Further remarks on persistence of straining and flow classification

We revisit the kinematic flow classification developed in Thompson and Souza Mendes (2005) [R.L. Thompson, P.R. de Souza Mendes, Persistence of straining and flow classification, Int. J. Engng. Sci. 43 (1-2) (2005) 79-105] and give an improved physical interpretation to the persistence-of-straining tensor. The criterion is, then, applied to an intrinsically unsteady viscometric flow proposed by Yin and Pipkin (1970) [W.L. Yin, A.C. Pipkin, Kinematics of viscometric flow, Arch. Rational Mech. Anal. 37 (1970) 111-135].