diff --git a/crates/specialized/rtx-cfd/src/kernels/cuda/e3_cut.cu b/crates/specialized/rtx-cfd/src/kernels/cuda/e3_cut.cu
index bb71d10..be125a7 100644
--- a/crates/specialized/rtx-cfd/src/kernels/cuda/e3_cut.cu
+++ b/crates/specialized/rtx-cfd/src/kernels/cuda/e3_cut.cu
@@ -235,14 +235,20 @@ __device__ double cut_face_update(const E3Params& g, const E3Ptrs& f, const E3Cu
         (void)n; (void)x;
         /* convection through the plus face */
         double u_plus, delta_plus;
-        if (f_up1 >= 0) {
+        /* bit 10 of wall_order (A2): a prescribed neighbour face exchanges no convective momentum */
+        const int* opn = c == 0 ? m.open_u : (c == 1 ? m.open_v : m.open_w);
+        int presc_up = (g.wall_order & 1024) && f_up1 >= 0 && !opn[f_up1];
+        int presc_dn = (g.wall_order & 1024) && f_dn1 >= 0 && !opn[f_dn1];
+        if (presc_up) { u_plus = u0; delta_plus = 0.0; }
+        else if (f_up1 >= 0) {
             if (g.scheme == SCHEME_UPWIND) delta_plus = 0.0;
             else if (m_plus >= 0.0) delta_plus = face_corr3(g.scheme, f_dn1 >= 0, dn1, u0, up1);
             else delta_plus = face_corr3(g.scheme, f_up2 >= 0, up2, up1, u0);
             u_plus = upwind3(m_plus, u0, up1);
         } else { u_plus = upwind3(m_plus, u0, beyond_p); delta_plus = 0.0; }
         double u_minus, delta_minus;
-        if (f_dn1 >= 0) {
+        if (presc_dn) { u_minus = u0; delta_minus = 0.0; }
+        else if (f_dn1 >= 0) {
             if (g.scheme == SCHEME_UPWIND) delta_minus = 0.0;
             else if (m_minus >= 0.0) delta_minus = face_corr3(g.scheme, f_dn2 >= 0, dn2, dn1, u0);
             else delta_minus = face_corr3(g.scheme, f_up1 >= 0, up1, u0, dn1);
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
index 8d4f60a..544687c 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
@@ -217,6 +217,7 @@ impl Mask {
             diffusion_transverse: false,
             distance_floor_fine: false,
             wall_advancing: false,
+            exchange_convection_off: false,
             wall_exchange_axis: false,
             grad_weights: None,
             diffusion_centroid: false,
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/exchange.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/exchange.rs
index 8a06e1d..0fcc1a2 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/exchange.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/exchange.rs
@@ -191,7 +191,9 @@ impl Mask {
                                         scheme.face_correction(up2, un, u0)
                                     };
                                     let u_face = upwind(m_plus, u0, un) + delta;
-                                    convective[c] -= -rho * m_plus * (u_face - u0);
+                                    if !self.exchange_convection_off {
+                                        convective[c] -= -rho * m_plus * (u_face - u0);
+                                    }
                                     force[c] -=
                                         mu * cv.ap[d][1] * a_d * (un - u0) / solid_spacing(1.0);
                                 }
@@ -208,7 +210,9 @@ impl Mask {
                                         scheme.face_correction(up1, u0, ud)
                                     };
                                     let u_face = upwind(m_minus, ud, u0) + delta;
-                                    convective[c] -= rho * m_minus * (u_face - u0);
+                                    if !self.exchange_convection_off {
+                                        convective[c] -= rho * m_minus * (u_face - u0);
+                                    }
                                     force[c] -=
                                         mu * cv.ap[d][0] * a_d * (ud - u0) / solid_spacing(-1.0);
                                 }
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/mod.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/mod.rs
index e91763b..5e440e7 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/mod.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/poisson/mod.rs
@@ -13,6 +13,7 @@ mod hierarchy;
 mod pcg;
 mod problem;
 
+#[cfg(feature = "cuda")]
 pub use device_cg::GuessBasis;
 pub use export::{LevelExport, export_hierarchy, vcycle_f32_reference};
 pub use hierarchy::Hierarchy;
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/cut_predictor.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/cut_predictor.rs
index 1209abd..d3aaaa0 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/cut_predictor.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/cut_predictor.rs
@@ -163,7 +163,19 @@ impl Solver {
                 }
             };
             // Convection through the plus face.
+            // A2: a prescribed neighbour face exchanges no convective momentum.
