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The University of Minnesota-Twin Cities (UM-TC) continues to offer support for graduate students in various space-related departments throughout the year. Below are the abstracts of papers recently presented by two of the current MnSGC Fellows (both students will complete the Ph.D. this year):
"The transient evolution of corotating disk flow"
The fluid spin-up from rest between corotating, shrouded disks was studied experimentally and numerically. Velocity measurememts were obtained using LDA and a facility consisting of a stack of 4 coaxial disks separated by solid spacers, 0.063 disk radii thick. The assembly was spun up to 1.05 rad/s in 0.2 seconds. Axial profiles of radial and tangential velocity were measured at three radii. The flow evolution is divided into two phases, boundary layer development followed by core spin-up. Calculations of the first phase assume axisymmetric flow with linear dependence on r . The tangential flow boundary layers are fully developed after ~1/3 disk revolution, and evolve similarly to the layer on a single disk. Geometric confinement induces secondary recirculation in the r-z plane, with negative radial velocities at the midplane. The secondary flow develops slower than the single disk radial pumping and the recirculation strength continues to grow long after the primary flow is stationary. Angular momentum is imparted to the fluid in the boundary layers and is convected into the core by the secondary flow. Core spin-up occurs by conservation of angular momentum as this fluid is convected towards the hub. Consequently, the near-hub flow is last to spin up. At each radial location, the strength of the secondary recirculation grows until the core flow begins to spin-up.
(Presented at the 48th Annual Meeting of the Division of Fluid Dynamics of the American Physical Society, held November 19-21, 1995 in Irvine, California.)
"Large electric field effects of ultrathin films of Bi/a Ge grown on single crystal SrTiO3 substrates."
A large electric field effect was observed in ultrathin films of Bi/a Ge, quench condensed at liquid helium temperatures onto single crystal SrTiO3 substrates. A Pt gate electrode was pre-evaporated on the back of the substrate. Insulating films exhibited a symmetric response to the gate voltage, with a maximum increase in conductivity of about 50% for the most insulating film. The symmetric effect disappeared as the superconductor insulator (SI) transition was approached. In the superconducting (metallic) state, the response was small and antisymmetric in the gate voltage. The variation of this effect with temperature, film thickness, and its relationship to the SI transition will be reported. The possible origins of this phenomena, including electron hole symmetry and electrostriction in SrTiO3 will be discussed.
(To be presented at the American Physical Society Meeting, March 18 - 26, 1996, St. Louis, MO.)
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