| 1. |
- Afonso, Marco Martins, et al.
(author)
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Kazantsev dynamo in turbulent compressible flows
- 2019
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In: Proceedings of the Royal Society. Mathematical, Physical and Engineering Sciences. - : ROYAL SOC. - 1364-5021 .- 1471-2946. ; 475:2223
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Journal article (peer-reviewed)abstract
- We consider the kinematic fluctuation dynamo problem in a flow that is random, white-in-time, with both solenoidal and potential components. This model is a generalization of the well-studied Kazantsev model. If both the solenoidal and potential parts have the same scaling exponent, then, as the compressibility of the flow increases, the growth rate decreases but remains positive. If the scaling exponents for the solenoidal and potential parts differ, in particular if they correspond to typical Kolmogorov and Burgers values, we again find that an increase in compressibility slows down the growth rate but does not turn it off. The slow down is, however, weaker and the critical magnetic Reynolds number is lower than when both the solenoidal and potential components display the Kolmogorov scaling. Intriguingly, we find that there exist cases, when the potential part is smoother than the solenoidal part, for which an increase in compressibility increases the growth rate. We also find that the critical value of the scaling exponent above which a dynamo is seen is unity irrespective of the compressibility. Finally, we realize that the dimension d = 3 is special, as for all other values of d the critical exponent is higher and depends on the compressibility.
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| 2. |
- Agrawal, Vipin, 1994-, et al.
(author)
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Active buckling of pressurized spherical shells : Monte Carlo simulation
- 2023
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In: Physical review. E. - : American Physical Society (APS). - 2470-0045 .- 2470-0053. ; 108:3
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Journal article (peer-reviewed)abstract
- We study the buckling of pressurized spherical shells by Monte Carlo simulations in which the detailed balance is explicitly broken—thereby driving the shell to be active, out of thermal equilibrium. Such a shell typically has either higher (active) or lower (sedate) fluctuations compared to one in thermal equilibrium depending on how the detailed balance is broken. We show that, for the same set of elastic parameters, a shell that is not buckled in thermal equilibrium can be buckled if turned active. Similarly a shell that is buckled in thermal equilibrium can unbuckle if sedated. Based on this result, we suggest that it is possible to experimentally design microscopic elastic shells whose buckling can be optically controlled.
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| 3. |
- Agrawal, Vipin, 1994-, et al.
(author)
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Chaos and irreversibility of a flexible filament in periodically driven Stokes flow
- 2022
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In: Physical review. E. - : American Physical Society (APS). - 2470-0045 .- 2470-0053. ; 106:2
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Journal article (peer-reviewed)abstract
- The flow of Newtonian fluid at low Reynolds number is, in general, regular and time-reversible due to absence of nonlinear effects. For example, if the fluid is sheared by its boundary motion that is subsequently reversed, then all the fluid elements return to their initial positions. Consequently, mixing in microchannels happens solely due to molecular diffusion and is very slow. Here, we show, numerically, that the introduction of a single, freely floating, flexible filament in a time-periodic linear shear flow can break reversibility and give rise to chaos due to elastic nonlinearities, if the bending rigidity of the filament is within a carefully chosen range. Within this range, not only the shape of the filament is spatiotemporally chaotic, but also the flow is an efficient mixer. Overall, we find five dynamical phases: the shape of a stiff filament is time-invariant-either straight or buckled; it undergoes a period-two bifurcation as the filament is made softer; becomes spatiotemporally chaotic for even softer filaments but, surprisingly, the chaos is suppressed if bending rigidity is decreased further.
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| 4. |
- Agrawal, Vipin, 1994-, et al.
(author)
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Flexible filament in time-periodic viscous flow: shape chaos and period three
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Other publication (other academic/artistic)abstract
- We study a single, freely--floating, inextensible, elastic filament in a linear shear flow: U0(x,y)=γyx̂ . In our model: the elastic energy depends only on bending; the rate-of-strain, γ=Ssin(ωt) is a periodic function of time, t; and the interaction between the filament and the flow is approximated by a local isotropic drag force. Based on the shape of the filament we find five different dynamical phases: straight, buckled, periodic (with period two, period three, period four, etc), chaotic, and one with chaotic transients. In the chaotic phase, we show that the iterative map for the angle, which the end-to-end vector of the filament makes with the tangent it's one end, has period three solutions; hence it is chaotic. Furthermore, in the chaotic phase, the flow is an efficient mixer.
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| 5. |
- Agrawal, Vipin, 1994-, et al.
