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  • 1.
    Agasthya, Lokahith
    et al.
    Indian Inst Sci Educ & Res, Pune 411008, Maharashtra, India.;Tata Inst Fundamental Res, Int Ctr Theoret Sci, Bangalore 560089, Karnataka, India.;Univ Roma Tor Vergata, Dept Phys, Via Ric Sci 1, I-00133 Rome, Italy.;Univ Roma Tor Vergata, Ist Nazl Fis Nucl, Via Ric Sci 1, I-00133 Rome, Italy..
    Picardo, Jason R.
    Tata Inst Fundamental Res, Int Ctr Theoret Sci, Bangalore 560089, Karnataka, India..
    Ravichandran, Siddharth
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, S-10691 Stockholm, Sweden..
    Govindarajan, Rama
    Tata Inst Fundamental Res, Int Ctr Theoret Sci, Bangalore 560089, Karnataka, India..
    Ray, Samriddhi Sankar
    Tata Inst Fundamental Res, Int Ctr Theoret Sci, Bangalore 560089, Karnataka, India..
    Understanding droplet collisions through a model flow: Insights from a Burgers vortex2019In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 99, no 6, article id 063107Article in journal (Refereed)
    Abstract [en]

    We investigate the role of intense vortical structures, similar to those in a turbulent flow, in enhancing collisions (and coalescences) which lead to the formation of large aggregates in particle-laden flows. By using a Burgers vortex model, we show, in particular, that vortex stretching significantly enhances sharp inhomogeneities in spatial particle densities, related to the rapid ejection of particles from intense vortices. Furthermore our work shows how such spatial clustering leads to an enhancement of collision rates and extreme statistics of collisional velocities. We also study the role of polydisperse suspensions in this enhancement. Our work uncovers an important principle, which, if valid for realistic turbulent flows, may be a factor in how small nuclei water droplets in warm clouds can aggregate to sizes large enough to trigger rain.

  • 2. Agosta, L.
    et al.
    Metere, A.
    Dzugutov, Mikhail
    KTH.
    Hexatic smectic phase with algebraically decaying bond-orientational order2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, no 5, article id 052702Article in journal (Refereed)
    Abstract [en]

    The hexatic phase predicted by the theories of two-dimensional melting is characterized by the power-law decay of the orientational correlations, whereas the in-layer bond orientational order in all the hexatic smectic phases observed so far was found to be long range. We report a hexatic smectic phase where the in-layer bond orientational correlations decay algebraically, in quantitative agreement with the hexatic ordering predicted by the theory for two dimensions. The phase was formed in a molecular dynamics simulation of a one-component system of particles interacting via a spherically symmetric potential. The present results thus demonstrate that the theoretically predicted two-dimensional hexatic order can exist in a three-dimensional system.

  • 3. Argun, Aykut
    et al.
    Soni, Jalpa
    Dabelow, Lennart
    Bo, Stefano
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Pesce, Giuseppe
    Eichhorn, Ralf
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Volpe, Giovanni
    Experimental realization of a minimal microscopic heat engine2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 5, article id 052106Article in journal (Refereed)
    Abstract [en]

    Microscopic heat engines are microscale systems that convert energy flows between heat reservoirs into work or systematic motion. We have experimentally realized a minimal microscopic heat engine. It consists of a colloidal Brownian particle optically trapped in an elliptical potential well and simultaneously coupled to two heat baths at different temperatures acting along perpendicular directions. For a generic arrangement of the principal directions of the baths and the potential, the symmetry of the system is broken, such that the heat flow drives a systematic gyrating motion of the particle around the potential minimum. Using the experimentally measured trajectories, we quantify the gyrating motion of the particle, the resulting torque that it exerts on the potential, and the associated heat flow between the heat baths. We find excellent agreement between the experimental results and the theoretical predictions.

  • 4.
    Aurell, Erik
    KTH, School of Electrical Engineering and Computer Science (EECS), Computational Science and Technology (CST). Aalto Univ, Dept Comp Sci, FIN-00076 Aalto, Finland.;Aalto Univ, Dept Appl Phys, FIN-00076 Aalto, Finland.
    Characteristic functions of quantum heat with baths at different temperatures2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, no 6, article id 062117Article in journal (Refereed)
    Abstract [en]

    This paper is about quantum heat defined as the change in energy of a bath during a process. The presentation takes into account recent developments in classical strong-coupling thermodynamics and addresses a version of quantum heat that satisfies quantum-classical correspondence. The characteristic function and the full counting statistics of quantum heat are shown to be formally similar. The paper further shows that the method can be extended to more than one bath, e.g., two baths at different temperatures, which opens up the prospect of studying correlations and heat flow. The paper extends earlier results on the expected quantum heat in the setting of one bath [E. Aurell and R. Eichhorn, New .J Phys. 17, 065007 (2015); E. Aurell, Entropy 19, 595 (2017)].

