Stream/Sediment Settling Velocity (Stokes) mechanism

Model scope

The equation is correct for ideal Stokes settling, but the specification is incomplete and potentially misleading because gravity is omitted from the input list, units and temperature selection are not explicit, and the approximately 0.1 mm validity statement is not a universal criterion. Exact ambiguity requiring review: whether gravity is a fixed constant or an omitted user input, and which temperature/reference values define water density and viscosity.

• The stated equation v=g(ρs−ρf)d²/(18μ) is the standard Stokes terminal settling velocity for an isolated spherical particle in creeping flow; the algebra is correct under those assumptions.
• Gravity g is required by the formula but is missing from the declared instrument inputs. The specification must either add g as an input or explicitly fix it, such as standard Earth gravity 9.80665 m/s².
• The stated approximately 0.1 mm validity limit is not universal: Stokes validity depends on particle Reynolds number Re=ρf|v|d/μ, which depends on density, viscosity, and velocity as well as diameter.
• The specification does not state the required assumptions of a spherical, isolated grain and an unbounded Newtonian fluid. Hindered settling, wall effects, nonspherical grains, and flocculation can invalidate the result.
• Mentioning a turbulent-regime limit without specifying an alternate drag law leaves the instrument undefined or misleading outside the Stokes regime.

Units and conventions

• Use SI units: d in meters, ρs and ρf in kg/m³, μ in Pa·s, g in m/s², and output v in m/s.
• If diameter is entered in millimeters or micrometers, convert it to meters before squaring it.
• ρs and ρf, and μ, must correspond to the same fluid temperature and composition; water reference values cannot be selected unambiguously without a temperature input or an explicit reference-temperature setting.
• The examples use positive viscosity and standard Earth gravity g=9.80665 m/s² unless otherwise shown.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "grain_diameter_m": 0.00004,
    "grain_density_kg_per_m3": 2650,
    "fluid_density_kg_per_m3": 998,
    "viscosity_Pa_s": 0.001002,
    "gravity_m_per_s2": 9.80665
  },
  "expected_outputs": {
    "settling_velocity_m_per_s": 0.0014371777
  },
  "working": "Δρ=2650−998=1652 kg/m³. d=40 μm=4.0×10^-5 m, so d²=1.6×10^-9 m². Numerator=9.80665×1652×1.6×10^-9=2.592093728×10^-5. Denominator=18×0.001002=0.018036. v=(2.592093728×10^-5)/0.018036=0.0014371777 m/s."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

The specification has no turbulent-regime settling formula, so inputs beyond the low-Reynolds-number Stokes regime cannot produce a valid velocity. It also requires explicit rejection of zero viscosity and explicit sign/unit conventions.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche5/tranche5-results.json
• BOUNDARY_HAZARD_CATALOG.json