(author)
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MeMC : A package for Monte Carlo simulations of spherical shells
- 2022
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In: Journal of Open Source Software. - : The Open Journal. - 2475-9066. ; 7:74
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Journal article (peer-reviewed)abstract
- The MeMC is an open-source software package for Monte Carlo simulation of elastic shells. It is designed as a tool to interpret the force-distance data generated by indentation of biological nano-vesicles by atomic force microscopes. The code is written in c++ and python. The code is customizable – new modules can be added in a straightforward manner.
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| 6. |
- Agrawal, Vipin, 1994-
(author)
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Shells and filament in flows
- 2022
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Doctoral thesis (other academic/artistic)abstract
- The motivation to study elastic structures such as filaments and shells stemmed from its application in the construction of tall buildings, bridges etc. Interest in this field has rekindled in the past decades due to growing interest in understanding biological materials and because of possible applications in nanoscience and medicine. This also poses new challenges as the biological materials show both solid and fluid-like behavior, and in addition, they are active. In this thesis, we study the mechanical properties of shells and filament and their interaction with fluid. The thesis is divided into two themes. First, how to model nano-vesicles and how are the mechanical properties affected if a spherical shell is thermal and active? Second, can non-linear interaction between fluid and filament generate turbulence and hence mixing in the Stokes flow?To model nano-vesicles, we develop an open-source software package - MeMC. The MeMC models nano-vesicles as an elastic objects. It interprets the force-distance data generated by indentation of biological nano-vesicles by atomic force microscopes and uses Monte-Carlo simulations to compute elastic coefficients of a nano-vesicle. Further, we use this code and break the detailed balance in Monte-Carlo simulation - thereby driving the shell active and out of thermal equilibrium - to study the effect of activity on mechanical properties of elastic shells, in particular, buckling. Such a shell typically has either higher (active) or lower (quiescent) fluctuations compared to one in thermal equilibrium depending on how the detailed balance is broken. We show that for the same set of elastic parameters, a shell that is not buckled in thermal equilibrium can be buckled if turned active. Similarly, a shell that is buckled in thermal equilibrium can unbuckle if turned quiescent. Based on this result, we suggest that it is possible to experimentally design microscopic elastic shells whose buckling can be optically controlled.In the next part of the thesis, we visit the problem of mixing in Stokes flow using elastic filament. We study the interaction of the filament with Stokes flow. As it is known, the flow of Newtonian fluid at low Reynolds number is, in general, regular, and time-reversible due to absence of nonlinear effects. For example, if the fluid is sheared by its boundary motion that is subsequently reversed, then all the fluid elements return to their initial positions. Consequently, mixing in microchannels happens solely due to molecular diffusion and is very slow. Here, we show, numerically, that the introduction of a single, freely floating, flexible filament in a time-periodic linear shear flow can break time reversibility and give rise to chaos due to elastic nonlinearities, if the bending rigidity of the filament is within a carefully chosen range. Within this range, not only the shape of the filament is spatiotemporally chaotic, but also the flow is an efficient mixer. We model the filament using the bead-rod model. We consider two different models for the viscous forces: (a) they are modelled by the Rotne-Prager tensor. This incorporates the hydrodynamic interaction between every pair of beads. (b) we consider only the diagonal term of the Rotne-Prager tensor which makes the viscous forces local. In both cases, we find the same qualitative result: the shape of a stiff filament is time-invariant -- either straight or buckled for large enough bending rigidity; it undergoes a period-n bifurcation (n = 2,3, 4, etc) as the filament is made softer; becomes spatiotemporally chaotic for even softer filaments. For case (a) but not for (b) we find that the chaos is suppressed if bending rigidity is decreased further. For (b), in the chaotic phase, we show that the iterative map for the angle, which the end–to–end vector of the filament makes with the tangent its one end, has period three solutions; hence it is chaotic.
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| 7. |
- Bagheri, Faranggis, et al.
(author)
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Statistics of polymer extensions in turbulent channel flow
- 2012
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In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 86:5, s. 056314-
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Journal article (peer-reviewed)abstract
- We present direct numerical simulations of turbulent channel flow with passive Lagrangian polymers. To understand the polymer behavior we investigate the behavior of infinitesimal line elements and calculate the probability distribution function (PDF) of finite-time Lyapunov exponents and from them the corresponding Cramer's function for the channel flow. We study the statistics of polymer elongation for both the Oldroyd-B model (for Weissenberg number Wi<1) and the FENE model. We use the location of the minima of the Cramer's function to define the Weissenberg number precisely such that we observe coil-stretch transition at Wi1. We find agreement with earlier analytical predictions for PDF of polymer extensions made by Balkovsky, Fouxon, and Lebedev for linear polymers (Oldroyd-B model) with Wi <1 and by Chertkov for nonlinear FENE-P model of polymers. For Wi >1 (FENE model) the polymer are significantly more stretched near the wall than at the center of the channel where the flow is closer to homogenous isotropic turbulence. Furthermore near the wall the polymers show a strong tendency to orient along the streamwise direction of the flow, but near the center line the statistics of orientation of the polymers is consistent with analogous results obtained recently in homogeneous and isotropic flows.
