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The goal of the present study is: (i) to demonstrate the two-dimensionalnature of the elasto-inertial instability in elasto-inertial turbulence (EIT),(ii) to identify the role of the bi-dimensional instability inthree-dimensional EIT flows and (iii) to establish the role of the smallelastic scales in the mechanism of self-sustained EIT. Direct numericalsimulations of FENE-P fluid flows are performed in two- and three-dimensionalchannels. The Reynolds number is set to $\mathrm{Re}_\tau = 85$ which issub-critical for 2D flows but beyond transition for 3D ones. The polymerproperties correspond to those of typical dilute polymer solutions and twomoderate Weissenberg numbers, $\mathrm{Wi}_\tau = 40, 100$, are considered. Thesimulation results show that sustained turbulence can be observed in 2Dsub-critical flows, confirming the existence of a bi-dimensionalelasto-inertial instability. The same type of instability is also observed in3D simulations where both Newtonian and elasto-inertial turbulent structuresco-exist. Depending on the Wi number, one type of structure can dominate anddrive the flow. For large Wi values, the elasto-inertial instability tends toprevail over the Newtonian turbulence. This statement is supported by (i) theabsence of the typical Newtonian near-wall vortices and (ii) strongsimilarities between two- and three-dimensional flows when considering largerWi numbers. The role of the small elastic scales is investigated by introducingglobal artificial diffusion in the hyperbolic transport equation for polymers.The study results show that the introduction of large polymer diffusion in thesystem strongly damps a significant part of the elastic scales that arenecessary to feed turbulence, eventually leading to the flow laminarization. Asufficiently high Schmidt number is necessary to allow self-sustainedturbulence to settle.

Two-dimensional dynamics of elasto-inertial turbulence and its role in polymer drag reduction