Turbulence turns elastic
May 5, 2000
It is over 100 years since Osborne Reynolds discovered that turbulence in fluid flow is controlled by the inertia or momentum of the fluid. Now two Israeli physicists have observed a form of turbulence that can occur in elastic fluids in the absence of inertial forces. Alexander Groisman and Victor Steinberg from the Weizmann Institute of Science in Rehevot observed so-called elastic turbulence in a polymer fluid trapped between two parallel disks (Nature 405 53). As the rotation speed of one of the disks was increased, it caused an abrupt change in the velocity of the fluid. Although the flow rate was still too small to generate much inertia, the fluid still acquired turbulent characteristics.
Polymer films are used in to make many mass-produced products, such as compact discs. It has been known for some time that the flow rate of liquid polymers can become irregular during the manufacturing process, despite the slow fluid speeds involved. This was surprising because these slow speeds should reduce the effects of inertia, and hence reduce the turbulence.
Researchers thought that by increasing the length of the polymer molecules, they would increase the viscosity of the polymer liquid, which should lead to regular or laminar flow, rather than turbulence. However, the velocity at which turbulent flow occurred became lower, rather than higher as expected. This behaviour appeared to be caused by the elasticity of the polymer chains -- which has now been confirmed by the observation of turbulence in the complete absence of inertial forces in the Weizmann experiments.