Polymer additives can dramatically reduce drag in turbulent flows. For example, in large oil pipelines, it helps achieve a smoother flow and, thereby, a larger flow rate. However, there is a limit to the possible reduction of drag, called maximum drag reduction asymptote. The opposite effect is observed in microchannels, where the viscoelasticity introduced by polymers creates instabilities and chaotic motion in flows that would otherwise be smooth and regular. The EU-funded project VISCELTURBFLOW (Computational study of macro- and microscopic turbulence controlled by polymer additives) was initiated to find out where the drag reduction onset and its maximum limit originate. Using numerical techniques, scientists showed that a new type of turbulence is involved. The VISCELTURBFLOW team and its collaborators demonstrated that an energy transfer from polymer elastic energy to turbulent kinetic energy takes place at small scales. This feeds the turbulence and prevents the flow to become fully laminar, potentially explaining the maximum drag reduction limit. Scientists studied in detail the interactions between inertial and elastic instabilities at different Reynolds numbers. The Reynolds number indicates the relative contribution of inertial to viscous forces. Their simulations showed that added polymers cause a delay in the transition from a smooth and laminar flow to a chaotic turbulent flow. However, if a sufficiently large perturbation is initially introduced in the flow, high elasticity promotes transition and leads to a self-sustained chaotic flow at subcritical Reynolds numbers. This new state was named elasto-inertial turbulence (EIT). The VISCELTURBFLOW team showed that EIT also exists in two dimensions. They suggested that EIT represents the ultimate maximum drag reduction state of the flow in the limit of very high elasticity. The VISCELTURBFLOW findings were presented at several conferences. The properties of the new type of turbulence have been described in a series of papers published in high-impact peer-reviewed journals.
Polymers, viscoelasticity, elasto-inertial turbulence (EIT), transition, VISCELTURBFLOW, Weissenberg number, Reynold number