4,827 research outputs found
Mean Temperature Profiles in Turbulent Thermal Convection
To predict the mean temperature profiles in turbulent thermal convection, the
thermal boundary layer (BL) equation including the effects of fluctuations has
to be solved. In Shishkina et al., Phys. Rev. Lett. 114 (2015), the thermal BL
equation with the fluctuations taken into account as an eddy thermal
diffusivity has been solved for large Prandtl-number fluids for which the eddy
thermal diffusivity and the velocity field can be approximated respectively as
a cubic and a linear function of the distance from the plate. In the present
work we make use of the idea of Prandtl's mixing length model and relate the
eddy thermal diffusivity to the stream function. With this proposed relation,
we can solve the thermal BL equation and obtain a closed-form expression for
the dimensionless mean temperature profile in terms of two independent
parameters for fluids with a general Prandtl number. With a proper choice of
the parameters, our predictions of the temperature profiles are in excellent
agreement with the results of our direct numerical simulations for a wide range
of Prandtl numbers from 0.01 to 2547.9 and Rayleigh numbers from 10^7 to 10^9.Comment: 8 pages, 4 figure
Law of the wall in an unstably stratified turbulent channel flow
We perform direct numerical simulations of an unstably stratified turbulent
channel flow to address the effects of buoyancy on the boundary layer dynamics
and mean field quantities. We systematically span a range of parameters in the
space of friction Reynolds number () and Rayleigh number (). Our
focus is on deviations from the logarithmic law of the wall due to buoyant
motion. The effects of convection in the relevant ranges are discussed
providing measurements of mean profiles of velocity, temperature and Reynolds
stresses as well as of the friction coefficient. A phenomenological model is
proposed and shown to capture the observed deviations of the velocity profile
in the log-law region from the non-convective case
New subgrid-scale models for large-eddy simulation of Rayleigh-Bénard 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 instabilities in a non-rotating turbulent convection
Formation of large-scale coherent structures in a turbulent convection via
excitation of large-scale instability is studied. The redistribution of the
turbulent heat flux due to non-uniform large-scale motions plays a crucial role
in the formation of the coherent large-scale structures in the turbulent
convection. The modification of the turbulent heat flux results in strong
reduction of the critical Rayleigh number (based on the eddy viscosity and
turbulent temperature diffusivity) required for the excitation of the
large-scale instability. The mean-field equations which describe the
large-scale instability, are solved numerically. We determine the key
parameters that affect formation of the large-scale coherent structures in the
turbulent convection. In particular, the degree of thermal anisotropy and the
lateral background heat flux strongly modify the growth rates of the
large-scale instability, the frequencies of the generated convective-shear
waves and change the thresholds required for the excitation of the large-scale
instability. This study elucidates the origins of the large-scale circulations
and rolls in the atmospheric convective boundary layers and the meso-granular
structures in the solar convection.Comment: 13 pages, 13 figures, Physics of Fluids, in pres
Scaling laws for convection and jet speeds in the giant planets
Three-dimensional studies of convection in deep spherical shells have been
used to test the hypothesis that the strong jet streams on Jupiter, Saturn,
Uranus, and Neptune result from convection throughout the molecular envelopes.
Due to computational limitations, these simulations must adopt viscosities and
heat fluxes many orders of magnitude larger than the planetary values. Several
numerical investigations have identified trends for how the mean jet speed
varies with heat flux and viscosity, but no previous theories have been
advanced to explain these trends. Here, we show using simple arguments that if
convective release of potential energy pumps the jets and viscosity damps them,
the mean jet speeds split into two regimes. When the convection is weakly
nonlinear, the equilibrated jet speeds should scale approximately with F/nu,
where F is the convective heat flux and nu is the viscosity. When the
convection is strongly nonlinear, the jet speeds are faster and should scale
approximately as (F/nu)^{1/2}. We demonstrate how this regime shift can
naturally result from a shift in the behavior of the jet-pumping efficiency
with heat flux and viscosity. Moreover, the simulations hint at a third regime
where, at sufficiently small viscosities, the jet speed becomes independent of
the viscosity. We show based on mixing-length estimates that if such a regime
exists, mean jet speeds should scale as heat flux to the 1/4 power. Our
scalings provide a good match to the mean jet speeds obtained in previous
Boussinesq and anelastic, three-dimensional simulations of convection within
giant planets over a broad range of parameters. When extrapolated to the real
heat fluxes, these scalings suggest that the mass-weighted jet speeds in the
molecular envelopes of the giant planets are much weaker--by an order of
magnitude or more--than the speeds measured at cloud level.Comment: 23 pages, 10 figures, in press at Icaru
Nonlinear diffusion model for Rayleigh-Taylor mixing
The complex evolution of turbulent mixing in Rayleigh-Taylor convection is
studied in terms of eddy diffusiviy models for the mean temperature profile. It
is found that a non-linear model, derived within the general framework of
Prandtl mixing theory, reproduces accurately the evolution of turbulent
profiles obtained from numerical simulations. Our model allows to give very
precise predictions for the turbulent heat flux and for the Nusselt number in
the ultimate state regime of thermal convection.Comment: 4 pages, 4 figure, PRL in pres
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