OneCAD-Simulation/docs/architecture.md

Architecture

ModelCraft turns geometry into finite element models, writes them for the solvers engineers already use, and solves linear problems itself.

SolidCraft B-rep ──tessellate──▶ boundary surface ──▶ volume mesh ──▶ FE model ──▶ Abaqus .inp
 (solidcraft-kernel)           (welded, watertight,   tet (Delaunay)   materials,     Nastran .bdf
                                face-tagged)          hex (core goal)  sections,      LS-DYNA .k
                                                                       BCs, loads,  ──▶ own linear
                                                                       steps            solver

Crates and layers

cargo xtask layers enforces the table in xtask/src/layers.rs: a crate may depend only on crates in strictly lower layers.

Layer	Crate	Role	Status
L0	modelcraft-model	Solver-neutral FE model: nodes and elements by label, their geometry tags, sets, materials, sections, couplings, BCs, loads and steps, the model check and the resolved view the writers read. Knows no file format and no geometry type.	started
L1	modelcraft-geometry	The only door to SolidCraft: opens .solidcraft designs (rebuilt from their timeline, bodies with persistent face names), B-rep bodies, measures, tessellation into a face-tagged SurfaceMesh; face-tagged 3MF files read into the same type without the kernel (docs/formats/tagged-mesh.md). No SolidCraft type in its API.	started
L1	modelcraft-mesh	Surface preparation (weld, watertightness, feature edges), robust predicates, Delaunay tetrahedralization and refinement, Voronoi-based optimization, hex meshing, quality metrics.	started
L1	modelcraft-fem	Element kernels shared by every structural solver: the element table (tet4, tet10, wedge6, hex8 full and incompatible modes, hex20 full and reduced), shape functions, quadrature, element stiffness, consistent and lumped mass, body and pressure loads, stresses and their extrapolation, isotropic and orthotropic elasticity, and the uniform-strain (one-point) hex8 and tet4 kinematics, hourglass shape vectors and element frequency bounds of the explicit solver. Pure functions, no solver loop.	started
L1	modelcraft-decks	Abaqus input, Nastran bulk data and LS-DYNA keyword readers and writers, to and from model. Text out, no file system; deck.write saves it.	started
L1	modelcraft-render	Display data without a GPU dependency: the visible faces, element edges and nodes of a model, the orbit camera, a CPU rasterizer for headless snapshots, geometry surfaces with their face boundaries, deformed shapes and banded contour plots of nodal results, picking.	started
L2	modelcraft-solver	Implicit solver: assembly from the resolved model, boundary conditions, sparse direct solution, linear static and modal analysis, stresses and reactions.	started
L2	modelcraft-explicit	Explicit dynamics: central-difference time integration with lumped mass, one-point hex8 with hourglass control, nodal-pressure tet4, bulk viscosity, rigid walls, energy account, histories and field frames.	started
L2	modelcraft-lbm	Lattice Boltzmann flow solver on the CPU: D3Q19 two-relaxation-time scheme, bounce-back and slip walls, Zou–He inlets and outlets, obstacles voxelised from the face-tagged surface with exact predicates, momentum-exchange forces per face. Uses mesh for the predicates and the boundary surface.	started
L3	modelcraft-fsi	Fluid–structure coupling between the L2 solvers (later).	planned
L4	modelcraft-doc	The document: geometry, mesh, model, results, history.	planned
L5	modelcraft-io	.modelcraft files (docs/formats/modelcraft.md): text model definition, binary mesh, geometry references with content hashes, the journal; atomic saves; autosave storage for crash recovery. Bytes in, bytes out; the file system only in fs and recovery. Result files later.	started
L6	modelcraft-engine	Commands (CommandSpec), Session::execute with rollback and the panic guard, scripts.	started
L7	modelcraft-ui-egui, modelcraft-mcp	Desktop UI (egui + wgpu viewport, model tree, commands, status bar, results display with legend, panel and probe, offscreen capture of the whole window) and control channel requests (docs/control-protocol.md); MCP server.	ui started
testkit	modelcraft-testkit	Test tooling, a dev-dependency only: the meshing test corpus (tests/corpus/*/recipe.json, analytic shapes) and its runner (cargo xtask corpus).	started
apps	modelcraft, modelcraft-cli, modelcraft-web	Desktop, command line, browser.	desktop, cli started

