Experimental observation of the origin and structure of elasto-inertial turbulence

Turbulence generally arises in shear flows if velocities and hence inertial forces are sufficiently large. In striking contrast, viscoelastic fluids can exhibit disordered motion even at vanishing inertia. Intermediate between these cases, a novel state of chaotic motion, `elasto-inertial turbulence' (EIT), has been observed in a narrow Reynolds number interval. We here determine the origin of EIT in experiments and show that characteristic EIT structures can be detected across an unexpectedly wide range of parameters. Close to onset a pattern of chevron shaped streaks emerges in excellent agreement with linear theory. However, the instability can be traced to far lower Reynolds numbers than permitted by theory. For increasing inertia, a secondary instability gives rise to a wall mode composed of inclined near wall streaks and shear layers. This mode persists to what is known as the `maximum drag reduction limit' and overall EIT is found to dominate viscoelastic flows across more than three orders of magnitude in Reynolds number.