| 1. |
- Doering, C. R., et al.
(författare)
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Absence of Evidence for the Ultimate Regime in Two-Dimensional Rayleigh-Benard Convection
- 2019
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 123:25
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Owing to technological advances, the number of exoplanets discovered has risen dramatically in the last few years. However, when trying to observe Earth analogs, it is often difficult to test the veracity of detection. We have developed a new approach to the analysis of exoplanetary spectral observations based on temporal multifractality, which identifies timescales that characterize planetary orbital motion around the host star and those that arise from stellar features such as spots. Without fitting stellar models to spectral data, we show how the planetary signal can be robustly detected from noisy data using noise amplitude as a source of information. For observation of transiting planets, combining this method with simple geometry allows us to relate the timescales obtained to primary and secondary eclipse of the exoplanets. Making use of data obtained with ground-based and space-based observations we have tested our approach on HD 189733b. Moreover, we have investigated the use of this technique in measuring planetary orbital motion via Doppler shift detection. Finally, we have analyzed synthetic spectra obtained using the SOAP 2.0 tool, which simulates a stellar spectrum and the influence of the presence of a planet or a spot on that spectrum over one orbital period. We have demonstrated that, so long as the signal-to-noise-ratio >= 75, our approach reconstructs the planetary orbital period, as well as the rotation period of a spot on the stellar surface.
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| 2. |
- Ravichandran, Sandhanakrishnan, et al.
(författare)
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The combined effects of buoyancy, rotation, and shear on phase boundary evolution
- 2022
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 941
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Tidskriftsartikel (refereegranskat)abstract
- We use well-resolved numerical simulations to study the combined effects of buoyancy, pressure-driven shear and rotation on the melt rate and morphology of a layer of pure solid overlying its liquid phase in three dimensions at a Rayleigh number Ra = 1.25 x 10(5) . During thermal convection, we find that the rate of melting of the solid phase varies non-monotonically with the strength of the imposed shear flow. In the absence of rotation, depending on whether buoyancy or shear dominates the flow, we observe either domes or ridges aligned in the direction of the shear flow, respectively. Furthermore, we show that the geometry of the phase boundary has important effects on the magnitude and evolution of the heat flux in the liquid layer. In the presence of rotation, the strength of which is characterized by the Rossby number, Ro, we observe that for Ro = O(1), the mean flow in the interior is perpendicular to the direction of the constant horizontal applied pressure gradient. As the magnitude of this pressure gradient increases, the geometry of solid-liquid interface evolves from the voids characteristic of melting by rotating convection, to grooves oriented perpendicular or obliquely to the direction of the pressure gradient.
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| 3. |
- Toppaladoddi, Srikanth, et al.
(författare)
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Penetrative convection at high Rayleigh numbers
- 2018
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Ingår i: Physical review fluids. - : American Physical Society. - 2469-990X. ; 3:4
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Tidskriftsartikel (refereegranskat)abstract
- We study penetrative convection of a fluid confined between two horizontal plates, the temperatures of which are such that a temperature of maximum density lies between them. The range of Rayleigh numbers studied is Ra = [10(6),10(8)] and the Prandtl numbers are Pr = 1 and 11.6. An evolution equation for the growth of the convecting region is obtained through an integral energy balance. We identify a new nondimensional parameter, Lambda, which is the ratio of temperature difference between the stable and unstable regions of the flow; larger values of Lambda denote increased stability of the upper stable layer. We study the effects of Lambda on the flow field using well-resolved lattice Boltzmann simulations and show that the characteristics of the flow depend sensitively upon it. For the range Lambda = [0.01,4], we find that for a fixed Ra the Nusselt number, Nu, increases with decreasing Lambda. We also investigate the effects of Lambda on the vertical variation of convective heat flux and the Brunt-Vaisala frequency. Our results clearly indicate that in the limit Lambda -> 0 the problem reduces to that of the classical Rayleigh-Benard convection.
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| 4. |
- Toppaladoddi, Srikanth, et al.
