Laboratory study of angular momentum transport in a rotating shear flow

The MagnetoRotational Instability (MRI) is widely accepted to be responsible for the angular momentum transport in accretion disks which power some of the most luminous objects in the universe. Conditions for instability to the MRI in ideal MHD are: (1) an angular velocity which decreases with radius and (2) a weak ambient magnetic field which allows the exchange of momentum between radially-separated fluid elements. The MRI has not been conclusively detected in the laboratory. Subcritical Hydrodynamic Instabilities have also received renewed interest for application to cool circumstellar disks which may be too poorly ionized to generate the MRI. Reports of purely hydrodynamic turbulence in subcritical flows lack transport measurements to support the hypothesis that angular velocity shear undergoes a spontaneous transition. A small aspect-ratio, wide gap circular-Couette experiment capable of operation at Reynolds number in excess of 106 is constructed to investigate these two mechanisms of angular momentum transport. The apparatus consists of two concentric co-rotating cylinders. To minimize the effect of the cylinder end caps, they are divided into nested differentially rotatable rings. Water and a water-glycerol mix are used as working fluids to study angular momentum transport in quasi-Keplerian flows and its scaling with Reynolds number. When the end rings speeds are optimized, large-scale advective transport due to the vertical boundaries is eliminated. The resulting flow is an excellent approximation to the ideal circular-Couette profile. Measurement of the r - o component of the Reynolds stress using Laser Doppler Velocimetry shows no indication of a subcritical instability. Pure hydrodynamic turbulence is an unlikely mechanism to transport angular momentum in accretion disks.