12. FLOW_BC Namelist
12.1. Overview
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 sides sets are imported into Truchas as face sets with the same IDs. 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,:math:hat{n}.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.
Tangential surface tension These boundary condition types are mutually exclusive: namely, no two types may be defined onoverlapping 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.
12.2. FLOW_BC Namelist Features
12.3. Components
name
face_set_ids
type
Option |
Description |
|---|---|
pressure |
Pressure is prescribed on the boundary. Use pressure or pressure_func to specify its value. |
velocity |
Velocity is prescribed on the boundary. Use velocity or velocity_func to specify its value. |
no-slip |
0-velocity is imposed on the boundary. This is incompatible with inviscid flow. |
free-slip |
No velocity normal to the boundary, but the tangential velocity is otherwise free (notraction forces). |
marangoni |
Like free-slip except a tangential traction is applied that is due to temperature dependence of surface tension. Use dsigma to specify the value of \(d\sigma/dT\). This is incompatible with inviscid flow. |