 # Calculate viscosity at each time_step in power-law viscoelastic system

Dear PyLith Users

hello
I applied the power law viscosity to the simulation. I think PyLith can calculating the power-law viscosity at each time. If so, I want to see the viscosity corresponding to each time as an output. How can i calculate viscosity at each time_step in power-law viscoelastic system.

Kim

The powerlaw viscoelastic formulation does not explicitly depend on an instantaneous viscosity. Instead the powerlaw viscoelastic formulation depends on the deviatoric stress and strain rate invariants (see section 5.3.4.1 of the PyLith v2.2.1 manual). The state variables include viscous strain and the current stress tensor. I believe you can compute the instantaneous relaxation time using equation 5.82 and the material property parameters and state variables, and then compute an instantaneous viscosity from the instantaneous relaxation time.

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Yes, PyLith does not output the effective viscosity (which I believe is what you want); however, it can be computed using the equations in the manual. You could do this either using a Python script or the Paraview calculator. You would have to use several Paraview calculators to compute all the variables, though. Another option is to use the Python filter within Paraview.

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Thank you

I will try to calculate viscosity from the instantaneous relaxation time.

Then, I have a question. The relaxation_time will be different for each cell and stable time_step is (relaxation time/ 5). When the pylith calculates, is the stable time_step minimum value or the average value?

It depends on what you want. If you want to see the effective viscosity for the entire domain (for visualization), you will want to just compute it at all of the element centroids. If you just need it for information, you probably want to compute the minimum value.

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