Immersed Boundary Options

The IBM block attaches surface boundary conditions (BCs) to geometry patches from Geometries, and sets sampling distances for the immersed-boundary stencil and wall-model queries. All values accept numbers or expressions (resolved against the Dictionary).

Structure

IBM
{
    sample_dist_wm   = <number | "expr">   # wall-model sampling distance
    sample_dist_bc   = <number | "expr">   # general IBM BC sampling distance

    BoundaryConditions
    {
        <bc_name_0> { ... }
        <bc_name_1> { ... }
        ...
    }
}

• sample_dist_wm — sampling distance (solver units) used by the wall model when it queries the flow.
• sample_dist_bc — sampling distance used by general IBM BC stencils (non-wall-model).
• Typical ranges (guidance):
  sample_dist_wm ≈ 2.5–4.5, sample_dist_bc ≈ 0.10–0.75. Choose relative to near-surface resolution.

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BoundaryConditions entries

Each entry targets one or more geometry patches and assigns a BC type via specifier, with optional type-specific parameters.

Supported Surface Boundary Conditions

For a full list of supported BCs, see Supported Immersed Boundary Conditions.

<bc_name>
{
    component          = "<patch-name | any>"
    geometries         = ["<geom-name>", ...] | "any"
    specifier          = "<bc-type>"

    # BC-type-specific parameters (numbers or expressions):
    <param_0>          = <...>
    <param_1>          = <...>
    ...
}

• geometries

  • Either "any" or a list of names that must exist under Geometries.
  • Matching is done per geometry at initialization.

• component

  • A patch name exported by the geometry’s Patches, or "any".
  • A named patch may map to multiple component IDs internally; the solver expands that mapping for you.

• specifier

  • The BC type identifier (e.g., wall_model, stagnation_subsonic_inflow, …).
  • Each type may require extra parameters (e.g., wall_temperature, total_pressure, …). We’ll document these in the separate IBM Surface Boundary Conditions reference.

• Values can be literals or expressions (e.g., total_pressure = "jetP0").

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Matching & Precedence

The solver builds BCs per geometry and then assigns one BC to each surface component that actually appears on that geometry’s immersed boundary. The rules are:

1. Exactly one "any" component rule is allowed across all entries. It acts as a default for any component that doesn’t receive a specific assignment.
2. Specific beats default, regardless of order.
   • If both a specific patch rule and the "any" default match a component, the specific rule is chosen.
   • In implementation: the default may be seen first, but a later specific will overwrite it; conversely, a later default will not overwrite an earlier specific.
3. Two specifics targeting the same component is an error.
   • If multiple non-"any" rules would assign different BCs to the same underlying component ID, the solver throws (duplicate assignment).
4. Missing coverage is an error.
   • If a component gets no matching rule and there is no "any" default, the solver throws (no BC found).
5. Unknown patch names are fatal.
   • If component names a patch that doesn’t exist on the targeted geometry, the solver throws.
6. Uniqueness requirements per BC type.
   • Some BC types declare must_be_unique_patch. If such a BC is applied to a patch name that expands to multiple IDs, the solver throws.

Practical recipe: define a single global default (component = "any", geometries = "any"), then add specific overrides for special patches. This guarantees coverage and avoids duplicate specifics.

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Inheritance with Subdomains

• IBM lives inside a Domain. A Subdomains entry inherits the parent domain’s IBM settings (distances and rules) unless it overrides them locally.
• This lets you use different sampling distances or BC parameters in a moving-frame subdomain without duplicating everything.

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Examples

1) Global default wall-model BC

IBM
{
    sample_dist_wm   = 3.0
    sample_dist_bc   = 0.5

    BoundaryConditions
    {
        default
        {
            component         = "any"
            geometries        = "any"
            specifier         = "wall_model"
        }
    }
}

2) Default + specific overrides on one geometry

IBM
{
    sample_dist_wm   = 2.8
    sample_dist_bc   = 0.9

    BoundaryConditions
    {
        wallBC
        {
            component         = "any"
            geometries        = ["main"]
            specifier         = "wall_model"
        }
        injectionBC
        {
            component          = "inlet"
            geometries         = ["main"]
            specifier          = "stagnation_subsonic_inflow"
            total_pressure     = "Ptot_jet"
            total_temperature  = "Ttot_jet"
        }
        ...
    }
}