  • 5.
    Aurell, Erik
    KTH. Aalto Univ, Dept Comp Sci & Appl Phys, FIN-00076 Espoo, Aalto, Finland..
    Unified picture of strong-coupling stochastic thermodynamics and time reversals2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, no 4, article id 042112Article in journal (Refereed)
    Abstract [en]

    Strong-coupling statistical thermodynamics is formulated as the Hamiltonian dynamics of an observed system interacting with another unobserved system (a bath). It is shown that the entropy production functional of stochastic thermodynamics, defined as the log ratio of forward and backward system path probabilities, is in a one-to-one relation with the log ratios of the joint initial conditions of the system and the bath. A version of strong-coupling statistical thermodynamics where the system-bath interaction vanishes at the beginning and at the end of a process is, as is also weak-coupling stochastic thermodynamics, related to the bath initially in equilibrium by itself. The heat is then the change of bath energy over the process, and it is discussed when this heat is a functional of the system history alone. The version of strong-coupling statistical thermodynamics introduced by Seifert and Jarzynski is related to the bath initially in conditional equilibrium with respect to the system. This leads to heat as another functional of the system history which needs to be determined by thermodynamic integration. The log ratio of forward and backward system path probabilities in a stochastic process is finally related to log ratios of the initial conditions of a combined system and bath. It is shown that the entropy production formulas of stochastic processes under a general class of time reversals are given by the differences of bath energies in a larger underlying Hamiltonian system. The paper highlights the centrality of time reversal in stochastic thermodynamics, also in the case of strong coupling.

  • 6.
    Aurell, Erik
    et al.
    KTH. Departments of Information and Computer Science and Applied Physics, Aalto University, FIN-00076 Espoo, Finland.
    Bo, Stefano
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Steady diffusion in a drift field: A comparison of large-deviation techniques and multiple-scale analysis2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 3, article id 032140Article in journal (Refereed)
    Abstract [en]

    A particle with internal unobserved states diffusing in a force field will generally display effective advection-diffusion. The drift velocity is proportional to the mobility averaged over the internal states, or effective mobility, while the effective diffusion has two terms. One is of the equilibrium type and satisfies an Einstein relation with the effective mobility while the other is quadratic in the applied force. In this contribution we present two new methods to obtain these results, on the one hand using large deviation techniques and on the other by a multiple-scale analysis, and compare the two. We consider both systems with discrete internal states and continuous internal states. We show that the auxiliary equations in the multiple-scale analysis can also be derived in second-order perturbation theory in a large deviation theory of a generating function (discrete internal states) or generating functional (continuous internal states). We discuss that measuring the two components of the effective diffusion give a way to determine kinetic rates from only first and second moments of the displacement in steady state.

  • 7.
    Aurell, Erik
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Computational Science and Technology (CST).
    Dominguez, Eduardo
    Univ Havana, Dept Theoret Phys, Grp Complex Syst & Stat Phys, Havana, Cuba..
    Machado, David
    Univ Havana, Dept Theoret Phys, Grp Complex Syst & Stat Phys, Havana, Cuba..
    Mulet, Roberto
    Univ Havana, Dept Theoret Phys, Grp Complex Syst & Stat Phys, Havana, Cuba..
    Exploring the diluted ferromagnetic p-spin model with a cavity master equation2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, no 5, article id 050103Article in journal (Refereed)
    Abstract [en]

    We introduce an alternative solution to Glauber multispin dynamics on random graphs. The solution is based on the recently introduced cavity master equation (CME), a time-closure turning the, in principle, exact dynamic cavity method into a practical method of analysis and of fast simulation. Running CME once is of comparable computational complexity as one Monte Carlo run on the same problem. We show that CME correctly models the ferromagnetic p-spin Glauber dynamics from high temperatures down to and below the spinoidal transition. We also show that CME allows an alternative exploration of the low-temperature spin-glass phase of the model.

  • 8.
    Aurell, Erik
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Computational Science and Technology (CST).
    Montana, Federica
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Thermal power of heat flow through a qubit2019In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 99, no 4, article id 042130Article in journal (Refereed)
    Abstract [en]

    In this paper we consider the thermal power of a heat flow through a qubit between two baths. The baths are modeled as a set of harmonic oscillators initially at equilibrium, at two temperatures. Heat is defined as the change of energy of the cold bath, and thermal power is defined as expected heat per unit time, in the long-time limit. The qubit and the baths interact as in the spin-boson model, i.e., through qubit operator sigma(z). We compute thermal power in an approximation analogous to a "noninteracting blip" (NIBA) and express it in the polaron picture as products of correlation functions of the two baths, and a time derivative of a correlation function of the cold bath. In the limit of weak interaction we recover known results in terms of a sum of correlation functions of the two baths, a correlation functions of the cold bath only, and the energy split.

