R8-h: a flat-tipped flag for the 3D solver (default off, byte-identical when off)

DeviceSdf gains `tip_corner: Option<f64>` (None = the capsule, as before; Some(r_c) = a FLAT tip through the centreline's last point, normal to the last segment, corners rounded to r_c): the last segment becomes a ray for the lateral distance and the strip is cut by the tip plane with the span cut's rounded intersection. Host twin (plate.rs: closest/tip_axial/ flat_cap, polyline and plate bodies) and the device φ and velocity (e3_geom.cu geom_phi_at / body_velocity / plate_dist; GeomSdf flat_tip + tip_corner) expression for expression. Knobs: flag test RTX_E3_FLAG_TIP=flat + RTX_E3_FLAG_TIP_CORNER (default 0.00125 m; the tip inset defaults to 0 with the flat tip; the host φ is the device form's); R8-a harness RTX_E3FSI_TIP=flat + RTX_E3FSI_TIP_CORNER (the centreline gains node A as a 36th station). New host test embedded3_flat_tip (G2 geometry: tip plane at the last point, r_c = half = the capsule pulled back by half to 4e-17, cut volume and wall area vs the analytic rounded rectangle at ny 62/124/248). Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
2026-09-25 22:32:31 -05:00
parent 171da41ed1
commit 02ab155022
9 changed files with 401 additions and 69 deletions
@@ -25,6 +25,8 @@ struct GeomSdf {
    int cyl_cut, flag_cut;  /* cut to the span */
    int npts;               /* polyline points (x, y interleaved) */
    int nst, ns;            /* R8-c plate: stations and points per station (nst 0: the polyline) */
+    int flat_tip;           /* R8-h: 1 = the flat tip (corners rounded to tip_corner); 0 = the capsule */
+    double tip_corner;
 };

 struct GeomGrid {
@@ -46,6 +48,24 @@ __device__ __forceinline__ double span_cut(double d2, double z, const GeomSdf& s
    return outside + rs_min(rs_max(q1, q2), 0.0) - r;
 }

+/* R8-h: the tip's axial distance along the last segment a→b from b (plate.rs tip_axial). */
+__device__ __forceinline__ double tip_axial(double x, double y, double ax, double ay, double bx, double by)
+{
+    double ex = bx - ax, ey = by - ay;
+    double l = sqrt(ex * ex + ey * ey);
+    return ((x - bx) * ex + (y - by) * ey) / l;
+}
+
+/* R8-h: the flat tip's rounded cap (plate.rs flat_cap). */
+__device__ __forceinline__ double flat_cap(double dl, double a, double r)
+{
+    double q1 = dl + r;
+    double q2 = a + r;
+    double m1 = rs_max(q1, 0.0), m2 = rs_max(q2, 0.0);
+    double outside = sqrt(m1 * m1 + m2 * m2);
+    return outside + rs_min(rs_max(q1, q2), 0.0) - r;
+}
+
 /*
 * R8-c: the deformed plate (`plate.rs`, the host twin expression for
 * expression): `poly` holds the stations' rows (x, y interleaved, row-major
@@ -57,7 +77,7 @@ __device__ __forceinline__ double span_cut(double d2, double z, const GeomSdf& s
 */
 __device__ double plate_dist(double x, double y, double z, const GeomSdf& s,
                             const double* __restrict__ P, const double* __restrict__ V,
-                             double* vx, double* vy)
+                             double* vx, double* vy, double* axial)
 {
    int nst = s.nst, ns = s.ns;
    const double* Z = P + 2 * (long long) nst * ns;
@@ -97,14 +117,14 @@ __device__ double plate_dist(double x, double y, double z, const GeomSdf& s,
        double l2 = ex * ex + ey * ey;
        double u = ((x - ax) * ex + (y - ay) * ey) / l2;
        if (u < 0.0) u = 0.0;
-        if (u > 1.0) u = 1.0;
+        if (u > 1.0 && !(s.flat_tip && m + 2 == ns)) u = 1.0;
        double px = ax + u * ex, py = ay + u * ey;
        double qx = x - px, qy = y - py;
        double d = sqrt(qx * qx + qy * qy);
        if (d < best) {
            best = d;
            mb = m;
-            ub = u;
+            ub = u > 1.0 ? 1.0 : u;
            qbx = qx;
            qby = qy;
        }
@@ -118,6 +138,20 @@ __device__ double plate_dist(double x, double y, double z, const GeomSdf& s,
        double qn = (qbx * cx + qby * cy) / best;
        best = best / sqrt(1.0 + qn * qn);
    }
+    if (s.flat_tip && axial) {
+        int m = ns - 2;
+        double ax, ay, bx, by;
+        if (interp) {
+            ax = R0[2 * m] + w * (R1[2 * m] - R0[2 * m]);
+            ay = R0[2 * m + 1] + w * (R1[2 * m + 1] - R0[2 * m + 1]);