Rules beyond the layer order:

• Nothing below L7 uses egui, eframe, winit, wgpu or rfd.
• Only modelcraft-geometry uses solidcraft-*, pinned by rev in the workspace Cargo.toml. SolidCraft's [patch.crates-io] (its fixed truck crates) does not reach dependents, so the workspace repeats it from the same rev; bump both together. The same goes for SolidCraft's patched egui-wgpu (MSAA clamped to what the device supports, 1x on OpenGL adapters, so the desktop app's 4x never fails pipeline validation at start).
• Everything below the apps builds for wasm32-unknown-unknown.
• modelcraft-geometry's non-default scripts feature builds designs from SolidCraft command scripts (script_design, through solidcraft-engine) for the test corpus; the library opens designs without it.

Meshing pipeline

1. Boundary surface. SolidCraft tessellates each body; every triangle carries its B-rep face, so feature edges and face-based loads and BCs survive meshing. Faces of an opened design also carry SolidCraft's persistent names (F2:side:l4, from the feature and sketch curve that made them), so a load on a named face follows it when the design is edited and opened again. Bodies of components are placed in the world through their occurrences (one body per placement), as SolidCraft displays and exports them.
2. Surface preparation (modelcraft-mesh::surface, mesh.surface.check). Weld the tessellation's per-face vertex copies (at twice SolidCraft's linear tolerance), check the surface is closed, manifold, consistently and outwardly oriented (voids inwards) and free of self-intersections (exact triangle–triangle tests), and find feature edges (face boundaries, sharp dihedral angles). Defects come back located by triangle, vertex and position.
3. Tetrahedral meshing. Bowyer–Watson Delaunay on Shewchuk's adaptive predicates (symbolic perturbation for degeneracies), boundary recovery by the robust constrained Delaunay method of Diazzi et al. 2023 (implicit Steiner points on indirect predicates, gift-wrapping facet recovery; modelcraft-mesh::cdt, command mesh.tet.generate), so every boundary triangle and its B-rep face tag survives. Then constrained Delaunay refinement to a size field, ODT smoothing interleaved as in Tournois et al. 2009, and the Klingner–Shewchuk 2007 improvement schedule against slivers.
4. Hexahedral meshing (the core). Octree/grid dual all-hex first, the one family that meshes any input validly (Livesu et al. 2021 dual schemes, Pitzalis et al. 2021 refinement, Gao et al. 2019 feature projection and padding, Garanzha et al. 2021 untangling). Then hex-dominant by tet recombination (Pellerin et al. 2018), then polycube (DualCube labelling) for blocky parts; frame-field/integer-grid-map methods stay research. Validity uses a certified trilinear Jacobian check (Marschner et al. 2020), not corner Jacobians alone.
5. Structured hex and face meshing. Faces get all-quad meshes on shared, prescribed edge nodes (modelcraft-mesh::quad): transfinite interpolation for four-sided faces (Gordon and Hall 1973), and for free faces a half-resolution constrained Delaunay triangulation, recombination (Lee and Lo 1994) and one uniform split that lands on the boundary nodes, then smoothing and untangling; every quad is checked with exact predicates. One SizeField trait (modelcraft-mesh::size) sets interior sizes and edge divisions, with even counts per loop. Mapped and swept hex build on these face meshes.
6. Quality. Scaled Jacobian, aspect ratio, dihedral angles; every generated mesh is checked for positive Jacobians and conserved volume before it reaches the model.

