11. FLOW_BC Namelist

The FLOW_BC namelist is used to define boundary conditions for the fluid flow model at external boundaries. At inflow boundaries it also specifies the value of certain intensive material quantities, like temperature, that may be associated with other physics models.

Each instance of the namelist defines a particular condition to impose over a subset \(\Gamma\) of the domain boundary. The boundary subset \(\Gamma\) is specified using mesh face sets. The namelist variable face_set_ids takes a list of face set IDs, and the boundary condition is imposed on all faces belonging to those face sets. Note that ExodusII mesh side sets are imported into Truchas as face sets with the same IDs.

Namelist Usage

Required/Optional:: Required
Single/Multiple Instances:: Multiple

11.1. Boundary Conditions

The following common types of boundary conditions can be defined:

Pressure. A pressure Dirichlet condition \(p=p_b\) on \(\Gamma\) is defined by setting type to “pressure”. The boundary value \(p_b\) is specified using either pressure for a constant value or pressure_func for a function.
Velocity. A velocity Dirichlet condition \(u=u_b\) on \(\Gamma\) is defined by setting type to “velocity”. The boundary value \(u_b\) is specified using either velocity for a constant value, or velocity_func for a function.
No Slip. The special velocity Dirichlet condition \(u = 0\) on \(\Gamma\) is defined by setting type to “no-slip”.
Free slip. A free-slip condition where fluid is not permitted to penetrate the boundary, \(\hat{n}\cdot u = 0\) on \(\Gamma\), where \(\hat{n}\) is the unit normal to \(\Gamma\), but is otherwise free to slide along the boundary (no tangential traction) is defined by setting type to “free-slip”. This is the default boundary condition.
Tangential surface tension

These boundary condition types are mutually exclusive: namely, no two types may be defined on overlapping subsets of the boundary. Any subset of the boundary not explicitly assigned a boundary condition will be implicitly assigned a free-slip condition.

Currently it is only possible to assign boundary conditions on the external mesh boundary. However in many multiphysics applications the boundary of the fluid flow domain will not coincide with the boundary of the larger problem mesh. In some cases the boundary will coincide with an internal mesh-conforming interface that separates fluid cells and solid (non-fluid) cells, where a boundary condition could conceivably be assigned. In other cases, typically those involving phase change, the boundary is only implicit, passing through mixed fluid/solid cells, and will not conform to the mesh. In either case, the flow algorithm aims to impose an effective no-slip condition for viscous flows, or a free-slip condition for inviscid flows. A possible modeling approach in the former mesh-conforming case is to define an internal mesh interface using the MESH namelist variable interface_side_sets. This effectively creates new external mesh boundary where flow boundary conditions can be assigned.

11.2. Namelist Variables

name 

A unique name used to identify a particular instance of this namelist.

Type:: string
Default:: none

face_set_ids 

A list of face set IDs that define the portion of the boundary \(\Gamma\) where the boundary condition will be imposed.

type 

The type of boundary condition. The available values are:

Value	Description	Associated Variables
“pressure”	Pressure is prescribed on the boundary.	pressure or pressure_func
“velocity”	Velocity is prescribed on the boundary.	velocity or velocity_func
“no-slip”	Zero velocity is imposed on the boundary. This is incompatible with inviscid flow.	(none)
“free-slip”	Zero velocity normal to the boundary, but the tangential velocity is otherwise free (no traction forces).	(none)
“marangoni”	Like “free-slip” except a tangential traction is applied that is due to temperature dependence of surface tension. This is incompatible with inviscid flow.	dsigma

pressure 

The constant value of boundary pressure for a pressure-type boundary condition. To specify a function, use pressure_func instead.

Default:: none
Type:: real

pressure_func 

The name of a FUNCTION namelist defining a function that gives the boundary pressure for a pressure-type boundary condition. The function is expected to be a function of \((t,x,y,z)\).

Default:: none
Type:: string

velocity 

The constant value of boundary velocity for a velocity-type boundary condition. To specify a function, use velocity_func instead.

Default:: none
Type:: real 3-vector

velocity_func 

The name of a VFUNCTION namelist defining a function that gives the boundary velocity for a velocity-type boundary condition. The function is expected to be a function of \((t,x,y,z)\).

Default:: none
Type:: string

dsigma 

The constant value of \(d\sigma/dT\) for the marangoni-type condition. Here \(\sigma(T)\) is the temperature-dependent surface tension coefficient.

Default:: none
Type:: real

inflow_material 

Velocity and pressure boundary conditions may result in fluid flow into the domain across the boundary. This optional variable specifies the name of the fluid material to flux in. If not specified, materials are fluxed into a cell through a boundary face in the same proportion as the material volume fractions present in the cell. To specify a function, use inflow_material_func instead.

Type:: string

inflow_material_func 

The name of a VFUNCTION namelist defining a function that gives the inflow material volume fractions for an inflow boundary condition. The function is expected to be a function of \((t,x,y,z)\), and return a vector of length equal to the number of liquid phases, plus void (if present). The order of the output components is the same as the input file ordering provided by materials.

Type:: string

inflow_temperature 

Velocity and pressure boundary conditions may result in fluid flow into the domain across the boundary. This optional variable specifies the temperature of the material fluxed in. If not specified, materials are fluxed into a call through a boundary face at the same temperature as the cell.

Type:: real

inflow_conc 

Velocity and pressure boundary conditions may result in fluid flow into the domain across the boundary. This optional variable specifies the concentration of the passively advected scalar fluxed into a cell through a boundary face. If not specified, the fluxed concentration is the same as that of the cell. To specify a function, use inflow_conc_func instead. This is a vector-valued variable with elements that correspond to the different species components of the species advection-diffusion model.

Type:: real

inflow_conc_func 

The names of FUNCTION namelists defining functions that give the inflow concentrations for an inflow boundary condition. The functions are expected to be functions of \((t,x,y,z)\). This is a vector-valued variable with elements that correspond to the different species components of the species advection-diffusion model.

Type:: string