+            bx = R0[2 * m + 2] + w * (R1[2 * m + 2] - R0[2 * m + 2]);
+            by = R0[2 * m + 3] + w * (R1[2 * m + 3] - R0[2 * m + 3]);
+        } else {
+            ax = R0[2 * m]; ay = R0[2 * m + 1];
+            bx = R0[2 * m + 2]; by = R0[2 * m + 3];
+        }
+        *axial = tip_axial(x, y, ax, ay, bx, by);
+    }
    if (V) {
        const double* V0 = V + 2 * (long long) k * ns;
        const double* V1 = interp ? V0 + 2 * ns : V0;
@@ -144,22 +178,29 @@ __device__ double geom_phi_at(double x, double y, double z, const GeomSdf& s, co
    double dc = sqrt(ex0 * ex0 + ey0 * ey0) - s.rc;
    if (s.cyl_cut) dc = span_cut(dc, z, s);
    /* the capsule: distance to the polyline (or the plate, R8-c) */
-    double best = 1.0 / 0.0;
-    if (s.nst > 0) best = plate_dist(x, y, z, s, poly, nullptr, nullptr, nullptr);
-    else for (int m = 0; m + 1 < s.npts; ++m) {
+    double best = 1.0 / 0.0, axial = 0.0;
+    if (s.nst > 0) best = plate_dist(x, y, z, s, poly, nullptr, nullptr, nullptr, &axial);
+    else {
+        for (int m = 0; m + 1 < s.npts; ++m) {
            double ax = poly[2 * m], ay = poly[2 * m + 1];
            double bx = poly[2 * m + 2], by = poly[2 * m + 3];
            double ex = bx - ax, ey = by - ay;
            double l2 = ex * ex + ey * ey;
            double u = ((x - ax) * ex + (y - ay) * ey) / l2;
            if (u < 0.0) u = 0.0;
-        if (u > 1.0) u = 1.0;
+            if (u > 1.0 && !(s.flat_tip && m + 2 == s.npts)) u = 1.0;
            double px = ax + u * ex, py = ay + u * ey;
            double qx = x - px, qy = y - py;
            double d = sqrt(qx * qx + qy * qy);
            if (d < best) best = d;
        }
+        if (s.flat_tip) {
+            int m = s.npts - 2;
+            axial = tip_axial(x, y, poly[2 * m], poly[2 * m + 1], poly[2 * m + 2], poly[2 * m + 3]);
+        }
+    }
    double df = best - s.half;
+    if (s.flat_tip) df = flat_cap(df, axial, s.tip_corner);
    if (s.flag_cut) df = span_cut(df, z, s);
    double r = s.fillet;
    if (r > 0.0 && dc < r && df < r) {
@@ -391,28 +432,36 @@ extern "C" __global__ void e3_geom_gather(
 __device__ double body_velocity(double x, double y, double z, int c, const GeomSdf& s,
                                const double* __restrict__ poly, const double* __restrict__ vel)
 {
-    double best = 1.0 / 0.0, vx = 0.0, vy = 0.0;
-    if (s.nst > 0) best = plate_dist(x, y, z, s, poly, vel, &vx, &vy);
-    else for (int m = 0; m + 1 < s.npts; ++m) {
+    double best = 1.0 / 0.0, vx = 0.0, vy = 0.0, axial = 0.0;
+    if (s.nst > 0) best = plate_dist(x, y, z, s, poly, vel, &vx, &vy, &axial);
+    else {
+        for (int m = 0; m + 1 < s.npts; ++m) {
            double ax = poly[2 * m], ay = poly[2 * m + 1];
            double bx = poly[2 * m + 2], by = poly[2 * m + 3];
            double ex = bx - ax, ey = by - ay;
            double l2 = ex * ex + ey * ey;
            double u = ((x - ax) * ex + (y - ay) * ey) / l2;
            if (u < 0.0) u = 0.0;
-        if (u > 1.0) u = 1.0;
+            if (u > 1.0 && !(s.flat_tip && m + 2 == s.npts)) u = 1.0;
            double px = ax + u * ex, py = ay + u * ey;
            double qx = x - px, qy = y - py;
            double d = sqrt(qx * qx + qy * qy);
            if (d < best) {
                best = d;
+                double uv = u > 1.0 ? 1.0 : u;
                double avx = vel[2 * m], avy = vel[2 * m + 1];
                double bvx = vel[2 * m + 2], bvy = vel[2 * m + 3];
-            vx = avx + u * (bvx - avx);
-            vy = avy + u * (bvy - avy);
+                vx = avx + uv * (bvx - avx);
+                vy = avy + uv * (bvy - avy);
+            }
+        }
+        if (s.flat_tip) {
+            int m = s.npts - 2;
+            axial = tip_axial(x, y, poly[2 * m], poly[2 * m + 1], poly[2 * m + 2], poly[2 * m + 3]);
        }
    }
    double df = best - s.half;
+    if (s.flat_tip) df = flat_cap(df, axial, s.tip_corner);
    if (s.flag_cut) df = span_cut(df, z, s);
    double ex0 = x - s.cx, ey0 = y - s.cy;
    double dc = sqrt(ex0 * ex0 + ey0 * ey0) - s.rc;
@@ -31,6 +31,12 @@ pub struct DeviceSdf {
    pub half: f64,
    /// The root fillet radius (0 = the plain union).