  • 9.
    Bhatnagar, Akshay
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Gupta, Anupam
    Mitra, Dhrubaditya
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Pandit, Rahul
    Heavy inertial particles in turbulent flows gain energy slowly but lose it rapidly2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, no 3, article id 033102Article in journal (Refereed)
    Abstract [en]

    We present an extensive numerical study of the time irreversibility of the dynamics of heavy inertial particles in three-dimensional, statistically homogeneous, and isotropic turbulent flows. We show that the probability density function (PDF) of the increment, W(tau), of a particle's energy over a time scale tau is non-Gaussian, and skewed toward negative values. This implies that, on average, particles gain energy over a period of time that is longer than the duration over which they lose energy. We call this slow gain and fast loss. We find that the third moment of W(tau) scales as tau(3) for small values of tau. We show that the PDF of power-input p is negatively skewed too; we use this skewness Ir as a measure of the time irreversibility and we demonstrate that it increases sharply with the Stokes number St for small St; this increase slows down at St similar or equal to 1. Furthermore, we obtain the PDFs of t(+) and t(-), the times over which p has, respectively, positive or negative signs, i.e., the particle gains or loses energy. We obtain from these PDFs a direct and natural quantification of the slow gain and fast loss of the energy of the particles, because these PDFs possess exponential tails from which we infer the characteristic loss and gain times t(loss) and t(gain), respectively, and we obtain t(loss) < t(gain) for all the cases we have considered. Finally, we show that the fast loss of energy occurs with greater probability in the strain-dominated region than in the vortical one; in contrast, the slow gain in the energy of the particles is equally likely in vortical or strain-dominated regions of the flow.

  • 10.
    Bhatnagar, Akshay
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Gustavsson, K.
    Gothenburg Univ, Dept Phys, S-41296 Gothenburg, Sweden..
    Mehlig, B.
    Gothenburg Univ, Dept Phys, S-41296 Gothenburg, Sweden..
    Mitra, Dhrubaditya
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Relative velocities in bidisperse turbulent aerosols: Simulations and theory2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 98, no 6, article id 063107Article in journal (Refereed)
    Abstract [en]

    We perform direct numerical simulations of a bidisperse suspension of heavy spherical particles in forced, homogeneous, and isotropic three-dimensional turbulence. We compute the joint distribution of relative particle distances and longitudinal relative velocities between particles of different inertia. For a pair of particles with small difference in their inertias we compare our results with recent theoretical predictions [Meibohm et al., Phys. Rev. E 96, 061102 (2017)] for the shape of this distribution. We also compute the moments of relative velocities as a function of particle separation and compare with the theoretical predictions. We observe good agreement. For a pair of particles that are very different from each other-one is heavy and the other one has negligible inertia-we give a theory to calculate their root-mean-square relative velocity. This theory also agrees well with the results of our simulations.

  • 11.
    Bhatnagar, Akshay
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.
    Gustavsson, K.
    Mitra, Dhrubaditya
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.
    Statistics of the relative velocity of particles in turbulent flows: Monodisperse particles2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, no 2, article id 023105Article in journal (Refereed)
    Abstract [en]

    We use direct numerical simulations to calculate the joint probability density function of the relative distance R and relative radial velocity component V-R for a pair of heavy inertial particles suspended in homogeneous and isotropic turbulent flows. At small scales the distribution is scale invariant, with a scaling exponent that is related to the particle-particle correlation dimension in phase space, D-2. It was argued [K. Gustavsson and B. Mehlig, Phys. Rev. E 84, 045304 (2011); J. Turbul. 15, 34 (2014)] that the scale invariant part of the distribution has two asymptotic regimes: (1) vertical bar V-R vertical bar << R, where the distribution depends solely on R, and (2) vertical bar V-R vertical bar >> R, where the distribution is a function of vertical bar V-R vertical bar alone. The probability distributions in these two regimes are matched along a straight line: vertical bar V-R vertical bar = z*R. Our simulations confirm that this is indeed correct. We further obtain D-2 and z* as a function of the Stokes number, St. The former depends nonmonotonically on St with aminimum at about St approximate to 0.7 and the latter has only a weak dependence on St.

  • 12.
    Bo, Stefano
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Max Planck Institute for the Physics of Complex Systems, Nöthnitzer Str. 38, Dresden, DE-01187, Germany.
    Schmidt, Falko
    Eichhorn, Ralf
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Volpe, Giovanni
    Measurement of anomalous diffusion using recurrent neural networks2019In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 100, no 1, article id 010102Article in journal (Refereed)
    Abstract [en]

    Anomalous diffusion occurs in many physical and biological phenomena, when the growth of the mean squared displacement (MSD) with time has an exponent different from one. We show that recurrent neural networks (RNNs) can efficiently characterize anomalous diffusion by determining the exponent from a single short trajectory, outperforming the standard estimation based on the MSD when the available data points are limited, as is often the case in experiments. Furthermore, the RNNs can handle more complex tasks where there are no standard approaches, such as determining the anomalous diffusion exponent from a trajectory sampled at irregular times, and estimating the switching time and anomalous diffusion exponents of an intermittent system that switches between different kinds of anomalous diffusion. We validate our method on experimental data obtained from subdiffusive colloids trapped in speckle light fields and superdiffusive microswimmers.

