HI Shadi Zekri

For surfaces in contact whit water fix pp and fix saturation and apply pp

For surfaces in contact whit Air and dry fix pp and free saturation

Thank you Mr Rashiddel

Does the command of fix pp mean no flow of the adjacent zones, or does it mean no flow to the outside environment?

Alireza Rashiddel

NO......

The opposite conditions, FIX pp and FIX saturation, may also be set at any gridpoint. In general, fluid may enter or leave the grid at an external boundary if either the pore pressure or the saturation is fixed.........

1-Pore pressure fixed; saturation free – This is the usual boundary for a free surface.

2-Pore pressure fixed; saturation fixed – This is the common boundary condition for an applied pore pressure

Thank you Alireza Rashiddel

Alireza Rashiddel what kind of boundary conditions with regard to pp and saturation should I give at infiltration boundary. Also if I am using van-genuchten model what should be the fmodulus for wetting fluid i.e. water. Is it equal to bulk modulus of water? What about non-wetting fluid i.e. air?

Hi Deepak Kumar Singh

Two-Phase Flow for fluid and Air (see manual, Two-Phase Flow part)

See the page 152 for water bulk modulus@ in dynamic part of manual

For fluid boundary condition:

1. Pore pressure free; saturation free – This is an impermeable boundary and is the default condition. There is no exchange between the grid and the outside world. Pressure and saturation changes are computed according to Eq. (1.28) or Eq. (1.29), depending on the current value of saturation and whether the fluid has “cavitated” (fallen below the tensile limit).

2. Pore pressure fixed; saturation free – This is the usual boundary for a free surface (i.e., at the top of the model). Fluid flows to and from the outside world. The saturation may vary only if the set pressure is exactly zero. Otherwise, saturation is forced to be 1 (to conform with FLAC ’s assumption, using the single-phase option, that pore pressures can only exist in a fully saturated material). Pore pressure cannot be fixed at a value less than the tensile limit; FLAC will reset any such pressures to the tensile limit.

3. Pore pressure free; saturation fixed – This is the condition at a surface with an applied discharge (e.g., with a specified surface infiltration rate). If the saturation is set to 1, then pore pressure is calculated in the normal way (no exchange with the outside world). However, if the pressure tends to fall below the tensile limit, fluid is “injected” from outside to keep the pressure at the tensile limit. If the saturation is set at less than 1, the pore pressure is forced to be exactly zero, which also implies that fluid is exchanged with the outside world.

4. Pore pressure fixed; saturation fixed – This is the common boundary condition for an applied pore pressure. If saturation is set to 1, then fluid is exchanged with the outside world; no computation of pressure or saturation is done. If saturation is set at less than 1, then the set pore pressure is redefined as zero. Pore pressure cannot be fixed at a value less than the tensile limit; FLAC will reset any such pressures to the tensile limit.

HI @Alireza Rashiddel, thank you for the previous answer. how to define parameters like van genuchten a, p0(reference capilary pressure), viscosity ratio. Is FLAC's vga same as the parameter m (in many texts). Also what about vgpcw and vgpcnw and also please tell about vg b. For example if i am trying to simulate in FLAC2D the results of a slope stability study done in FEM. How to define these van genuchten parameters like vga, vgb, vgc, vgp0, vgpcnw, vgpcw, visrat, rsat, udcoe.

Please elaborate, if you know.

HI Alireza Rashiddel thank you for the previous answer. how to define parameters like van genuchten a, p0(reference capilary pressure), viscosity ratio. Is FLAC's vga same as the parameter m (in many texts). Also what about vgpcw and vgpcnw and also please tell about vg b. For example if i am trying to simulate in FLAC2D the results of a slope stability study done in FEM. How to define these van genuchten parameters like vga, vgb, vgc, vgp0, vgpcnw, vgpcw, visrat, rsat, udcoe.

Please elaborate, if you know.

Sorry I did not work with van genuchten model

Hello Alireza Rashiddel. Thank you for your reply. Can you tell me how to impose constant air pressure boundary conditions in FLAC 2D.