(författare)
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Roughness as a Route to the Ultimate Regime of Thermal Convection
- 2017
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 118:7
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Tidskriftsartikel (refereegranskat)abstract
- We use highly resolved numerical simulations to study turbulent Rayleigh-Benard convection in a cell with sinusoidally rough upper and lower surfaces in two dimensions for Pr = 1 and Ra = [4 x 10(6), 3 x 10(9)]. By varying the wavelength. at a fixed amplitude, we find an optimal wavelength lambda(opt) for which the Nusselt-Rayleigh scaling relation is (Nu - 1 proportional to Ra-0.483), maximizing the heat flux. This is consistent with the upper bound of Goluskin and Doering [J. Fluid Mech. 804, 370 (2016)] who prove that Nu can grow no faster than O(Ra-1/2) as Ra -> infinity, and thus with the concept that roughness facilitates the attainment of the so-called ultimate regime. Our data nearly achieve the largest growth rate permitted by the bound. When lambda << lambda(opt) and lambda >> lambda(opt), the planar case is recovered, demonstrating how controlling the wall geometry manipulates the interaction between the boundary layers and the core flow. Finally, for each Ra, we choose the maximum Nu among all., thus optimizing over all lambda, to find Nu(opt) - 1 = 0.01xRa(0.444).
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| 5. |
- Toppaladoddi, S., et al.
(författare)
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Seasonal Evolution of the Arctic Sea Ice Thickness Distribution
- 2023
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Ingår i: Journal of Geophysical Research - Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 128:5
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Tidskriftsartikel (refereegranskat)abstract
- The Thorndike et al. (1975, ) theory of the ice thickness distribution, g(h), treats the dynamic and thermodynamic aggregate properties of the ice pack in a novel and physically self-consistent manner. Therefore, it has provided the conceptual basis of the treatment of sea-ice thickness categories in climate models. The approach, however, is not mathematically closed due to the treatment of mechanical deformation using the redistribution function ?, the authors noting "The present theory suffers from a burdensome and arbitrary redistribution function ?." Toppaladoddi and Wettlaufer (2015, ) showed how ? can be written in terms of g(h), thereby solving the mathematical closure problem and writing the theory in terms of a Fokker-Planck equation, which they solved analytically to quantitatively reproduce the observed winter g(h). Here, we extend this approach to include open water by formulating a new boundary condition for their Fokker-Planck equation, which is then coupled to the observationally consistent sea-ice growth model of Semtner (1976, ) to study the seasonal evolution of g(h). We find that as the ice thins, g(h) transitions from a single- to a double-peaked distribution, which is in agreement with observations. To understand the cause of this transition, we construct a simpler description of the system using the equivalent Langevin equation formulation and solve the resulting stochastic ordinary differential equation numerically. Finally, we solve the Fokker-Planck equation for g(h) under different climatological conditions to study the evolution of the open-water fraction.Plain Language Summary A quantitative understanding of the evolution of the thickness distribution of sea ice is necessary to accurately predict changes in the Arctic ice cover. In the original formulation of the governing equation for the thickness distribution by Thorndike et al., the treatment of the redistribution term-which represents the mechanical deformation of ice by rafting and ridging-is referred to as "arbitrar y" and "burdensome." Using an analogy with Brownian motion, we have recast the redistribution term, closed the original theory and incorporated the process of open water formation to produce seasonal predictions of the thickness distribution. Using our theory we show that a second peak in the thickness distribution emerges in summer, which is consistent with observations. Furthermore, we explore how the greenhouse gas and oceanic heat flux forcings impact the open-water fraction and mean thickness, and the relative sensitivities to these forcings.
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| 6. |
- Toppaladoddi, Srikanth, et al.
(författare)
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Statistical Mechanics and the Climatology of the Arctic Sea Ice Thickness Distribution
- 2017
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Ingår i: Journal of statistical physics. - : Springer. - 0022-4715 .- 1572-9613. ; 167:3-4, s. 683-702
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Tidskriftsartikel (refereegranskat)abstract
- We study the seasonal changes in the thickness distribution of Arctic sea ice, g(h), under climate forcing. Our analytical and numerical approach is based on a Fokker-Planck equation for g(h) (Toppaladoddi and Wettlaufer in Phys Rev Lett 115(14): 148501, 2015), in which the thermodynamic growth rates are determined using observed climatology. In particular, the Fokker-Planck equation is coupled to the observationally consistent thermodynamic model of Eisenman and Wettlaufer (Proc Natl Acad Sci USA 106: 28-32, 2009). We find that due to the combined effects of thermodynamics and mechanics, g(h) spreads during winter and contracts during summer. This behavior is in agreement with recent satellite observations from CryoSat-2 (Kwok and Cunningham in Philos Trans R Soc A 373(2045): 20140157, 2015). Because g(h) is a probability density function, we quantify all of the key moments (e. g., mean thickness, fraction of thin/thick ice, mean albedo, relaxation time scales) as greenhouse-gas radiative forcing, Delta F-0, increases. The mean ice thickness decays exponentially with Delta F-0, but much slower than do solely thermodynamic models. This exhibits the crucial role that ice mechanics plays in maintaining the ice cover, by redistributing thin ice to thick ice-far more rapidly than can thermal growth alone.
