Help regarding Meaning of Usage of Scaling while defining the displacement variables in MOOSE SolidMechanics Physics type problem

[shrituntunroy]

Hi Moose Team,

I am a new Moose user and currently learning to use Moose.

I have a doubt regarding defination of variables in the Moose input file.

Ref : https://github.com/apatra6/rhocp/blob/main/examples/tantalum/strain_rate_effect/1e-4ps/bcc_pxtal.i

[Variables]
  [./disp_x]
    order = FIRST
    family = LAGRANGE
    scaling = 1e-4
  [../]
  [./disp_y]
    order = FIRST
    family = LAGRANGE
    scaling = 1e-4
  [../]
  [./disp_z]
    order = FIRST
    family = LAGRANGE
    scaling = 1e-4
  [../]
[]

I am unable to understand what is the function of scaling parameter while defining the variables for FE problem.

I have referred the MOOSE documentation and source files still didn't understood the purpose of scaling parameter in the defination of variables.

And How do we decide ideal value of scaling parameter.

Kindly help me understanding this concept.

[GiudGiud]

Hello

Scaling is used as a simple preconditioning of the equations. When there are multiple equations, we weight the residuals of the equation for each variable by this scaling factor to make the convergence criterion more fair and/or help with matrix conditioning

This page on porous flow has an explanation
https://mooseframework.inl.gov/modules/porous_flow/convergence.html#46b5c8c6-401d-44a8-80b8-1ae589dcd695

And How do we decide ideal value of scaling parameter.

Experimentally most of the time. You look at the residuals and see if they are orders of magnitude apart, maybe it's worth bringiing them closer together.

[shrituntunroy]

Hi Guillaume,

Thank you for your response.

From what I understand, the scaling parameter is particularly useful when solving multiphysics problems in MOOSE. Would it also be beneficial when dealing with single-physics problems, such as mechanics, in MOOSE?

Thank you.

[GiudGiud]

It would because the displacements (the equation for each component) might have very different residuals still. For example a traction test might have much larger residual in the direction of the traction

[shrituntunroy]

Thankyou Guillaume, for your help