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The transition to turbulence in a plane Poiseuille flow of dilute polymersolutions is studied by direct numerical simulations of a FENE-P fluid. A rangeof Reynolds number ($Re$) in $2000 \le Re \le 5000$ is studied but with thesame level of elasticity in viscoelastic flows. The evolution of afinite-amplitude perturbation and its effects on transition dynamics areinvestigated. A viscoelastic flow begins transition at an earlier time than itsNewtonian counterparts, but the transition time appears to be insensitive topolymer concentration at dilute or semi-dilute regimes studied. Increasingpolymer concentration, however, decreases the maximum attainable energy growthduring the transition process. The critical or minimum perturbation amplituderequired to trigger transition is computed. Interestingly, both Newtonian andviscoelastic flows follow almost the same power-law scaling of $Re^\gamma$ withthe critical exponent $\gamma \approx -1.25$, which is in close agreement withprevious studies. However, a shift downward is observed for viscoelastic flow,suggesting that smaller perturbation amplitudes are required for thetransition. A mechanism of the early transition is investigated by theevolution of wall-normal and spanwise velocity fluctuations and flow structure.The early growth of these fluctuations and formation of quasi-streamwisevortices around low-speed streaks are promoted by polymers, hence causing anearly transition. These vortical structures are found to support the criticalexponent $\gamma \approx -1.25$. Once the transition process is completed,polymers play a role in dampening the wall-normal and spanwise velocityfluctuations and vortices to attain a drag-reduced state in viscoelasticturbulent flows.

Transition to turbulence in viscoelastic channel flow of dilute polymer solutions