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

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

Jan 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000pepi..117..153s&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 117, Issue 1-4, p. 153-170.

Physics

19

Scientific paper

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 Rac. Three major regimes are found. For Ra/Rac<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 18 (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.

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