--- title: Choosing boundary conditions section: guide group: Choosing… order: 60 --- # Choosing boundary conditions Which boundary condition should each boundary of the mesh take? Give every solid surface a wall, and choose its wall model separately. Close an external flow with a farfield on the compressible solver, and an internal flow with an inflow and an outflow; the incompressible solver always uses the latter pair. Put a symmetry condition on a plane of mirror symmetry. Where a boundary is not closed at all but joined to another part of the domain, whether an overlapping mesh, an abutting rotating mesh or its own periodic partner, make it an interface. | `type` | Use it for | Selected by `kind` | Names its flow state with | | ------------------------------------------------------------- | --------------------------------------------------------------------- | ----------------------------------------------------- | ------------------------- | | [`wall`](/reference/model/boundary-conditions/wall) | solid surfaces; see [Choosing a wall model](choosing-a-wall-model.md) | `slip`, `no slip`, `wall function` | — | | [`farfield`](/reference/model/boundary-conditions/farfield) | open boundaries of an external flow, compressible solver only | `riemann`, `pressure`, `supersonic`, `preconditioned` | `condition` | | [`inflow`](/reference/model/boundary-conditions/inflow) | inlets | `pressure`, `massflow`, `default` (incompressible) | `reference` | | [`outflow`](/reference/model/boundary-conditions/outflow) | outlets | `pressure`, `massflow`, `radial pressure gradient` | `reference` | | [`symmetry`](/reference/model/boundary-conditions/symmetry) | planes of mirror symmetry | — | — | | [`interface`](/reference/model/boundary-conditions/interface) | overset boundaries, sliding interfaces and periodic pairs | `overset`, `sliding`, `periodic` | — | ## Declaring a boundary condition Boundary conditions are a dictionary of named blocks under the mesh's model entry. Each block is keyed by a name of your choosing, conventionally `BC_1`, `BC_2` and so on, and describes one boundary condition: ```python parameters = { "config_version": 2, "solver": {...}, "model": { "naca0012": { "mesh": "naca0012.h5", "boundary_conditions": { "BC_1": {"zones": [2, 3], "type": "symmetry"}, "BC_2": {"zones": [4], "type": "wall", "kind": "no slip"}, "BC_3": { "zones": [1], "type": "farfield", "condition": "IC_1", "kind": "riemann", }, }, } }, } ``` Every block requires a `type`, which selects the boundary condition applied to its zones, and `zones`, which lists those zones. Most types also take a `kind`, which selects a variant of the type with its own further keys. A wall's `kind`, for example, chooses between `slip`, `no slip` and `wall function`, and the keys available below it change with the choice. ## Zones [`zones`](/reference/model/boundary-conditions) is a list of the zone identifiers of the mesh to which the boundary condition applies. The converters record which source zone name became which identifier; see [Zone identifiers](../setting-up/preparing-a-mesh.md#zone-identifiers). ## Wall See [Choosing a wall model](choosing-a-wall-model.md). ## Farfield A farfield boundary closes an external flow, and switches between inflow and outflow face by face according to the local flow direction. It names its flow state through `condition`, a [reference condition](/reference/solver/reference-conditions) entry. The incompressible solver has no farfield boundary; close an incompressible domain with inflow and outflow boundaries instead. The [`kind`](/reference/model/boundary-conditions/farfield#kind) sets how each face is treated: | `kind` | Treatment | | ---------------- | -------------------------------------------------------------------------------------------------- | | `riemann` | Riemann invariants for a non-reflecting inflow; a viscous outflow takes the pressure from outside. | | `pressure` | a pressure boundary for both inflow and outflow. | | `supersonic` | the upstream state on inflow and outflow alike. | | `preconditioned` | the reference state imposed as a supersonic inflow on every face. | A farfield's [`target`](/reference/model/boundary-conditions/farfield#target) turns its angle of attack until a lift or drag coefficient reaches a set value; see [Trimming to a target](../working-with/trimming-to-a-target.md). ## Inflow An inflow boundary takes one of three kinds, selected by [`kind`](/reference/model/boundary-conditions/inflow#kind): | `kind` | The