P6-b design 2: the Robin wall on the patch's Inner side — RobinWall { alpha, datum } with the wall velocity u_s + (t_f − datum)/α (explicit, damped by the wall's own viscous gain μ/(α d)) and the compliant-wall pressure term |S| p'/α implicit in the projection (the added-mass operator in the fluid's own response); Dirichlet bit for bit when off; MMS pin on the skewed annulus (orders 2.46/2.13 at α = 10 and 100 μ/h, the Dirichlet values; a 1e12 wall reproduces Dirichlet to 2e-6); OversetPisoSolver::patch_mut; harness knob RTX_FSI2O_ROBIN_ALPHA with the previous pass's tractions as the datum, printed in the header

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YJPeT6WA2e7YvAnS875AHL

crates/specialized/rtx-fsi/tests/fsi2_harness/overset.rs

@@ -18,7 +18,7 @@ use rtx_cfd::solvers::incompressible::{
     AleBoundaries, ConvectionScheme, CurvilinearParameters, CurvilinearPisoSolver,
     EmbeddedParameters, EmbeddedPisoSolver, FlowField, MgPrecision, NormalDiffusion, OversetField,
     OversetParameters, OversetPisoSolver, OversetResult, OversetSolverState, PatchConvection,
-    PatchField, PoissonSolverKind, SideBoundary,
+    PatchField, PoissonSolverKind, RobinWall, SideBoundary,
 };
 use rtx_cfd::{CfdConfig, CfdResult};
 use rtx_fea::mesh::{Mesh, NodeId};
@@ -967,6 +967,24 @@ impl OversetFluid {
         Ok(())
     }
 
+    /// The fluid's traction on every Inner (wall) face, in the patch's
+    /// Inner-face order — the Robin wall's datum for the next pass.
+    pub fn inner_tractions(&self) -> Vec<[f64; 2]> {
+        self.solver
+            .patch()
+            .wall_tractions(&self.field.patch, PatchSide::Inner, self.solver.time())
+            .into_iter()
+            .map(|(_, _, _, t)| t)
+            .collect()
+    }
+    /// Put the Robin wall (impedance `alpha`, datum per Inner face) on the
+    /// patch, or remove it.
+    pub fn set_robin(&mut self, alpha: f64, datum: Option<Vec<[f64; 2]>>) {
+        self.solver
+            .patch_mut()
+            .set_robin_wall(datum.map(|d| RobinWall { alpha, datum: d }));
+    }
+
     /// Drag and lift on cylinder + flag from the patch's wall stress.
     pub fn measure_force(&self) -> (f64, f64) {
         let f = self