    pub fillet: f64,
+    /// R8-h: the tip. `None` = the capsule (the semicircular tip around the
+    /// last point, as before); `Some(r_c)` = a FLAT tip: the flag ends in
+    /// the plane through the last point normal to the last segment, its two
+    /// corners rounded to radius `r_c` (0 ≤ r_c ≤ `half`; `r_c = half` is
+    /// the capsule's semicircle pulled back by `half`, geometrically).
+    pub tip_corner: Option<f64>,
    /// The centreline polyline's points (x, y) at `t`.
    pub poly: Vec<[f64; 2]>,
    /// R6-2 step 2: the centreline's velocity (vx, vy) per point at `t`, the
@@ -708,6 +708,7 @@ mod tests {
            r_edge: 0.0,
            half,
            fillet: 0.0,
+            tip_corner: None,
            poly: Vec::new(),
            vel: Vec::new(),
            plate: Some(surf),
@@ -131,12 +131,16 @@ fn span_cut(d2: f64, z: f64, s: &DeviceSdf) -> f64 {
 }

 /// The closest segment of a polyline given by `pt(m)`, `m < n`: the
-/// in-plane distance, the segment, its parameter and the offset `(qx, qy)`.
+/// in-plane distance, the segment, its parameter (clamped to the segment)
+/// and the offset `(qx, qy)`. R8-h: with `ray_last` the last segment is a
+/// RAY beyond its end point (its parameter unclamped above for the
+/// distance) — the lateral distance the flat tip's cap needs.
 #[inline]
 fn closest<P: Fn(usize) -> [f64; 2]>(
    x: f64,
    y: f64,
    n: usize,
+    ray_last: bool,
    pt: P,
 ) -> (f64, usize, f64, [f64; 2]) {
    let mut best = f64::INFINITY;
@@ -151,7 +155,7 @@ fn closest<P: Fn(usize) -> [f64; 2]>(
        if u < 0.0 {
            u = 0.0;
        }
-        if u > 1.0 {
+        if u > 1.0 && !(ray_last && m + 2 == n) {
            u = 1.0;
        }
        let px = ax + u * ex;
@@ -162,25 +166,63 @@ fn closest<P: Fn(usize) -> [f64; 2]>(
        if d < best {
            best = d;
            mb = m;
-            ub = u;
+            ub = if u > 1.0 { 1.0 } else { u };
            qb = [qx, qy];
        }
    }
    (best, mb, ub, qb)
 }

+/// R8-h: the signed distance along the last segment's unit tangent from
+/// its end point `b` (the flat tip's plane; > 0 beyond the tip).
+#[inline]
+fn tip_axial(x: f64, y: f64, a: [f64; 2], b: [f64; 2]) -> f64 {
+    let ex = b[0] - a[0];
+    let ey = b[1] - a[1];
+    let l = (ex * ex + ey * ey).sqrt();
+    ((x - b[0]) * ex + (y - b[1]) * ey) / l
+}
+
+/// R8-h: the flat tip — the strip of lateral distance `dl` (distance to
+/// the centreline with its last segment a ray, minus the half-thickness)
+/// cut by the tip's plane (axial distance `a`) with the two corners rounded
+/// to radius `r` (the span cut's rounded intersection). Exact near the tip
+/// where the centreline is straight over the corner (the last segment).
+#[inline]
+fn flat_cap(dl: f64, a: f64, r: f64) -> f64 {
+    let q1 = dl + r;
+    let q2 = a + r;
+    let m1 = rs_max(q1, 0.0);
+    let m2 = rs_max(q2, 0.0);
+    let outside = (m1 * m1 + m2 * m2).sqrt();
+    outside + rs_min(rs_max(q1, q2), 0.0) - r
+}
+
 /// The plate's in-plane closest point at `(x, y, z)`, the slope-corrected
 /// distance to the mid-surface, and the interpolated velocity there
 /// (zero without velocities).
