Boundary Conditions

Boundary conditions act on the boundary (ghost) particles of the narrow band around the physical region. They are registered per boundary tag in the domain's bc_map and applied by apply_boundary_conditions!, which iterates over the map and calls each condition's functor.

The time stepper applies them to every state it builds: each stage state $Y_i$ (after the implicit solve) and the final state. MOOD re-forms a candidate after an order drop, and the boundary conditions are re-applied to that candidate as well. Interior particles are never touched by a boundary condition, and boundary particles are never updated by the spatial scheme; their right-hand side is zero.

On moving grids, boundary particles keep their positions (their grid velocity is zero).

Built-in conditions

FixedDirichlet sets every boundary particle with the given tag back to its value in pg.rhos. Because boundary particles are never advanced, this is their initial state.
OutflowBC is a zero-gradient (transmissive) condition using a nearest-donor sweep. Interior particles act as donors; in up to five passes, each unresolved boundary particle with the tag copies the state of its nearest already-resolved neighbour. A boundary particle that finds no donor keeps its initial state.

Writing your own

Subtype AbstractBoundaryCondition and implement the functor:

struct MyBC <: AbstractBoundaryCondition end

function (::MyBC)(pg::ParticleGrid, rhos_buffer, tag::Int, ts, eq, t)
    @inbounds for i in 1:pg.meta.N
        if pg.core.is_boundary[i] && pg.core.tags[i] == tag
            rhos_buffer[i] = ...   # write in place
        end
    end
    return nothing
end

Argument	Meaning
`pg`	The grid; positions, tags, flags and neighbour lists are available
`rhos_buffer`	The state vector being built (a stage buffer, the MOOD candidate, or the final state). Modify this one, not `pg.rhos`.
`tag`	The boundary tag this call is responsible for
`ts`	The time stepper (useful for type-based dispatch)
`eq`	The PDE
`t`	Time of the state being built (stage time, or `t + dt`)

Register the condition with get_rectangular_domain(...; bc_map = Dict(1 => MyBC())). Use pg.shared buffers for scratch space (as OutflowBC does with pg.shared.int_buffer) rather than allocating.

Reference

HyperCloudCore.FixedDirichlet — Type

FixedDirichlet <: AbstractBoundaryCondition
(::FixedDirichlet)(pg::ParticleGrid, rhos_buffer, tag::Int, ts, eq, t)

A strict, unchanging boundary condition strategy.

Callable / Functor

Enforces a static Dirichlet condition upon boundary particles. Iterates over the grid and resets the state of any boundary particle matching the specified tag back to its initial original state stored in pg.rhos.

source

HyperCloudCore.FixedDirichlet — Method

(::FixedDirichlet)(pg::ParticleGrid, rhos_buffer, tag::Int, ts, eq, t)

Enforces a static Dirichlet condition upon boundary particles.

Iterates over the grid and resets the state of any particle matching the specified boundary tag back to its initial original state stored in pg.rhos.

source

HyperCloudCore.OutflowBC — Type

OutflowBC <: AbstractBoundaryCondition
(::OutflowBC)(pg::ParticleGrid, rhos_buffer, tag::Int, ts, eq, t)

A zero-gradient, transmissive boundary condition strategy.

Callable / Functor

Enforces a zero-gradient outflow utilizing a multi-pass nearest-donor algorithm.

Initiates all active interior particles as valid state donors.
Executes up to 5 symmetrical passes outward, dynamically locating the nearest resolved neighbor using squared distances and copying its state to the target particle.
Safely falls back to the original initial state for any completely orphaned boundary particles that fail to resolve a donor.

source

HyperCloudCore.OutflowBC — Method

(::OutflowBC)(pg::ParticleGrid, rhos_buffer, tag::Int, ts, eq, t)

Enforces a zero-gradient outflow boundary condition using a multi-pass nearest-donor algorithm.

Initiates all active interior particles as valid state donors, and marks boundary particles matching the specific tag as requiring resolution.
Executes up to 5 symmetrical passes outward, dynamically locating the nearest resolved neighbor using squared distances and copying its state to the target particle.
Safely falls back to the original initial state for any completely orphaned boundary particles that failed to resolve a donor.

source