•Bénard-Marangoni convection cells as well as hydrothermal waves were observed in an evaporating sessile droplet.•Marangoni instabilities occur even when substrate temperature is lower than room ...temperature.•The critical conditions for the incipience of Marangoni instabilities were determined.
Marangoni instabilities of a sessile droplet of 0.65 cSt silicone oil evaporating at constant contact line mode are experimentally investigated in a wide range of the substrate temperatures (Tw) from those lower than room temperature (Ta) to those higher than it. For Tw > Ta, the Bénard-Marangoni convection cells are observed and the cell patterns vary from the quasi-steady state to irregular oscillatory state with the droplet evaporation. For Tw < Ta, the irregular oscillatory Bénard-Marangoni cells are observed when Tw is higher than 10.6 °C. The cell size is larger while there are less cell numbers compared to that of Tw > Ta. When Tw is higher than 14.3 °C three kinds of convection mode, i.e. the travelling hydrothermal waves, the coexistence of hydrothermal waves and irregular oscillatory Bénard-Marangoni convection, and the irregular oscillatory Bénard-Marangoni convection, occur successively with droplet evaporating. The critical contact angles for the incipience of these Marangoni instabilities are determined.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
32.
Thermal convection over fractal surfaces Toppaladoddi, Srikanth; Wells, Andrew J.; Doering, Charles R. ...
Journal of fluid mechanics,
2020, Volume:
907
Journal Article
Peer reviewed
Open access
We use well resolved numerical simulations with the lattice Boltzmann method to study Rayleigh–Bénard convection in cells with a fractal boundary in two dimensions for $Pr = 1$ and $Ra \in \left ...10^7, 10^{10}\right $, where Pr and Ra are the Prandtl and Rayleigh numbers. The fractal boundaries are functions characterized by power spectral densities $S(k)$ that decay with wavenumber, $k$, as $S(k) \sim k^{p}$ ($p < 0$). The degree of roughness is quantified by the exponent $p$ with $p < -3$ for smooth (differentiable) surfaces and $-3 \le p < -1$ for rough surfaces with Hausdorff dimension $D_f=\frac {1}{2}(p+5)$. By computing the exponent $\beta$ using power law fits of $Nu \sim Ra^{\beta }$, where $Nu$ is the Nusselt number, we find that the heat transport scaling increases with roughness through the top two decades of $Ra \in \left 10^8, 10^{10}\right $. For $p$ $= -3.0$, $-2.0$ and $-1.5$ we find $\beta = 0.288 \pm 0.005, 0.329 \pm 0.006$ and $0.352 \pm 0.011$, respectively. We also find that the Reynolds number, $Re$, scales as $Re \sim Ra^{\xi }$, where $\xi \approx 0.57$ over $Ra \in \left 10^7, 10^{10}\right $, for all $p$ used in the study. For a given value of $p$, the averaged $Nu$ and $Re$ are insensitive to the specific realization of the roughness.
Macroscopic and microscopic drying patterns were observed on a cover glass, a watch glass, and a Petri glass dish during the course of dryness of aqueous solutions of I--cyclodextrin (I-CD), beta ...-cyclodextrin ( beta CD), and I3-cyclodextrin (I3CD), i.e., cone shape oligomers of polysaccharide. For all CD molecules, two kinds of macroscopic patterns, outside and inner broad rings and spoke lines formed. Multi-broad rings were formed for beta CD in the inner region of the main broad ring at the outside edge especially at the high concentrations. Cooperative drying processes of the convection, sedimentation, and solidification were clarified. Microscopic drying patterns showing the formation of rod-like and/or sward-like crystals were observed mainly in the direction along the spoke lines. The microscopic patterns of beta CD were similar to those of some of polysaccharides and polynucleotides the authors studied previously. I-- and I3-cyclodextrins were slightly hygroscopic, and clear-cut drying patterns were not observed.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
•CFD simulation of a finned triplex tube heat exchanger with PCM under simultaneous charging and discharging.•Developed fin configurations for SCD, compatible with natural convection.•More fins ...enhanced the heat transfer as long as natural convection was not suppressed.•Longer fins enhanced the heat transfer as long as natural convection was not suppressed.•The effect of fin thickness was negligible, similar to non-SCD conditions.