  • 13.
    Borlenghi, Simone
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Boman, Magnus
    KTH, School of Electrical Engineering and Computer Science (EECS), Software and Computer systems, SCS. RISE SICS, Electrum 229, SE-16429 Kista, Sweden..
    Delin, Anna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Modeling reservoir computing with the discrete nonlinear Schrodinger equation2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 98, no 5, article id 052101Article in journal (Refereed)
    Abstract [en]

    We formulate, using the discrete nonlinear Schrodinger equation (DNLS), a general approach to encode and process information based on reservoir computing. Reservoir computing is a promising avenue for realizing neuromorphic computing devices. In such computing systems, training is performed only at the output level by adjusting the output from the reservoir with respect to a target signal. In our formulation, the reservoir can be an arbitrary physical system, driven out of thermal equilibrium by an external driving. The DNLS is a general oscillator model with broad application in physics, and we argue that our approach is completely general and does not depend on the physical realization of the reservoir. The driving, which encodes the object to be recognized, acts as a thermodynamic force, one for each node in the reservoir. Currents associated with these thermodynamic forces in turn encode the output signal from the reservoir. As an example, we consider numerically the problem of supervised learning for pattern recognition, using as a reservoir a network of nonlinear oscillators.

  • 14.
    Cozzani, Giulia
    et al.
    Univ Paris Sud, Sorbonne Univ, Observ Paris, Lab Phys Plasmas,CNRS,Ecole Polytech, F-91128 Palaiseau, France.;Univ Pisa, Dipartimento Fis E Fermi, I-56127 Pisa, Italy..
    Retino, A.
    Univ Paris Sud, Sorbonne Univ, Observ Paris, Lab Phys Plasmas,CNRS,Ecole Polytech, F-91128 Palaiseau, France..
    Califano, F.
    Univ Pisa, Dipartimento Fis E Fermi, I-56127 Pisa, Italy..
    Alexandrova, A.
    Univ Paris Sud, Sorbonne Univ, Observ Paris, Lab Phys Plasmas,CNRS,Ecole Polytech, F-91128 Palaiseau, France..
    Contel, O. Le
    Univ Paris Sud, Sorbonne Univ, Observ Paris, Lab Phys Plasmas,CNRS,Ecole Polytech, F-91128 Palaiseau, France..
    Khotyaintsev, Y.
    Swedish Inst Space Phys, SE-75121 Uppsala, Sweden..
    Vaivads, Andris
    Swedish Inst Space Phys, SE-75121 Uppsala, Sweden..
    Fu, H. S.
    Beihang Univ, Sch Space & Environm, Beijing 100083, Peoples R China..
    Catapano, F.
    Univ Paris Sud, Sorbonne Univ, Observ Paris, Lab Phys Plasmas,CNRS,Ecole Polytech, F-91128 Palaiseau, France.;Univ Calabria, Dipartimento Fis, I-87036 Arcavacata Di Rende, CS, Italy..
    Breuillard, H.
    Univ Paris Sud, Sorbonne Univ, Observ Paris, Lab Phys Plasmas,CNRS,Ecole Polytech, F-91128 Palaiseau, France.;Univ Orleans, UMR 7328, CNRS, Lab Phys & Chim Environm & Espace, F-45071 Orleans, France..
    Ahmadi, N.
    Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80309 USA..
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Ergun, R. E.
    Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80309 USA..
    Torbert, R. B.
    Univ New Hampshire, Space Sci Ctr, Durham, NH 03824 USA..
    Giles, B. L.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA..
    Russell, C. T.
    Univ Calif Los Angeles, Dept Earth & Space Sci, Los Angeles, CA 90095 USA..
    Nakamura, R.
    Austrian Acad Sci, Space Res Inst, A-8042 Graz, Austria..
    Fuseher, S.
    Southwest Res Inst, San Antonio, TX 78238 USA.;Univ Texas San Antonio, San Antonio, TX 78238 USA..
    Mauk, B. H.
    Johns Hopkins Univ, Appl Phys Lab, Johns Hopkins Rd, Laurel, MD 20723 USA..
    Moore, T.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA..
    Burch, J. L.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    In situ spacecraft observations of a structured electron diffusion region during magnetopause reconnection2019In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 99, no 4, article id 043204Article in journal (Refereed)
    Abstract [en]

