Rigid Bodies

Rigid bodies in TrixiParticles.jl are represented by particles whose motion is evolved with rigid-body translation and rotation. This allows fluid-structure interaction while keeping the structure kinematics rigid.

API

TrixiParticles.RigidBodySystem — Type

RigidBodySystem(initial_condition;
               boundary_model=nothing,
               contact_model=nothing,
               acceleration=ntuple(_ -> 0.0, ndims(initial_condition)),
               particle_spacing=initial_condition.particle_spacing,
               max_manifolds=8,
               source_terms=nothing, adhesion_coefficient=0.0,
               color_value=0)

System for particles of a rigid structure.

The rigid body is represented by particles and advanced with rigid-body translation and rotation. Fluid-structure interaction forces are reduced to resultant force and torque and applied consistently to all rigid particles.

Arguments

initial_condition: Initial condition representing the rigid particles.

Keywords

boundary_model: Boundary model for fluid-structure interaction (see Boundary Models).
contact_model: Optional rigid contact model. If specified, rigid-wall and rigid-rigid collisions are enabled.
acceleration: Global acceleration vector applied to all rigid particles.
particle_spacing: Reference particle spacing used for time-step estimation.
max_manifolds: Maximum number of wall-contact manifolds cached per rigid particle.
source_terms: Optional source terms of the form (coords, velocity, density, pressure, t) -> source.
adhesion_coefficient: Wall-adhesion strength used by Akinci-type surface tension models when fluids interact with this rigid body. This is only evaluated for fluid-structure interaction with surface-tension-enabled fluid systems.
color_value: Integer label stored as system.cache.color. Currently this is used with BoundaryModelDummyParticles during colorfield initialization so fluids using ColorfieldSurfaceNormal can detect contact with rigid bodies, it participates in the multi-system color sanity check for surface-tension setups, and it is written to VTK output as "color".

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Contact Models

Rigid contact is configured through the contact model. This is separate from the boundary model used for fluid-structure interaction; see Boundary Models for that part of the rigid-body setup.

RigidContactModel currently defines a normal spring-dashpot contact law with the parameters normal_stiffness, normal_damping, and contact_distance.

The current implementation uses the same model for rigid-wall and rigid-rigid contact:

rigid-wall contact groups penetrating wall neighbors into a small number of contact manifolds per rigid particle and applies one normal contact force per manifold,
rigid-rigid contact evaluates direct pairwise normal contact forces between rigid particles,
and both paths are currently normal-only, i.e. there are no tangential/frictional forces or contact-history terms yet.

Here, a contact manifold is a discrete approximation of one locally smooth contact patch. A rigid particle touching a flat wall will usually produce one manifold, while corners or edges can produce several.

The number of cached rigid-wall manifolds per rigid particle is controlled by the RigidBodySystem(...; max_manifolds=8) keyword argument. If more wall-contact patches are detected than cached manifold slots are available, the implementation falls back to the best-matching existing manifold for that particle.

contact_distance defines when contact starts. If contact_distance == 0, the particle spacing of the RigidBodySystem is used.

If no contact_model is specified for a rigid body, rigid-wall and rigid-rigid contact for that system are disabled.

For output and postprocessing, rigid bodies also expose the diagnostics contact_count and max_contact_penetration. They are available through rigid-body system data and VTK output.

TrixiParticles.RigidContactModel — Type

RigidContactModel(; normal_stiffness,
                  normal_damping=0.0,
                  contact_distance=0.0)

Shared rigid-contact model used by the active rigid-wall and rigid-rigid contact paths. The current contact force consists of a linear normal spring-dashpot contribution only. If contact_distance == 0, the particle spacing of the RigidBodySystem will be used as contact distance.

Experimental implementation

This is an experimental feature and may change in future releases.

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