Novel Convective Instabilities of Low- to Moderate-prandtl Number Fluids

LOWTO MODERATE-PRANDTL NUMBER FLUIDS Guy Metcalfe Division of Building, Construction and Engineering CSIRO Highett, Victoria AUSTRALIA R.P. Behringer Dept. of Physics and Center for Nonlinear and Complex Systems Duke University Durham, North Carolina USA to be given at the 12th Australasian Fluid Mechanics Conference, Sydney Australia December 10{15 1995 ABSTRACT Super uid mixtures have Prandtl numbers tunable between those of liquid metals and water: 0:04 < Pr < 2. Moreover, super uid mixtures convect as regular uids, i.e. classical Rayleigh-B enard convection. With this unique Pr range and a variable aspect ratio ( ) experiment we survey convective instabilities in the largely unexplored space 0:12 < Pr < 1:4 and 2 < < 95. Among the novel behaviour found in the survey are: Instability competition greatly increases the complexity of convective states, but a heat-pulse method allows state selection. And, as becomes large (> 44), the onset of convection changes from stationary to time-dependent. As increases, oscillations arbitrarily close to the onset of convection arise, then give way to large-amplitude irregular uctuations. INTRODUCTION The dynamics of Rayleigh-B enard convection (RBC) rolls|convection in a horizontal layer of uid subject to a destabilizing density gradient|have been one of the most valuable experimental arenas for generating and testing ideas on linear and nonlinear stability, dynamical systems, chaos, and the transition to turbulence. Super uid mixture convection (SMC) is an analogous but much less studied form of convection that occurs when the density gradient is created in a layer of a dilute mixture of 3He in super uid 4He. mercury superfluid mixtures