    The electron diffusion region (EDR) is the region where magnetic reconnection is initiated and electrons are energized. Because of experimental difficulties, the structure of the EDR is still poorly understood. A key question is whether the EDR has a homogeneous or patchy structure. Here we report Magnetospheric Multiscale (MMS) spacecraft observations providing evidence of inhomogeneous current densities and energy conversion over a few electron inertial lengths within an EDR at the terrestrial magnetopause, suggesting that the EDR can be rather structured. These inhomogenenities are revealed through multipoint measurements because the spacecraft separation is comparable to a few electron inertial lengths, allowing the entire MMS tetrahedron to be within the EDR most of the time. These observations are consistent with recent high-resolution and low-noise kinetic simulations.

  • 15. Elperin, T.
    et al.
    Kleeorin, N.
    Liberman, Michael A.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholms University, Sweden.
    Lipatnikov, A. N.
    Rogachevskii, I.
    Yu, R.
    Turbulent diffusion of chemically reacting flows: Theory and numerical simulations2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 5, article id 053111Article in journal (Refereed)
    Abstract [en]

    The theory of turbulent diffusion of chemically reacting gaseous admixtures developed previously [T. Elperin et al., Phys. Rev. E 90, 053001 (2014)] is generalized for large yet finite Reynolds numbers and the dependence of turbulent diffusion coefficient on two parameters, the Reynolds number and Damkohler number (which characterizes a ratio of turbulent and reaction time scales), is obtained. Three-dimensional direct numerical simulations (DNSs) of a finite-thickness reaction wave for the first-order chemical reactions propagating in forced, homogeneous, isotropic, and incompressible turbulence are performed to validate the theoretically predicted effect of chemical reactions on turbulent diffusion. It is shown that the obtained DNS results are in good agreement with the developed theory.

  • 16. Fouxon, Itzhak
    et al.
    Ge, Zhouyang
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Leshansky, Alexander
    Integral representation of channel flow with interacting particles2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 6, article id 063110Article in journal (Refereed)
    Abstract [en]

    We construct a boundary integral representation for the low-Reynolds-number flow in a channel in the presence of freely suspended particles (or droplets) of arbitrary size and shape. We demonstrate that lubrication theory holds away from the particles at horizontal distances exceeding the channel height and derive a multipole expansion of the flow which is dipolar to the leading approximation. We show that the dipole moment of an arbitrary particle is a weighted integral of the stress and the flow at the particle surface, which can be determined numerically. We introduce the equation of motion that describes hydrodynamic interactions between arbitrary, possibly different, distant particles, with interactions determined by the product of the mobility matrix and the dipole moment. Further, the problem of three identical interacting spheres initially aligned in the streamwise direction is considered and the experimentally observed "pair exchange" phenomenon is derived analytically and confirmed numerically. For nonaligned particles, we demonstrate the formation of a configuration with one particle separating from a stable pair. Our results suggest that in a dilute initially homogenous particulate suspension flowing in a channel the particles will eventually separate into singlets and pairs.

  • 17.
    Gao, Chen-Yi
    et al.
    Chinese Acad Sci, Inst Theoret Phys, Key Lab Theoret Phys, Beijing 100190, Peoples R China.;Univ Chinese Acad Sci, Sch Phys Sci, Beijing 100049, Peoples R China..
    Zhou, Hai-Jun
    Chinese Acad Sci, Inst Theoret Phys, Key Lab Theoret Phys, Beijing 100190, Peoples R China.;Univ Chinese Acad Sci, Sch Phys Sci, Beijing 100049, Peoples R China.;Hunan Normal Univ, Synerget Innovat Ctr Quantum Effects & Applicat, Changsha 410081, Hunan, Peoples R China..
    Aurell, Erik
    KTH, School of Computer Science and Communication (CSC), Computational Biology, CB. Aalto Univ, Dept Appl Phys, Aalto 00076, Finland.;Aalto Univ, Dept Comp Sci, Aalto 00076, Finland..
    Correlation-compressed direct-coupling analysis2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 98, no 3, article id 032407Article in journal (Refereed)
    Abstract [en]

    Learning Ising or Potts models from data has become an important topic in statistical physics and computational biology, with applications to predictions of structural contacts in proteins and other areas of biological data analysis. The corresponding inference problems are challenging since the normalization constant (partition function) of the Ising or Potts distribution cannot be computed efficiently on large instances. Different ways to address this issue have resulted in a substantial amount of methodological literature. In this paper we investigate how these methods could be used on much larger data sets than studied previously. We focus on a central aspect, that in practice these inference problems are almost always severely under-sampled, and the operational result is almost always a small set of leading predictions. We therefore explore an approach where the data are prefiltered based on empirical correlations, which can be computed directly even for very large problems. Inference is only used on the much smaller instance in a subsequent step of the analysis. We show that in several relevant model classes such a combined approach gives results of almost the same quality as inference on the whole data set. It can therefore provide a potentially very large computational speedup at the price of only marginal decrease in prediction quality. We also show that the results on whole-genome epistatic couplings that were obtained in a recent computation-intensive study can be retrieved by our approach. The method of this paper hence opens up the possibility to learn parameters describing pairwise dependences among whole genomes in a computationally feasible and expedient manner.