Due to the inherent intermittency of renewable energy sources such as solar, latent heat thermal energy storage in phase change materials (PCMs) has received considerable attention. Among several techniques to enhance PCMs’ thermal conductivity, the majority of studies have focused on fin integration due to its simplicity, ease of manufacturing, and low cost. In this study, utilization of extended surfaces (by longitudinal fins) was investigated by development of a numerical model to study the performance of a triplex tube heat exchanger (TTHX) equipped with a PCM under simultaneous charging and discharging (SCD). Governing equations were developed and numerically solved using ANSYS Fluent v16.2. Three conventional fin geometries and six developed fin configurations were compared based on the temperature, liquid fraction, and natural convection behavior under both SCD and non-SCD conditions. The intensity of natural convection was investigated for different fins for the inside heating/outside cooling scenario based on the solid–liquid interface evolution over time. The results indicated that since the buoyancy forces induce upward melted PCM motion, the inner hot tube requires fins on its lower half, while the outer cold one should be extended from its upper half. It was concluded that the case with 3 hot tube fins and 1 cold tube fin is most compatible with natural convection and provides the best performance under SCD conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
35.
Rotation suppresses giant-scale solar convection Vasil, Geoffrey M.; Julien, Keith; Featherstone, Nicholas A.
Proceedings of the National Academy of Sciences - PNAS,
08/2021, Volume:
118, Issue:
31
Journal Article
Peer reviewed
Open access
Significance
The entire Sun completes a full rotation in roughly 28 d. Within the outer 30% of the solar interior, turbulent thermal convection powers fluid outward. Rotation deflects the fluid and ...determines the morphology of eddies and large-scale shear. Such flows are the ultimate agents of astrophysical and planetary magnetic field generation, one of the most important open problems in all of science. Our results make theoretical predictions regarding the Sun’s internal flow structure and rotational constraint. We predict tall and slender vortices persisting throughout much of the convection zone under the sway of strong rotation. We also clarify previous observational discrepancies and explain why such structures have been hard to reproduce in numerical simulations.
The observational absence of giant convection cells near the Sun’s outer surface is a long-standing conundrum for solar modelers. We herein propose an explanation. Rotation strongly influences the internal dynamics, leading to suppressed convective velocities, enhanced thermal-transport efficiency, and (most significantly) relatively smaller dominant length scales. We specifically predict a characteristic convection length scale of roughly 30-Mm throughout much of the convection zone, implying weak flow amplitudes at 100- to 200-Mm giant cells scales, representative of the total envelope depth. Our reasoning is such that Coriolis forces primarily balance pressure gradients (geostrophy). Background vortex stretching balances baroclinic torques. Both together balance nonlinear advection. Turbulent fluxes convey the excess part of the solar luminosity that radiative diffusion cannot. We show that these four relations determine estimates for the dominant length scales and dynamical amplitudes strictly in terms of known physical quantities. We predict that the dynamical Rossby number for convection is less than unity below the near-surface shear layer, indicating rotational constraint.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Purpose of Review
Global cloud-resolving models (GCRMs) are a new type of atmospheric model which resolve nonhydrostatic accelerations globally with kilometer-scale resolution. This review explains ...what distinguishes GCRMs from other types of models, the problems they solve, and the questions their more commonplace use is raising.
Recent Findings
GCRMs require high-resolution discretization over the sphere but can differ in many other respects. They are beginning to be used as a main stream research tool. The first GCRM intercomparison studies are being coordinated, raising new questions as to how best to exploit their advantages.