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| 8. |
- Bhatnagar, Akshay, et al.
(author)
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Deviation-angle and trajectory statistics for inertial particles in turbulence
- 2016
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In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1539-3755 .- 1550-2376. ; 94:6
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Journal article (peer-reviewed)abstract
- Small particles in suspension in a turbulent fluid have trajectories that do not follow the pathlines of the flow exactly. We investigate the statistics of the angle of deviation φ between the particle and fluid velocities. We show that, when the effects of particle inertia are small, the probability distribution function (PDF) Pφ of this deviation angle shows a power-law region in which Pφ∼φ-4. We also find that the PDFs of the trajectory curvature κ and modulus θ of the torsion have power-law tails that scale, respectively, as Pκ∼κ-5/2, as κ→∞, and Pθ∼θ-3, as θ→∞: These exponents are in agreement with those previously observed for fluid pathlines. We propose a way to measure the complexity of heavy-particle trajectories by the number NI(t,St) of points (up until time t) at which the torsion changes sign. We present numerical evidence that nI(St)≡limt→∞NI(t,St)t∼St-Δ for large St, with Δ≃0.5.
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| 9. |
- Bhatnagar, Akshay, et al.
(author)
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Heavy inertial particles in turbulent flows gain energy slowly but lose it rapidly
- 2018
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In: Physical review. E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 97:3
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Journal article (peer-reviewed)abstract
- 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.
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| 10. |
- Bhatnagar, Akshay, et al.
(author)
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How long do particles spend in vortical regions in turbulent flows?
- 2016
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In: Physical Review E. - 2470-0045. ; 94:5
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Journal article (peer-reviewed)abstract
- We obtain the probability distribution functions (PDFs) of the time that a Lagrangian tracer or a heavy inertial particle spends in vortical or strain-dominated regions of a turbulent flow, by carrying out direct numerical simulations of such particles advected by statistically steady, homogeneous, and isotropic turbulence in the forced, three-dimensional, incompressible Navier-Stokes equation. We use the two invariants, Q and R, of the velocity-gradient tensor to distinguish between vortical and strain-dominated regions of the flow and partition the Q-R plane into four different regions depending on the topology of the flow; out of these four regions two correspond to vorticity-dominated regions of the flow and two correspond to strain-dominated ones. We obtain Q and R along the trajectories of tracers and heavy inertial particles and find out the time t(pers) for which they remain in one of the four regions of the Q-R plane. We find that the PDFs of tpers display exponentially decaying tails for all four regions for tracers and heavy inertial particles. From these PDFs we extract characteristic time scales, which help us to quantify the time that such particles spend in vortical or strain-dominated regions of the flow.
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| 11. |
- Bhatnagar, Akshay, et al.
(author)
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Rate of formation of caustics in heavy particles advected by turbulence
- 2022
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In: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 380:2219
-
Journal article (peer-reviewed)abstract
- The rate of collision and the relative velocities of the colliding particles in turbulent flows are a crucial part of several natural phenomena, e.g. rain formation in warm clouds and planetesimal formation in protoplanetary discs. The particles are often modelled as passive, but heavy and inertial. Within this model, large relative velocities emerge due to formation of singularities (caustics) of the gradient matrix of the velocities of the particles. Using extensive direct numerical simulations of heavy particles in both two (direct and inverse cascade) and three-dimensional turbulent flows, we calculate the rate of formation of caustics, J as a function of the Stokes number (St). The best approximation to our data is J similar to exp(-C/St), in the limit St -> 0 where C is a non-universal constant. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.
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| 12. |
- Bhatnagar, Akshay, et al.
(author)
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Relative velocities in bidisperse turbulent aerosols : Simulations and theory
- 2018
-
In: Physical review. E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 98:6
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Journal article (peer-reviewed)abstract
- 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.
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| 13. |
- Bhatnagar, Akshay, et al.
(author)
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Statistics of the relative velocity of particles in turbulent flows: Monodisperse particles
- 2018
-
In: Physical Review E. - : AMER PHYSICAL SOC. - 2470-0045 .- 2470-0053. ; 97:2
-
Journal article (peer-reviewed)abstract
- 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.
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| 14. |
- Bhatnagar, A., et al.
