[CIG-SHORT] PyLith hint - nondimensionalization

[Brad Aagaard]

PyLith users:

This is a friendly reminder to PyLith users that it is VERY IMPORTANT to 
make sure that the scales used in the nondimensionalization are 
appropriate for your problem. PyLith can solve problems across an 
extremely wide range of spatial and temporal scales if the appropriate 
scales are used in the nondimensionalization.

Due to roundoff errors and convergence tolerances in the iterative 
solvers, PyLith relies on reasonable scales in the solution in 
constructing the friction criterion and preconditioning the system. 
Failure to set appropriate scales in the nondimensionalization will 
cause the solution to be garbage.

Quasi-static problems

   Default values:
     relaxation_time = 1.0*year
     length_scale = 1.0*km
     pressure_scale = 3.0e+10*Pa

   Recommended values:
     relaxation_time = TIME_STEP
     length_scale = DISCRETIZATION_SIZE or DISPLACEMENT_MAGNITUDE
     pressure_scale = SHEAR_MODULUS

Dynamic problems

   Default values:
     shear_wave_speed = 3.0*km/s
     density = 3000.0*kg/m**3
     wave_period = 1.0*s

   Recommended values:
     shear_wave_speed = MINIMUM_SHEAR_WAVE_SPEED
     density = DENSITY
     wave_period = MINIMUM_WAVE_PERIOD


This tip has been added to the PyLith wiki, which we are beginning to 
construct based on questions from users.

http://www.geodynamics.org/cig/community/workinggroups/short/workarea/pylith-wiki/

Brad