-fn plate_closest(p: &PlateSurface, x: f64, y: f64, z: f64) -> (f64, [f64; 2]) {
+///
+/// R8-h: with `flat` the last segment is a ray (see [`closest`]) and the
+/// third value is the tip's axial distance on the interpolated polyline
+/// (0 otherwise).
+fn plate_closest(p: &PlateSurface, x: f64, y: f64, z: f64, flat: bool) -> (f64, [f64; 2], f64) {
    let ns = p.ns;
    let (k, w) = p.bracket(z);
    let row = |k: usize, m: usize| p.xy[k * ns + m];
    let lerp2 =
        |a: [f64; 2], b: [f64; 2], w: f64| [a[0] + w * (b[0] - a[0]), a[1] + w * (b[1] - a[1])];
    let (best, m, u, q) = match w {
-        None => closest(x, y, ns, |m| row(k, m)),
-        Some(w) => closest(x, y, ns, |m| lerp2(row(k, m), row(k + 1, m), w)),
+        None => closest(x, y, ns, flat, |m| row(k, m)),
+        Some(w) => closest(x, y, ns, flat, |m| lerp2(row(k, m), row(k + 1, m), w)),
+    };
+    let axial = if flat {
+        let at = |m: usize| match w {
+            None => row(k, m),
+            Some(w) => lerp2(row(k, m), row(k + 1, m), w),
+        };
+        tip_axial(x, y, at(ns - 2), at(ns - 1))
+    } else {
+        0.0
    };
    let mut d = best;
    if let Some(_w) = w {
@@ -211,7 +253,7 @@ fn plate_closest(p: &PlateSurface, x: f64, y: f64, z: f64) -> (f64, [f64; 2]) {
        let (a, b) = (at(m), at(m + 1));
        [a[0] + u * (b[0] - a[0]), a[1] + u * (b[1] - a[1])]
    };
-    (d, v)
+    (d, v, axial)
 }

 impl DeviceSdf {
@@ -231,20 +273,30 @@ impl DeviceSdf {
    /// and its surface velocity at the closest point.
    #[must_use]
    pub fn flag_distance_host(&self, x: f64, y: f64, z: f64) -> (f64, [f64; 2]) {
-        let (best, v) = match self.plate.as_ref() {
-            Some(p) => plate_closest(p, x, y, z),
+        let flat = self.tip_corner.is_some();
+        let (best, v, axial) = match self.plate.as_ref() {
+            Some(p) => plate_closest(p, x, y, z, flat),
            None => {
-                let (best, m, u, _) = closest(x, y, self.poly.len(), |m| self.poly[m]);
+                let n = self.poly.len();
+                let (best, m, u, _) = closest(x, y, n, flat, |m| self.poly[m]);
                let v = if self.vel.len() == self.poly.len() && !self.vel.is_empty() {
                    let (a, b) = (self.vel[m], self.vel[m + 1]);
                    [a[0] + u * (b[0] - a[0]), a[1] + u * (b[1] - a[1])]
                } else {
                    [0.0, 0.0]
                };
-                (best, v)
+                let axial = if flat {
+                    tip_axial(x, y, self.poly[n - 2], self.poly[n - 1])
+                } else {
+                    0.0
+                };
+                (best, v, axial)
            }
        };
        let mut df = best - self.half;
+        if let Some(rc) = self.tip_corner {
+            df = flat_cap(df, axial, rc);
+        }
        if self.flag_cut {
            df = span_cut(df, z, self);
        }
@@ -344,6 +396,7 @@ mod tests {
            r_edge: 0.0066,
            half: 0.01,
            fillet: 0.0,
+            tip_corner: None,
            poly,
            vel,
            plate,
@@ -406,7 +459,7 @@ mod tests {
            vel: Vec::new(),
        };
        for &(y, zz) in &[(0.53, 0.05), (0.47, 0.15), (0.6, 0.12)] {
-            let (d, _) = plate_closest(&p, 0.1, y, zz);
+            let (d, _, _) = plate_closest(&p, 0.1, y, zz, false);
            let exact = (y - (0.5 + a * (zz - 0.1))).abs() / (1.0 + a * a).sqrt();
            assert!((d - exact).abs() < 1e-15, "{d} vs {exact}");
        }
@@ -75,6 +75,9 @@ struct GeomSdf {
    npts: i32,
    nst: i32,
    ns: i32,
+    /// R8-h: the flat tip (1) and its corner radius.