(author)
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Universal statistical properties of inertial-particle trajectories in three-dimensional, homogeneous, isotropic, fluid turbulence
- 2015
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In: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
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Conference paper (peer-reviewed)abstract
- We obtain new universal statistical properties of heavy-particle trajectories in three-dimensional, statistically steady, homogeneous, and isotropic turbulent flows by direct numerical simulations. We show that the probability distribution functions (PDFs) P(φ), of the angle φ between the Eulerian velocity u and the particle velocity v, at a point and time, scales as P(φ) ∼ φ−γ, with a new universal exponent γ ≃ 4. The PDFs of the trajectory curvature κ and modulus θ of the torsion ϑ scale, respectively, as P(κ) ∼ κ−hκ, as κ → ∞, and P(θ) ∼ θ−hθ, as θ → ∞, with exponents hκ ≃ 2.5 and hθ ≃ 3 that do not depend on the Stokes number St. We also show that γ, hκ and hθ can be obtained by using simple stochastic models. We show that the number NI(t,St) of points (up until time t), at which ϑ changes sign, is such that nI(St) ≡ limt→∞ NI(tSt) ∼ St−∆, with ∆ ≃ 0.4 a universal exponent. t
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| 15. |
- Bonanno, Alfio, et al.
(author)
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Breakdown of chiral symmetry during saturation of the Tayler instability
- 2012
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In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 86:1, s. 016313-
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Journal article (peer-reviewed)abstract
- We study spontaneous breakdown of chiral symmetry during the nonlinear evolution of the Tayler instability. We start with an initial steady state of zero helicity. Within linearized perturbation calculations, helical perturbations of this initial state have the same growth rate for either sign of helicity. Direct numerical simulations (DNS) of the fully nonlinear equations, however, show that an infinitesimal excess of one sign of helicity in the initial perturbation gives rise to a saturated helical state. We further show that this symmetry breaking can be described by weakly nonlinear finite-amplitude equations with undetermined coefficients which can be deduced solely from symmetry consideration. By fitting solutions of the amplitude equations to data from DNS, we further determine the coefficients of the amplitude equations.
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| 16. |
- Bonfils, Anthony F., 1994-, et al.
(author)
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Asymptotic interpretation of the Miles mechanism of wind-wave instability
- 2022
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In: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 944
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Journal article (peer-reviewed)abstract
- When wind blows over water, ripples are generated on the water surface. These ripples can be regarded as perturbations of the wind field, which is modelled as a parallel inviscid flow. For a given wavenumber k, the perturbed streamfunction of the wind field and the complex phase speed are the eigenfunction and the eigenvalue of the so-called Rayleigh equation in a semi-infinite domain. Because of the small air-water density ratio, rho(a)/rho(w) epsilon << 1, the wind and the ripples are weakly coupled, and the eigenvalue problem can be solved perturbatively. At the leading order, the eigenvalue is equal to the phase speed c(0) of surface waves. At order epsilon, the eigenvalue has a finite imaginary part, which implies growth. Miles (J. Fluid Mech., vol. 3, 1957, pp. 185-204) showed that the growth rate is proportional to the square modulus of the leading-order eigenfunction evaluated at the so-called critical level z = z
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| 17. |
- Bonfils, Anthony, 1994-, et al.
(author)
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Flow-driven interfacial waves : An inviscid asymptotic study
- 2023
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In: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 976
-
Journal article (peer-reviewed)abstract
- Motivated by wind blowing over water, we use asymptotic methods to study the evolution of short wavelength interfacial waves driven by the combined action of these flows. We solve the Rayleigh equation for the stability of the shear flow, and construct a uniformly valid approximation for the perturbed streamfunction, or eigenfunction. We then expand the real part of the eigenvalue, the phase speed, in a power series of the inverse wavenumber and show that the imaginary part is exponentially small. We give expressions for the growth rates of the Miles (J. Fluid Mech., vol. 3, 1957, pp. 185-204) and rippling (e.g. Young & Wolfe, J. Fluid Mech., vol. 739, 2014, pp. 276-307) instabilities that are valid for an arbitrary shear flow. The accuracy of the results is demonstrated by a comparison with the exact solution of the eigenvalue problem in the case when both the wind and the current have an exponential profile.
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| 18. |
- Bonfils, Anthony, 1994-, et al.
(author)
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Flow driven interfacial waves: an asymptotic study
-
Other publication (other academic/artistic)abstract
- We use asymptotic methods to study the evolution of short wavelength interfacial waves driven by the combined action of wind and current. We solve the Rayleigh equation for the stability of the shear flow, and construct a uniformly valid approximation for the perturbed streamfunction, or eigenfunction. We then expand the real part of the eigenvalue, the phase speed, in a power series of the inverse wavenumber and show that the imaginary part is exponentially small. We give expressions for the growth rates of the Miles (1957) and rippling (e.g., Young & Wolfe 2013) instabilities that are valid for an arbitrary shear flow. The accuracy of the results is demonstrated by a comparison with the exact solution of the eigenvalue problem in the case when both the wind and the current have an exponential profile.
