From d6a25f5df6798d0e2a29f10cf669a8b0f718417c Mon Sep 17 00:00:00 2001
From: Jeremy Lore <lorejd@ornl.gov>
Date: Tue, 8 Sep 2026 08:52:28 -0400
Subject: [PATCH] Preserve face orientation in intflux

Use the signed fcQalf projection in the primal and differentiated intflux routines so face orientation is retained across topology cuts.
---
 src/differentiation/adjoint/intflux_b.F90         | 11 ++---------
 src/differentiation/hessian_tgt/intflux_dv_dv.F90 | 11 ++---------
 src/differentiation/tangent/intflux_dv.F90        | 11 ++---------
 src/utility/intflux.F                             |  6 ++++--
 4 files changed, 10 insertions(+), 29 deletions(-)

diff --git a/src/differentiation/adjoint/intflux_b.F90 b/src/differentiation/adjoint/intflux_b.F90
index ae57880d3..ac4d4b456 100644
--- a/src/differentiation/adjoint/intflux_b.F90
+++ b/src/differentiation/adjoint/intflux_b.F90
@@ -53,8 +53,6 @@ SUBROUTINE INTFLUX_NODIFF(nfc, ncv, mpg, geo, faceflux, density, cvflux)
 !
   INTEGER :: idir, ifc
   REAL(kind=r8) :: qf(nfc), wrkf(nfc, 0:1), wrkc(ncv, 0:1)
-  INTRINSIC ABS
-  REAL(kind=r8) :: abs0
 !
 ! ..interpolate the quantity to cell faces
   CALL INTFACE(ncv, nfc, mpg%fccv, geo%fcvol, density, qf)
@@ -64,13 +62,8 @@ SUBROUTINE INTFLUX_NODIFF(nfc, ncv, mpg, geo, faceflux, density, cvflux)
     DO ifc=1,nfc
       IF (geo%fcqalf(ifc, idir) .NE. 0.0_R8 .AND. qf(ifc) .NE. 0.0_R8) &
 &     THEN
-        IF (geo%fcqalf(ifc, idir) .GE. 0.) THEN
-          abs0 = geo%fcqalf(ifc, idir)
-        ELSE
-          abs0 = -geo%fcqalf(ifc, idir)
-        END IF
-        wrkf(ifc, idir) = faceflux(ifc, idir)/qf(ifc)/(geo%fcs(ifc)*abs0&
-&         )
+        wrkf(ifc, idir) = faceflux(ifc, idir)/qf(ifc)/(geo%fcs(ifc)*geo%&
+&         fcqalf(ifc, idir))
       ELSE
         wrkf(ifc, idir) = 0.0_R8
       END IF
diff --git a/src/differentiation/hessian_tgt/intflux_dv_dv.F90 b/src/differentiation/hessian_tgt/intflux_dv_dv.F90
index bd49a96e5..3cfe2d693 100644
--- a/src/differentiation/hessian_tgt/intflux_dv_dv.F90
+++ b/src/differentiation/hessian_tgt/intflux_dv_dv.F90
@@ -59,8 +59,6 @@ SUBROUTINE INTFLUX_NODIFF_NODIFF(nfc, ncv, mpg, geo, faceflux, density, &
 !
   INTEGER :: idir, ifc
   REAL(kind=r8) :: qf(nfc), wrkf(nfc, 0:1), wrkc(ncv, 0:1)
-  INTRINSIC ABS
-  REAL(kind=r8) :: abs0
 !
