[aspect-devel] Question on Thermal Viscosity thresholds

[Lev Karatun]

Hi Felipe,

just a small addition:
Note that in the H(x) formula, x is the exponent value, not T.

Best regards,
Lev Karatun.

2018-04-26 11:01 GMT-04:00 Lev Karatun <lev.karatun at gmail.com>:

> 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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>>>
>>>
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>>
>>
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