rtx-cfd/rtx-fsi: overset A-P0 GATED + M1 precision probe — curvilinear collocated PISO: relative-reduction pressure stop (the absolute stop floored |du/dt| at 2e-4 on 64²), line-implicit-n sign fix, adjustPhi; gates: Cartesian reduction 1.37–1.40x the staggered error at orders 0.83/0.90; skewed stretched periodic annulus Stokes orders 2.30/2.06 (explicit and line-implicit), upwind 1.08/0.80; Poiseuille exact to 1e-9 on Cartesian and affine-sheared periodic channels (both diffusion variants), varying-skew channel order 2.02 (v 1.9), cell mass 1e-14; divergence ≤ 1e-11 relative every step; snapshot/restore bit-identical. M1: poisson.rs multigrid hierarchy generic over MgScalar (f32/f64), f64 CG keeps its own fine level; MgPrecision on MultigridParameters/EmbeddedParameters/PisoParameters, set_poisson_precision, harness RTX_FSI2_POISSON_F32 (march + noise probe, printed marker); f64 arm bit-identical in vivo (FSI2 default line-for-line with 08-31), f32 arm holds the noise floor and stall pins and the FSI2 band; poisson_equivalence f32 arm

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01X2GmJXeQ2njUecEKiJZ1G2

crates/specialized/rtx-cfd/src/solvers/incompressible/piso.rs

@@ -37,7 +37,9 @@
 //! - Convective face fluxes fell back to the centre value at the sweep edges
 //!   instead of using the prescribed boundary faces that exist there.
 
-use super::poisson::{MultigridParameters, PoissonProblem, PoissonSolverKind, solve_multigrid_pcg};
+use super::poisson::{
+    MgPrecision, MultigridParameters, PoissonProblem, PoissonSolverKind, solve_multigrid_pcg,
+};
 use super::{BoundaryConditions, FlowField, IncompressibleSolver, SolverResult};
 use crate::{CfdConfig, CfdResult};
 use async_trait::async_trait;
@@ -59,6 +61,9 @@ pub struct PisoParameters {
     /// [`PoissonSolverKind::Sor`]). Both solve the same system to the same
     /// true-residual stop; multigrid's cost is mesh-independent.
     pub poisson_solver: PoissonSolverKind,
+    /// Precision of the multigrid V-cycle (default [`MgPrecision::F64`] =
+    /// bit-identical; `F32` = the M1 precision probe, no effect with SOR).
+    pub poisson_precision: MgPrecision,
 }
 
 impl Default for PisoParameters {
@@ -68,6 +73,7 @@ impl Default for PisoParameters {
             time_step: 0.001,
             tolerance: 1e-6,
             poisson_solver: PoissonSolverKind::Sor,
+            poisson_precision: MgPrecision::F64,
         }
     }
 }
@@ -398,7 +404,10 @@ impl PisoSolver {
             let solution = solve_multigrid_pcg(
                 &problem,
                 &mut p_prime,
-                &MultigridParameters::default(),
+                &MultigridParameters {
+                    precision: self.parameters.poisson_precision,
+                    ..MultigridParameters::default()
+                },
                 inner_stop,
                 Some(nx + 1),
             );