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| 19. |
- Bonfils, Anthony, 1994-
(author)
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Waves and instabilities through the lens of asymptotic analysis
- 2023
-
Doctoral thesis (other academic/artistic)abstract
- Understanding the interaction of water waves with winds and marine currents is a fundamental problem in geophysical fluid dynamics. From the point of view of hydrodynamic stability, surface waves are regarded as perturbations of an inviscid parallel shear flow modeling the wind in the air and the current in the water. For small two-dimensional perturbations, the linearization of the Euler equation of motion yields an eigenvalue problem to be solved for a given wavenumber k. The eigenfunction is a streamfunction obeying the so-called Rayleigh equation. The eigenvalue is a complex phase speed, c, whose real part is the actual phase speed of sheared waves while the imaginary part of kc is the growth rate of the wave amplitude. Using the smallness of the air/water density ratio and assuming no flow in the water, Miles solved this eigenvalue problem perturbatively in 1957. He uncovered an instability of the wind field due to a critical layer in the air, where the wind speed equals the phase speed of free surface waves, and showed that the growth rate of wind-waves is proportional to the square modulus of the solution of the Rayleigh equation at the critical level. This level is a regular singular point, which makes the resolution of the Rayleigh equation challenging. For that reason, an explicit expression of the growth rate of the Miles instability as a function of the wavenumber was lacking. Firstly, I designed a numerical scheme to solve the Rayleigh equation for an arbitrary monotonic wind profile. Secondly, I solved it analytically using asymptotic methods for long and short waves.In physical oceanography, a standard model for the mean turbulent wind field is the logarithmic profile, which contains only one length scale: the roughness length, z0 ~1 mm, accounting for the presence of waves on the water surface. I am interested in waves propagating due to gravity and surface tension, which have wavelengths ranging from a few millimeters to hundreds of meters. Hence, a natural small parameter is kz0, which I used to obtain long wave solutions of the Rayleigh equation, and subsequently the growth rate of the Miles instability. The comparison with both numerical and measured growth rates is excellent. Furthermore, I approximated the maximum growth rate in the strong wind limit, and inferred that the fastest growing wave is such that the aerodynamic pressure is in phase with the wave slope.I also considered the short wave limit of the eigenvalue problem. Using 1/(kL) as a small parameter, where L is a characteristic length scale of the shear, I found general asymptotic solutions for interfacial waves in presence of a wind and a current, where the density ratio does not need to be small. One application concerns the mixing of elements at the surface of white dwarfs. Moreover, short wave asymptotics provide insights on another instability. When waves have a phase speed that matches the current speed, there is another critical layer, in the water, which is responsible for the so-called rippling instability. I obtained a general asymptotic formula for the growth rate of this instability.Finally, I used my experience in solving eigenvalue problems to study, in collaboration with other researchers, wrinkles in thin elastic sheets floating on a liquid foundation. We had to solve a fourth order eigenvalue problem where the eigenvalue is the compressive load imposed on the sheet and the eigenfunction is the vertical displacement. For homogeneous sheets, the bending stiffness of the sheet is constant and the eigenvalue problem could be solved analytically. We found that the buckling shape has a symmetric and an antisymmetric mode. The mode associated with the minimum compressive load depends on the size of the confined sheet. Hence, there are changes of symmetry at certain confinement sizes for which the buckling shape is degenerate. We numerically showed that this degeneracy disappears for composite sheets, whose bending stiffness depends on space due to the presence of liquid inclusions.
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| 20. |
- Brandenburg, Axel, et al.
(author)
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Detection of Negative Effective Magnetic Pressure Instability in Turbulence Simulations
- 2011
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In: Astrophysical Journal, ISSN 0004-637X, EISSN 1538-4357. ; 740:2, s. L50-
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Journal article (peer-reviewed)abstract
- We present the first numerical demonstration of the negative effective magnetic pressure instability in direct numerical simulations of stably stratified, externally forced, isothermal hydromagnetic turbulence in the regime of large plasma beta. By the action of this instability, initially uniform horizontal magnetic field forms flux concentrations whose scale is large compared to the turbulent scale. We further show that the magnetic energy of these large-scale structures is only weakly dependent on the magnetic Reynolds number. Our results support earlier mean-field calculations and analytic work that identified this instability. Applications to the formation of active regions in the Sun are discussed.
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| 21. |
- Candelaresi, Simon, 1982-, et al.