Model definition

modelcraft-model holds the analysis model in no solver's vocabulary (the writers map it to element names, cards and keywords):

• Geometry tags. Geometry is referred to by name only: a body and, inside it, the persistent name of a face, edge or vertex ({"body": "Body1", "face": "F2:top"}). A mesher tags what it makes: each element with the body (solids), face (shells) or edge (beams) it fills, each boundary element face with its geometry face, element edges with geometry edges, nodes with vertices (Model::geometry_tag, tag_element, tag_element_face, …; mesh.tag by hand). Replacing or removing an element drops its tags; a remesh clears them and writes new ones.
• Sets are defined by geometry names, explicit labels or both, and resolve against the current mesh into sorted nodes, elements and element faces. A face name also selects the pieces a kernel splits it into (name#0, name#1). Because the definition names geometry, a remesh, or an edit of the design followed by a remesh, keeps every set, section and load in place.
• Element faces are numbered per kind (ElementKind::faces): solids' faces are oriented with the right-hand normal pointing out; shells have a positive and a negative side.
• Definitions (materials, sections with their element formulation, reference points, couplings, boundary conditions, loads, steps) refer to each other by name. A reference must exist when it is made, and a definition still referred to (a load listed by a step included) cannot be deleted; what depends on the mesh is checked when the model is resolved.
• Steps list the boundary conditions and loads active in them. Nothing propagates from one step to the next.
• Units are a label (mm-t-s-N-MPa, …); numbers are never converted, and the check warns when a modulus, density or gravity is implausible for the chosen system.
• Check and resolve. Model::check lists errors and warnings, each naming its item (a set member not on the mesh, an element with no section or in two, a moment on solid nodes, a missing density for a frequency step, …). Model::resolve returns what writers and the solver read besides the definitions: every set resolved (each element then lies in exactly one section's set), the node label of every reference point, and every force, moment and traction as nodal values. It fails when the check finds an error.
• Loads that keep their meaning on any mesh. A force or moment on a set is a total by default: split by the consistent nodal weights ∫ N_i dA of the set's faces (Gauss quadrature on each face's own linear or quadratic geometry), or equally over a set of nodes only. Applying it at every node is an explicit, mesh-dependent opt-in (per_node). A traction is force per area on faces. The expansion lives in resolve, so the deck writers and the solver share it.
• Reference point labels are stored with the point: chosen from the top of the label range when it is created, so remeshing or adding nodes and points never renumbers them.
• Files. A model deserializes by replaying its contents through the same checked methods the commands use, so a file cannot hold what commands would refuse.

Implicit solver

modelcraft-solver solves a model's linear static and frequency steps (solve.static, solve.frequency; solve.probe reads the stored results). Methods and papers are cited in the module docs; ADR 0001 records why the linear algebra is our own.

1. System. Model::resolve(); each solid section's elements take their material's elasticity matrix and the section's formulation (modelcraft_fem::element::ELEMENT_TYPES, one row per element kind: tet4, tet10, wedge6, hex8, hex20); 2D sections (plane stress, plane strain, axisymmetric) take the 2D table (modelcraft_fem::plane::PLANE_TYPES: tri3, tri6, quad4, quad8), chosen by the section kind, and the model then has two degrees of freedom per node: the 2D elements declare ElementDofs::new(DofSet::PLANE) (Ux, Uy) to the DofLayout. A 2D model lies in the XY plane (axisymmetric: x the radius, Y the axis; nodes on the axis are held radially), is loaded on element edges, and cannot mix with solids or couplings yet. Every reference point a coupling ties to the mesh gets six degrees of freedom (translations, rotations) after the mesh's. Shells and beams return "not supported yet". The step's boundary conditions prescribe translations (rotations do not exist on solid nodes) and a coupled point's six directions; its loads become a load vector: forces and tractions from Resolved::nodal_loads, forces and moments on coupled reference points, pressure as consistent face loads, gravity and body forces through the element shape functions.
2. Assembly on the node graph (3 × 3 blocks for solids, 2 × 2 in 2D models, elements in label order, so sums are reproducible bit for bit).
3. Ordering and elimination (constraints). Nested dissection of the nodes by coordinate bisection. Couplings become linear constraint rows: kinematic = rigid link of each coupled translation to the point's translation and rotation; distributing = the point follows the least-squares rigid fit of the set's translations (equal weights, the deck writers' choice). Prescribed values and rows are eliminated master–slave in a fixed order, one dependent degree of freedom per row (the coupled node or the distributing point while it is free, else the largest coefficient), giving u = T_f q + T_p u_p; the reduced matrix T_fᵀ K T_f stays symmetric positive definite (no penalty, no multipliers). Free mesh equations follow the node order, reference point equations come last. Without couplings T is a selection and the numbers are those of plain elimination, bit for bit.
4. Static solution. Up-looking sparse Cholesky (the factor size is computed and capped before any value), reactions T_pᵀ (K u − f) (at nodes and at prescribed reference points, forces and moments), integration-point stresses extrapolated to the nodes and averaged, von Mises. A zero pivot names the node or reference point and direction that can move freely.
5. Frequency. Shift-invert block Lanczos (block 6, full reorthogonalization) on K − σM, σ = 0 for a supported structure and slightly negative for a free one, so rigid-body modes come out at zero; mass-normalized shapes, participation factors and effective masses.
6. Results stay in the session per step, stamped with the session revision they were solved at; Session::result hands out only those of the current revision, so an edit makes them stale and undoing back to the solved state makes them current again.