 ! ..interpolate the quantity to cell faces
   CALL INTFACE(ncv, nfc, mpg%fccv, geo%fcvol, density, qf)
@@ -70,13 +68,8 @@ SUBROUTINE INTFLUX_NODIFF_NODIFF(nfc, ncv, mpg, geo, faceflux, density, &
     DO ifc=1,nfc
       IF (geo%fcqalf(ifc, idir) .NE. 0.0_R8 .AND. qf(ifc) .NE. 0.0_R8) &
 &     THEN
-        IF (geo%fcqalf(ifc, idir) .GE. 0.) THEN
-          abs0 = geo%fcqalf(ifc, idir)
-        ELSE
-          abs0 = -geo%fcqalf(ifc, idir)
-        END IF
-        wrkf(ifc, idir) = faceflux(ifc, idir)/qf(ifc)/(geo%fcs(ifc)*abs0&
-&         )
+        wrkf(ifc, idir) = faceflux(ifc, idir)/qf(ifc)/(geo%fcs(ifc)*geo%&
+&         fcqalf(ifc, idir))
       ELSE
         wrkf(ifc, idir) = 0.0_R8
       END IF
diff --git a/src/differentiation/tangent/intflux_dv.F90 b/src/differentiation/tangent/intflux_dv.F90
index ea839d1ac..7f7fa20d7 100644
--- a/src/differentiation/tangent/intflux_dv.F90
+++ b/src/differentiation/tangent/intflux_dv.F90
@@ -55,8 +55,6 @@ SUBROUTINE INTFLUX_NODIFF(nfc, ncv, mpg, geo, faceflux, density, cvflux)
 !
   INTEGER :: idir, ifc
   REAL(kind=r8) :: qf(nfc), wrkf(nfc, 0:1), wrkc(ncv, 0:1)
-  INTRINSIC ABS
-  REAL(kind=r8) :: abs0
 !
 ! ..interpolate the quantity to cell faces
   CALL INTFACE(ncv, nfc, mpg%fccv, geo%fcvol, density, qf)
@@ -66,13 +64,8 @@ SUBROUTINE INTFLUX_NODIFF(nfc, ncv, mpg, geo, faceflux, density, cvflux)
     DO ifc=1,nfc
       IF (geo%fcqalf(ifc, idir) .NE. 0.0_R8 .AND. qf(ifc) .NE. 0.0_R8) &
 &     THEN
-        IF (geo%fcqalf(ifc, idir) .GE. 0.) THEN
-          abs0 = geo%fcqalf(ifc, idir)
-        ELSE
-          abs0 = -geo%fcqalf(ifc, idir)
-        END IF
-        wrkf(ifc, idir) = faceflux(ifc, idir)/qf(ifc)/(geo%fcs(ifc)*abs0&
-&         )
+        wrkf(ifc, idir) = faceflux(ifc, idir)/qf(ifc)/(geo%fcs(ifc)*geo%&
+&         fcqalf(ifc, idir))
       ELSE
         wrkf(ifc, idir) = 0.0_R8
       END IF
diff --git a/src/utility/intflux.F b/src/utility/intflux.F
index 8c2933d12..862775912 100644
--- a/src/utility/intflux.F
+++ b/src/utility/intflux.F
@@ -22,7 +22,9 @@ subroutine intflux(nFc,nCv,mpg,geo,faceflux,density,cvflux)
 c Provides an interpolation of face flux quantities to cell centers
 c The flux carries the cell-centered density of the quantity being transported
 c We first interpolate the density to cell faces
-c We then compute an effective velocity at cell faces
+c We then compute an effective velocity at cell faces.  The projected area
+c retains the sign of fcQalf because face orientation can reverse across
+c topology cuts.
 C We interpolate this velocity to cell centers
 c We multiply the interpolated velocity by the density and the projected area
       use b2mod_types
@@ -47,7 +49,7 @@ subroutine intflux(nFc,nCv,mpg,geo,faceflux,density,cvflux)
         do iFc = 1, nFc
           if (geo%fcQalf(iFc,idir).ne.0.0_R8.and.qf(iFc).ne.0.0_R8) then
             wrkf(iFc,idir) = faceflux(iFc,idir)/qf(iFc)/
-     .                      (geo%fcS(iFc)*abs(geo%fcQalf(iFc,idir)))
+     .                      (geo%fcS(iFc)*geo%fcQalf(iFc,idir))
           else
             wrkf(iFc,idir) = 0.0_R8
           end if