(author)
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Magnetic helicity transport in the advective gauge family
- 2011
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In: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 18:1, s. 012903-
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Journal article (peer-reviewed)abstract
- Magnetic helicity fluxes are investigated in a family of gauges in which the contribution from ideal magnetohydrodynamics takes the form of a purely advective flux. Numerical simulations of magnetohydrodynamic turbulence in this advective gauge family exhibit instabilities triggered by the build-up of unphysical irrotational contributions to the magnetic vector potential. As a remedy, the vector potential is evolved in a numerically well behaved gauge, from which the advective vector potential is obtained by a gauge transformation. In the kinematic regime, the magnetic helicity density evolves similarly to a passive scalar when resistivity is small and turbulent mixing is mild, i.e., when the fluid Reynolds number is not too large. In the dynamical regime, resistive contributions to the magnetic helicity flux in the advective gauge are found to be significant owing to the development of small length scales in the irrotational part of the magnetic vector potential.
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| 22. |
- Cedenblad, Lukas, et al.
(author)
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Planetesimals on Eccentric Orbits Erode Rapidly
- 2021
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 921:2
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Journal article (peer-reviewed)abstract
- We investigate the possibility of erosion of planetesimals in a protoplanetary disk. We use theory and direct numerical simulations (lattice Boltzmann method) to calculate the erosion of large-much larger than the mean-free path of gas molecules-bodies of different shapes in flows. We find that erosion follows a universal power law in time, at intermediate times, independent of the Reynolds number of the flow and the initial shape of the body. Consequently, we estimate that planetesimals in eccentric orbits, of even very small eccentricity, rapidly (in about 100 yr) erodes away if the semimajor axis of their orbit lies in the inner disk-less than about 10 au. Even planetesimals in circular orbits erode away in approximately 10,000 yr if the semimajor axis of their orbits are <0.6 au.
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| 23. |
- Chan, Chi-kwan, et al.
(author)
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Dynamics of saturated energy condensation in two-dimensional turbulence
- 2012
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In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 85:3, s. 036315-
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Journal article (peer-reviewed)abstract
- In two-dimensional forced Navier-Stokes turbulence, energy cascades to the largest scales in the system to form a pair of coherent vortices known as the Bose condensate. We show, both numerically and analytically, that the energy condensation saturates and the system reaches a statistically stationary state. The time scale of saturation is inversely proportional to the viscosity and the saturation energy level is determined by both the viscosity and the force. We further show that, without sufficient resolution to resolve the small-scale enstrophy spectrum, numerical simulations can give a spurious result for the saturation energy level. We also find that the movement of the condensate is similar to the motion of an inertial particle with an effective drag force. Furthermore, we show that the profile of the saturated coherent vortices can be described by a Gaussian core with exponential wings.
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| 24. |
- Chatterjee, Piyali, et al.
(author)
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Spontaneous chiral symmetry breaking by hydromagnetic buoyancy
- 2011
-
In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 84:2, s. 25403(R)-
-
Journal article (peer-reviewed)abstract
- Evidence for the parity-breaking nature of the magnetic buoyancy instability in a stably stratified gas is reported. In the absence of rotation, no helicity is produced, but the nonhelical state is found to be unstable to small helical perturbations during the development of the instability. The parity-breaking nature of this magnetohydrodynamic instability appears to be the first of its kind and has properties similar to those in chiral symmetry breaking in biochemistry. Applications to the production of mean fields in galaxy clusters are discussed.
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| 25. |
- De, Sadhitro, et al.
(author)
-
Dynamic multiscaling in stochastically forced Burgers turbulence
- 2023
-
In: Scientific Reports. - : Springer Nature. - 2045-2322. ; 13:1
-
Journal article (peer-reviewed)abstract
- We carry out a detailed study of dynamic multiscaling in the turbulent nonequilibrium, but statistically steady, state of the stochastically forced one-dimensional Burgers equation. We introduce the concept of interval collapse time, which we define as the time taken for a spatial interval, demarcated by a pair of Lagrangian tracers, to collapse at a shock. By calculating the dynamic scaling exponents of the moments of various orders of these interval collapse times, we show that (a) there is not one but an infinity of characteristic time scales and (b) the probability distribution function of the interval collapse times is non-Gaussian and has a power-law tail. Our study is based on (a) a theoretical framework that allows us to obtain dynamic-multiscaling exponents analytically, (b) extensive direct numerical simulations, and (c) a careful comparison of the results of (a) and (b). We discuss possible generalizations of our work to higher dimensions, for the stochastically forced Burgers equation, and to other compressible flows that exhibit turbulence with shocks.
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| 26. |
- De, Sadhitro, et al.
(author)
-
Uncovering the multifractality of Lagrangian pair dispersion in shock-dominated turbulence
- 2024
-
In: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 6:2
-
Journal article (peer-reviewed)abstract
- Lagrangian pair dispersion provides insights into mixing in turbulent flows. By direct numerical simulations (DNSs) we show that the statistics of pair dispersion in the randomly forced two-dimensional Burgers equation, which is a typical model of shock-dominated turbulence, is very different from its incompressible counterpart because Lagrangian particles get trapped in shocks. We develop a heuristic theoretical framework that accounts for this - a generalization of the multifractal model - whose prediction of the scaling of Lagrangian exit times agrees well with our DNS.
