| 1. |
- Weady, S., et al.
(författare)
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Circuit bounds on stochastic transport in the Lorenz equations
- 2018
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Ingår i: Physics Letters A. - : Elsevier. - 0375-9601 .- 1873-2429. ; 382:26, s. 1731-1737
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Tidskriftsartikel (refereegranskat)abstract
- In turbulent Rayleigh–Bénard convection one seeks the relationship between the heat transport, captured by the Nusselt number, and the temperature drop across the convecting layer, captured by the Rayleigh number. In experiments, one measures the Nusselt number for a given Rayleigh number, and the question of how close that value is to the maximal transport is a key prediction of variational fluid mechanics in the form of an upper bound. The Lorenz equations have traditionally been studied as a simplified model of turbulent Rayleigh–Bénard convection, and hence it is natural to investigate their upper bounds, which has previously been done numerically and analytically, but they are not as easily accessible in an experimental context. Here we describe a specially built circuit that is the experimental analogue of the Lorenz equations and compare its output to the recently determined upper bounds of the stochastic Lorenz equations [1]. The circuit is substantially more efficient than computational solutions, and hence we can more easily examine the system. Because of offsets that appear naturally in the circuit, we are motivated to study unique bifurcation phenomena that arise as a result. Namely, for a given Rayleigh number, we find a reentrant behavior of the transport on noise amplitude and this varies with Rayleigh number passing from the homoclinic to the Hopf bifurcation.
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| 2. |
- Hansen-Goos, H., et al.
(författare)
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On the edge of habitability and the extremes of liquidity
- 2014
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 98:Special Issue: SI, s. 169-181
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Tidskriftsartikel (refereegranskat)abstract
- The physical and biological mechanisms that extend the equilibrium domain of liquid water into the ice region of the bulk phase diagram are examined in view of their importance for the enhancement of planetary habitability. The physical phenomena studied are the premelting of ice, which allows for films of liquid water at temperatures well below freezing, and the wetting of hygroscopic salts with the persistence of briny films even for thermodynamic conditions remote from those of bulk liquid water. Organisms are known to produce a variety of frost-suppressing substances, one of which, the anti-freeze protein, is described here. In this article, we provide a synthesis of theoretical and experimental studies whilst extending ideas into new territory as we address the question of habitability. (C) 2013 Elsevier Ltd. All rights reserved.
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| 3. |
- Thomson, Erik S, et al.
(författare)
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Grain boundary melting in ice
- 2013
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 138:12
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Tidskriftsartikel (refereegranskat)abstract
- We describe an optical scattering study of grain boundary premelting in water ice. Ubiquitous long ranged attractive polarization forces act to suppress grain boundary melting whereas repulsive forces originating in screened Coulomb interactions and classical colligative effects enhance it. The liquid enhancing effects can be manipulated by adding dopant ions to the system. For all measured grain boundaries this leads to increasing premelted film thickness with increasing electrolyte concentration. Although we understand that the interfacial surface charge densities qs and solute concentrations can potentially dominate the film thickness, we cannot directly measure them within a given grain boundary. Therefore, as a framework for interpreting the data we consider two appropriate qs dependent limits; one is dominated by the colligative effect and other is dominated by electrostatic interactions.
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| 4. |
- Worster, M. Grae, et al.
(författare)
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Colloidal mushy layers
- 2021
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Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 914
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Tidskriftsartikel (refereegranskat)abstract
- We consider the solidification of idealised two-component mixtures comprising a solvent or suspending fluid and dissolved solute molecules or suspended colloidal particles, each considered as hard spheres. We review some fundamental thermodynamic ideas regarding relative motion between species and phase equilibria in such mixtures to show how the related solid-liquid phase diagrams depend on the size of the spheres. Using similarity solutions, we first describe freezing of the solvent to form a pure solid (here referred to as 'ice'), with the solute rejected from the solid forming a boundary layer or dense particle layer ahead of the freezing front. We extend ideas of constitutional supercooling to the case of colloidal suspensions and show that, for a given temperature difference driving solidification, constitutional supercooling occurs only for an intermediate range of particle sizes. Constitutional supercooling promotes the formation of a mushy layer in which segregated ice separates regions of concentrated solute or particles on the microscale. We formulate a continuum model of the mushy layer that relies on a key observation that the regelative motion of concentrated clusters of particles is independent of the size and geometry of the cluster. Our modelling begins with a description of relative motion as a Fickian diffusive process. However, at high particle concentrations, we show that it is more convenient and more computationally tractable to use an equivalent formulation in terms of Darcy flow of the solvent. Within a mushy layer these diffusive fluxes correspond directly to the regelative flux of particle clusters at a rate determined by the local temperature and temperature gradient.
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| 5. |
- Agarwal, Sahil, et al.
(författare)
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EXOPLANETARY DETECTION BY MULTIFRACTAL SPECTRAL ANALYSIS
- 2017
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Ingår i: Astronomical Journal. - : Institute of Physics (IOP). - 0004-6256 .- 1538-3881. ; 153:1
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Tidskriftsartikel (refereegranskat)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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| 6. |
- Agarwal, Sahil, et al.
