[aspect-devel] Question on Thermal Viscosity thresholds

[Wolfgang Bangerth]

Felipe,

>         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.
It may be instructive to look at the implementation to understand what the 
code actually does:

https://github.com/geodynamics/aspect/blob/master/source/material_model/simple.cc#L37

The implementation first computes something that is the min or max of an 
expression -- this yields a continuous function. (You can graphically 
understand this: draw two functions that are both continuous, and then in a 
different color draw that function that is the minimum of the first two -- 
it's always a continuous function.)

The rest is then just an interpolation, which in this case is also continuous.

Best
  W.


-- 
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Wolfgang Bangerth          email:                 bangerth at colostate.edu
                            www: http://www.math.colostate.edu/~bangerth/