  • 18.
    Kleeorin, Nathan
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Ben Gurion Univ Negev, Dept Mech Engn, POB 653, IL-8410530 Beer Sheva, Israel.;Nordita Stockholm Univ, S-10691 Stockholm, SwedenUniv Helsinki, Inst Atmospher & Earth Syst Res, Helsinki 00014, Finland..
    Rogachevskii, lgor
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Ben Gurion Univ Negev, Dept Mech Engn, POB 653, IL-8410530 Beer Sheva, Israel.;Nordita Stockholm Univ, S-10691 Stockholm, SwedenUniv Helsinki, Inst Atmospher & Earth Syst Res, Helsinki 00014, Finland..
    Soustova, I. A.
    Russian Acad Sci, Inst Appl Phys, Nizhnii Novgorod 603950, Russia..
    Troitskaya, Yu, I
    Russian Acad Sci, Inst Appl Phys, Nizhnii Novgorod 603950, Russia..
    Ermakova, O. S.
    Russian Acad Sci, Inst Appl Phys, Nizhnii Novgorod 603950, Russia..
    Zilitinkevich, S.
    Univ Helsinki, Inst Atmospher & Earth Syst Res, Helsinki 00014, Finland.;Finnish Meteorol Inst, Helsinki 00101, Finland.;Moscow MV Lomonosov State Univ, Fac Geog, Moscow 119234, Russia.;Tyumen State Univ, Tyumen 625003, Russia..
    Internal gravity waves in the energy and flux budget turbulence-closure theory for shear-free stably stratified flows2019In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 99, no 6, article id 063106Article in journal (Refereed)
    Abstract [en]

    We have advanced the energy and flux budget turbulence closure theory that takes into account a two-way coupling between internal gravity waves (IGWs) and the shear-free stably stratified turbulence. This theory is based on the budget equation for the total (kinetic plus potential) energy of IGWs, the budget equations for the kinetic and potential energies of fluid turbulence, and turbulent fluxes of potential temperature for waves and fluid flow. The waves emitted at a certain level propagate upward, and the losses of wave energy cause the production of turbulence energy. We demonstrate that due to the nonlinear effects more intensive waves produce more strong turbulence, and this, in turn, results in strong damping of IGWs. As a result, the penetration length of more intensive waves is shorter than that of less intensive IGWs. The anisotropy of the turbulence produced by less intensive IGWs is stronger than that caused by more intensive waves. The low-amplitude IGWs produce turbulence consisting up to 90% of turbulent potential energy. This resembles the properties of the observed high-altitude tropospheric strongly anisotropic (nearly two-dimensional) turbulence.

  • 19.
    Liberman, Michael
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Kleeorin, Nathan
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden; Ben-Gurion University of the Negev, Israel.
    Rogachevskii, Igor
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden; Ben-Gurion University of the Negev, Israel.
    Haugen, Nils Erland L.
    Mechanism of unconfined dust explosions: Turbulent clustering and radiation-induced ignition2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 95, no 5, article id 051101Article in journal (Refereed)
    Abstract [en]

    It is known that unconfined dust explosions typically start off with a relatively weak primary flame followed by a severe secondary explosion. We show that clustering of dust particles in a temperature stratified turbulent flow ahead of the primary flame may give rise to a significant increase in the radiation penetration length. These particle clusters, even far ahead of the flame, are sufficiently exposed and heated by the radiation from the flame to become ignition kernels capable to ignite a large volume of fuel-air mixtures. This efficiently increases the total flame surface area and the effective combustion speed, defined as the rate of reactant consumption of a given volume. We show that this mechanism explains the high rate of combustion and overpressures required to account for the observed level of damage in unconfined dust explosions, e.g., at the 2005 Buncefield vapor-cloud explosion. The effect of the strong increase of radiation transparency due to turbulent clustering of particles goes beyond the state of the art of the application to dust explosions and has many implications in atmospheric physics and astrophysics.

