# Impossible to set more than one thermal viscosity exponent in ASPECT

Dear developers. In CITCOM is possible to use more than one thermal viscosity exponent, for example:

``````num_mat=4 # number of material group with possibly different rheology
visc0=1.0,1.0,1.0,50.0 	# pre-exponential constant
viscE=9.21,9.21,9.21,11.51	#activation energy
viscZ=0.0,0.0,0.0,0.0
``````

I checked the aspect manual and in ASPECT you can set only one thermal viscosity coefficient for your planet, it would be fantastic if, in future update, i could set more than one value, for example:

``````set Thermal viscosity exponents = 9.21,9.21,9.21,11.51
set Viscosity prefactors = 1.0, 1.0, 1.0, 50.0
``````

I mean, i can use 10 as a value, i donâ€™t ithink that my simulation will change a lot
Francesco

all of this depends on what material model you are using. Can you explain which model you use? Separately, what would these multiple exponents correspond to? The viscosities of different phases? Different compositions?
Best
W.

Thank you Bangerth for your answer. Actually iâ€™m using a multicomponent model. So iâ€™m doing a mix between Steinberger, ASCII and simple:

``````   subsection Compositing
set Viscosity = ascii reference profile
set Density = Steinberger
set Thermal expansion coefficient = Steinberger
set Specific heat = Steinberger
set Thermal conductivity = Steinberger
set Compressibility = Steinberger
set Entropy derivative pressure = Steinberger
set Entropy derivative temperature = Steinberger
set Reaction terms = Steinberger
end
``````

According to the paper that i want to emulate, the thermal viscosity exponent, called also â€śActivation energyâ€ť, correspond the the term A of this equation:

The ASPECTâ€™s manual refer to it as the thermal viscosity exponent:

The point is that in CITCOM you can choose as many thermal viscosity exponent as many viscosity prefactors you have, in ASPECT you can only choose one. So, going back to my post, i was asking if in a future update will be possible to set up more than one thermal viscosity exponent like in Citcom.
Best regards
Francesco

Francesco,
Iâ€™m still not clear what the multiple exponents would correspond to. If you provide multiple exponents, how does CITCOM define the overall viscosity?

The longer answer of course is that, as in most volunteer projects, functionality appears if someone implements it. In your case, if there is no material model that does what you want to do, you can always implement your own material model that does what you want it to do.

Best
W.

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