+    flat_tip: i32,
+    tip_corner: f64,
 }
 unsafe impl DeviceRepr for GeomSdf {}
 unsafe impl ValidAsZeroBits for GeomSdf {}
@@ -151,6 +154,8 @@ fn geom_sdf(sdf: &DeviceSdf) -> GeomSdf {
        npts: sdf.poly.len() as i32,
        nst: sdf.plate.as_ref().map_or(0, |p| p.z.len() as i32),
        ns: sdf.plate.as_ref().map_or(0, |p| p.ns as i32),
+        flat_tip: i32::from(sdf.tip_corner.is_some()),
+        tip_corner: sdf.tip_corner.unwrap_or(0.0),
    }
 }

@@ -96,8 +96,35 @@ fn root_fillet() -> f64 {
 /// record before this date had the apex 10 mm beyond A (`=0` restores
 /// them): at ny 62 on the recorded motion that was drag 253.3 → 240.0 and
 /// the lift swing 1,005 → 836 (the overset's 867).
+///
+/// R8-h: with the flat tip the default inset is 0 (the flat face through A).
 fn tip_inset() -> f64 {
-    env_f("RTX_E3_FLAG_TIP_INSET", FLAG_HALF)
+    env_f(
+        "RTX_E3_FLAG_TIP_INSET",
+        if flat_tip().is_some() { 0.0 } else { FLAG_HALF },
+    )
+}
+
+/// R8-h (2026-09-25): the tip's shape. `RTX_E3_FLAG_TIP=flat` gives the
+/// flag a FLAT tip through the centreline's last point (A, the inset
+/// defaulting to 0), its corners rounded to `RTX_E3_FLAG_TIP_CORNER`
+/// metres (default 0.00125, the 2D overset's recommended line; at most
+/// `FLAG_HALF`); unset or `capsule` = the capsule (the semicircular tip).
+/// The flat tip's host φ and surface velocity are the device form's
+/// (`DeviceSdf::phi_host`, the kernel's arithmetic).
+fn flat_tip() -> Option<f64> {
+    match std::env::var("RTX_E3_FLAG_TIP").as_deref() {
+        Err(_) | Ok("capsule") => None,
+        Ok("flat") => {
+            let rc = env_f("RTX_E3_FLAG_TIP_CORNER", 0.00125);
+            assert!(
+                (0.0..=FLAG_HALF).contains(&rc),
+                "RTX_E3_FLAG_TIP_CORNER {rc} outside [0, {FLAG_HALF}]"
+            );
+            Some(rc)
+        }
+        Ok(v) => panic!("RTX_E3_FLAG_TIP={v}: flat or capsule"),
+    }
 }

 /// Smooth union with a concave fillet of radius `r` (the plain `min` at r = 0).
@@ -491,6 +518,7 @@ fn flag_wake_on_the_device() {
            r_edge,
            half: FLAG_HALF,
            fillet: r_fillet,
+            tip_corner: flat_tip(),
            poly: match (&plate, recorded()) {
                (Some(_), _) => Vec::new(),
                (None, Some(rec)) => recorded_polyline(rec, t)
@@ -527,9 +555,9 @@ fn flag_wake_on_the_device() {
            c.1.clone().expect("sdf")
        })
    };
-    let body = if plate_body() {
+    let body = if plate_body() || flat_tip().is_some() {
        // R8-c: the host φ and surface velocity ARE the device form's (the
-        // kernel's arithmetic on the host).
+        // kernel's arithmetic on the host); R8-h: the flat tip too.
        Body::from_sdf(move |x, y, z, t| sdf_at(t).phi_host(x, y, z))
            .with_surface_velocity(move |x, y, z, t| sdf_at(t).velocity_host(x, y, z))
    } else {
@@ -587,6 +615,9 @@ fn flag_wake_on_the_device() {
        g.cells(),
        (t_end / dt).ceil() as usize
    );
+    if let Some(rc) = flat_tip() {
+        println!("  R8-h: FLAT tip through the centreline's last point, corner radius {rc:.5} m");
+    }
    unsafe { std::env::set_var("RTX_PROFILE", "1") };
    let mut device = DeviceStep::new(solver, g);
    device.upload(&field);
@@ -0,0 +1,138 @@
+//! R8-h (2026-09-25): the flat-tipped flag's geometry (`DeviceSdf::tip_corner`,
+//! host twin `DeviceSdf::phi_host` = the kernel's `geom_phi_at`).
+//!
