Elastic turbulence in a polymer solution flow

Turbulence is a ubiquitous phenomenon that is not fully understood. It is k nown that the flow of a simple, newtonian fluid is likely to be turbulent w hen the Reynolds number is large (typically when the velocity is high, the viscosity is low and the size of the tank is large(1,2)). In contrast, visc oelastic fluids(3) such as solutions of flexible long-chain polymers have n onlinear mechanical properties and therefore may be expected to behave diff erently. Here we observe experimentally that the flow of a sufficiently ela stic polymer solution can become irregular even at low velocity, high visco sity and in a small tank. The fluid motion is excited in a broad range of s patial and temporal scales, and we observe an increase in the flow resistan ce by a factor of about twenty. Although the Reynolds number may be arbitra rily low, the observed flow has all the main features of developed turbulen ce. A comparable state of turbulent flow for a newtonian fluid in a pipe wo uld have a Reynolds number as high as 10(5) (refs 1, 2). The low Reynolds n umber or 'elastic' turbulence that we observe is accompanied by significant stretching of the polymer molecules, resulting in an increase in the elast ic stresses of up to two orders of magnitude.