Verification (crates/solver/tests/verification.rs, analytic only): displacement and traction patch tests for every element and formulation (exact to round-off), cantilever tip deflection against Timoshenko with convergence rates of the linear elements, Lamé's cylinder, Kirsch's plate with a hole (Kt → 3), cantilever and free–free beam frequencies with six rigid-body modes. crates/solver/tests/plane.rs: the 2D elements' displacement patch tests in plane stress, plane strain and axisymmetry, uniform traction, a plane stress cantilever against Timoshenko with convergence rates, Kirsch's plate, Lamé's cylinder in plane strain and axisymmetric, a rotating disc (plane stress) and a rotating cylinder (axisymmetric) against Timoshenko & Goodier, masses and rigid-body modes. crates/solver/tests/couplings.rs: a block given a rigid motion through a coupled point (kinematic and distributing), clamping through a coupled point equals clamping (static and modes), tension through a distributing coupling against the equivalent pressure away from the end, pure bending by a moment through a distributing coupling, torsion of a tube by a moment on a kinematically coupled point (twist and shear stress).

Explicit solver

modelcraft-explicit integrates a model's explicit_dynamic steps in time (solve.explicit; solve.history and solve.probe read the stored histories and frames). Methods and papers are cited in the module docs and in docs/solvers/literature.md (Part H).

1. System. Model::resolve(); hex8 sections with reduced integration become one-point uniform-strain hexahedra with Flanagan–Belytschko hourglass control, tet4 sections Bonet–Burton average nodal pressure tetrahedra (modelcraft_fem::uniform holds their kinematics). Other element kinds and formulations, couplings, reference points and moments return "not supported yet". Masses are row-sum lumped. A zero-valued boundary condition holds a degree of freedom; a displacement value is reached at constant velocity over the step. Loads act in full from the start. The step lists initial velocities and fixed frictionless rigid planes by set.
2. Time step. Each element's highest frequency is bounded in closed form (exact for a cube); the increment is the step's scale (0.9 by default) times the smallest element increment with Wilkins' bulk-viscosity correction, recomputed every increment. The ten controlling elements are reported; a step that would need more increments than its cap is refused before it runs.
3. Integration. Central differences in the half-step form, elements in label order, no parallel reductions (bit-for-bit repeatable). Rigid walls and prescribed degrees of freedom are kinematic velocity constraints whose impulses are the reactions.
4. Energy account. Kinetic, internal (stress work), hourglass, bulk viscosity, external and constraint work, and their balance; a run warns when the balance is off by more than 1% of the largest energy or the hourglass energy exceeds 10% of the internal energy.
5. Results. Histories (energies, momentum, constraint force, chosen nodes) at the history interval and field frames (displacement, velocity, element stress and strain) at the output interval, capped in size; kept in the session like the implicit results.

The first version is geometrically linear with linear elastic materials. Verification (crates/explicit/tests/verification.rs, analytic only): stress waves in hex8 and tet4 bars (front at c·t, σ = ρ c v, work input), a bar striking a rigid wall (contact time 2L/c, rebound speed, plateau force ρ c v₀ A), the energy balance, bit-for-bit repeatability; in the engine (solve.rs), a cantilever's free-vibration period against solve.frequency.