inflow state is set from | | ---------- | ---------------------------------------------------------------------------------------- | | `default` | the reference condition alone; this is the incompressible solver's velocity inlet | | `pressure` | a total pressure ratio and a total temperature ratio relative to the reference condition | | `massflow` | a mass flow rate or a mass flow ratio | The `default` kind is for the incompressible solver. A compressible deck uses `pressure` or `massflow`, and validation rejects `default` there. All three name their flow state through [`reference`](/reference/model/boundary-conditions/inflow#reference), a [reference condition](/reference/solver/reference-conditions) entry. The key is `reference`, not `condition`, which belongs to the farfield. On the compressible solver, the `pressure` and `massflow` kinds take a [`direction`](/reference/model/boundary-conditions/inflow#direction) for the flow to enter along; without one, the flow enters normal to the boundary. A `pressure` inflow also takes a [`mach`](/reference/model/boundary-conditions/inflow#mach), which fixes the inlet Mach number: the inlet static pressure then follows from the total pressure ratio rather than from the flow inside the domain. ::: {.note} A mass flow rate is given in kg/s, and can be used only if the mesh is in metres. A mass flow ratio is non-dimensional, with $A_{in}$ in mesh units: $massflowratio = \rho_{in} U_{in} A_{in}/(\rho_{ref} U_{ref})$. Both must be positive: the boundary type sets the direction of the flow, so zero or a negative value is rejected at validation. ::: ## Outflow An outflow boundary is a pressure, mass flow or radial-pressure-gradient outlet, selected by [`kind`](/reference/model/boundary-conditions/outflow#kind) as for an [inflow](#inflow). All three kinds name their flow state through [`reference`](/reference/model/boundary-conditions/outflow#reference). A radial-pressure-gradient outlet holds its [`static_pressure_ratio`](/reference/model/boundary-conditions/outflow#static-pressure-ratio) at the [`reference_radius`](/reference/model/boundary-conditions/outflow#reference-radius); elsewhere the exit pressure follows radial equilibrium about the axis of the frame the outlet's cells are solved in: the model's rotating `motion`, whether a moving mesh or a frame, or the [`rotating` fluid zone](choosing-a-fluid-zone-model.md#rotating-zones) that holds those cells. An outflow sets no temperature, so it takes no total temperature ratio; that belongs on a `pressure` or `massflow` inflow. ::: {.note} A mass flow rate is given in kg/s, and can be used only if the mesh is in metres. A mass flow ratio is non-dimensional, with $A_{out}$ in mesh units: $massflowratio = \rho_{out} U_{out} A_{out}/(\rho_{ref} U_{ref})$. Both must be positive: the boundary type sets the direction of the flow, so zero or a negative value is rejected at validation. ::: ::: {.demo setting="reference/model/boundary-conditions/outflow#kind" values="massflow, pressure" case="pipe" figure="pipe-outflow-kind"} ![Outlet mass flow against cycle in a turbulent pipe, for a mass-flow outlet and a static-pressure outlet, with the imposed mass flow marked](/images/pipe-outflow-kind.svg "A mass-flow outlet scales the exit velocity until the flow through it matches the request. It settles at 9.989 kg/s, 0.1 per cent under the 10 kg/s requested. A pressure outlet fixes the exit pressure, and the flow follows from the pressure drop along the pipe. At a ratio of 101238/101325 it settles at 8.888 kg/s, 11 per cent lower, with a drop from inlet to outlet of 87 Pa against 109 Pa.") Both runs use the same mesh, Menter SST with wall functions and a total-pressure inlet; only the outlet changes. The pressure ratio is not tuned to the 10 kg/s target: a pressure outlet sets the pressure, and the flow follows. ::: ## Interface An interface boundary, `{"type": "interface"}`, joins a boundary to another part of the domain rather than closing it: the solution on one side is supplied by the other. The [`kind`](/reference/model/boundary-conditions/interface#kind) selects how the two sides are joined: | `kind` | Use it where | What crosses the join | | ---------- | --------------------------------------------------------------------------------------- | ---------------------------------------------- | | `overset` | this mesh overlaps another | fringe values, taken from the mesh beneath | | `sliding` | two meshes meet on a