How to specify boundary conditions

Problem-specific Dirichlet BCs

An example of problem-specific Dirichlet boundary conditions is given below

solver = "riecg"

problem = {
  name = "vortical_flow"
}

bc_dir = {
  { 1, 1, 1, 1, 1, 1 },
  { 2, 1, 1, 1, 1, 1 }
}

The above example selects the RieCG solver, which solves for the variables . The above block configuring Dirichlet BCs contains two blocks with exactly 6 integers. Of these 6 integers the first one is the side set id which specifies the exodus side set (given in the input mesh file) to which the given Dirichlet BC should be applied – in this case side sets 1 and 2. In this example, the following 5 integers are all specified as ones. Since a specific problem type is also specified vortical_flow, used for verification with a known analytic solution, the 5 ones following the side set ids prescribe evaluation of the analytic solution specific to this problem at each time step for each variable solved for. If a zero is given instead of a one for a variable, the enforcement of the Dirichlet BC for that variable is skipped. Dirichlet BCs are enforced in mesh nodes of the side set. For a full control file employing problem-specific Dirichlet BCs see RieCG: Vortical flow.

Dirichlet BCs with a user-defined problem

An example of Dirichlet boundary conditions used with a user-defined problem is given below

solver = "lohcg"

-- Note: a 'problem' block is not specified -> default: 'userdef'

ic = {
  velocity = { 1.0, 0.0, 0.0 }
}

bc_dir = {
  { 6, 0, 1, 1, 1 },
  { 7, 1, 0, 0, 0 }
}

The above example selects the LohCG solver, which solves for the variables . The above block configuring Dirichlet BCs contains two blocks with exactly 5 integers. Of these 5 integers the first one is the side set id which specifies the exodus side set (given in the input mesh file) to which the given Dirichlet BC should be applied – in this case side sets 6 and 7. The following 4 integers are specified as zeros and ones. A zero means that the Dirichlet BC is skipped for that variable, while one means apply. According to the above, on side set 6 the pressure is left alone and the velocity is set to its initial value, presribed by the preceding ic block. On side set 7, the velocity is left unmodified and the pressure is set to its initial value which is zero by default. Dirichlet BCs are enforced in mesh nodes of the side set. For a full control file employing Dirichlet BCs with a user-defined problem, see LohCG: Inviscid flow past a sphere.

Inhomogeneous Dirichlet BCs

An example of inhomogeneous Dirichlet boundary conditions, specifying constant values at a side set, is given below

pressure = {
  bc_dir = { { 1, 2 },
             { 2, 2 } },
  bc_dirval = { { 1, 2.4 },
                { 2, 0.0 } }
}

The above example specifies two Dirichlet BC blocks, bc_dir and bc_dirval that together specify inhomgeneous (nonzero) constant values for the pressure. The bc_dir block contains integers, where the first one specifies the side set id and the second one here is given as the integer code 2 for the scalar quantity for which the BC is given, in this case pressure. The integer code 2 in the bc_dir block means that a corresponding bc_dirval block will specify the inhomgeneous value for the BC. The bc_dirval block contains an integer in the first position, specifying the side set id, followed by real values for the pressure BC: on side set 1 the pressure will be set to 2.4, while on side set 2, it will be set to 0.0. Dirichlet BCs are enforced in mesh nodes of the side set. For a full control file employing such Dirichlet BCs for the pressure solve, see ChoCG: Poiseuille flow.

Another example for specifying inhomogeneous Dirichlet BCs is the following.

solver = "chocg"
ic = {
  velocity = { 0.0, 0.0, 0.0 }
}
bc_dir = {
  { 4, 2, 2, 2 }
}
bc_dirval = {
  { 4, 1.0, 0.0, 0.0 }
}

The above selects the ChoCG solver, which solves for the prognostic variables . It specifies quiescent initial conditions via the ic block and uses inhomogeneous Dirichlet BCs to prescribe the constant values of components of the velocity vector on side set 4. Dirichlet BCs are enforced in mesh nodes of the side set. See ChoCG: Lid-driven cavity for the full example. See also LohCG: Lid-driven cavity for a similar setup but for LohCG which skips the pressure as the prognostic variable in the first position.

Symmetry BCs

An example specifying symmetry (free-slip) conditions is given below

bc_sym = {
  sideset = { 1, 2, 3 }
}

The block lists 3 side set ids at which symmetry (free slip) boundary conditions are specified. The ids must correspond to those in the input exodus file. The symmetry conditions remove the normal component of the velocity at a surface. Symmetry BCs are enforced in nodes of triangle surface elements that belong to the side set. If a node is shared by multiple side sets, the condition is enforced indepently for each surface element the node belongs to using the normal vector for the surface element. For a full example, see RieCG: Sedov blast.

No-slip BCs

An example specifying no-slip/no-penetration conditions is given below

bc_noslip = {
  sideset = { 3, 4 }
}

The block lists 2 side set ids at which no-slip/no-penetration boundary conditions are specified. The ids must correspond to those in the input exodus file. No slip conditions prescribe zero velocity at mesh nodes that belong to a side set. For a full example, see ChoCG: Poiseuille flow.

Farfield BCs

An example specifying farfield conditions is given below

bc_far = {
  pressure = 1.0e+5,
  density = 1.225,
  velocity = { 283.57, 15.159, 0.0 },
  sideset = { 4 }
}

The above block specfies explicit values for the flow variables to be applied in farfield boundaries for transconic flows. The side set id is also given, which is the exodus side set id, appearing in the input mesh file. For a full example, see RieCG: ONERA wing.

Pressure BCs

The example below specifies pressure conditions on an inlet and outlet, each given by a list of side sets.

bc_pre = {
  inlet = {
    density = 1.996,
    pressure = 1.005e5,
    sideset = { 2 }
  },
  outlet = {
    density = 1.1996,
    pressure = 87200,
    sideset = { 1 }
  }
}

The above example specifies the fluid density and pressure conditions at an inlet and outlet, each given by a list of exodus side set ids that must match those in the input mesh file. For a full example, see RieCG: Diesel engine.