# GENERAL

Defines options for the General subsurface flow mode.

## Options

GAS_COMPONENT_FORMULA_WEIGHT <float>
Specifies the molecular weight of the gas component in grams per mole.
TWO_PHASE_ENERGY_DOF <string>
Specifies the unknown solved for the energy degree of freedom. Options are: AIR_PRESSURE, TEMPERATURE.
ISOTHERMAL
Disables the energy calculation.
NO_AIR
Disables the gas component calculation.
MAX_PRESSURE_CHANGE <float>
Attempts to control subsequent time steps so that maximum pressure change is <float>.
MAX_TEMPERATURE_CHANGE <float>
Attempts to control subsequent time steps so that maximum temperature change is <float>.
MAX_CONCENTRATION_CHANGE
Attempts to control subsequent time steps so that maximum change in dissolved air mole fraction is <float>.
MAX_SATURATION_CHANGE
Attempts to control subsequent time steps so that maximum saturation change is <float>.
SATURATION_CHANGE_LIMIT
Not yet supported.
PRESSURE_CHANGE_LIMIT
Not yet supported.
TEMPERATURE_CHANGE_LIMIT
Not yet supported.
MAX_CFL
Maximum CFL permitted (local pore water velocity * dt / local grid spacing). Recommend setting to 1.0 for better convergence of Darcy fluxes. This limits flow distance to a single grid cell per time step.
IMMISCIBLE
Toggles on multiphase flow in a two-phase state without miscibility of phase components.

## Options that may improve convergence

### Notes

• ITOL refers to a tolerance based on an infinity norm: max|xi|
MAX_ITERATION_BEFORE_DAMPING <integer>
Specifies the threshold number of Newton iterations before damping is imposed.
DAMPING_FACTOR <float>
The scalar value by which the solution is damped once damping is imposed.
MAXIMUM_PRESSURE_CHANGE <float>
Truncates maximum pressure change in a Newton iteration. (Not to be confused with MAX_PRESSURE_CHANGE in OPTIONS of process model block.)
WINDOW_EPSILON <float>
Specifies the tolerance or window that must be exceeded before phase change will occur.
ITOL_SCALED_RESIDUAL <float>
Convergence based scaling the residual by the accumulation term: max|Ri/Ai| < tolerance where A is approximately vol * por * sat * den * xmol / dt for phase mass conservation and vol * (por * sat * den * U + (1-por) * denrock * Cp * T) / dt for energy conservation calculated at the previous time level (t).
ITOL_RELATIVE_UPDATE <float>
Convergence based on relative update of primary dependent variables: max|dxi/Xi|
TOUGH2_ITOL_SCALED_RESIDUAL <float> <optional float>
Similar to ITOL_SCALED_RESIDUAL except Ai is calculated at the previous Newton iteration. The first float (e1) is the tolerance. Second optional float (e2) needs further explanation.
NO_TEMP_DEPENDENT_DIFFUSION
Turns off temperature-dependent diffusion coefficients.
DIFFUSE_XMASS
Diffusion is based on gradients in mass fraction, not mole fraction.
HARMONIC_GAS_DIFFUSIVE_DENSITY
Use harmonic distance weighted average for gas density in gas diffusion calculation.
ARITHMETIC_GAS_DIFFUSIVE_DENSITY
Use arithmetic average for gas density in gas diffusion calculation.
ANALYTICAL_DERIVATIVES
Turns on analytical derivatives.

## Examples

...
PROCESS_MODELS
SUBSURFACE_FLOW flow
MODE GENERAL
OPTIONS
!WINDOW_EPSILON 1.d-4
ISOTHERMAL
TWO_PHASE_ENERGY_DOF TEMPERATURE
GAS_COMPONENT_FORMULA_WEIGHT 2.01588D0 ! kg/kmol
MAXIMUM_PRESSURE_CHANGE 1.0D6 ! truncates pressure change
/
/
/
...


Updated