  • 20.
    Lindahl, V.
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Physics.
    Lidmar, Jack
    KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hess, Berk
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Physics.
    Riemann metric approach to optimal sampling of multidimensional free-energy landscapes2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 98, no 2, article id 023312Article in journal (Refereed)
    Abstract [en]

    Exploring the free-energy landscape along reaction coordinates or system parameters λ is central to many studies of high-dimensional model systems in physics, e.g., large molecules or spin glasses. In simulations this usually requires sampling conformational transitions or phase transitions, but efficient sampling is often difficult to attain due to the roughness of the energy landscape. For Boltzmann distributions, crossing rates decrease exponentially with free-energy barrier heights. Thus, exponential acceleration can be achieved in simulations by applying an artificial bias along λ tuned such that a flat target distribution is obtained. A flat distribution is, however, an ambiguous concept unless a proper metric is used and is generally suboptimal. Here we propose a multidimensional Riemann metric, which takes the local diffusion into account, and redefine uniform sampling such that it is invariant under nonlinear coordinate transformations. We use the metric in combination with the accelerated weight histogram method, a free-energy calculation and sampling method, to adaptively optimize sampling toward the target distribution prescribed by the metric. We demonstrate that for complex problems, such as molecular dynamics simulations of DNA base-pair opening, sampling uniformly according to the metric, which can be calculated without significant computational overhead, improves sampling efficiency by 50%-70%.

  • 21. Pramanik, S
    et al.
    Wettlaufer, John
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Confinement effects in premelting dynamics2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 5, article id 052801Article in journal (Refereed)
    Abstract [en]

    We examine the effects of confinement on the dynamics of premelted films driven by thermomolecular pressure gradients. Our approach is to modify a well-studied setting in which the thermomolecular pressure gradient is driven by a temperature gradient parallel to an interfacially premelted elastic wall. The modification treats the increase in viscosity associated with the thinning of films, studied in a wide variety of materials, using a power law and we examine the consequent evolution of the confining elastic wall. We treat (1) a range of interactions that are known to underlie interfacial premelting and (2) a constant temperature gradient wherein the thermomolecular pressure gradient is a constant. The difference between the cases with and without the proximity effect arises in the volume flux of premelted liquid. The proximity effect increases the viscosity as the film thickness decreases thereby requiring the thermomolecular pressure driven flux to be accommodated at higher temperatures where the premelted film thickness is the largest. Implications for experiment and observations of frost heave are discussed.

  • 22.
    Radiom, Milad
    et al.
    KTH, School of Chemical Science and Engineering (CHE).
    Borkovec, Michal
    Influence of ligand-receptor interactions on force-extension behavior within the freely jointed chain model2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 6, article id 062501Article in journal (Refereed)
    Abstract [en]

    We study the influence of receptor-ligand interactions on the force response of single polymer chains theoretically. The extension of the chain is modeled in terms of freely jointed chain or elastic freely jointed chain (EFJC) models. The situation involving noninteracting bonds is solved exactly, while effects of interactions are treated within a mean-field approximation. The form with shorter bonds governs the low force situation, while the form with longer bonds is relevant in the high force regime. We further discuss the accuracy of approximate relations, which were used to describe the response of the EFJC model.

  • 23.
    Rosén, Tomas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Kotsubo, Yusuke
    Aidun, Cyrus K.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Orientational dynamics of a triaxial ellipsoid in simple shear flow: Influence of inertia2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 1, article id 013109Article in journal (Refereed)
    Abstract [en]

    The motion of a single ellipsoidal particle in simple shear flow can provide valuable insights toward understanding suspension flows with nonspherical particles. Previously, extensive studies have been performed on the ellipsoidal particle with rotational symmetry, a so-called spheroid. The nearly prolate ellipsoid (one major and two minor axes of almost equal size) is known to perform quasiperiodic or even chaotic orbits in the absence of inertia. With small particle inertia, the particle is also known to drift toward this irregular motion. However, it is not previously understood what effects from fluid inertia could be, which is of highest importance for particles close to neutral buoyancy. Here, we find that fluid inertia is acting strongly to suppress the chaotic motion and only very weak fluid inertia is sufficient to stabilize a rotation around themiddle axis. Themechanism responsible for this transition is believed to be centrifugal forces acting on fluid, which is dragged along with the rotational motion of the particle. With moderate fluid inertia, it is found that nearly prolate triaxial particles behave similarly to the perfectly spheroidal particles. Finally, we also are able to provide predictions about the stable rotational states for the general triaxial ellipsoid in simple shear with weak inertia.

  • 24. Schiulaz, M.
    et al.
    Laumann, C. R.
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Spivak, B. Z.
    Theory of deflagration in disordered media2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 95, no 3, article id 032103Article in journal (Refereed)
    Abstract [en]

    The conventional theory of burning works well in the case of uniform media where all system parameters are spatially independent. We develop a theory of burning in disordered media. In this case, rare regions (hot spots) where the burning process is more effective than on average may control the heat propagation in an explosive sample. We show that most predictions of the theory of burning are quite different from the conventional case. In particular, we show that a system of randomly distributed hot spots exhibits a dynamic phase transition, which is similar to the percolation transition. Depending on parameters of the system the phase transition can be either first or second order. These two regimes are separated by a tricritical point. The above results may be applicable to dynamics of any overheated disordered system with a first order phase transition.

