From c9bf66a632429d5a19fd8a7e88cb9923f267afed Mon Sep 17 00:00:00 2001
From: Will Bainbridge <http://cfd.direct>
Date: Fri, 12 May 2023 11:45:51 +0100
Subject: [PATCH] fvMeshStitcher: Stabilise calculation of error face areas

---
 src/fvMeshStitchers/moving/fvMeshStitchersMoving.C | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/fvMeshStitchers/moving/fvMeshStitchersMoving.C b/src/fvMeshStitchers/moving/fvMeshStitchersMoving.C
index e1b7d12fa5..c141a44833 100644
--- a/src/fvMeshStitchers/moving/fvMeshStitchersMoving.C
+++ b/src/fvMeshStitchers/moving/fvMeshStitchersMoving.C
@@ -987,11 +987,14 @@ void Foam::fvMeshStitchers::moving::unconformErrorFaceCorrectMeshPhi
             const label errorPatchFacei0 = 2*origPatchFacei;
             const label errorPatchFacei1 = 2*origPatchFacei + 1;
 
-            const vector errorSf =
-                origPp.faceNormals()[origPatchFacei]
-               *phi.boundaryField()[errorPatchi][errorPatchFacei0]
+            const scalar a =
+                phi.boundaryField()[errorPatchi][errorPatchFacei0]
                /max(meshMagUfb[origPatchi][origPatchFacei], vSmall);
 
+            const vector errorSf =
+                sign(a)*min(mag(a), origPp.magFaceAreas()[origPatchFacei])
+               *origPp.faceNormals()[origPatchFacei];
+
             fvsPatchField<vector>& Sfp =
                 SfSf.boundaryFieldRef()[errorPatchi];
             Sfp[errorPatchFacei0] += errorSf;