[aspect-devel] Question on Thermal Viscosity thresholds

[Lev Karatun]

Hi Felipe,

I checked the formulas again and I really don't think there will be a
discontinuity in the resulting prefactor for any limiter values > 0. I
suspect you might have made a mistake when you plotted it, or there might
be some sort or a visualization glitch.

Best regards,
Lev Karatun.

2018-04-26 2:27 GMT-04:00 FELIPE ORELLANA ROVIROSA <f_orellana at berkeley.edu>
:

>
>  Hi Lev,
>
>       Thanks a lot for replying.
>
>        I am referring to section A.77 Parameters in section Material model
>
>              then subsection model 'simple'
>
> Temperature prefactor for viscosity:
>
>       there is a definition tau(T) = H(exp...), H(x) = { .. 3 cases..}
>
>      I have inspected the formula several times and it seems to me that it
> would create a discontinuity if the values {tau min, tau max} are not {0.01
> and 100}.
>
>      I plotted the formula on matlab using values different than those,
> and indeed created a discontinuity. ALternatively, depending on how you
> program the formula, it will leave an unspecified interval.
>
>      It's nothing grave I think, but if one is not wary when using the
> model parameters tau max-min, the issue could get under the hood.
>
> cheers,
> thank you.
>
> Felipe
>
>
> On Wed, Apr 25, 2018 at 10:44 PM, Lev Karatun <lev.karatun at gmail.com>
> wrote:
>
>> Hi Felippe,
>>
>> I didn't write the plugin, but I'll try to give you an answer based on
>> looking at the code and the manual.
>> As long as temperature range is continuous, I don't think there will be
>> any discontinuities in the viscosity. Thermal prefactors (which are set
>> using the "Maximum thermal prefactor" and "Minimum thermal prefactor"
>> parameters) are just limiters, they don't add any non-linearity into the
>> calculations. If you make the range too wide, the solver might have
>> problems converging, but other than that it should be fine. Is this what
>> you were asking?
>>
>> As a side note, a new version of the manual is generated every day, so
>> page numbers aren't too helpful to locate the part you're referring to.
>> Maybe you could quote it instead or specify a section title?
>>
>> Best regards,
>> Lev Karatun.
>>
>> 2018-04-25 16:50 GMT-04:00 FELIPE ORELLANA ROVIROSA <
>> f_orellana at berkeley.edu>:
>>
>>>
>>>  Hi,
>>>
>>>       Hope you guys are having a nice day.
>>>
>>>        I am crafting and trying different models on a 3D convecting
>>> system, and considering Temperature-dependent viscosity among other things.
>>>
>>>        For the material model 'simple', the viscosity depends on
>>> temperature as a negative exponential (a classic), but some
>>> thresholds/bounds are imposed on it.
>>>
>>>        According to Aspect manual 1.5.0, page 289; if the exponential is
>>> out of the bounds [ 0.01 , 100 ], that part of the viscosity will assume
>>> the user-defined threshold values given by (Tau min, Tau max). Later the
>>> amplitude will be given by the scaling constant eta0.
>>>
>>>        But, if a user chooses values for {Tau min, Tau max} different
>>> than {0.01 , 100}, then discontinuities in viscosity will be produced over
>>> the temperature range.
>>>
>>>        The question is the following: are those limits 10^-2 < x < 10^2
>>> only a handbook example?, or will those limits assume the user-specified
>>> values of Taumin, Taumax?
>>>
>>> warm regards,
>>> Felipe
>>>
>>>
>>>
>>>
>>>
>>>
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>>> Aspect-devel at geodynamics.org
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>>>
>>
>>
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>
>
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