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| 27. |
- Devlen, Ebru, et al.
(author)
-
A mean field dynamo from negative eddy diffusivity
- 2013
-
In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 432:2, s. 1651-1657
-
Journal article (peer-reviewed)abstract
- Using direct numerical simulations, we verify that Roberts-IV flow exhibits dynamo action dominated by horizontally averaged large-scale magnetic field. With the test-field method, we compute the turbulent magnetic diffusivity and find that it is negative and overcomes the molecular diffusivity, thus explaining quantitatively the large-scale dynamo for magnetic Reynolds numbers above approximate to 8. As expected for a dynamo of this type, but contrary to alpha-effect dynamos, the two horizontal field components grow independently of each other and have arbitrary amplitude ratios and phase differences. Small length-scales of the mean magnetic field are shown to be stabilized by the turbulent magnetic diffusivity becoming positive at larger wavenumbers. Oscillatory decaying or growing solutions have also been found in certain wavenumber intervals and sufficiently large values of the magnetic Reynolds number. For magnetic Reynolds numbers below approximate to 0.5, the turbulent magnetic diffusivity is confirmed to be positive, as expected for all incompressible flows. Earlier claims of a dynamo driven by a modified Taylor-Green flow through negative eddy diffusivity could not be confirmed.
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| 28. |
- Dikpati, Mausumi, et al.
(author)
-
DATA ASSIMILATION IN A SOLAR DYNAMO MODEL USING ENSEMBLE KALMAN FILTERS : SENSITIVITY AND ROBUSTNESS IN RECONSTRUCTION OF MERIDIONAL FLOW SPEED
- 2016
-
In: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 828:2
-
Journal article (peer-reviewed)abstract
- We implement an Ensemble Kalman Filter procedure using the. Data Assimilation Research Testbed for assimilating "synthetic" meridional flow-speed data in a Babcock-Leighton-type flux-transport solar dynamo model. By performing several "observing system simulation experiments," we reconstruct time. variation in meridional flow. speed and analyze sensitivity and robustness of reconstruction. Using 192 ensemble members including 10 observations, each with 4% error, we find that flow. speed is reconstructed best if observations of near-surface poloidal fields from low. latitudes and tachocline toroidal fields from midlatitudes are assimilated. If observations include a mixture of poloidal and toroidal fields from different latitude locations, reconstruction is reasonably good for. <= 40% error in low-latitude data, even if observational error in polar region data becomes 200%, but deteriorates when observational error increases in low- and midlatitude data. Solar polar region observations are known to contain larger errors than those in low latitudes; our forward operator (a flux-transport dynamo model here) can sustain larger errors in polar region data, but is more sensitive to errors in low- latitude data. An optimal reconstruction is obtained if an assimilation interval of 15 days is used; 10- and 20-day assimilation intervals also give reasonably good results. Assimilation intervals <5 days do not produce faithful reconstructions of flow. speed, because the system requires a minimum time to develop dynamics to respond to flow. variations. Reconstruction also deteriorates if an assimilation interval >45 days is used, because the system's inherent memory interferes with its short-term dynamics during a substantially long run without updating.
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| 29. |
- Dikpati, Mausumi, et al.
(author)
-
Ensemble Kalman filter data assimilation in a Babcock-Leighton solar dynamo model : An observation system simulation experiment for reconstructing meridional flow speed
- 2014
-
In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:15, s. 5361-5369
-
Journal article (peer-reviewed)abstract
- Accurate knowledge of time variation in meridional flow speed and profile is crucial for estimating the solar cycle's features, which are ultimately responsible for causing space climate variations. However, no consensus has been reached yet about the Sun's meridional circulation pattern observations and theories. By implementing an ensemble Kalman filter (EnKF) data assimilation in a Babcock-Leighton solar dynamo model using Data Assimilation Research Testbed framework, we find that the best reconstruction of time variation in meridional flow speed can be obtained when 10 or more observations are used with an updating time of 15 days and a 10% observational error. Increasing ensemble size from 16 to 160 improves reconstruction. Comparison of reconstructed flow speed with true state reveals that EnKF data assimilation is very powerful for reconstructing meridional flow speeds and suggests that it can be implemented for reconstructing spatiotemporal patterns of meridional circulation.
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| 30. |
- Dikpati, Mausumi, et al.
