Influence of container shape on scaling of turbulent fluctuations in convection

Foroozani, Najmeh; Niemela, Joseph; Armenio, Vincenzo; Sreenivasan, K. R.

doi:10.1103/physreve.90.063003

Cited by 21 publications

(18 citation statements)

References 14 publications

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“…LSC states with the qualitative features described above have previously been found in laboratory experiments (Bai et al 2016), as well as in large-eddy simulations (Foroozani et al 2014(Foroozani et al , 2017, of turbulent Rayleigh-Bénard convection in a cubical domain, both at higher Rayleigh numbers. In addition, the experiments were run over longer time intervals than our DNS study.…”

Section: Primary Eigenfunctionssupporting

confidence: 70%

“…While the LSC roll drifts slowly in the azimuthal direction in a cylinder (Shi et al 2012) -the only remaining statistically homogeneous direction -it gets locked in diagonal corners or parallel to the side walls in cubic and rectangular containers. In a cubic container, the LSC switches rather rapidly from one into another of the eight possible macroscopic flow states (Foroozani et al 2014;Bai et al 2016;Foroozani et al 2017). The dynamics is then determined by discrete symmetries such as rotations by 90, 180, 270, and 360 degrees.…”

Section: Introductionmentioning

confidence: 99%

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Koopman analysis of the long-term evolution in a turbulent convection cell

et al. 2018

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We analyse the long-time evolution of the three-dimensional flow in a closed cubic turbulent Rayleigh-Bénard convection cell via a Koopman eigenfunction analysis. A datadriven basis derived from diffusion kernels known in machine learning is employed here to represent a regularized generator of the unitary Koopman group in the sense of a Galerkin approximation. The resulting Koopman eigenfunctions can be grouped into subsets in accordance with the discrete symmetries in a cubic box. In particular, a projection of the velocity field onto the first group of eigenfunctions reveals the four stable large-scale circulation (LSC) states in the convection cell. We recapture the preferential circulation rolls in diagonal corners and the short-term switching through roll states parallel to the side faces which have also been seen in other simulations and experiments. The diagonal macroscopic flow states can last as long as a thousand convective free-fall time units. In addition, we find that specific pairs of Koopman eigenfunctions in the secondary subset obey enhanced oscillatory fluctuations for particular stable diagonal states of the LSC. The corresponding velocity field structures, such as corner vortices and swirls in the midplane, are also discussed via spatiotemporal reconstructions.

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Section: Primary Eigenfunctionssupporting

confidence: 70%

Section: Introductionmentioning

confidence: 99%

Koopman analysis of the long-term evolution in a turbulent convection cell

et al. 2018

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“…Based on the geometry of the cell (shape and aspect ratio) and the tendency of the LSC to extend along its largest spatial distance that corresponds to the most stable mode [37], the large-scale coherent flow evolves along a diagonal plane of the cell [24,28,[38][39][40][41]. Therefore, the reported planar PIV flow measurements are projections of the diagonal flow field on the measurement plane, as seen through the square window depicted in Fig.…”

Section: Resultsmentioning

confidence: 99%

Assessing the flow characteristics of nanofluids during turbulent natural convection

Kouloulias

Sergis

Hardalupas

2018

J Therm Anal Calorim

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High-performance cooling is of vital importance for the cutting-edge technology of today, from nanoelectronic mechanical systems to nuclear reactors. Advances in nanotechnology have allowed the development of a new category of coolants, termed nanofluids that have the potential to enhance the thermal performance of conventional heat transfer fluids. At the present time, nanofluids are a controversial research theme, since there is yet no conclusive answer to explain the underlying physical mechanisms of heat transfer. The current study investigates experimentally the heat and mass transfer behaviour of dilute Al 2 O 3 -H 2 O nanofluids under turbulent natural convection-Rayleigh number of the order of 10 9 -in a cubic Rayleigh-Bénard cell with optical access. Traditional heat transfer measurements were combined with a velocimetry method to obtain a deeper understanding of the impact of nanoparticles on the heat transfer performance of the base fluid. Particle image velocimetry was employed to quantify the resulting mean velocity field and flow structures in dilute nanofluids under natural convection, at three parallel planes inside the cubic cell. All the results were compared with that for the base fluid, i.e. deionised water. It was observed that the presence of a minute amount of Al 2 O 3 nanoparticles in deionised water, u v = 0.00026 vol.%, considerably modifies the mass transfer behaviour of the fluid in the bulk region of turbulent Rayleigh-Bénard convection. Simultaneously, the general heat transport, as expressed by the Nusselt number, remained unaffected within the experimental uncertainty.

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“…The former can afford low-to-moderate values of Rayleigh number, the latter can push the limit of Ra to larger values. LES of RBC in cubic cavities have been performed by [39] and by [40] in the presence of grooves.…”

Section: Les Of Rayleigh-benard Convectionmentioning

confidence: 99%

Assessment of Solution Algorithms for LES of Turbulent Flows Using OpenFOAM

Castaño

Petronio²,

Armenio

2019

Fluids

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We validate and test two algorithms for the time integration of the Boussinesq form of the Navier—Stokes equations within the Large Eddy Simulation (LES) methodology for turbulent flows. The algorithms are implemented in the OpenFOAM framework. From one side, we have implemented an energy-conserving incremental-pressure Runge–Kutta (RK4) projection method for the solution of the Navier–Stokes equations together with a dynamic Lagrangian mixed model for momentum and scalar subgrid-scale (SGS) fluxes; from the other side we revisit the PISO algorithm present in OpenFOAM (pisoFoam) in conjunction with the dynamic eddy-viscosity model for SGS momentum fluxes and a Reynolds Analogy for the scalar SGS fluxes, and used for the study of turbulent channel flows and buoyancy-driven flows. In both cases the validity of the anisotropic filter function, suited for non-homogeneous hexahedral meshes, has been studied and proven to be useful for industrial LES. Preliminary tests on energy-conservation properties of the algorithms studied (without the inclusion of the subgrid-scale models) show the superiority of RK4 over pisoFoam, which exhibits dissipative features. We carried out additional tests for wall-bounded channel flow and for Rayleigh–Bènard convection in the turbulent regime, by running LES using both algorithms. Results show the RK4 algorithm together with the dynamic Lagrangian mixed model gives better results in the cases analyzed for both first- and second-order statistics. On the other hand, the dissipative features of pisoFoam detected in the previous tests reflect in a less accurate evaluation of the statistics of the turbulent field, although the presence of the subgrid-scale model improves the quality of the results compared to a correspondent coarse direct numerical simulation. In case of Rayleigh–Bénard convection, the results of pisoFoam improve with increasing values of Rayleigh number, and this may be attributed to the Reynolds Analogy used for the subgrid-scale temperature fluxes. Finally, we point out that the present analysis holds for hexahedral meshes. More research is need for extension of the methods proposed to general unstructured grids.

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Influence of container shape on scaling of turbulent fluctuations in convection

Cited by 21 publications

References 14 publications

Koopman analysis of the long-term evolution in a turbulent convection cell

Koopman analysis of the long-term evolution in a turbulent convection cell

Assessing the flow characteristics of nanofluids during turbulent natural convection

Assessment of Solution Algorithms for LES of Turbulent Flows Using OpenFOAM

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