Re: [deal.II] step-22 partial boundary conditions

2017-11-29 Thread Jane Lee 
Hi Wolfgang, thank you for your reply. 

I've had another look at the functions available and i'm stuck on how to 
implement these in this case. 

The whole point of not imposing zero for all components in a Dirichlet 
sense is that I am testing for a case where I have inhomogeneous normal 
component of the normal stress, with the tangential stress being zero on 
some boundaries. On some boundaries, I have zero normal flux and zero 
tangential stress but I'm pretty sure this doesn't amount to the actual 
components being zero anyway.

Can you suggest a way to implement this if I algorithmically, what I was 
doing previously wouldn't work?

Thank you - I will keep having a look mathematically if there is some 
manipulation I might be able to do. 

Many thanks

On Monday, November 27, 2017 at 10:53:14 PM UTC, Wolfgang Bangerth wrote:
>
> On 11/27/2017 01:16 PM, Jane Lee wrote: 
> > I'm trying to apply some partial boundary conditions to the step-22 
> > stokes problem. I can't seem to find much further help on this and when 
> > I try and implement it, it solves but solution is clearly unstable/blows 
> > up. 
> > 
> > I am trying the basics before i impose inhomogeneous quantities, and 
> > using no normal flux on the boundary, which constrains one component, 
> > and then allow no tangential stresses either, which should constrain the 
> > other two. Can anyone spot where I'm going wrong? 
>
> I don't think you can do it that way -- this would constrain the normal 
> component in terms of the tangential components, and then somehow try to 
> find a coordinate system in which to constrain the tangential 
> components, but I can completely see how this leads to circular 
> dependencies and all sorts of other weirdness. If you want a zero 
> boundary condition, then just impose zero for all components. 
>
> I'll add that *theoretically* things should work this way -- you are 
> constraining all components. But *algorithmically*, I don't think that's 
> a useful approach. 
>
> Best 
>   W. 
>
> -- 
>  
> Wolfgang Bangerth  email: bang...@colostate.edu 
>  
> www: http://www.math.colostate.edu/~bangerth/ 
>

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Re: [deal.II] Assemble the rhs of time-dependent linear elasticity

2017-11-29 Thread Jie Cheng 
Hi Wolfgang

Thanks for your reply. I carefully tested these two methods and found that 
they produce two different rhs vectors given identical previous solution. 
There is no non-zero boundary values in my case, and the constrained values 
are indeed zero. It seems that the unconstrained components in the rhs 
vector are wrong. 

Here is what I suspect: Method 1 uses the stiffness_matrix that has been 
modified during the assembly, while method 2 is using the original 
stiffness. Although method 2 will eventually distribute the rhs vector as 
well as the matrix, it is distributed along with system_matrix, not 
stiffness_matrix. In the time-dependent simulation, those two are 
different. I don't know if the modifications to the rhs vector depends on 
the matrix or not.

Thanks
Jie


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Re: [deal.II] Assemble the rhs of time-dependent linear elasticity

2017-11-29 Thread Wolfgang Bangerth 

On 11/28/2017 02:33 PM, Jie Cheng wrote:


Because the global stiffness matrix is the sum of local matrices, ideally this 
approach should work as the first one. But the solution turned out to be 
garbage. Could anybody see why the second approach is wrong?


Can you be more specific what "garbage" means? Are the boundary values wrong? 
The difference between your two methods is what happens to non-zero boundary 
values in the rhs vector.


Best
 W.


--

Wolfgang Bangerth  email: bange...@colostate.edu
   www: http://www.math.colostate.edu/~bangerth/

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