Horizontally extended turbulent convection, termed mesoscale convection in natural systems, remains a challenge to investigate in both experiments and simulations. This is particularly so for very ...low molecular Prandtl numbers, such as occur in stellar convection and in the Earth's outer core. The present study reports three-dimensional direct numerical simulations of turbulent Rayleigh–Bénard convection in square boxes of side length $L$ and height $H$ with the aspect ratio $\varGamma =L/H$ of 25, for Prandtl numbers that span almost 4 orders of magnitude, $10^{-3}\le Pr \le 7$, and Rayleigh numbers $10^5 \le Ra \le 10^7$, obtained by massively parallel computations on grids of up to $5.36\times 10^{11}$ points. The low end of this $Pr$-range cannot be accessed in controlled laboratory measurements. We report the essential properties of the flow and their trends with the Rayleigh and Prandtl numbers, in particular, the global transport of momentum and heat – the latter decomposed into convective and diffusive contributions – across the convection layer, mean vertical profiles of the temperature and temperature fluctuations and the kinetic energy and thermal dissipation rates. We also explore the degree to which the turbulence in the bulk of the convection layer resembles classical homogeneous and isotropic turbulence in terms of spectra, increment moments and dissipative anomaly, and find close similarities. Finally, we show that a characteristic scale of the order of the mesoscale seems to saturate to a wavelength of $\lambda \gtrsim 3H$ for $Pr\lesssim 0.005$. We briefly discuss possible implications of these results for the development of subgrid-scale parameterization of turbulent convection.
The interplay between convective, rotational and magnetic forces defines the dynamics within the electrically conducting regions of planets and stars. Yet their triadic effects are separated from one ...another in most studies, arguably due to the richness of each subset. In a single laboratory experiment, we apply a fixed heat flux, two different magnetic field strengths and one rotation rate, allowing us to chart a continuous path through Rayleigh–Bénard convection (RBC), two regimes of magnetoconvection, rotating convection and two regimes of rotating magnetoconvection, before finishing back at RBC. Dynamically rapid transitions are determined to exist between jump rope vortex states, thermoelectrically driven magnetoprecessional modes, mixed wall- and oscillatory-mode rotating convection and a novel magnetostrophic wall mode. Thus, our laboratory ‘pub crawl’ provides a coherent intercomparison of the broadly varying responses arising as a function of the magnetorotational forces imposed on a liquid-metal convection system.
We consider a finite element method which couples the continuous Galerkin method away from internal and boundary layers with a discontinuous Galerkin method in the vicinity of layers. We prove that ...this consistent method is stable in the streamline diffusion norm if the convection field flows non-characteristically from the region of the continuous Galerkin to the region of the discontinuous Galerkin method. The stability properties of the coupled method are illustrated with a numerical experiment.
We studied the influence of the diffusion contrast between species on the dynamics of Rayleigh‐Bénard (RB) convection in porous media. The onset time of buoyancy‐driven instabilities and convective ...dissolution flux was quantified using linear stability analysis and direct numerical simulations. The parametric analysis indicates eight distinct instability regions. Different stability mechanisms were characterized over the given range of diffusivity and relative buoyancy ratios. In particular, transition from instabilities solely by double diffusion to RB convection was identified using linear stability analysis and confirmed using nonlinear simulations. The parametric analysis on the onset also indicates that double diffusion has a potential to accelerate or slow down the RB convection depending on the solutes diffusion contrast. This study provides new insight into the effect of diffusion contrast and can be used to develop strategies for acceleration and deceleration of buoyancy‐driven instabilities.
Key Points
The effect of the diffusion contrast on the dynamics of Rayleigh‐Benard (RB) convection in porous media is studied
Eight stability regions have been identified using linear stability analysis (LSA) and direct numerical simulations (DNS)
Transition from instabilities solely by double diffusion to Rayleigh‐Benard (RB) convection was identified
We numerically study the melting process of a solid layer heated from below such that a liquid melt layer develops underneath. The objective is to quantitatively describe and understand the emerging ...topography of the structures (characterized by the amplitude and wavelength), which evolve out of an initially smooth surface. By performing both two-dimensional (achieving Rayleigh number up to $Ra=10^{11}$) and three-dimensional (achieving Rayleigh number up to $Ra=10^9$) direct numerical simulations with an advanced finite difference solver coupled to the phase-field method, we show how the interface roughness is spontaneously generated by thermal convection. With increasing height of the melt the convective flow intensifies and eventually even becomes turbulent. As a consequence, the interface becomes rougher but is still coupled to the flow topology. The emerging structure of the interface coincides with the regions of rising hot plumes and descending cold plumes. We find that the roughness amplitude scales with the mean height of the liquid layer. We derive this scaling relation from the Stefan boundary condition and relate it to the non-uniform distribution of heat flux at the interface. For two-dimensional cases, we further quantify the horizontal length scale of the morphology, based on the theoretical upper and lower bounds given for the size of convective cells known from Wang et al. (Phys. Rev. Lett., vol. 125, 2020, 074501). These bounds agree with our simulation results. Our findings highlight the key connection between the morphology of the melting solid and the convective flow structures in the melt beneath.
