Can the Visco-Plastic Material Model Support Partially Constant and Partially Variable Densities?

ASPECT
1

Hi everyone,

I have some questions about the visco plastic model. I’d like to fix the densities of two compositional fields—the upper crust and the lower crust—but allow the background material (the mantle) to vary with temperature and pressure.

Below are my current settings. However, they also fix the mantle density at 3300 kg/m³:
I’d like to know whether the visco-plastic model can support this behavior, or if I need to switch to a different material model.

Thanks for your time reading this, and please let me know if there are any other details I can provide!

    set Densities = 3300, 2608., 2915.
    set Heat capacities = 1250, 800, 800
    set Thermal conductivities = 3.3, 2.5, 2.5

Best,
Haolin

2

Hi Haolin,

Thanks for posting on the forum! Unless you are using phase transitions for the density, then the Visco Plastic material model will only vary the density according to the temperature with the following expression:

rho = rho_0 (1 - alpha(T - T_0))

where rho is the model density, rho_0 is the reference density (which you set in the parameter file using the set Densities line) at reference temperature T_0, and alpha is the thermal expansivity and T is the model temperature. Therefore, in order to keep the density of a given field constant you just have to set the thermal expansivities to 0 for the fields which you want to have a fixed density, and non-zero for fields which you want to have a variable density. I hope this helps!

Cheers,

Daniel

3

Hi Daniel,

Thank you so much for your clear and helpful explanation! I now fully understand how density variation works in the Visco Plastic material model and how to keep the density constant for specific fields by setting their thermal expansivities to zero. Your guidance is greatly appreciated!

Cheers,

Haolin