In fluid dynamics, the Keulegan–Carpenter number, also called the period number, is a dimensionless quantity describing the relative importance of the drag forces over inertia forces for bluff objects in an oscillatory fluid flow.
Or similarly, for objects that oscillate in a fluid at rest.
A closely related parameter, also often used for sediment transport under water waves, is the displacement parameter δ:[1] with A the excursion amplitude of fluid particles in oscillatory flow and L a characteristic diameter of the sediment material.
For sinusoidal motion of the fluid, A is related to V and T as A = VT/(2π), and: The Keulegan–Carpenter number can be directly related to the Navier–Stokes equations, by looking at characteristic scales for the acceleration terms: Dividing these two acceleration scales gives the Keulegan–Carpenter number.
Conversely, the Keulegan–Carpenter number is related to the oscillation frequency of an unsteady flow into which the object is placed.