Summary
GCRMs are designed to resolve the multiscale nature of moist convection in the global dynamics context, without using cumulus parameterization. Clouds are simulated with cloud microphysical schemes in ways more comparable to observations. Because they do not suffer from ambiguity arising from cumulus parameterization, as computational resources increase, GCRMs are the promise of a new generation of global weather and climate simulations.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The dominant modes of convection (vertical motion) in the tropical convective regions were explored using the EOF analysis method and the ERA-interim reanalysis data, with an emphasis on comparing ...their roles in transporting moisture and moist static energy (MSE). Two major types of convection orthogonal to each other were identified in the deep tropics, including deep and shallow convection. The deep convection, derived from the 1st EOF mode of vertical motion, shows different vertical structures (e.g., top-heavy versus bottom-heavy) under different thermodynamic environments (e.g., west versus east Pacific); while the shallow convection, derived from the 2nd EOF mode of vertical motion, exhibits opposite signs of vertical structure (e.g., positive and negative) between upper and lower troposphere. The two subtypes of deep convection exhibit distinct tendencies in moisture and MSE transport, i.e., while both import moisture into the atmosphere, the top-heavy one tends to export MSE, leading to a stabilized atmosphere; whereas the bottom-heavy counterpart inclines to import MSE, resulting in a destabilized atmosphere. On the contrary, the two subtypes of shallow convection display a consistent tendency in moisture and MSE transport, i.e., the positive shallow convection imports moisture and MSE; whereas the negative counterpart exports moisture and MSE. More importantly, the MSE transport by shallow convection (including positive and negative ones) can be comparable in size with the MSE transport by deep convection (including top-heavy and bottom-heavy ones), implying a non-negligible role played by shallow convection in charging and discharging the atmosphere in addition to the commonly recognized deep convection.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
This work reviews present knowledge of double-diffusive convection at low Prandtl number obtained using direct numerical simulations, in both the fingering regime and the oscillatory regime. ...Particular emphasis is given to modeling the induced turbulent mixing and its impact in various astrophysical applications. The nonlinear saturation of fingering convection at low Prandtl number usually drives small-scale turbulent motions whose transport properties can be predicted reasonably accurately using a simple semi-analytical model. In some instances, large-scale internal gravity waves can be excited by a collective instability and eventually cause layering. The nonlinear saturation of oscillatory double-diffusive convection exhibits much more complex behavior. Weakly stratified systems always spontaneously transition into layered convection associated with very efficient mixing. More strongly stratified systems remain dominated by weak wave turbulence unless they are initialized into a layered state. The effects of rotation, shear, lateral gradients, and magnetic fields are briefly discussed.
A theoretical model is derived to predict the heat fluxes at the upper and lower horizontal surfaces of an internally heated (IH) convection cell by extending the well-known Grossmann & Lohse (Phys. ...Rev. Lett., vol. 86, 2001, pp. 3316–3319) theory. The approach of Creyssels (J. Fluid Mech., vol. 900, 2020, p. A39) is generalized for a fluid heated internally and uniformly, confined between top and bottom plates of equal temperature. For each plate, a Nusselt number is defined and an analytical formula is given to predict its variations with the Rayleigh and Prandtl numbers. The turbulent flow produced in the upper half of the IH convection cell is very similar to that observed in standard Rayleigh–Bénard convection. On the contrary, the lower plate is swept by the large scale flow that circulates through the entire cell. The corresponding boundary layer is therefore modelled by a laminar boundary layer of the Blasius type. These predictions are consistent with the independent theoretical scalings proposed by Wang et al. (Geophys. Res. Lett., vol. 48, issue 4, 2021, p. e2020GL091198) and they are confirmed by the numerical results obtained by Goluskin & van der Poel (J. Fluid Mech., vol. 791, 2016, p. R6) and Wang et al. (Geophys. Res. Lett., vol. 48, issue 4, 2021, p. e2020GL091198).
We studied the flow organization and heat transfer properties in two-dimensional and three-dimensional Rayleigh–Bénard cells that are imposed with different types of wall shear. The external wall ...shear is added with the motivation of manipulating flow mode to control heat transfer efficiency. We imposed three types of wall shear that may facilitate the single-roll, the horizontally stacked double-roll, and the vertically stacked double-roll flow modes, respectively. Direct numerical simulations are performed for fixed Rayleigh number $Ra = 10^{8}$ and fixed Prandtl number $Pr = 5.3$, while the wall-shear Reynolds number ($Re_{w}$) is in the range $60 \leqslant Re_{w} \leqslant 6000$. Generally, we found enhanced heat transfer efficiency and global flow strength with the increase of $Re_{w}$. However, even with the same magnitude of global flow strength, the heat transfer efficiency varies significantly when the cells are under different types of wall shear. An interesting finding is that by increasing the wall-shear strength, the thermal turbulence is relaminarized, and more surprisingly, the heat transfer efficiency in the laminar state is higher than that in the turbulent state. We found that the enhanced heat transfer efficiency at the laminar regime is due to the formation of more stable and stronger convection channels. We propose that the origin of thermal turbulence laminarization is the reduced amount of thermal plumes. Because plumes are mainly responsible for turbulent kinetic energy production, when the detached plumes are swept away by the wall shear, the reduced number of plumes leads to weaker turbulent kinetic energy production. We also quantify the efficiency of facilitating heat transport via external shearing, and find that for larger $Re_{w}$, the enhanced heat transfer efficiency comes at a price of a larger expenditure of mechanical energy.
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