The Womersley number arises in the solution of the linearized Navier–Stokes equations for oscillatory flow (presumed to be laminar and incompressible) in a tube. It expresses the ratio of the transient or oscillatory inertia force to the shear force.
The Womersley number is a quantity that appears when solving simplified Navier-Stokes equations for steady, laminar, and incompressible flow within a tube under oscillatory conditions.  It represents the balance between inertial forces and shear forces in this type of flow.