+//! G2: a straight flag (a five-point polyline, root a semicircle, tip flat
+//! with corners rounded to `r_c`) at two orientations, cut on the cubic grid
+//! at three resolutions: the solid volume and the wall area of the cut
+//! geometry against the analytic rounded rectangle; the tip plane through
+//! the last point; `r_c = half` = the capsule pulled back by `half`.
+
+use rtx_cfd::solvers::incompressible::embedded3::{Body, CutGeometry, DeviceSdf, Grid};
+
+const HALF: f64 = 0.01;
+const LEN: f64 = 0.35;
+const P0: [f64; 2] = [0.05, 0.08];
+
+fn flag(theta: f64, tip: Option<f64>, pull_back: f64) -> DeviceSdf {
+    let (c, s) = (theta.cos(), theta.sin());
+    let len = LEN - pull_back;
+    let poly = (0..5)
+        .map(|m| {
+            let f = len * m as f64 / 4.0;
+            [P0[0] + f * c, P0[1] + f * s]
+        })
+        .collect();
+    DeviceSdf {
+        // the circle far outside the domain
+        cyl: [-10.0, -10.0, 0.01],
+        cyl_cut: false,
+        flag_cut: false,
+        zc: 0.0,
+        span: 1.0,
+        r_edge: 0.0,
+        half: HALF,
+        fillet: 0.0,
+        tip_corner: tip,
+        poly,
+        vel: Vec::new(),
+        plate: None,
+    }
+}
+
+/// The analytic cross-section: the strip `LEN × 2 HALF`, the root's half
+/// disk, less the two rounded corners' `(1 − π/4) r²` (capsule: a second
+/// half disk instead of the flat end).
+fn analytic(tip: Option<f64>) -> (f64, f64) {
+    let pi = std::f64::consts::PI;
+    match tip {
+        Some(r) => (
+            LEN * 2.0 * HALF + 0.5 * pi * HALF * HALF - 2.0 * (1.0 - 0.25 * pi) * r * r,
+            2.0 * (LEN - r) + (2.0 * HALF - 2.0 * r) + pi * r + pi * HALF,
+        ),
+        None => (
+            LEN * 2.0 * HALF + pi * HALF * HALF,
+            2.0 * LEN + 2.0 * pi * HALF,
+        ),
+    }
+}
+
+fn cut(sdf: DeviceSdf, h: f64) -> (f64, f64) {
+    let (nx, ny, nz) = ((0.46 / h).ceil() as usize, (0.26 / h).ceil() as usize, 2);
+    let g = Grid::cubic(nx, ny, nz, h);
+    let body = Body::from_sdf(move |x, y, z, _t| sdf.phi_host(x, y, z));
+    let geo = CutGeometry::build(&body, g, 0.0);
+    let depth = nz as f64 * h;
+    let solid = (nx * ny * nz) as f64 * h * h * h - geo.fluid_volume();
+    let (wall, _) = geo.wall_area_and_closure();
+    (solid / depth, wall / depth)
+}
+
+#[test]
+fn flat_tip_plane_passes_through_the_last_point() {
+    for theta in [0.0, 0.3] {
+        for rc in [0.0, 0.00125, 0.005, HALF] {
+            let s = flag(theta, Some(rc), 0.0);
+            let tip = *s.poly.last().unwrap();
+            let (c, sn) = (f64::cos(theta), f64::sin(theta));
+            // the face centre on the surface; 1 mm beyond it at 1 mm, inside by 1 mm
+            assert!(s.phi_host(tip[0], tip[1], 0.0).abs() < 1e-15);
+            if rc < HALF {
+                let beyond = s.phi_host(tip[0] + 1e-3 * c, tip[1] + 1e-3 * sn, 0.0);
+                assert!((beyond - 1e-3).abs() < 1e-12, "{beyond}");
+            }
+            let inside = s.phi_host(tip[0] - 1e-3 * c, tip[1] - 1e-3 * sn, 0.0);
+            assert!(inside < 0.0);
+            // the corner's apex region: the rounded corner's arc
+            let (px, py) = (
+                tip[0] - rc * c - (HALF - rc) * sn,
+                tip[1] - rc * sn + (HALF - rc) * c,
+            );
+            let d = 2e-3;
+            let (dx, dy) = ((c - sn) / 2f64.sqrt(), (sn + c) / 2f64.sqrt());
