Add rotating drum angle of repose benchmark

Cargo.toml

@@ -126,6 +126,10 @@ path = "examples/example_template/main.rs"
 name = "cone_hopper"
 path = "examples/cone_hopper/main.rs"
 
+[[example]]
+name = "bench_rotating_drum"
+path = "examples/bench_rotating_drum/main.rs"
+
 [profile.release]
 opt-level = 3
 lto = "fat"

examples/bench_rotating_drum/README.md

@@ -0,0 +1,79 @@
+# Rotating Drum Angle of Repose Benchmark
+
+## Physics
+
+This benchmark validates the **dynamic angle of repose** in a 2D rotating drum
+against expected trends from DEM literature. In the rolling regime (Froude
+number Fr = omega^2 * R / g << 1), granular material in a rotating drum forms a
+flat free surface tilted at the **dynamic angle of repose**. This angle
+depends primarily on the inter-particle friction coefficient.
+
+## Setup
+
+- **Geometry**: 2D (quasi-3D) drum of radius R = 50 mm, modeled as a 48-sided
+  polygon of plane walls (to capture tangential friction at the drum wall).
+  Periodic in y to enforce a single-layer quasi-2D configuration.
+- **Particles**: 200 monodisperse spheres, radius = 2 mm, density = 2500 kg/m^3
+  (glass-like), Young's modulus = 5 MPa (softened for fast simulation).
+- **Rotation**: Instead of rotating the drum, gravity is rotated at omega = 1.5
+  rad/s (Fr = 0.011). This is physically equivalent for Fr << 1 where
+  centrifugal/Coriolis effects are negligible.
+- **Fill fraction**: ~40% of the drum cross-section.
+
+## Measurement
+
+After an 80,000-step settling phase and one full rotation of transient, the
+free surface angle is measured every 500 steps by:
+
+1. Rotating particle positions into a gravity-aligned frame
+2. Binning particles by horizontal position
+3. Finding the highest particle in each bin (free surface)
+4. Fitting a line through the surface points using least squares
+5. Computing the angle from the slope
+
+## Validation
+
+The benchmark runs 4 simulations with friction coefficients mu = 0.2, 0.3, 0.5, 0.7.
+
+**Expected results** (from Li & Cleary 2015, Zhou et al. 2002, Wensrich & Katterfeld 2012):
+- The angle of repose should fall within 15-45 degrees for typical DEM parameters
+- The angle should **increase monotonically** with friction coefficient
+- Typical values: mu=0.3 -> ~24 deg, mu=0.5 -> ~30 deg, mu=0.7 -> ~35 deg
+
+## How to Run
+
+```bash
+# Single run with default friction (mu=0.5):
+cargo run --release --example bench_rotating_drum --no-default-features \
+    -- examples/bench_rotating_drum/config.toml
+
+# Full validation (4 friction values, ~15 min total):
+cd examples/bench_rotating_drum
+python validate.py
+
+# Generate plots:
+python plot.py
+```
+
+## Output Files
+
+- `output/surface_angle.txt` — Time series of measured surface angle (step, time, angle_deg)
+- `results.txt` — Summary: friction vs mean angle (generated by validate.py)
+- `angle_vs_friction.png` — Angle of repose vs friction coefficient plot
+- `angle_time_series.png` — Time series of surface angle for each friction value
+
+## Assumptions and Simplifications
+
+- **2D (quasi-3D)**: Thin slab periodic in y; single particle layer
+- **Monodisperse**: All particles have the same radius
+- **Soft particles**: E = 5 MPa (much softer than real glass) for reasonable timestep
+- **Rotating gravity**: Equivalent to rotating drum for Fr << 1
+- **Polygon drum**: 48-sided polygon approximates a smooth cylinder
+- **Restitution = 0.5**: High damping for fast energy dissipation during settling
+- **Rolling friction = 0.1**: Small but nonzero for realistic packing
+
+## References
+
+- Li, T., & Cleary, P. W. (2015). *Granular Matter*, 17, 367-382.
+- Zhou, Y. C., et al. (2002). *Chemical Engineering Science*, 57, 3621-3638.
+- Wensrich, C. M., & Katterfeld, A. (2012). *Granular Matter*, 14, 467-482.

examples/bench_rotating_drum/config.toml

@@ -0,0 +1,85 @@
+# ============================================================================
+# Rotating Drum Angle of Repose Benchmark
+# ============================================================================
+#
+# 2D (quasi-3D) rotating drum with monodisperse particles.
+# The drum wall is a ring of frozen particles rotated kinematically.
+# Full Hertz-Mindlin contact (with tangential friction) between wall and mobile
+# particles drives the cascading flow.
+#
+# Froude number: Fr = omega^2 * R / g = 1.5^2 * 0.05 / 9.81 = 0.011
+# This is well within the rolling regime (Fr < 0.1).
+
+# ── Domain ────────────────────────────────────────────────────────────────────
+[domain]
+x_low = -0.01
+x_high = 0.11         # drum fits in [0, 0.10]
+y_low = 0.0
+y_high = 0.005        # thin slab (~1.25 particle diameters)
+z_low = -0.01
+z_high = 0.11
+periodic_x = false
+periodic_y = true     # quasi-2D: periodic in y
+periodic_z = false
+
+# ── Neighbor list ─────────────────────────────────────────────────────────────
+[neighbor]
+skin_fraction = 1.5
+bin_size = 0.005      # ~1.25 particle diameters (m)
+every = 10
+check = true
+
+# ── DEM materials ─────────────────────────────────────────────────────────────
+[dem]
+contact_model = "hertz"
+
+# Type 0: mobile particles
+[[dem.materials]]
+name = "particles"
+youngs_mod = 5.0e6    # Pa — soft particles for fast simulation
+poisson_ratio = 0.3
+restitution = 0.5     # moderate damping for fast settling
+friction = 0.5        # sliding friction coefficient
+rolling_friction = 0.1
+
+# Type 1: wall particles (same properties; friction between wall & mobile
+# particles is the mixed value from the material table)
+[[dem.materials]]
+name = "wall"
+youngs_mod = 5.0e6
+poisson_ratio = 0.3
+restitution = 0.5
+friction = 0.5
+rolling_friction = 0.1
+
+# ── Particle insertion ────────────────────────────────────────────────────────
+# Insert mobile particles in a cylinder region inside the drum.
+# Wall particles are created programmatically by WallParticlePlugin.
+[[particles.insert]]
+material = "particles"
+count = 200           # ~40% fill of drum cross-section
+radius = 0.002        # 2 mm radius (4 mm diameter)
+density = 2500.0      # kg/m³ (glass-like)
+velocity = 0.0
+region = { type = "cylinder", center = [0.05, 0.05], radius = 0.042, axis = "y", lo = 0.0, hi = 0.005 }
+
+# ── Gravity ───────────────────────────────────────────────────────────────────
+[gravity]
+gx = 0.0
+gy = 0.0
+gz = -9.81            # m/s²
+
+# ── Run settings ──────────────────────────────────────────────────────────────
+[run]
+steps = 300000        # total steps (15.0 s at dt=5e-5)
+thermo = 5000
+dt = 5.0e-5           # s
+dump_interval = 10000
+
+# ── Dump output ───────────────────────────────────────────────────────────────
+[dump]
+format = "text"
+
+# ── Output directory ──────────────────────────────────────────────────────────
+[output]
+directory = "examples/bench_rotating_drum/output"