The Tibetan Plateau (TP) plays an essential role in influencing the global climate, and precipitation is one of its most important water‐cycle components. However, accurately simulating precipitation ...over the TP is a long‐standing challenge. In this study, a convection‐permitting model (CPM; with 4 km grid spacing) that covers the entire TP was conducted and compared to two mesoscale models (MSMs; with model horizontal resolutions of 13 and 35 km) over the course of a summer. The results showed that the two MSMs have notable wet biases over the TP and can overestimate the summer precipitation by more than 4.0 mm·day−1 in some parts of the Three Rivers Source region. Moreover, both MSMs have more frequent light rainfall; increasing horizontal resolution of the MSMs alone does not reduce the excessive precipitation. Further investigation reveals that the MSMs have a spurious early‐afternoon rainfall peak, which can be linked to a strong dependence on convective available potential energy (CAPE) that dominates the wet biases. Herein, we highlight that the sensitivity of CAPE to surface temperatures may cause the MSMs to have a spurious hydrological response to surface warming. Users of climate projections should be aware of this potential model uncertainty when investigating future hydrological changes over the TP. In comparison, the CPM removes the spurious afternoon rainfall and thus significantly reduces the wet bias simulated by the MSMs. In addition, the CPM also better depicts the precipitation frequency and intensity, and is therefore a promising tool for dynamic downscaling over the TP.
A convection‐permitting model (CPM) covering the whole Tibetan Plateau is compared with two mesoscale models to investigate the added values of CPM in simulating precipitation through an entire summer.
CPM better reproduces summer precipitation characteristics over the Tibetan Plateau.
Spurious afternoon precipitation is traced to a strong dependence on convective available potential energy which leads to wet biases over the Tibetan Plateau in mesoscale models.
Convection‐permitting modelling improves simulated precipitation over the central and eastern Tibetan Plateau
•Newly developed solar dryer reduces the drying time of stevia leaves by 62%.•Exergy efficiency of solar dryer and drying process are estimated.•Overall solar dryer efficiency was estimated as ...33.5%.•Quality of stevia leaves dried in the solar dryer is better than open sun drying.•Estimated payback period of the newly developed dryer is 0.65 yr.
In the present work, experimental investigations carried out on drying of Stevia leaves in a newly developed solar dryer of mixed mode forced convection type (MFSCD) and open sun drying (OSD) are presented. Experiments have been performed under the average solar radiation of 567 W/m2, ambient temperature of 30 °C and drying air flow rate of 0.049 kg/s. The safe (final) moisture content of stevia leaves 0.053 (d.b) has reached in 330 min and 870 min of drying time in MFSCD and OSD, respectively. The overall dryer efficiency and average exergy efficiency of the MFSCD were found as 33.5% and 59.1%, respectively. Quality analyses were carried out for fresh, open sun-dried and solar dried stevia leave samples. It was found that the anti-oxidant and the flavonoids were rich in solar dried samples compared to that of OSD samples. The color preservation is good in solar dried samples compared to OSD. Sensory analysis (flavor, aroma and taste) carried out on stevia leaves indicated that the solar dried stevia leaves provided better score compared to OSD samples. The estimated payback period of the newly developed dryer was found as 0.65 yr.