+            let got = s.phi_host(px + (rc + d) * dx, py + (rc + d) * dy, 0.0);
+            assert!((got - d).abs() < 1e-12, "corner {rc}: {got} vs {d}");
+        }
+    }
+}
+
+#[test]
+fn flat_tip_at_half_is_the_capsule_pulled_back() {
+    let a = flag(0.2, Some(HALF), 0.0);
+    let b = flag(0.2, None, HALF);
+    let mut worst = 0.0f64;
+    for i in 0..400 {
+        for j in 0..200 {
+            let (x, y) = (0.30 + 0.0003 * i as f64, 0.10 + 0.0010 * j as f64);
+            let (pa, pb) = (a.phi_host(x, y, 0.0), b.phi_host(x, y, 0.0));
+            if pa.abs().min(pb.abs()) < 0.02 {
+                worst = worst.max((pa - pb).abs());
+            }
+        }
+    }
+    println!("  r_c = half vs capsule pulled back by half: max |Δφ| {worst:.2e}");
+    assert!(worst < 1e-12, "{worst}");
+}
+
+#[test]
+fn flat_tip_cut_volume_and_area_converge_to_the_rounded_rectangle() {
+    println!("  theta  tip        h          solid err   wall err");
+    for theta in [0.0, 0.3] {
+        for tip in [None, Some(0.0), Some(0.00125), Some(0.005)] {
+            let (va, pa) = analytic(tip);
+            let mut errs = Vec::new();
+            for ny in [62.0, 124.0, 248.0] {
+                let h = 0.41 / ny;
+                let (v, p) = cut(flag(theta, tip, 0.0), h);
+                let (ev, ep) = ((v - va) / va, (p - pa) / pa);
+                println!(
+                    "  {theta:.1}   {:>9}  {h:.3e}  {ev:+.3e}  {ep:+.3e}",
+                    tip.map_or("capsule".to_string(), |r| format!("{r:.5}"))
+                );
+                assert!(ev.abs() < 1e-2 && ep.abs() < 2e-2, "{ev} {ep} at h {h}");
+                errs.push((ev, ep));
+            }
+            // the volume converges (the rung ny 248 below ny 62's error)
+            assert!(errs[2].0.abs() < errs[0].0.abs(), "volume {errs:?}");
+        }
+    }
+}
@@ -56,6 +56,27 @@ const X0: f64 = 0.25;
 /// last 10 mm is the capsule's cap).
 const STATIONS: usize = 35;

+/// R8-h: the flat tip (`RTX_E3FSI_TIP=flat`, corner radius
+/// `RTX_E3FSI_TIP_CORNER`, default 0.00125 m): the centreline gains the
+/// tip node A (x 0.60) as its last station and the body's tip is flat
+/// through it (`DeviceSdf::tip_corner`); unset = the capsule.
+pub fn flat_tip() -> Option<f64> {
+    match std::env::var("RTX_E3FSI_TIP").as_deref() {
+        Err(_) | Ok("capsule") => None,
+        Ok("flat") => {
+            let rc = env_f("RTX_E3FSI_TIP_CORNER", 0.00125);
+            assert!((0.0..=HALF).contains(&rc), "RTX_E3FSI_TIP_CORNER {rc}");
+            Some(rc)
+        }
+        Ok(v) => panic!("RTX_E3FSI_TIP={v}: flat or capsule"),
+    }
+}
+
+/// The centreline's stations (35, or 36 with the flat tip's A).
+fn stations() -> usize {
+    STATIONS + usize::from(flat_tip().is_some())
+}
+
 /// The flag's structure-side bookkeeping: the centreline nodes, the wetted
 /// edges with their reference coordinates, the tip node A.
 struct Flag {
@@ -82,7 +103,7 @@ impl Flag {
                .expect("node")
                .0
        };
-        let centre = (0..STATIONS)
+        let centre = (0..stations())
            .map(|k| find(X0 + 0.01 * k as f64, 0.2))
            .collect();
        let interface = Interface::build(mesh);
@@ -217,10 +238,10 @@ fn distribute(
 /// The fluid's centreline from the structure's centreline displacement `c`
 /// (2 per station) with velocities `(c − c_prev) / dt`.