+            let prescribed = |q: [i64; 3]| -> bool {
+                mask.exchange_convection_off
+                    && lat.face(c, q).is_some_and(|f| {
+                        (match c {
+                            0 => mask.u_kind(f),
+                            1 => mask.v_kind(f),
+                            _ => mask.w_kind(f),
+                        }) != super::super::wall::FaceKind::Fluid
+                    })
+            };
             let (u_plus, delta_plus) = match up1 {
+                Some(_) if prescribed(add(p, ed, 1)) => (u0, 0.0),
                 Some(un) => {
                     let delta = if scheme == ConvectionScheme::Upwind {
                         0.0
@@ -177,6 +189,7 @@ impl Solver {
                 None => (Self::upwind(m_plus, u0, beyond(true)), 0.0),
             };
             let (u_minus, delta_minus) = match dn1 {
+                Some(_) if prescribed(add(p, ed, -1)) => (u0, 0.0),
                 Some(ud) => {
                     let delta = if scheme == ConvectionScheme::Upwind {
                         0.0
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/device.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/device.rs
index cd1ba8e..e28165e 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/device.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/device.rs
@@ -304,7 +304,9 @@ impl DeviceStep {
                     )
                 + 256 * i32::from(self.solver.params.distance_floor_fine)
                 // bit 9: the advancing-wall friction closure (A3-i).
-                + 512 * i32::from(self.solver.params.wall_advancing),
+                + 512 * i32::from(self.solver.params.wall_advancing)
+                // bit 10: no convective exchange with prescribed faces (A2).
+                + 1024 * i32::from(self.solver.params.exchange_convection_off),
             dx: g.dx,
             dy: g.dy,
             dz: g.dz,
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/mod.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/mod.rs
index 53caba5..42f152c 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/mod.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/mod.rs
@@ -120,6 +120,12 @@ pub struct Parameters {
     /// normal to itself sets up (thinner than a cell on the fast flag). f(0) = 1
     /// exactly: static cases unchanged. `RTX_E3_WALL_ADVANCING=1`.
     pub wall_advancing: bool,
+    /// A2: no convective momentum exchange with a PRESCRIBED neighbour face —
+    /// the flux through a control-volume side that adjoins one advects the
+    /// volume's own value (the upwinded exchange was a first-order momentum
+    /// sink the wall collected: 4 % of the flag's drag at ny 62).
+    /// `RTX_E3_EXCHANGE_CONVECTION=off`.
+    pub exchange_convection_off: bool,
     /// The diffusive exchange of a fluid face with a SOLID neighbour face
     /// over the axis distance to the wall, `δ = min(h, d_f/|n_d|)`, and
     /// implicit — instead of the full `h`, which places the no-slip value
@@ -171,6 +177,8 @@ impl Default for Parameters {
             distance_floor_fine: std::env::var("RTX_E3_DISTANCE_FLOOR")
                 .map_or(true, |v| v != "coarse"),
             wall_advancing: std::env::var("RTX_E3_WALL_ADVANCING").is_ok_and(|v| v == "1"),
+            exchange_convection_off: std::env::var("RTX_E3_EXCHANGE_CONVECTION")
+                .is_ok_and(|v| v == "off"),
             wall_exchange_axis: std::env::var("RTX_E3_WALL_EXCHANGE").is_ok_and(|v| v == "axis"),
             pressure_centroid: std::env::var("RTX_E3_PRESSURE_CENTROID").is_ok_and(|v| v == "1"),
             // ON by default since S2-5 (`=0` reproduces the records before it).
@@ -311,6 +319,7 @@ impl Solver {
                 self.params.diffusion_transverse && self.params.diffusion_centroid;
             m.distance_floor_fine = self.params.distance_floor_fine;
             m.wall_advancing = self.params.wall_advancing;
+            m.exchange_convection_off = self.params.exchange_convection_off;
             m.diffusion_centroid = self.params.diffusion_centroid;
             if self.params.diffusion_centroid {
                 m.compute_face_shifts();
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs
index 6ef8c11..bd7a30a 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/wall.rs
@@ -105,6 +105,7 @@ pub struct Mask {
     pub(super) diffusion_transverse: bool,
     pub(super) distance_floor_fine: bool,
     pub(super) wall_advancing: bool,
+    pub(super) exchange_convection_off: bool,
     /// The axis-distance implicit wall exchange (S2-5).
     pub(super) wall_exchange_axis: bool,
     /// The centroid prototype's pressure-gradient weights per u / v / w face.
@@ -531,6 +532,7 @@ impl Mask {
             diffusion_transverse: false,
             distance_floor_fine: false,
             wall_advancing: false,
+            exchange_convection_off: false,
             wall_exchange_axis: false,
             grad_weights: None,
             diffusion_centroid: false,