[aspect-devel] Problem: Iterative advection solver does not converge

[FELIPE ORELLANA ROVIROSA]

 Hi John,

    I understand your points.

    I have tried tolerances in the range 10 ^ (-6 to -8 ).

    I am trying to combine them with max number of iterations (10,20,100),
but there are many combinations, many I haven't really tried.

    My system is isoviscous 10^21 Pa.s, so I kind of expect not to have
dramatic spatial variations on the strain rate..

    I am indeed planning to use temperature-dependent viscosity, and
perhaps non-Newtonian stuff, in the future. Regarding that, I think I first
have to understand the more basic case I am dealing with now.

    I could start just taking my unitary-dimensions model (which runs
successfully) and upgrade its dimensions, but doing one-by-one might take
me weeks.. So I am trying to make an educated guess on what to change,
somehow a shortcut..

thanks a lot for your comments,

Felipe

On Mon, Jun 6, 2016 at 1:46 PM, John Naliboff <jbnaliboff at ucdavis.edu>
wrote:

> Hi Felipe,
>
> In general, it is best not to ‘force’ a solution to converge by decreasing
> the solver tolerances significantly.
>
> Personally, I never decrease the Linear solver tolerance below 1e-7 … I
> have used values between 1e-7 and 1e-9.
>
> The nonlinear solver tolerance is a bit more tricky if you using highly
> non-linear constitutive relationships (i.e. strain-rate dependent
> viscosity, plasticity, etc).  In reproducing some published shear band
> (plasticity) models, I can only get the non-linear residual to converge
> down to 1e-5 or 1e-6 six if I’m lucky.
>
> In general, convergence behavior may vary significantly from problem to
> problem.  If you are using a highly non-linear material model, you will
> need to try varying the convergence parameters to ensure your key findings
> are not highly dependent on these values (they may well be).
>
> Does this help?
>
> Cheers,
> John
>
> *************************************************
> Assistant Project Scientist, CIG
> Earth & Planetary Sciences Dept., UC Davis
>
>
>
>
>
> On Jun 6, 2016, at 1:25 PM, FELIPE ORELLANA ROVIROSA <
> f_orellana at berkeley.edu> wrote:
>
>
>   Hi guys,
>
>    I still have a quick question:
>
>    In your experience, what should be the typical values for the solver
> parameters
>
>      Linear solver tolerance             ..not far from the default 1e-7 ?
>
>      Max nonlinear iterations            (I have used 10,20, 100)?
>
> Felipe
>
> On Mon, Jun 6, 2016 at 1:11 PM, Wolfgang Bangerth <bangerth at tamu.edu>
> wrote:
>
>>
>> However, I think the definition of the residual may be a bit different
>>> than
>>> what I’m used to:
>>> r = || v_i - v_i-1  ||  / || v_i  ||
>>> where v_i is the current velocities and v_i-1 are the previous values.
>>>
>>
>> The "residual" (literally: "what is left") in iterative solver methods is
>> typically the quantity
>>   r_i = A x_i - b
>> (or the norm of this vector), where x_i is the solution vector in the
>> i-th iteration. It can be "large" because it carries physical units. As a
>> consequence, its numerical size is meaningless unless compared to, say, the
>> norm of the right hand side itself.
>>
>> For example, we typically terminate iterative solver if the condition
>>   || r_i ||    <=    10^-6  || b ||
>> is satisfied. What is relevant is not whether the numeric value on the
>> left is large or small, but whether it is large or small compared to ||b||.
>>
>> Does that help?
>> Cheers
>>  W.
>>
>>
>> --
>> ------------------------------------------------------------------------
>> Wolfgang Bangerth               email:            bangerth at math.tamu.edu
>>                                 www: http://www.math.tamu.edu/~bangerth/
>>
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