 fn line_of(t: f64, c: &[f64], c_prev: &[f64], dt: f64) -> Line {
-    let pts = (0..STATIONS)
+    let pts = (0..stations())
        .map(|k| [X0 + 0.01 * k as f64 + c[2 * k], 0.2 + c[2 * k + 1]])
        .collect();
-    let vel = (0..STATIONS)
+    let vel = (0..stations())
        .map(|k| {
            [
                (c[2 * k] - c_prev[2 * k]) / dt,
@@ -252,7 +273,7 @@ fn fsi2_on_embedded3() {

    let mesh = flag_mesh(35, 2);
    let flag_geo = Flag::build(&mesh);
-    let zero_c = vec![0.0; 2 * STATIONS];
+    let zero_c = vec![0.0; 2 * stations()];
    let rest = line_of(0.0, &zero_c, &zero_c, 1.0);
    // `RTX_E3FSI_LOAD=<dir>`: continue from a saved coupled state (extruded
    // onto the full duct when the saved nz differs); `RTX_E3FSI_SAVE=<dir>`
@@ -381,7 +402,7 @@ fn fsi2_on_embedded3() {
        .collect();
    let a_dofs = flag.borrow().node_dofs(flag_geo.a_node);
    let extract = |s: &DynamicState| -> Vec<f64> {
-        let mut c = vec![0.0; 2 * STATIONS];
+        let mut c = vec![0.0; 2 * stations()];
        for (k, d) in centre_dofs.iter().enumerate() {
            c[2 * k] = s.displacement[d[0]];
            c[2 * k + 1] = s.displacement[d[1]];
@@ -486,8 +507,8 @@ fn fsi2_on_embedded3() {
                    .sqrt();
                println!(
                    "    step {step} pass: |c_new − c_cand| {res:.3e}, tip cand ({:+.4e}, {:+.4e}), load flag ({:+.3}, {:+.3}) cyl ({:+.3}, {:+.3}) total ({:+.3}, {:+.3}), residual {:.1e}, fresh {}",
-                    cand[2 * STATIONS - 2],
-                    cand[2 * STATIONS - 1],
+                    cand[2 * stations() - 2],
+                    cand[2 * stations() - 1],
                    on_flag[0],
                    on_flag[1],
                    on_cyl[0],
@@ -227,17 +227,9 @@ impl E3Fluid {
        }));
        VERSION.fetch_add(1, Ordering::AcqRel);
        let (l1, l2, l3) = (lines.clone(), lines.clone(), lines.clone());
-        let body = Body::from_sdf(move |x, y, _z, t| cylinder(x, y).min(capsule(&l1, x, y, t).0))
-            .with_surface_velocity(move |x, y, _z, t| {
-                let (df, (vx, vy)) = capsule(&l2, x, y, t);
-                if df <= cylinder(x, y) {
-                    (vx, vy, 0.0)
-                } else {
-                    (0.0, 0.0, 0.0)
-                }
-            });
        let width = nz as f64 * h;
-        let body = body.with_device_sdf(move |t| {
+        let tip_corner = super::flat_tip();
+        let device_sdf = move |t: f64| {
            let line = l3.read().expect("lines").at(t);
            DeviceSdf {
                cyl: [CX, CY, R_CYL],
@@ -248,12 +240,48 @@ impl E3Fluid {
                r_edge: h,
                half: HALF,
                fillet: 0.0,
+                tip_corner,
                poly: line.pts,
                vel: line.vel,
                // R8-c's plate body (merged alongside): the span-uniform harness keeps the polyline.
                plate: None,
            }
-        });
+        };
+        let body = if tip_corner.is_some() {
+            // R8-h: the flat tip's host φ and velocity are the device form's
+            // (the kernel's arithmetic), cached per thread, time and lines.
+            let ds = device_sdf.clone();
+            let sdf_at = move |t: f64| -> Arc<DeviceSdf> {
+                thread_local! {
+                    static SDF: RefCell<(u64, u64, Option<Arc<DeviceSdf>>)> =
+                        const { RefCell::new((u64::MAX, u64::MAX, None)) };
+                }
+                let ver = VERSION.load(Ordering::Acquire);
+                SDF.with(|cell| {
+                    let mut c = cell.borrow_mut();
+                    if c.0 != t.to_bits() || c.1 != ver || c.2.is_none() {
+                        c.2 = Some(Arc::new(ds(t)));
+                        c.0 = t.to_bits();
+                        c.1 = ver;
+                    }
+                    c.2.clone().expect("sdf")
+                })
+            };
+            let sdf_v = sdf_at.clone();
+            Body::from_sdf(move |x, y, z, t| sdf_at(t).phi_host(x, y, z))
+                .with_surface_velocity(move |x, y, z, t| sdf_v(t).velocity_host(x, y, z))
+        } else {
+            Body::from_sdf(move |x, y, _z, t| cylinder(x, y).min(capsule(&l1, x, y, t).0))
+                .with_surface_velocity(move |x, y, _z, t| {
+                    let (df, (vx, vy)) = capsule(&l2, x, y, t);
+                    if df <= cylinder(x, y) {
+                        (vx, vy, 0.0)
+                    } else {
+                        (0.0, 0.0, 0.0)
+                    }
+                })
+        };
+        let body = body.with_device_sdf(device_sdf);
        solver.set_moving_body(body);
        let g = Grid::cubic(nx, ny, nz, h);
        let mut field = Field::new(g);