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  • 1.
    Brandenburg, Axel
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA; Carnegie Mellon University, USA.
    Kahniashvili, Tina
    Mandal, Sayan
    Pol, Alberto Roper
    Tevzadze, Alexander G.
    Vachaspati, Tanmay
    Dynamo effect in decaying helical turbulence2019In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 2, article id 024608Article in journal (Refereed)
    Abstract [en]

    We show that in decaying hydromagnetic turbulence with initial kinetic helicity, a weak magnetic field eventually becomes fully helical. The sign of magnetic helicity is opposite to that of the kinetic helicity-regardless of whether the initial magnetic field was helical. The magnetic field undergoes inverse cascading with the magnetic energy decaying approximately like t(-1/2). This is even slower than in the fully helical case, where it decays like t(-2/3). In this parameter range, the product of magnetic energy and correlation length raised to a certain power slightly larger than unity is approximately constant. This scaling of magnetic energy persists over long timescales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a crossover to the t(-2/3) decay law that is commonly observed for fully helical magnetic fields. Regardless of the presence or absence of initial kinetic helicity, the magnetic field experiences exponential growth during the first few turnover times, which is suggestive of small-scale dynamo action. Our results have applications to a wide range of experimental dynamos and astrophysical time-dependent plasmas, including primordial turbulence in the early universe.

  • 2. Picardo, Jason R.
    et al.
    Bhatnagar, Akshay
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Ray, Samriddhi Sankar
    Lagrangian irreversibility and Eulerian dissipation in fully developed turbulence2020In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, no 4, article id 042601Article in journal (Refereed)
    Abstract [en]

    We revisit the issue of Lagrangian irreversibility in the context of recent results [H. Xu et al., Proc. Natl. Acad. Sci. USA, 111, 7558 (2014)] on flight-crash events in turbulent flows and show how extreme events in the Eulerian dissipation statistics are related to the statistics of power fluctuations for tracer trajectories. Surprisingly, we find that particle trajectories in intense dissipation zones are dominated by energy gains sharper than energy losses, contrary to flight crashes, through a pressure-gradient driven take-off phenomenon. Our conclusions are rationalized by analyzing data from simulations of three-dimensional intermittent turbulence, as well as from nonintermittent decimated flows. Lagrangian irreversibility is found to persist even in the latter case, wherein fluctuations of the dissipation rate are shown to be relatively mild and to follow probability distribution functions with exponential tails.

  • 3. Rana, Chinar
    et al.
    Pramanik, Satyajit
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Indian Institute of Technology Ropar, India.
    Martin, Michel
    De Wit, A.
    Mishra, Manoranjan
    Influence of Langmuir adsorption and viscous fingering on transport of finite size samples in porous media2019In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 10, article id 104001Article in journal (Refereed)
    Abstract [en]

    We examine the transport in a homogeneous porous medium of a finite slice of a solute which adsorbs on the porous matrix following a Langmuir adsorption isotherm and can influence the dynamic viscosity of the solution. In the absence of any viscosity variation, the Langmuir adsorption induces the formation of a shock layer wave at the frontal interface and of a rarefaction wave at the rear interface of the sample. For a finite width sample, these waves interact after a given time that varies nonlinearly with the adsorption properties to give a triangle-like concentration profile in which the mixing efficiency of the solute is larger in comparison to the linear or no-adsorption cases. In the presence of a viscosity contrast such that a less viscous carrier fluid displaces the more viscous finite slice, viscous fingers are formed at the rear rarefaction interface. The fingers propagate through the finite sample to preempt the shock layer at the viscously stable front. In the reverse case, i.e., when the shock layer front features viscous fingering, the fingers are unable to intrude through the rarefaction zone and the qualitative properties of the expanding rear wave are preserved. A nonmonotonic dependence with respect to the Langmuir adsorption parameter b is observed in the onset time of interaction between the nonlinear waves and viscous fingering. The coupled effect of viscous fingering at the rear interface and of Langmuir adsorption provides a powerful mechanism to enhance the mixing efficiency of the adsorbed solute.

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