shared surface of revolution with non-matching faces | a conservative flux | | `periodic` | two boundaries of the same mesh are one surface, related by a rotation or a translation | the flow leaving one boundary enters the other | Every interface names what it connects to in its [`connection`](/reference/model/boundary-conditions/interface#connection) block. What the connection holds depends on the `kind`, and is described under each heading below. ::: {.note} A case with an overset or sliding interface requires an implicit pseudo-time scheme, `"implicit euler"`, `"lu-sgs"` or `"mf-gmres"`; an explicit scheme alongside either is rejected at validation. See [Choosing a time-marching scheme](choosing-a-time-marching-scheme.md#scheme). ::: ### Overset An overset interface goes on the outer boundaries of a mesh that is overset on one or more other meshes. Its connection names the model or models whose meshes provide donor data; without a connection, the solver maps against every preceding model. See [Overset meshes](../working-with/overset-meshes.md) for how an overset case is set up and solved. ```python "BC_2": { "zones": [10], "type": "interface", "kind": "overset", "connection": {"model": ["background"]}, }, ``` ### Sliding A sliding interface joins two models whose meshes abut without overlapping on a shared cylindrical, conical or annular surface, typically a rotating region and the stationary region around it. The faces on the two sides need not match. The two face sets are clipped against each other into a common set of intersection polygons, one numerical flux is computed for each polygon, and that flux is applied to the cell on each side. The interface is therefore conservative, not interpolated. A sliding interface is declared on both models, and each names the other. The connection gives the model and the boundary on the other side; that boundary must itself be a sliding interface whose connection names this one back. The interface must be a surface of revolution about the rotation axis of the side that rotates. That axis comes from the model's rotating [`motion`](/reference/model/transforms#motion), with `"moving mesh": True` for a turning rotor or `"moving mesh": False` for a frozen rotor; the interface itself takes no `axis` or `origin`. When neither side rotates, the two meshes are joined directly, and the axis is found from the shape of the interface itself. The interface may be a cylinder, a cone, a flat annulus, a spherical band, a disc or spherical cap that reaches the axis, or a whole sphere centred on the axis. It must cover the full circle about the axis; a periodic sector is not supported. The summary printed at the start of the run names the shape the solver found, which is a quick check that the right zones were listed. A closed or compound surface, such as the two faces and the side of a drum, can be declared either way: - as one sliding interface on each side that lists every zone. The solver groups the zones into single surfaces, a cylinder meshed as two zones counting as one, and pairs each surface with the matching one on the other side; - as one sliding interface per surface, each naming its partner, when the pairing should be stated explicitly. A zone that matches no surface on the other side stops the run at setup, with the zone, the model and the shape it was found to have. ```python "outer": {"boundary_conditions": { "BC_2": {"zones": [10, 63, 62], "type": "interface", "kind": "sliding", "connection": {"model": "disc", "boundary": "BC_1"}}}}, "disc": {"boundary_conditions": { "BC_1": {"zones": [21, 22, 23], "type": "interface", "kind": "sliding", "connection": {"model": "outer", "boundary": "BC_2"}}}}, ``` A rotor turning inside the stator around it, each connected to the other: ```python "model": { "rotor": { "mesh": "rotor.h5", "transforms": { "motion": { "type": "rotating", "axis": [0.0, 0.0, 1.0], "origin": [0.0, 0.0, 0.0], "omega": 100.0, "moving mesh": True, }, }, "boundary_conditions": { "BC_3": { "zones": [21], "type": "interface", "kind": "sliding", "connection": {"model": "stator", "boundary": "BC_2"}, }, }, }, "stator": { "mesh": "stator.h5", "boundary_conditions": { "BC_2": { "zones": [20], "type": "interface", "kind": "sliding", "connection": {"model": "rotor", "boundary": "BC_3"}, }, }, }, }, ``` Two settings trade conservation against other properties of the interface: | Setting | `conservative` / `segment` (the defaults) | `free