Froude number

The Froude number is a dimensionless ratio of inertial effects to gravitational effects in fluid flow. It helps predict when gravity-driven wave behavior becomes important.

Critical flow: Flow in which a surface wave travels at the same speed as the fluid. At Froude number one, flow reaches the boundary between subcritical and supercritical behavior.
Froude scaling: A method of model testing that preserves the Froude number between a model and its full-scale counterpart. Matching the ratio preserves gravity-wave behavior in scaled free-surface experiments.
William Froude: A nineteenth-century English engineer who studied ship resistance and developed methods for testing scale models. The dimensionless number bears his name because of his work on ship-model similarity.
Subcritical flow: Free-surface flow slower than the speed of gravity waves traveling upstream relative to the channel. Its Froude number is below one, allowing disturbances to travel upstream.
Shallow-water equations: Equations describing fluid motion when horizontal scales greatly exceed fluid depth. Their wave speed sets the characteristic velocity used in the Froude number.
Ship model basin: A long, controlled tank used to test scale models of ships and their hydrodynamic performance. Model speed is selected to match the ship's Froude number and wave pattern.
Ship resistance: The force opposing a ship's motion through water, arising from effects such as friction and wave creation. Froude's model experiments separated wave-making resistance from other contributions.
Supercritical flow: Free-surface flow faster than the speed of gravity waves traveling upstream relative to the channel. Its Froude number exceeds one, so disturbances cannot propagate upstream.
Gravity wave: A wave whose restoring force is gravity acting on a displaced fluid surface or interface. The ratio compares flow inertia with the gravity that governs these waves.
Open-channel flow: Fluid flow with a free surface exposed to atmospheric pressure, such as flow in a river or canal. The Froude number classifies its gravity-driven flow regimes.

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