Consider a flow whose typical kinematics is given by 
, and let λ be the relaxation time of the
fluid. The Weissenberg number is given by
 | (9–1) |
This number indicates the elasticity involved in a flow.
The Weissenberg number is sometimes understood as the ratio between a normal-stress difference and a shear stress. Such an interpretation raises questions about the relevance of Weissenberg numbers as high as 20 or even 100. Indeed, this would involve a very high normal stress, and besides a few biological fluids, the majority of macromolecular fluids would not sustain such a high stress.
Important: Note that a polymer melt is characterized by a relaxation spectrum, rather than by
a single relaxation time. Hence, when evaluating 
, it is important to consider an adequate relaxation time
λ.