Spacelab Experiments on Convection in A Rotating Spherical Shell with Radial Gravity

Abstract

Experiments on thermal convection in a rotating, differentially-heated hemispherical shell of fluid with a radial gravity field were carried out in the microgravity environment of Spacelab 3 which was flown on the space shuttle Challenger in May 1985. Such global-scale convection interacting with rotation has bearing on flows within the Sun and the giant planets, all of which rotate and possess deep shells of convection. The radial force of gravity is modelled by imposing a strong electric field across the shell, with dielectric polarization forces producing radial accelerations proportional to temperature. The thermally-driven circulations result from imposed radial and latitudinal temperature gradients on the bounding surfaces. When the rotation is rapid and its influence strong, the experiments with spherically-symmetric heating yield north-south oriented columnar convection (“banana cells”) in equatorial and sub-equatorial regions. As the radial heating is increased, these roll-like cells interact with mid-latitude waves, ultimately being destroyed by more isotropic convection that moves down from the pole. When a significant equator-to-pole temperature difference is imposed, “spiral waves” develop on top of a strong meridional circulation. Intricate, nonaxisymmetric, convective patterns that propagate in longitude and evolve in time are observed, including some “triangular waves” that couple mid-latitude and equatorial disturbances. Schlieren visualizations of these laboratory flows are compared briefly to three-dimensional nonlinear simulations that can be conducted at the more modest heating rates.