research: Dütsch driver — exact-point body mask, periodic geometry from step mod T

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

research/duetsch_re100_kc5/protocol.py

@@ -25,12 +25,15 @@ def amplitude(D):
 
 
 def position(s, T, A):
-    """Centre position relative to the mean: x = A cos(2 pi s / T)."""
+    """Centre position relative to the mean: x = A cos(2 pi s / T), evaluated from the
-    return A * math.cos(2.0 * math.pi * s / T)
+    native step modulo T so every cycle repeats the same geometry bit for bit."""
+    return A * math.cos(2.0 * math.pi * (int(s) % T) / T)
 
 
 def phase(s, T):
-    return (360.0 * s / T - 90.0) % 360.0
+    """Paper phase in degrees from the native step modulo T (the unwrapped cycle
+    index is kept separately by ``cycle``)."""
+    return (360.0 * (int(s) % T) / T - 90.0) % 360.0
 
 
 def cycle(s, T):
@@ -63,6 +66,18 @@ def morison_fit(f_norm, b1, b2):
     return float(c_d), float(c_i)
 
 
+def inside_circle(x, y, centre, radius):
+    return (x - centre[0]) ** 2 + (y - centre[1]) ** 2 < radius * radius
+
+
+def sample(field, fluid, x, y, centre, radius):
+    """Profile sample: MISSING if the exact point is inside the body (checked before
+    any corner interpolation), else ``bilinear_fluid``."""
+    if inside_circle(x, y, centre, radius):
+        return None, (), 0.0
+    return bilinear_fluid(field, fluid, x, y)
+
+
 def bilinear_fluid(field, fluid, x, y):
     """Bilinear value at (x, y) in cell coordinates from fluid cells only.
 

research/duetsch_re100_kc5/run.py

@@ -70,11 +70,12 @@ def main():
                 fields = sim.get_macroscopic()
                 fluid = (sim.get_flags().reshape(n, n) & 1) != 0
                 ux, uy = (np.asarray(fields[k]).reshape(n, n) for k in ("ux", "uy"))
+                centre = (xm + p.position(s, T, A), ym)  # analytic centre after the step
                 for xs in p.STATIONS:
                     for x2 in p.X2:
                         x, y = xm + xs * D, ym + x2 * D
-                        u, wu, wsum = p.bilinear_fluid(ux, fluid, x, y)
+                        u, wu, wsum = p.sample(ux, fluid, x, y, centre, R)
-                        v, _, _ = p.bilinear_fluid(uy, fluid, x, y)
+                        v, _, _ = p.sample(uy, fluid, x, y, centre, R)
                         samples.append((s, p.cycle(s, T), ph, xs, x2,
                                         math.nan if u is None else u, math.nan if v is None else v,
                                         wsum, json.dumps(wu)))

tests/unit/test_duetsch_protocol.py

@@ -62,3 +62,25 @@ def test_interval_bases_and_morison_fit_recover_known_coefficients():
     assert np.allclose(b[:, 1], np.sin(phi), atol=1e-5)          # midpoint ~ interval mean
     f = 2.09 * b[:, 0] - (math.pi ** 2 * 1.45 / p.KC) * b[:, 1]
     assert p.morison_fit(f, b[:, 0], b[:, 1]) == pytest.approx((2.09, 1.45), abs=1e-9)
+
+
+def test_geometry_repeats_exactly_across_cycles():
+    A = p.amplitude(20)
+    for m in (0, 1, T // 4, T // 2 - 1, 7 * T // 12):
+        for c in (1, 5, 11):
+            assert p.position(m + c * T, T, A) == p.position(m, T, A)
+            assert p.phase(m + c * T, T) == p.phase(m, T)
+            v0 = p.position(m + 1, T, A) - p.position(m, T, A)
+            assert p.position(m + 1 + c * T, T, A) - p.position(m + c * T, T, A) == v0
+    assert p.cycle(T // 4 + 5 * T, T) == 6  # unwrapped index stays separate
+
+
+def test_exact_point_inside_body_is_missing_even_with_fluid_corners():
+    field = np.zeros((6, 6))
+    fluid = np.ones((6, 6), bool)
+    fluid[3, 3] = False                         # only one corner solid
+    centre, radius = (3.0, 3.0), 0.3
+    value, used, total = p.sample(field, fluid, 3.2, 3.1, centre, radius)   # inside the circle
+    assert value is None and used == () and total == 0.0
+    value, used, total = p.sample(field, fluid, 3.35, 3.0, centre, radius)  # just outside
+    assert value is not None and total > 0.0