Calculate the strain rate ( or rate of strain, or rate of deformation)

The equation is:
where is the stress
in the -direction on a face normal to the -axis,\n
is the non-equilibrium pdf.\n
For more information, please refer to: equation 45 in\n
Krueger T, Varnik F, Raabe D. Shear stress in lattice Boltzmann
simulations. Physical Review E. 2009;79(4):1-14.\n

For multi-level mesh, Omega on finer level needs to be adjusted in order to
get the correct shearstress calculation.\n
First, we defines c as the dx ratio between finer and coarse level.\n
Then the viscosity on the different levels must satisfy:\n
This constrain leads to a relationship of omega on different levels:\n
For more information, please refer to:\n
Manuel H, Harald K, Joerg B, Sabine R. Aeroacoustic validation of the
lattice boltzmann method on non-uniform grids. ECCOMAS 2012

description of the method to obtain the variables, here some preset
values might be stored, like the space time function to use or the
required variables.

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