55 research outputs found
Inhomogeneous turbulence in the vicinity of a large scale coherent vortex
We study the statistics of turbulent velocity fluctuations in the
neighbourhood of a strong large scale vortex at very large Reynolds number. At
each distance from the vortex core, we observe that the velocity spectrum has a
power law ``inertial range'' of scales and that intermittency -- defined as the
variation of the probability density function (PDF) of velocity increments as
the length of the increment is varied -- is also present. We show that the
spectrum scaling exponents and intermittency characteristics vary with the
distance to the vortex. They are also influenced by the large scale dynamics of
the vortex.Comment: submitted to europhys lett, 6 pages, 5 figure
New subgrid-scale models for large-eddy simulation of Rayleigh-BĂ©nard convection
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.At the crossroad between flow topology analysis and the theory of turbulence, a new eddy-viscosity model for Large-eddy simulation has been recently proposed by Trias et al.[PoF, 27, 065103 (2015)]. The S3PQR-model has the proper cubic near-wall behaviour and no intrinsic limitations for statistically inhomogeneous flows. In this work, the new model has been tested for an air turbulent Rayleigh-Benard convection in a rectangular cell of aspect ratio unity and n span-wise open-ended distance. To do so, direct numerical simulation has been carried out at two Rayleigh numbers Ra = 108 and 1010, to assess the model performance and investigate a priori the effect of the turbulent Prandtl number. Using an approximate formula based on the Taylor series expansion, the turbulent Prandtl number has been calculated and revealed a constant and Ra-independent value across the bulk region equals to 0.55. It is found that the turbulent components of eddy-viscosity and eddy-diffusivity are positively prevalent to maintain a turbulent wind essentially driven by the mean buoyant force at the sidewalls. On the other hand, the new eddy-viscosity model is preliminary tested for the case of Ra = 108 and showed overestimation of heat flux within the boundary layer but fairly good prediction of turbulent kinetics at this moderate turbulent flow.Peer ReviewedPostprint (published version
Large scale dynamics in turbulent Rayleigh-Benard convection
The progress in our understanding of several aspects of turbulent
Rayleigh-Benard convection is reviewed. The focus is on the question of how the
Nusselt number and the Reynolds number depend on the Rayleigh number Ra and the
Prandtl number Pr, and on how the thicknesses of the thermal and the kinetic
boundary layers scale with Ra and Pr. Non-Oberbeck-Boussinesq effects and the
dynamics of the large-scale convection-roll are addressed as well. The review
ends with a list of challenges for future research on the turbulent
Rayleigh-Benard system.Comment: Review article, 34 pages, 13 figures, Rev. Mod. Phys. 81, in press
(2009
A rotating annulus driven by localized convective forcing: a new atmosphere-like experiment
We present an experimental study of flows in a
cylindrical rotating annulus convectively forced by local heating
in an annular ring at the bottom near the external wall
and via a cooled circular disk near the axis at the top surface
of the annulus. This new configuration is distinct from
the classical thermally-driven annulus analogue of the atmosphere
circulation, in which thermal forcing is applied
uniformly on the sidewalls, but with a similar aim to investigate
the baroclinic instability of a rotating, stratified
flow subject to zonally symmetric forcing. Two vertically
and horizontally displaced heat sources/sinks are arranged
so that, in the absence of background rotation, statically unstable
Rayleigh-BĂ©nard convection would be induced above
the source and beneath the sink, thereby relaxing strong constraints
placed on background temperature gradients in previous
experimental configurations based on the conventional
rotating annulus. This better emulates local vigorous convection
in the tropics and polar regions of the atmosphere
whilst also allowing stably-stratified baroclinic motion in
the central zone of the annulus, as in midlatitude regions in
the Earth’s atmosphere. Regimes of flow are identified, depending
mainly upon control parameters that in turn depend
on rotation rate and the strength of differential heating. Several
regimes exhibit baroclinically unstable flows which are
qualitatively similar to those previously observed in the classical
thermally-driven annulus, However, in contrast to the
classical configuration, they typically exhibit more spatiotemporal
complexity. Thus, several regimes of flow demonstrate the equilibrated co-existence of, and interaction between,
free convection and baroclinic wave modes. These
new features were not previously observed in the classical
annulus and validate the new setup as a tool for exploring
fundamental atmosphere-like dynamics in a more realistic
framework. Thermal structure in the fluid is investigated and
found to be qualitatively consistent with previous numerical
results, with nearly isothermal conditions respectively above
and below the heat source and sink, and stably-stratified,
sloping isotherms in the near-adiabatic interior
New perspectives in turbulent Rayleigh-BĂ©nard convection
Recent experimental, numerical and theoretical advances in turbulent Rayleigh-BĂ©nard convection are presented. Particular emphasis is given to the physics and structure of the thermal and velocity boundary layers which play a key role for the better understanding of the turbulent transport of heat and momentum in convection at high and very high Rayleigh numbers. We also discuss important extensions of Rayleigh-BĂ©nard convection such as non-Oberbeck-Boussinesq effects and convection with phase changes
Transmission of sound through a single vortex
We investigate experimentally the deformation of acoustic wavefronts after crossing of a single, isolated vortex in free space. The incident sound wavelength can be varied in a large domain. We study the wavefronts at variable distance after transmission through the vortex, when the wavelength and the vortex strength are varied. For small wavelength (, the vortex core size) our results are in very good agreement with predictions and simulations based on geometrical acoustics principles. However, as the sound wavelength increases to value comparable with the vortex diameter, the deformation of the wavefronts show the development of scattering contributions, with characteristics in agreement with recent theoretical and numerical studies
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