| 1. |
- O'Rourke, J. G., et al.
(författare)
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Mushy-layer dynamics in micro and hyper gravity
- 2012
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 24:10, s. 103305-
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Tidskriftsartikel (refereegranskat)abstract
- We describe the results of experiments on mushy layers grown from aqueous ammonium chloride solution in normal, micro, and hyper gravity environments. In the fully developed chimney state, the chimney plume dynamics differ strikingly when conditions change from micro to hyper gravity. In microgravity, we find fully arrested plume motion and suppressed convection. As gravity exceeds Earth conditions, we observe a host of phenomena, ranging from arched plumes that undergo forced Rayleigh-Taylor instabilities to in-phase multiple plume oscillatory behavior. For the same initial solute concentrations and fixed boundary cooling temperatures, we find that, in runs of over two hours, the averaged effects of microgravity and hypergravity result in suppressed growth of the mushy layers, a phenomenon caused by a net enhancement of convective heat and solute transport from the liquid to the mushy layers. These behaviors are placed in the context of the theory of convecting mushy layers as studied under normal laboratory conditions.
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| 2. |
- Agarwal, S., et al.
(författare)
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Fluctuations in Arctic sea-ice extent : Comparing observations and climate models
- 2018
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Ingår i: Philosophical Transactions. Series A. - : Royal Society Publishing. - 1364-503X .- 1471-2962. ; 376:2129
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Tidskriftsartikel (refereegranskat)abstract
- The fluctuation statistics of the observed sea-ice extent during the satellite era are compared with model output from CMIP5 models using a multifractal time series method. The two robust features of the observations are that on annual to biannual time scales the ice extent exhibits white noise structure, and there is a decadal scale trend associated with the decay of the ice cover. It is shown that (i) there is a large inter-model variability in the time scales extracted from the models, (ii) none of the models exhibits the decadal time scales found in the satellite observations, (iii) five of the 21 models examined exhibit the observed white noise structure, and (iv) the multi-model ensemble mean exhibits neither the observed white noise structure nor the observed decadal trend. It is proposed that the observed fluctuation statistics produced by this method serve as an appropriate test bed for modelling studies.
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| 3. |
- Wells, Andrew J., et al.
(författare)
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Nonlinear mushy-layer convection with chimneys : stability and optimal solute fluxes
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 716, s. 203-227
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Tidskriftsartikel (refereegranskat)abstract
- We model buoyancy-driven convection with chimneys - channels of zero solid fraction - in a mushy layer formed during directional solidification of a binary alloy in two dimensions. A large suite of numerical simulations is combined with scaling analysis in order to study the parametric dependence of the flow. Stability boundaries are calculated for states of finite-amplitude convection with chimneys, which for a narrow domain can be interpreted in terms of a modified Rayleigh number criterion based on the domain width and mushy-layer permeability. For solidification in a wide domain with multiple chimneys, it has previously been hypothesized that the chimney spacing will adjust to optimize the rate of removal of potential energy from the system. For a wide variety of initial liquid concentration conditions, we consider the detailed flow structure in this optimal state and derive scaling laws for how the flow evolves as the strength of convection increases. For moderate mushy-layer Rayleigh numbers these flow properties support a solute flux that increases linearly with Rayleigh number. This behaviour does not persist indefinitely, however, with porosity-dependent flow saturation resulting in sublinear growth of the solute flux for sufficiently large Rayleigh numbers. Finally, we consider the influence of the porosity dependence of permeability, with a cubic function and a Carman-Kozeny permeability yielding qualitatively similar system dynamics and flow profiles for the optimal states.
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| 4. |
- Giorgini, Ludovico Theo, 1994-, et al.
(författare)
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Modeling the El Niño Southern Oscillation with Neural Differential Equations
- 2021
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Konferensbidrag (refereegranskat)abstract
- We use a Neural Ordinary Differential Equation to model and predict the seasonal to interannual variability of El Niño Southern Oscillation (ENSO). We train our neural network model using partial observations involving only sea surface temperature data. Our approach is computationally inexpensive, it reproduces the main seasonal features of ENSO, and exhibits robust predictions skills.
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| 5. |
- Guarnieri, F., et al.
(författare)
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Solution of the Fokker-Planck equation with a logarithmic potential and mixed eigenvalue spectrum
- 2017
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Ingår i: Journal of Mathematical Physics. - : AIP Publishing. - 0022-2488 .- 1089-7658. ; 58:9
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by a problem in climate dynamics, we investigate the solution of a Bessel-like process with a negative constant drift, described by a Fokker-Planck equation with a potential V(x) = -[b ln(x) + a x], for b > 0 and a < 0. The problem belongs to a family of Fokker-Planck equations with logarithmic potentials closely related to the Bessel process that has been extensively studied for its applications in physics, biology, and finance. The Bessel-like process we consider can be solved by seeking solutions through an expansion into a complete set of eigenfunctions. The associated imaginary-time Schrodinger equation exhibits a mix of discrete and continuous eigenvalue spectra, corresponding to the quantum Coulomb potential describing the bound states of the hydrogen atom. We present a technique to evaluate the normalization factor of the continuous spectrum of eigenfunctions that relies solely upon their asymptotic behavior. We demonstrate the technique by solving the Brownian motion problem and the Bessel process both with a constant negative drift. We conclude with a comparison to other analytical methods and with numerical solutions.
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| 6. |
- Style, Robert W., et al.
(författare)
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Universal Deformation of Soft Substrates Near a Contact Line and the Direct Measurement of Solid Surface Stresses
- 2013
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 110:6, s. 066103-
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Tidskriftsartikel (refereegranskat)abstract
- Droplets deform soft substrates near their contact lines. Using confocal microscopy, we measure the deformation of silicone gel substrates due to glycerol and fluorinated-oil droplets for a range of droplet radii and substrate thicknesses. For all droplets, the substrate deformation takes a universal shape close to the contact line that depends on liquid composition, but is independent of droplet size and substrate thickness. This shape is determined by a balance of interfacial tensions at the contact line and provides a novel method for direct determination of the surface stresses of soft substrates. Moreover, we measure the change in contact angle with droplet radius and show that Young's law fails for small droplets when their radii approach an elastocapillary length scale. For larger droplets the macroscopic contact angle is constant, consistent with Young's law. DOI: 10.1103/PhysRevLett.110.066103
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| 7. |
- 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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| 8. |
- 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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