# [aspect-devel] Questions on Boundary temperature model/Function

Magali Billen mibillen at ucdavis.edu
Fri Feb 27 10:16:59 PST 2015

```Hello Timo,

Thanks for your response to Tim about setting the boundary indicators. I'm still confused about a couple things with different input options,
so I'll try to explain more clearly what we are trying to do.

We have a box (3000 x 3000 km). At the top (z=3000) we want the temperature to be constant (Ttop=273 K) and fixed for the whole run.
At the bottom (z=0), we want the temperature to be function of position ( Tbot (x,0)). For the initial condition, the temperature will be linear
with depth, but varies as a function of x, because the bottom boundary temperature changes T(x,z) = (Tbot(x,0) - Ttop)*(L-z)/L + Ttop.

So, to implement this, I define the initial temperature with:
subsection Initial conditions
set Model name = function
subsection Function
set Variable names      = x,z
set Function constants  = Tcon=1673, xstep=0.5, dT=100, L=3000, sm=0.02, Ttop=273
set Function expression = (((Tcon - (1-xstep)*dT) + dT*(0.5 + (1/pi)*atan( (x - xstep*L)./(sm*L) ))) - Ttop)*(L-z)/L + Ttop;
end
end

Then I tell Aspect that the top and bottom boundaries have fixed temperature (sides are insulating be default):

subsection Model settings
set Fixed temperature boundary indicators   = bottom, top
end

So, where I get confused is how do I assigned the constant temperature to the top and the functional temperature to the bottom?

I feel like I can either enter the function for the bottom or the constant for the top, but not both. If I enter the same function as
the initial conditions (as below), does it just evaluate this function on each of the boundaries?  OR, do I need to use two separate
functions (one that is just T=273) with an if statement (if z=0 T=T(x,0), if z=3000 T=273? If I need to use an if statement, can you
give me an example of what this looks like?

subsection Boundary temperature model
set Model name = Function

subsection Function
set Variable names      = x,z
set Function constants  = Tcon=1673, xstep=0.5, dT=100, L=3000, sm=0.02, Ttop=273
set Function expression = (((Tcon - (1-xstep)*dT) + dT*(0.5 + (1/pi)*atan( (x - xstep*L)./(sm*L) ))) - Ttop)*(L-z)/L + Ttop;
end

end

Magali

On Feb 21, 2015, at 6:13 AM, Timo Heister <timo.heister at gmail.com> wrote:

> Hey Tim,
>
>> I am trying to figure out how to assign the function to a specific boundary.
>> Below is the current function. I would like to have it only pertain to the
>> bottom boundary. Thank you!
>
> subsection Model settings
>  set Fixed temperature boundary indicators   = bottom
> end
>
> causes your boundary condition for the temperature to be applied to
> the bottom only (the others will be insulating).
>
> For your initial condition you have to supply a value everywhere. Do
> you want it to be zero except at the bottom boundary?
> _______________________________________________
> Aspect-devel mailing list
> Aspect-devel at geodynamics.org
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Earth & Planetary Sciences Dept., UC Davis
Davis, CA 95616
Room 2129 Earth & Physical Sciences Bldg.
Office Phone: (530) 752-4169
http://mygeologypage.ucdavis.edu/billen/
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