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This comment proves that Balan's (2023) formulation of impulsive Couette start-up flow for viscoelastic fluids imposes initial conditions incompatible with a nonzero retardation time $\lambda_2$. An exact analytical derivation reveals that the original paper solved a plate velocity ramped as $1 - \mathrm{e}^{-t/\lambda_2}$ rather than a true impulsive step, perfectly matching Balan's digitized data while exposing the reported anomalous wall normal stresses as grid-dependent numerical artifacts. Correcting these closed-form solutions resolves key baseline discrepancies for non-Newtonian flow solvers and provides fully reproducible, open-source verification notebooks.
Flawed initial boundary data turned a standard viscoelastic flow benchmark into an accidental exponential ramp, exposing supposed anomalous wall normal stresses as pure numerical artifacts.
I show that the initial conditions imposed by Balan [Phys.\ Fluids \textbf{35}, 113108 (2023)] are incompatible with a nonzero retardation time, so that the problem solved is not impulsive start-up, but, as I prove exactly, the start-up of a plate ramped as $1 - \mathrm{e}^{-t/\lambda_2}$ on the retardation timescale $\lambda_2$ (a material parameter, not a setting in a rheometer). Values digitized from that paper's viscoelastic figures fall on the ramped-plate solution, not on Tanner's start-up solution. The large negative wall normal stress reported for the corotational model is a grid-dependent artifact due to the same error in the initial data. An open-source code repository provides annotated notebooks reproducing every figure and number in this Comment.