diff --git a/crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs b/crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs
index 51dcec8..a2b4c09 100644
--- a/crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs
+++ b/crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs
@@ -200,6 +200,10 @@ fn flag_wake_on_the_device() {
     // flag as a 2D problem, minutes per rung: the instrument for the load routes' parts.
     let slab_nz = env_f("RTX_E3_FLAG_NZ", 0.0) as usize;
     let nz = if slab_nz > 0 { slab_nz } else { ny };
+    // S2-9b: the 3D run with the slab's 2D inflow (uniform in z) and/or slip
+    // side walls — the decomposition of the 3D-over-slab drag.
+    let inflow_2d = std::env::var("RTX_E3_FLAG_INFLOW").is_ok_and(|v| v == "2d");
+    let z_slip = std::env::var("RTX_E3_FLAG_ZSIDES").is_ok_and(|v| v == "slip");
     let r_edge = h;
     // S2-9: with a recorded kinematics the period and the speed bound are the record's.
     let rec_period = env_f("RTX_E3_FLAG_KIN_PERIOD", 0.5225);
@@ -233,6 +237,14 @@ fn flag_wake_on_the_device() {
                     z1: Side::Periodic,
                     ..Boundaries::default()
                 }
+            } else if z_slip {
+                // S2-9b: slip side walls (the 3D solver on a spanwise-uniform problem).
+                Boundaries {
+                    x1: Side::PressureOutlet,
+                    z0: Side::SlipWall,
+                    z1: Side::SlipWall,
+                    ..Boundaries::default()
+                }
             } else {
                 Boundaries {
                     x1: Side::PressureOutlet,
@@ -247,7 +259,7 @@ fn flag_wake_on_the_device() {
         },
     );
     let inflow_at = move |y: f64, z: f64| {
-        if slab_nz > 0 {
+        if slab_nz > 0 || inflow_2d {
             6.0 * y * (H - y) / (H * H)
         } else {
             inflow(y, z)
@@ -283,8 +295,16 @@ fn flag_wake_on_the_device() {
     }
     solver.initialize(&mut field);
     println!(
-        "  flag wake ny {ny} (span {}): {nx}×{ny}×{nz} = {} cells, h {h:.4e}, dt {dt:.3e}, {periods} periods = {t_end:.3} s, {} steps",
+        "  flag wake ny {ny} (span {}; inflow {}, z sides {}): {nx}×{ny}×{nz} = {} cells, h {h:.4e}, dt {dt:.3e}, {periods} periods = {t_end:.3} s, {} steps",
         flag_span(),
+        if slab_nz > 0 || inflow_2d { "2d" } else { "3d" },
+        if slab_nz > 0 {
+            "periodic"
+        } else if z_slip {
+            "slip"
+        } else {
+            "wall"
+        },
         g.cells(),
         (t_end / dt).ceil() as usize
     );