(författare)
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Maximal stochastic transport in the Lorenz equations
- 2016
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Ingår i: Physics Letters A. - : Elsevier. - 0375-9601 .- 1873-2429. ; 380:1-2, s. 142-146
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Tidskriftsartikel (refereegranskat)abstract
- We calculate the stochastic upper bounds for the Lorenz equations using an extension of the background method. In analogy with Rayleigh-Benard convection the upper bounds are for heat transport versus Rayleigh number. As might be expected, the stochastic upper bounds are larger than the deterministic counterpart of Souza and Doering [1], but their variation with noise amplitude exhibits interesting behavior. Below the transition to chaotic dynamics the upper bounds increase monotonically with noise amplitude. However, in the chaotic regime this monotonicity depends on the number of realizations in the ensemble; at a particular Rayleigh number the bound may increase or decrease with noise amplitude. The origin of this behavior is the coupling between the noise and unstable periodic orbits, the degree of which depends on the degree to which the ensemble represents the ergodic set. This is confirmed by examining the close returns plots of the full solutions to the stochastic equations and the numerical convergence of the noise correlations. The numerical convergence of both the ensemble and time averages of the noise correlations is sufficiently slow that it is the limiting aspect of the realization of these bounds. Finally, we note that the full solutions of the stochastic equations demonstrate that the effect of noise is equivalent to the effect of chaos.
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| 7. |
- Agarwal, Sahil, et al.
(författare)
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The Statistical Properties of Sea Ice Velocity Fields
- 2017
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Ingår i: Journal of Climate. - : American Meteorological Society. - 0894-8755 .- 1520-0442. ; 30:13, s. 4873-4881
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Tidskriftsartikel (refereegranskat)abstract
- By arguing that the surface pressure field over the Arctic Ocean can be treated as an isotropic, stationary, homogeneous, Gaussian random field, Thorndike estimated a number of covariance functions from two years of data (1979 and 1980). Given the active interest in changes of general circulation quantities and indices in the polar regions during the recent few decades, the spatial correlations in sea ice velocity fields are of particular interest. It is thus natural to ask, How persistent are these correlations?'' To this end, a multifractal stochastic treatment is developed to analyze observed Arctic sea ice velocity fields from satellites and buoys for the period 1978-2015. Since it was previously found that the Arctic equivalent ice extent (EIE) has a white noise structure on annual to biannual time scales, the connection between EIE and ice motion is assessed. The long-term stationarity of the spatial correlation structure of the velocity fields and the robustness of their white noise structure on multiple time scales is demonstrated; these factors (i) combine to explain the white noise characteristics of the EIE on annual to biannual time scales and (ii) explain why the fluctuations in the ice velocity are proportional to fluctuations in the geostrophic winds on time scales of days to months. Moreover, it is shown that the statistical structure of these two quantities is commensurate from days to years, which may be related to the increasing prevalence of free drift in the ice pack.
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| 8. |
- Bar-Dolev, Maya, et al.
(författare)
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New insights into ice growth and melting modifications by antifreeze proteins
- 2012
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Ingår i: Journal of the Royal Society Interface. - : The Royal Society. - 1742-5689 .- 1742-5662. ; 9:77, s. 3249-3259
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Tidskriftsartikel (refereegranskat)abstract
- Antifreeze proteins (AFPs) evolved in many organisms, allowing them to survive in cold climates by controlling ice crystal growth. The specific interactions of AFPs with ice determine their potential applications in agriculture, food preservation and medicine. AFPs control the shapes of ice crystals in a manner characteristic of the particular AFP type. Moderately active AFPs cause the formation of elongated bipyramidal crystals, often with seemingly defined facets, while hyperactive AFPs produce more varied crystal shapes. These different morphologies are generally considered to be growth shapes. In a series of bright light and fluorescent microscopy observations of ice crystals in solutions containing different AFPs, we show that crystal shaping also occurs during melting. In particular, the characteristic ice shapes observed in solutions of most hyperactive AFPs are formed during melting. We relate these findings to the affinities of the hyperactive AFPs for the basal plane of ice. Our results demonstrate the relation between basal plane affinity and hyperactivity and show a clear difference in the ice-shaping mechanisms of most moderate and hyperactive AFPs. This study provides key aspects associated with the identification of hyperactive AFPs.
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| 9. |
- 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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| 10. |
- French, Roger H., et al.
(författare)
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Long range interactions in nanoscale science
- 2010
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Ingår i: Reviews of Modern Physics. - 0034-6861. ; 82:2, s. 1887-1944
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Forskningsöversikt (refereegranskat)abstract
- Our understanding of the “long range” electrodynamic, electrostatic, and polar interactions that dominate the organization of small objects at separations beyond an interatomic bond length is reviewed. From this basic-forces perspective, a large number of systems are described from which one can learn about these organizing forces and how to modulate them. The many practical systems that harness these nanoscale forces are then surveyed. The survey reveals not only the promise of new devices and materials, but also the possibility of designing them more effectively.
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