  • 25.
    Schiulaz, Mauro
    et al.
    Boston Univ, Dept Phys, Boston, MA 02215 USA.;Yeshiva Univ, Dept Phys, New York, NY 10016 USA..
    Laumann, Christopher R.
    Boston Univ, Dept Phys, Boston, MA 02215 USA..
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Spivak, Boris Z.
    Univ Washington, Dept Phys, Seattle, WA 98195 USA..
    Theory of combustion in disordered media2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, no 6, article id 062133Article in journal (Refereed)
    Abstract [en]

    The conventional theory of combustion describes systems where all of the parameters are spatially homogeneous. On the other hand, combustion in disordered explosives has long been known to occur after local regions of the material, called hot spots, are ignited. In this article we show that a system of randomly distributed hot spots exhibits a dynamic phase transition, which, depending on parameters of the system, can be either first or second order. These two regimes are separated by a tricritical point. We also show that on the qualitative level the phase diagram of the system is universal. It is valid in two and three dimensions, in the cases when the hot spots interact either by heat or sound waves, and in a broad range of microscopic disorder models.

  • 26.
    Wang, Wenlong
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Díaz-Méndez, Rogelio
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Melting of a two-dimensional monodisperse cluster crystal to a cluster liquid2019In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 99, no 4, article id 042140Article in journal (Refereed)
    Abstract [en]

    Monodisperse ensembles of particles that have cluster crystalline phases at low temperatures can model a number of physical systems, such as vortices in type-1.5 superconductors, colloidal suspensions, and cold atoms. In this work, we study a two-dimensional cluster-forming particle system interacting via an ultrasoft potential. We present a simple mean-field characterization of the cluster-crystal ground state, corroborating with Monte Carlo simulations for a wide range of densities. The efficiency of several Monte Carlo algorithms is compared, and the challenges of thermal equilibrium sampling are identified. We demonstrate that the liquid to cluster-crystal phase transition is of first order and occurs in a single step, and the liquid phase is a cluster liquid. © 2019 American Physical Society.

  • 27.
    Wang, Wenlong
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics. Texas A&M University, USA.
    Moore, M. A.
    Katzgraber, Helmut G.
    Fractal dimension of interfaces in Edwards-Anderson spin glasses for up to six space dimensions2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, no 3, article id 032104Article in journal (Refereed)
    Abstract [en]

    The fractal dimension of domain walls produced by changing the boundary conditions from periodic to antiperiodic in one spatial direction is studied using both the strong-disorder renormalization group algorithm and the greedy algorithm for the Edwards-Anderson Ising spin-glass model for up to six space dimensions. We find that for five or fewer space dimensions, the fractal dimension is lower than the space dimension. This means that interfaces are not space filling, thus implying that replica symmetry breaking is absent in space dimensions fewer than six. However, the fractal dimension approaches the space dimension in six dimensions, indicating that replica symmetry breaking occurs above six dimensions. In two space dimensions, the strong-disorder renormalization group results for the fractal dimension are in good agreement with essentially exact numerical results, but the small difference is significant. We discuss the origin of this close agreement. For the greedy algorithm there is analytical expectation that the fractal dimension is equal to the space dimension in six dimensions and our numerical results are consistent with this expectation.

  • 28.
    Wang, Wenlong
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Physics.
    Chaotic temperature and bond dependence of four-dimensional Gaussian spin glasses with partial thermal boundary conditions2018In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 98, no 6, article id 062122Article in journal (Refereed)
    Abstract [en]

    Spin glasses have competing interactions and complex energy landscapes that are highly susceptible to perturbations, such as the temperature or the bonds. The thermal boundary condition technique is an effective and visual approach for characterizing chaos and has been successfully applied to three dimensions. In this paper, we tailor the technique to partial thermal boundary conditions, where the thermal boundary condition is applied in a subset (three out of four in this work) of the dimensions for better flexibility and efficiency for a broad range of disordered systems. We use this method to study both temperature chaos and bond chaos of the four-dimensional Edwards-Anderson model with Gaussian disorder to low temperatures. We compare the two forms of chaos, with chaos of three dimensions, and also the four-dimensional +/- J model. We observe that the two forms of chaos are characterized by the same set of scaling exponents, bond chaos is much stronger than temperature chaos, and the exponents are also compatible with the +/- J model. Finally, we discuss the effects of chaos on the number of pure states in the thermal boundary condition ensemble.

  • 29. Zhang, Jun
    et al.
    Önskog, Thomas
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematical Statistics.
    Langevin equation elucidates the mechanism of the Rayleigh-Benard instability by coupling molecular motions and macroscopic fluctuations2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 4, article id 043104Article in journal (Refereed)
1 - 29 of 29
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