to any value you like (but as mentioned 
> in the comment normally one would just set it to zero). If you want to 
> change the value, just change the "10" in the line above.
>
> David