Laboratory experiments on high Rayleigh number thermal convection in a rapidly rotating hemispherical shell

Abstract

We report the results of laboratory experiments on high Rayleigh number thermal convection in a rotating hemispherical shell at Ekman number of Ek=4.7×10 −6. We use the combined effect of centrifugal acceleration and laboratory gravity in the lower hemisphere of a spherical shell to simulate the gravity in the Earth's core. Visualization and recording of the pattern and flow, together with the measurements of temperature time series, are used to quantify the motions. We study the change in the convective pattern and temperature structure vs. the Rayleigh number, Ra, up to 45 times the critical value Ra c. Three major regimes are found. For Ra/ Ra c<1 (regime i), the hemisphere is stably stratified. Zonal motion exists in this regime in the form of interleaved lenses of retrograde and prograde flows. For 1< Ra/ Ra c<8 (regime ii), we observe penetrative convection, driven by prograde spiralling cold plumes that originate from the inner core and form a closely spaced columnar structure. These plumes slowly drift retrograde, advected by the mean zonal flow. For Ra/ Ra c>8 (regime iii), we observe dual convection, driven by cold plumes from the inner boundary and by warm plumes from the outer boundary. This produces a very fine-scaled geostrophic turbulence. A retrograde flow, fastest near the inner boundary, is present in this regime, perhaps driven by Reynolds stresses. The transition from (ii) to (iii) is caused by difference in the wave number of the plumes originating from the inner and outer boundaries, i.e., the cold plumes from the inner boundary have smaller wave numbers as compared to warm plumes from the outer boundary, owing to differences in the outer boundary slope. Temperature measurements by thermistor probes indicate oscillations with a typical period corresponding to the azimuthal drift of columnar structures past the fixed probes, as well as nonsinusoidal features arising from nonlinearity. Convection is the main form of heat transfer in most of the range covered, and is pronounced in the equatorial region. Under the assumption of geostrophic balance, we suggest several new interpretations for convection in the Earth's core.