Lattice Boltzmann flow on the CPU: model, solver, engine (M12) (#36)

Model: Newtonian fluid materials; fluid domains (box, spacing, conditions on
the box faces, obstacle bodies with conditions on their named faces, initial
velocity, body acceleration); fluid steps with caps. Deck writers refuse
fluid steps; a fluid-only model needs no mesh in model.check; one step
selector (Model::select_step) for every solver.

New L2 crate modelcraft-lbm: D3Q19 two-relaxation-time collision with Guo
forcing; halfway bounce-back walls (moving walls after Ladd), slip faces,
periodic pairs; Zou–He velocity and pressure faces with the D3Q19
transverse correction; obstacles voxelised from the face-tagged boundary
surface with exact predicates and symbolic perturbation; momentum-exchange
forces per box face and per B-rep face; unit conversion, automatic time step
and Mach/tau limits; bounded runs with frames and force histories.

Engine: model.fluid_domain.create/edit/delete, model.fluid_domain.boundary,
solve.fluid (obstacles from the open design or imported surfaces, lowerable
caps), solve.probe for fluid fields and wall force histories. Fluid Domain,
Inlet & Outlet and Lattice Boltzmann toolbar entries are ready.

Verification (analytic): plane Poiseuille exact with Λ = 3/16 and second
order (2.000) with Λ = 1/4; Couette exact with the wall shear; Taylor–Green
decay second order (2.01, 2.00); staircase pipe; Stokes drag of a sphere
array within 1.7% of Hasimoto at R = 6; Zou–He channel; mass conserved to
round-off; bit-for-bit reproducibility. Benchmark: Schäfer–Turek 2D-1.


Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/model/tests/definition.rs

@@ -56,7 +56,8 @@ fn defined() -> Model {
     m.add_set("all", Set::of_geometry(vec![GeometryRef::body("Bar").unwrap()])).unwrap();
     m.add_set("root", Set::of_geometry(vec![GeometryRef::face("Bar", "left").unwrap()])).unwrap();
     m.add_set("end", Set::of_geometry(vec![GeometryRef::face("Bar", "right").unwrap()])).unwrap();
-    m.add_material("Steel", Material { elastic: Some(Elastic::Isotropic { young: 2.1e5, poisson: 0.3 }), density: Some(7.85e-9) }).unwrap();
+    m.add_material("Steel", Material { elastic: Some(Elastic::Isotropic { young: 2.1e5, poisson: 0.3 }), density: Some(7.85e-9), fluid: None })
+        .unwrap();
     m.add_section("bar", Section { set: "all".into(), material: "Steel".into(), kind: SectionKind::Solid {}, formulation: Formulation::default() })
         .unwrap();
     m.add_reference_point("RP", ReferencePoint { xyz: [3.0, 0.5, 0.5], label: 0 }).unwrap();
@@ -120,7 +121,7 @@ fn the_check_names_what_is_missing() {
     m.add_set("side", Set { faces: vec![ElementFace { element: 1, face: 3 }], ..Set::default() }).unwrap();
     m.add_load("ghost-load", Load::Pressure { set: "ghost".into(), magnitude: 1.0 }).unwrap();
     m.add_load("twist", Load::Moment { target: Target::Set("end".into()), vector: [1.0, 0.0, 0.0], distribution: Distribution::Total }).unwrap();
-    m.add_material("Soft", Material { elastic: Some(Elastic::Isotropic { young: 2.1e11, poisson: 0.3 }), density: None }).unwrap();
+    m.add_material("Soft", Material { elastic: Some(Elastic::Isotropic { young: 2.1e11, poisson: 0.3 }), density: None, fluid: None }).unwrap();
     m.add_section("again", Section { set: "side".into(), material: "Soft".into(), kind: SectionKind::Solid {}, formulation: Formulation::default() })
         .unwrap();
     m.add_section(
@@ -326,7 +327,8 @@ fn a_total_force_on_a_face_is_the_same_on_every_mesh() {
     m.set_units(UnitSystem::MmTonneS);
     m.add_set("all", Set::of_geometry(vec![GeometryRef::body("Block").unwrap()])).unwrap();
     m.add_set("end", Set::of_geometry(vec![GeometryRef::face("Block", "end").unwrap()])).unwrap();
-    m.add_material("Steel", Material { elastic: Some(Elastic::Isotropic { young: 2.1e5, poisson: 0.3 }), density: Some(7.85e-9) }).unwrap();
+    m.add_material("Steel", Material { elastic: Some(Elastic::Isotropic { young: 2.1e5, poisson: 0.3 }), density: Some(7.85e-9), fluid: None })
+        .unwrap();
     m.add_section("block", Section { set: "all".into(), material: "Steel".into(), kind: SectionKind::Solid {}, formulation: Formulation::default() })
         .unwrap();
     let force = |distribution| Load::Force { target: Target::Set("end".into()), vector: [100.0, 0.0, -50.0], distribution };