Questions About Fully Coupled Flow–Mechanical Analysis and Solution-Control Parameters in FLAC3D

FLAC3D
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Hello @everyone, @itascaYang @isaac @itsmoho_sen @ITE @Inwoo @Inu @itasca @dafo407_itasca @itascan

I am planning to perform a fully coupled flow–mechanical analysis in FLAC3D. My current understanding of the command sequence is as follows:

model configure fluid

Then build the geometry, assign mechanical models/properties, interfaces, geogrids, and construction stages.

Next, assign the fluid model and properties:

zone fluid cmodel assign isotropic range group 'X1' or 'X2'
zone fluid property permeability 1.02e-9 porosity 0.365 range group 'X1'
zone fluid property permeability 1.0e-12 porosity 0.365 range group 'X2'
zone gridpoint initialize fluid-modulus 2.2e9 range group 'X1' or 'X2'

zone fluid biot on
zone fluid property biot 0.9931 range group 'X1'
zone gridpoint initialize biot-modulus 5.46e9 range group 'X1'

Then define water density and water table:

zone water density 1000
zone water plane origin (x,y,z_water) normal (0,0,1)

Then initialize saturation:

zone gridpoint initialize saturation 1.0 range group 'X1' or 'X2'

Then bring the model to coupled equilibrium:

model fluid active on
zone mechanical active on

model mechanical substep 100
model fluid substep 10
model mechanical follower on

model solve fluid time-total 1.0e4 or mechanical convergence 10

After that, external loading will be applied and the coupled analysis will continue using:

model fluid active on
zone mechanical active on

model mechanical substep 100
model fluid substep 10
model mechanical follower on

model history fluid time-total

zone history pore-pressure position x y z
zone history displacement-z position x y z

model solve fluid time-total 3.0e6 or mechanical convergence 10

I would like to ask whether this overall approach for fully coupled analysis is reasonable or if there are any important corrections I should make.

I also want to better understand the selection of the following commands and values:

model mechanical substep 100
model fluid substep 10
model mechanical follower on
model solve fluid time-total ...

Specifically:

  1. How are the values for model mechanical substep and model fluid substep selected?

  2. Is there any physical or numerical basis for choosing values like 100 and 10, or are they mainly trial-and-error control parameters?

  3. What is the exact purpose of model mechanical follower on in fully coupled analysis?

  4. In fully coupled flow–mechanical simulations, should model mechanical follower on generally always remain on?

  5. How can model solve fluid time-total be estimated or calculated for a coupled consolidation-type problem?

  6. Is there a recommended procedure to determine appropriate fluid time-total values for a model?

If possible, could anyone also please provide example problems, documentation links, or references related to fully coupled flow–mechanical analysis and the selection of these solution-control parameters?

Any guidance would be greatly appreciated.

Thanks in advance.