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 <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

crates/explicit/tests/verification.rs

@@ -77,7 +77,8 @@ impl Bar {
             }
         }
         m.add_set("end_face", Set { faces, ..Set::default() }).unwrap();
-        m.add_material("Mat", Material { elastic: Some(Elastic::Isotropic { young: YOUNG, poisson: 0.0 }), density: Some(DENSITY) }).unwrap();
+        m.add_material("Mat", Material { elastic: Some(Elastic::Isotropic { young: YOUNG, poisson: 0.0 }), density: Some(DENSITY), fluid: None })
+            .unwrap();
         let integration = if kind == ElementKind::Hex8 { Integration::Reduced } else { Integration::Full };
         let formulation = Formulation { integration, ..Formulation::default() };
         m.add_section("bar", Section { set: "all".into(), material: "Mat".into(), kind: SectionKind::Solid {}, formulation }).unwrap();
@@ -333,10 +334,13 @@ fn caps_and_unsupported_models_are_errors() {
     assert!(solve_explicit(&full.model, None, &Limits::default()).unwrap_err().to_string().contains("not supported yet"));
     let mut ortho = Bar::new(ElementKind::Tet4, 4, ExplicitDynamic::new(end));
     let constants = Elastic::EngineeringConstants { young: [YOUNG; 3], poisson: [0.0; 3], shear: [YOUNG / 2.0; 3] };
-    ortho.model.replace_material("Mat", Material { elastic: Some(constants), density: Some(DENSITY) }).unwrap();
+    ortho.model.replace_material("Mat", Material { elastic: Some(constants), density: Some(DENSITY), fluid: None }).unwrap();
     assert!(solve_explicit(&ortho.model, None, &Limits::default()).unwrap_err().to_string().contains("orthotropic"));
     let mut light = Bar::new(ElementKind::Tet4, 4, ExplicitDynamic::new(end));
-    light.model.replace_material("Mat", Material { elastic: Some(Elastic::Isotropic { young: YOUNG, poisson: 0.0 }), density: None }).unwrap();
+    light
+        .model
+        .replace_material("Mat", Material { elastic: Some(Elastic::Isotropic { young: YOUNG, poisson: 0.0 }), density: None, fluid: None })
+        .unwrap();
     assert!(solve_explicit(&light.model, None, &Limits::default()).is_err());
     // A node behind its wall at the start.
     let mut behind = ExplicitDynamic::new(end);