We report the observation of superstructures, i.e. very large-scale and long living coherent structures in highly turbulent Rayleigh-Bénard convection up to Rayleigh Ra = 10 9 . We perform direct numerical simulations in horizontally periodic domains with aspect ratios up to Γ = 128. In the considered Ra number regime the thermal superstructures have a horizontal extend of six to seven times the height of the domain and their size is independent of Ra. Many laboratory experiments and numerical simulations have focused on small aspect ratio cells in order to achieve the highest possible Ra. However, here we show that for very high Ra integral quantities such as the Nusselt number and volume averaged Reynolds number only converge to the large aspect ratio limit around Γ ≈ 4, while horizontally averaged statistics such as standard deviation and kurtosis converge around Γ ≈ 8, and the integral scale converges around Γ ≈ 32, and the peak position of the temperature variance and turbulent kinetic energy spectra only around Γ ≈ 64.

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Abstract

Abstract

Oceans play a big role in the nature of our planet, about 70% of our earth is covered by water [1]. Strong currents are transporting warm water around the world making life possible, and allowing us to harvest its power producing energy. Yet, oceans also carry a much more deadly side. Floods and tsunamis can easily annihilate whole cities and destroy life in seconds. The earth's climate system is also very much linked to the currents in the ocean due to its large coverage of the earth's surface, thus, gaining scientific insights into the mechanisms and effects through simulations is of high importance. Deep ocean currents can be simulated by means of wall-bounded turbulent flow simulations. To support these very large scale numerical simulations and enable the scientists to interpret their output, we deploy an interactive visualization framework to study sheared thermal convection. The visualizations are based on volume rendering of the temperature field. To address the needs of supercomputer users with different hardware and software resources, we evaluate different volume rendering implementations supported in the ParaView [2] environment: two GPU-based solutions with Kitware's native volume mapper or NVIDIA's IndeX library, and a CPU-only Intel OSPRay-based implementation. Figure 1: Snapshot of the three-dimensional temperature field of sheared thermal convection at Ra = 4.6 × 10 6 and Re w = 6000 [3].

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