stream` / `parent` | | ------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------- | | [`area`](/reference/model/boundary-conditions/interface#area) | both sides use the mean of the two parent area maps: conservative to round-off, closure error first order in cell width | each side uses its own parent map: closure and free-stream preservation exact, conservation deficit second order | | [`flux`](/reference/model/boundary-conditions/interface#flux) | one flux per intersection polygon, with a shared area vector, so the two sides cancel exactly | one flux per parent face, against the area-weighted average of its donors: quieter as cells pass, but not conservative | The two `area` settings are identical on a planar interface and for matching meshes. Under `parent` the flux does not change when the intersection pattern changes, so the noise a rotating interface makes as cells pass one another is lower; but each side fluxes its own faces and nothing cancels. `segment` is the default for that reason. [`segment_capacity_factor`](/reference/model/boundary-conditions/interface#segment-capacity-factor), [`sliver_tolerance`](/reference/model/boundary-conditions/interface#sliver-tolerance) and [`closure_tolerance`](/reference/model/boundary-conditions/interface#closure-tolerance) are rarely needed. The first sizes the segment storage reserved for each interface face; setup sweeps a full revolution, and stops with the factor actually needed if it is too small. The closure tolerance is the largest fraction of a face's area that may be left uncovered before setup stops, so that a real gap between the two sides, from mismatched radii, a partial annulus or a wrong axis, shows at setup rather than as a silent leak. The rotor's `motion` turns its mesh every time step and requires an unsteady run. A frozen rotor, steady or unsteady, sets `"moving mesh": False` on the same `motion`, which solves the rotor in a rotating frame over every cell of its mesh. Both are described in [Placing and moving a mesh](../working-with/placing-and-moving-a-mesh.md). ### Periodic A periodic interface treats two boundaries of the same mesh as one surface: the flow leaving one boundary enters the other. It is declared in pairs. Each boundary's connection names the other boundary of the pair, and that boundary must name this one back. Each side carries the [`transform`](/reference/model/boundary-conditions/interface#transform) that maps it onto the other, so the two transforms must be inverses of each other. A rotation of θ about an axis pairs with a rotation of −θ about the same axis line, or equivalently θ about the reversed axis; a translation by a vector pairs with the translation by its negative. Both sides of a pair use the same transform `type`, `'linear'` or `'rotated'`. ::: {.warning} The incompressible solver supports only the `'linear'`, translational, periodic transform; `'rotated'` periodic interfaces need the compressible solver. A rotated pair on an incompressible case, `{"solver_settings": {"type": "incompressible"}}`, passes validation but is ignored, with a warning in the log. ::: A linear periodic pair: ```python "BC_3": { "zones": [11], "type": "interface", "kind": "periodic", "connection": {"boundary": "BC_4"}, "transform": {"type": "linear", "vector": [0.0, 0.0, 1.0]}, }, "BC_4": { "zones": [12], "type": "interface", "kind": "periodic", "connection": {"boundary": "BC_3"}, "transform": {"type": "linear", "vector": [0.0, 0.0, -1.0]}, }, ``` A rotated periodic pair for a 120° sector: ```python "BC_5": { "zones": [5], "type": "interface", "kind": "periodic", "connection": {"boundary": "BC_6"}, "transform": { "type": "rotated", "theta": math.radians(120.0), "axis": [1.0, 0.0, 0.0], "origin": [0.0, 0.0, 0.0], }, }, "BC_6": { "zones": [6], "type": "interface", "kind": "periodic", "connection": {"boundary": "BC_5"}, "transform": { "type": "rotated", "theta": math.radians(-120.0), "axis": [1.0, 0.0, 0.0], "origin": [0.0, 0.0, 0.0], }, }, ``` ## Reference entries - [Boundary conditions](/reference/model/boundary-conditions), with `type` and `zones` - [`wall`](/reference/model/boundary-conditions/wall), [`farfield`](/reference/model/boundary-conditions/farfield), [`inflow`](/reference/model/boundary-conditions/inflow), [`outflow`](/reference/model/boundary-conditions/outflow), [`symmetry`](/reference/model/boundary-conditions/symmetry) and [`interface`](/reference/model/boundary-conditions/interface) - [Reference conditions](/reference/solver/reference-conditions)