Lattice Boltzmann solver

modelcraft-lbm solves a model's fluid steps (solve.fluid; solve.probe reads velocity and pressure at points and wall force histories). Papers are cited in the module docs and in docs/solvers/literature.md (Part I).

1. Model. A fluid material (density, Newtonian viscosity), a fluid domain (box, cubic cell spacing, a condition on each box face: wall, moving wall, slip, velocity inlet with a uniform or parabolic profile, pressure outlet, periodic pair; obstacle bodies and moving-wall conditions on their named faces; initial velocity; body acceleration) and a fluid step (end time or steps, optional time step, frames, the TRT parameter Λ). Deck writers refuse fluid steps.
2. Setup. Lattice units from the spacing, the time step and the density. Without a given time step the solver keeps the characteristic lattice velocity at 0.05 and τ ≤ 1. A lattice Mach number above 0.3, τ below 0.505 or a lattice acceleration above 0.01 is refused with the numbers and what to change; caps on cells, steps, cell updates and frame memory are checked before anything is allocated.
3. Grid. The engine triangulates each obstacle body (or takes an imported surface) and prepares its BoundarySurface; cells whose centres have a positive winding number are solid, decided with exact orient2d/orient3d and one symbolic perturbation, so faces through cell centres and lines through edges are classified consistently.
4. Stepping. TRT collision with Guo forcing in place; pull streaming as one shifted copy per direction with per-link rules for the cells next to walls and box faces; Zou–He on inlet and outlet layers. Wall links give the momentum exchanged with every box face and B-rep face (the face of the triangle nearest the link). A non-positive density or a lattice speed above 0.5 stops the run with an error.
5. Results. Frames of velocity and gauge pressure per cell (single precision), force histories per wall and the fluid mass, in model units, stored per step like the implicit results.

Verification (crates/lbm/tests/verification.rs, analytic): plane Poiseuille exact to round-off with Λ = 3/16 and second order with Λ = 1/4; Couette exact with the wall shear μU/H; Taylor–Green decay second order; Poiseuille in a staircase pipe; Stokes drag of a simple cubic sphere array against Hasimoto (1959); a parabolic-inlet, pressure-outlet channel against Poiseuille; mass conserved to round-off; bit-for-bit reproducibility. crates/lbm/tests/benchmark.rs (ignored, published reference values): Schäfer–Turek 2D-1.

Commands

Every user-visible action is a CommandSpec in crates/engine/src/cmd/<area>.rs with a JSON form. Session::execute clones the model, runs the command under catch_unwind, and restores the model on an error or a panic; commands that changed the model are journalled, so the journal replays as a script. Each journalled command is also an undo step (the state before it): edit.undo restores that state and moves the command off the journal onto the redo stack, edit.redo puts it back, so the journal always rebuilds the current model. History is bounded by step count and by an estimate of the memory the kept states hold (oldest steps go first). Commands of kind Session (file.*, edit.*) are never journalled. file.save and file.open write and read .modelcraft files; the desktop app autosaves unsaved work for file.recover.

Test corpus

Meshers are measured on our own parts (docs/adr/0001-libraries.md, test geometry). Each part is a SolidCraft command script in tests/corpus/<name>/recipe.json (the file solidcraft-cli run takes, plus a corpus object with its categories and, when it has one, its closed-form volume) with a one-line README.md; analytic shapes (sphere, torus, cylinder, thick-walled pipe, plate with a hole) are generated in code. modelcraft-testkit runs a recipe in SolidCraft's engine, saves the design and opens it through open_design, the path a person's design takes, so no geometry is stored. cargo xtask corpus (a release build of the testkit's corpus binary) builds every part, tessellates it and runs each stage of modelcraft_testkit::runner, writing docs/meshing/corpus.md. A mesher joins by implementing runner::Stage (its columns, and a run over the part's tessellated surfaces) and adding itself to runner::stages().