Beyond a single entrance length: Multiscale development of velocity and stress in viscoelastic pipe flow

Fully developed flow is a standard assumption in theoretical and numerical descriptions of internal viscoelastic flows, yet the entrance region is typically reduced to a single development length, marking the validity of this assumption. Here, development is reinterpreted as a multiscale diagnostic of the evolving viscoelastic state rather than a geometric correction. Using axisymmetric simulations of an exponential Phan–Thien–Tanner fluid with Navier slip, we introduce complementary development measures based on the velocity field, the first normal stress difference, and their radial fluctuations. These measures reveal distinct regimes in which inertia, elasticity, shear thinning, and wall slip reorganize the ordering and relative magnitude of velocity and stress development. We identify an inertial crossover where stress develops before velocity, a transitional window in which mean and fluctuating stresses equilibrate nearly simultaneously, an elasticity-dominated asymptotic regime independent of inertia, and slip-induced inversions that decouple kinematic and stress development scales. By elevating development length from a geometric marker to a physical diagnostic, this framework provides a new insight into the axial evolution of velocity and polymeric stress fields in viscoelastic pipe flow and clarifies their role in the pathways leading to elastic flow transitions.