# Re: [deal.II] Issue with convergence of iterative linear solver for system matrix in modified step-57 with no-normal flux constraints

```I have found my problem. A boundary condition in my mesh was ill-defined
and this lead to my error.
Sorry for the confusion.
Thanks```
```

On Monday, 19 February 2018 13:41:35 UTC-5, Bruno Blais wrote:
>
> Hello,
> Sorry I feel I have not explained myself correctly. Here is a drawing of
> the case:
>
>
>
> With Ux may be a profile or a constant.
> Initially I had set-up that case with Ux as a Poiseuille flow profile and
> the Top and Bottom as no slip boundary conditions. In that case I got the
> solution I was expecting. Convergence was good and the solution was what I
> Then what I wish to do is set Ux as a constant and apply slip at the top
> and bottom boundary conditions. When I do this is when I am unable to reach
> convergence in my iterative solution, yet I do not understand why because
> the problem should be well posed.
> I don't think that leaving my right boundary as a free traction boundary
> condition is problematic since it is by all mean an outlet boundary. This
> worked well for instance in a backward facing step case.
> Thank you for your time, sorry for the confusion.
> Best regards
>
>
>
> On Monday, 19 February 2018 10:50:48 UTC-5, Timo Heister wrote:
>>
>> > Does the order in which I apply the nonzero and zero constraints
>> matter?
>>
>> These are two independent objects, so no.
>>
>> > Currently I apply the inlet and then the no-slip in the
>> nonzero_constraints,
>> > thus the bottom and top wall appear after the inlet. Afterward the
>> cylinder
>> > is put in the zero constraints.
>>
>> I don't understand. You need boundary conditions in
>> nonzero_constraints and zero_constraints for all boundaries. The
>> reason for these two sets is that one is used for the initial solve,
>> while the second one is used for the Newton updates.
>>
>>
>> --
>> Timo Heister
>> http://www.math.clemson.edu/~heister/
>>
>

--
The deal.II project is located at http://www.dealii.org/
For mailing list/forum options, see