Adolfo Ribeiro

Approaching the asymptotic regime of rapidly rotating convection: Boundary layers versus interior dynamics

Rapidly rotating Rayleigh-Bénard convection is studied by combining results from direct numerical simulations (DNS), laboratory experiments, and asymptotic modeling. The asymptotic theory is shown to provide a good description of the bulk dynamics at low, but finite Rossby number. However, large deviations from the asymptotically predicted heat transfer scaling are found, with laboratory experiments and DNS consistently yielding much larger Nusselt numbers than expected. These deviations are traced down to dynamically active Ekman boundary layers, which are shown to play an integral part in controlling heat transfer even for Ekman numbers as small as 10^{-7}. By adding an analytical parametrization of the Ekman transport to simulations using stress-free boundary conditions, we demonstrate that the heat transfer jumps from values broadly compatible with the asymptotic theory to states of strongly increased heat transfer, in good quantitative agreement with no-slip DNS and compatible with the experimental data. Finally, similarly to nonrotating convection, we find no single scaling behavior, but instead that multiple well-defined dynamical regimes exist in rapidly rotating convection systems.

Performance benchmarks for a next generation numerical dynamo model

Numerical simulations of the geodynamo have successfully represented many observable characteristics of the geomagnetic field, yielding insight into the fundamental processes that generate magnetic fields in the Earths core. Because of limited spatial resolution, however, the diffusivities in numerical dynamo models are much larger than those in the Earths core, and consequently, questions remain about how realistic these models are. The typical strategy used to address this issue has been to continue to increase the resolution of these quasi-laminar models with increasing computational resources, thus pushing them toward more realistic parameter regimes. We assess which methods are most promising for the next generation of supercomputers, which will offer access to O(106) processor cores for large problems. Here we report performance and accuracy benchmarks from 15 dynamo codes that employ a range of numerical and parallelization methods. Computational performance is assessed on the basis of weak and strong scaling behavior up to 16,384 processor cores. Extrapolations of our weak-scaling results indicate that dynamo codes that employ two-dimensional or three-dimensional domain decompositions can perform efficiently on up to ∼106 processor cores, paving the way for more realistic simulations in the next model generation.

Experimental study of internal wave generation by convection in water

We experimentally investigate the dynamics of water cooled from below at 0 C ° and heated from above. Taking advantage of the unusual property that water’s density maximum is at about 4 C ° , this set-up allows us to simulate in the laboratory a turbulent convective layer adjacent to a stably stratified layer, which is representative of atmospheric and stellar conditions. High precision temperature and velocity measurements are described, with a special focus on the convectively excited internal waves propagating in the stratified zone. Most of the convective energy is at low frequency, and corresponding waves are localized to the vicinity of the interface. However, we show that some energy radiates far from the interface, carried by shorter horizontal wavelength, higher frequency waves. Our data suggest that the internal wave field is passively excited by the convective fluctuations, and the wave propagation is correctly described by the dissipative linear wave theory. S Online supplementary data available from stacks.iop.org/fdr/47/045502/ mmedia