Weissenberg number

Summary

The Weissenberg number $\mathrm{Wi}$ is a dimensionless number. It compares the relaxation time of a viscoelastic fluid with the time scale of the deformation in a flow. It measures how strong elastic effects are compared with viscous effects. When $\mathrm{Wi}$ is small the fluid behaves almost like a Newtonian fluid. When it is large, elastic effects such as normal stresses and elastic instabilities dominate.