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| 7. |
- Toppaladoddi, Srikanth, et al.
(författare)
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Tailoring boundary geometry to optimize heat transport in turbulent convection
- 2015
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 111:4
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Tidskriftsartikel (refereegranskat)abstract
- By tailoring the geometry of the upper boundary in turbulent Rayleigh-Benard convection we manipulate the boundary layer-interior flow interaction, and examine the heat transport using the lattice Boltzmann method. For fixed amplitude and varying boundary wavelength., we find that the exponent beta in the Nusselt-Rayleigh scaling relation, Nu - 1 proportional to Ra-beta, is maximized at lambda =lambda(max) approximate to ( 2 pi)(-1), but decays to the planar value in both the large (lambda >> lambda(max)) and small (lambda << lambda(max)) wavelength limits. The changes in the exponent originate in the nature of the coupling between the boundary layer and the interior flow. We present a simple scaling argument embodying this coupling, which describes the maximal convective heat flux.
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| 8. |
- Toppaladoddi, S., et al.
(författare)
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The combined effects of shear and buoyancy on phase boundary stability
- 2019
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Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 868, s. 648-665
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Tidskriftsartikel (refereegranskat)abstract
- We study the effects of externally imposed shear and buoyancy driven flows on the stability of a solid-liquid interface. A linear stability analysis of shear and buoyancy-driven flow of a melt over its solid phase shows that buoyancy is the only destabilizing factor and that the regime of shear flow here, by inhibiting vertical motions and hence the upward heat flux, stabilizes the system. It is also shown that all perturbations to the solid-liquid interface decay at a very modest shear flow strength. However, at much larger shear-flow strength, where flow instabilities coupled with buoyancy might enhance vertical motions, a re-entrant instability may arise.
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| 9. |
- Toppaladoddi, Srikanth, et al.
(författare)
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Theory of the Sea Ice Thickness Distribution
- 2015
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 115:14
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Tidskriftsartikel (refereegranskat)abstract
- We use concepts from statistical physics to transform the original evolution equation for the sea ice thickness distribution g(h) from Thorndike et al. into a Fokker-Planck-like conservation law. The steady solution is g(h) = N(q)h(q)e(-h/H), where q and H are expressible in terms of moments over the transition probabilities between thickness categories. The solution exhibits the functional form used in observational fits and shows that for h << 1, g(h) is controlled by both thermodynamics and mechanics, whereas for h >> 1 only mechanics controls g(h). Finally, we derive the underlying Langevin equation governing the dynamics of the ice thickness h, from which we predict the observed g(h). The genericity of our approach provides a framework for studying the geophysical-scale structure of the ice pack using methods of broad relevance in statistical mechanics.
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| 10. |
- Toppaladoddi, Srikanth, et al.
(författare)
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Thermal convection over fractal surfaces
- 2020
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 907
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Tidskriftsartikel (refereegranskat)abstract
- We use well resolved numerical simulations with the lattice Boltzmann method to study Rayleigh-Benard convection in cells with a fractal boundary in two dimensions for Pr = 1 and Ra is an element of [10(7), 10(10)], 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) similar to kp (p < 0). The degree of roughness is quantified by the exponent p with p < -3 for smooth (differentiable) surfaces and -3 <= p < -1 for rough surfaces with Hausdorff dimension D-f = 1/2 ( p + 5). By computing the exponent beta using power law fits of Nu similar to 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 is an element of [10(8), 10(10)]. For p = -3.0, -2.0 and -1.5 we find beta = 0.288 +/- 0.005, 0.329 +/- 0.006 and 0.352 +/- 0.011, respectively. We also find that the Reynolds number, Re, scales as Re similar to Ra-xi, where xi approximate to 0.57 over Ra is an element of [10(7), 10(10)], 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.
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