Recently, in Zhang et al. (Phys. Rev. Lett., vol. 124, 2020, 084505), it was found that, in rapidly rotating turbulent Rayleigh–Bénard convection in slender cylindrical containers (with ...diameter-to-height aspect ratio $\varGamma =1/2$) filled with a small-Prandtl-number fluid (${Pr}\approx 0.8$), the large-scale circulation is suppressed and a boundary zonal flow (BZF) develops near the sidewall, characterized by a bimodal probability density function of the temperature, cyclonic fluid motion and anticyclonic drift of the flow pattern (with respect to the rotating frame). This BZF carries a disproportionate amount (${>}60\,\%$) of the total heat transport for ${Pr} < 1$, but decreases rather abruptly for larger ${Pr}$ to approximately $35\,\%$. In this work, we show that the BZF is robust and appears in rapidly rotating turbulent Rayleigh–Bénard convection in containers of different $\varGamma$ and over a broad range of ${Pr}$ and ${Ra}$. Direct numerical simulations for Prandtl number $0.1 \leq {\textit {Pr}} \leq 12.3$, Rayleigh number $10^7 \leq {Ra} \leq 5\times 10^{9}$, inverse Ekman number $10^{5} \leq 1/{\textit {Ek}} \leq 10^{7}$ and $\varGamma = 1/3$, 1/2, 3/4, 1 and 2 show that the BZF width $\delta _0$ scales with the Rayleigh number ${Ra}$ and Ekman number ${\textit {Ek}}$ as $\delta _0/H \sim \varGamma ^{0} Pr^{\{-1/4, 0\}} {Ra}^{1/4} {\textit {Ek}}^{2/3}$ ($\{{\textit {Pr}}<1, {\textit {Pr}}>1\}$) and with the drift frequency scales as $\omega /\varOmega \sim \varGamma ^{0} Pr^{-4/3} {Ra}\,{\textit {Ek}}^{5/3}$, where $H$ is the cell height and $\varOmega$ the angular rotation rate. The mode number of the BZF is 1 for $\varGamma \lesssim 1$ and $2 \varGamma$ for $\varGamma = \{1,2\}$ independent of ${Ra}$ and ${Pr}$. The BZF is quite reminiscent of wall mode states in rotating convection.
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Analytical solutions may not always be applicable in the calculation of the turbulent flow convective heat transfer rate, unlike the radiative and the conductive ones. Therefore, ...experimental correlations on convective heat transfer coefficient have been developed in enclosures. Convective heat transfer in a cavity is classified as natural, forced, and mixed convection on the basis of the driving forces (buoyant or mechanical). In recent years, there has been an increasing interest in the mixed convection, particularly in cooled ceiling – displacement ventilation indoor applications. It seems this interest is tending to increase while contamination of viruses and energy saving are growing as health and environmental concerns worldwide. Hence, the reasons behind the interest in mixed convection applications have been investigated along the paper.
This comparative study seeks to explain the progress of convective heat transfer at indoor applications by reviewing mostly experimental correlation studies in time. The mixed convection has not been widely studied experimentally in indoor applications in comparison to natural and forced convection. Therefore, this study is devoted to indicate this gap in the literature on this issue and it includes all convection types with a wide and up-to-date review, descriptions, explanations, and comparisons, as well. Moreover, almost all empirical correlations on the topic are given in tables in detail. It can be concluded that general correlations for mixed convection applications is needed. Correlations related to radiant floor cooling applications are nearly non-existent. Additionally, more experimental studies are required for various split air conditioner cases. These gaps in the literature are unveiled and comparison of applications with various convection types have been made as a first comparative study in the literature.
We report on turbulent thermal convection experiments in a rotating cylinder with a diameter ($D$) to height ($H$) aspect ratio of $\varGamma =D/H=0.5$. Using nitrogen and pressurised sulphur ...hexafluoride we cover Rayleigh numbers (Ra) from $8\times 10^{9}$ to $8\times 10^{14}$ at Prandtl numbers $0.72\lesssim {\textit {Pr}}\lesssim 0.94$. For these Ra we measure the global vertical heat flux (i.e. the Nusselt number – Nu), as well as statistical quantities of local temperature measurements, as a function of the rotation rate, i.e. the inverse Rossby number – 1/Ro. In contrast to measurements in fluids with a higher Pr we do not find a heat transport enhancement, but only a decrease of Nu with increasing 1/Ro. When normalised with Nu(0) for the non-rotating system, data for all different Ra collapse and, for sufficiently large 1/Ro, follow a power law ${\textit {Nu}}/{\textit {Nu}}_0\propto (1/{\textit {Ro}})^{-0.43}$. Furthermore, we find that both the heat transport and the temperature field qualitatively change at rotation rates $1/{\textit {Ro}}^*_1=0.8$ and $1/{\textit {Ro}}^*_2=4$. We interpret the first transition at $1/{\textit {Ro}}^*_1$ as change from a large-scale circulation roll to the recently discovered boundary zonal flow (BZF). The second transition at rotation rate $1/{\textit {Ro}}^*_2$ is not associated with a change of the flow morphology, but is rather the rotation rate for which the BZF is at its maximum. For faster rotation the vertical transport of warm and cold fluid near the sidewall within the BZF decreases and hence so does Nu.
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