(author)
-
Role of response time of a Babcock-Leighton solar dynamo model in meridional flow-speed reconstruction by EnKF data assimilation
- 2016
-
In: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 58:8, s. 1589-1595
-
Journal article (peer-reviewed)abstract
- Ensemble Kalman Filter in the framework of Data Assimilation Research Testbed (DART) has been successfully implemented into a 2D kinematic flux-transport dynamo model by Dikpati and colleagues in order to do a parameter estimation, the parameter being the meridional flow-speed as function of time. They performed several 'Observing System Simulation Experiments' (OSSEs), and showed that an optimal reconstruction of time-series of meridional flow-speed can be obtained by using 16 ensemble members and only one surface magnetic observation with 30% observational error. Error in reconstruction can be reduced by increasing the ensemble size and number of observations. However, this parameter reconstruction has been found to be sensitive to locations from where observational data are taken. While assimilation of low-latitudes' surface poloidal magnetic field data can produce good reconstruction, medium frequency oscillations appear in time-series of reconstructed flow-speed if tachocline toroidal field data are assimilated. These oscillations occur primarily because tachocline toroidal fields change very little during an assimilation interval taken to be 15 days, due to changes in meridional flow. A Babcock-Leighton dynamo model's response time to changes in meridional flow-speed is a few months. We show here that rms error in reconstruction can be significantly reduced if model's response time is taken into consideration in assimilation of tachocline toroidal field data.
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| 31. |
- Fontcuberta, Aleix Espuna, et al.
(author)
-
Forecasting Solar Cycle 25 with Physical Model-Validated Recurrent Neural Networks
- 2023
-
In: Solar Physics. - : Springer Nature. - 0038-0938 .- 1573-093X. ; 298:1
-
Journal article (peer-reviewed)abstract
- The Sun's activity, which is associated with the solar magnetic cycle, creates a dynamic environment in space known as space weather. Severe space weather can disrupt space-based and Earth-based technologies. Slow decadal-scale variations on solar-cycle timescales are important for radiative forcing of the Earth's atmosphere and impact satellite lifetimes and atmospheric dynamics. Predicting the solar magnetic cycle is therefore of critical importance for humanity. In this context, a novel development is the application of machine-learning algorithms for solar-cycle forecasting. Diverse approaches have been developed for this purpose; however, with no consensus across different techniques and physics-based approaches. Here, we first explore the performance of four different machine-learning algorithms - all of them belonging to a class called Recurrent Neural Networks (RNNs) - in predicting simulated sunspot cycles based on a widely studied, stochastically forced, nonlinear time-delay solar dynamo model. We conclude that the algorithm Echo State Network (ESN) performs the best, but predictability is limited to only one future sunspot cycle, in agreement with recent physical insights. Subsequently, we train the ESN algorithm and a modified version of it (MESN) with solar-cycle observations to forecast Cycles 22 - 25. We obtain accurate hindcasts for Solar Cycles 22 - 24. For Solar Cycle 25 the ESN algorithm forecasts a peak amplitude of 131 +/- 14 sunspots around July 2024 and indicates a cycle length of approximately 10 years. The MESN forecasts a peak of 137 +/- 2 sunspots around April 2024, with the same cycle length. Qualitatively, both forecasts indicate that Cycle 25 will be slightly stronger than Cycle 24 but weaker than Cycle 23. Our novel approach bridges physical model-based forecasts with machine-learning-based approaches, achieving consistency across these diverse techniques.
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| 32. |
- Fornari, Walter, et al.
(author)
-
Rheology of Confined Non-Brownian Suspensions
- 2016
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 116:1
-
Journal article (peer-reviewed)abstract
- We study the rheology of confined suspensions of neutrally buoyant rigid monodisperse spheres in plane-Couette flow using direct numerical simulations. We find that if the width of the channel is a (small) integer multiple of the sphere diameter, the spheres self-organize into two-dimensional layers that slide on each other and the effective viscosity of the suspension is significantly reduced. Each two-dimensional layer is found to be structurally liquidlike but its dynamics is frozen in time.
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| 33. |
- Fornari, Walter, 1989-, et al.
(author)
-
Rheology of extremely confined non-Brownian suspensions
- 2016
-
In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 116:1
-
Journal article (other academic/artistic)abstract
- We study the rheology of confined suspensions of neutrally buoyant rigid monodisperse spheres in plane-Couetteflow using Direct Numerical Simulations.We find that if the width of the channel is a (small) integer multiple of the spherediameter, the spheres self-organize into two-dimensional layersthat slide on each other and the effective viscosity of the suspension issignificantly reduced. Each two-dimensional layer is found to be structurallyliquid-like but its dynamics is frozen in time.
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| 34. |
- Haugen, Nils Erland L., et al.
(author)
-
The effect of turbulence on mass transfer rates of small inertial particles with surface reactions
- 2017
-
In: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 836, s. 932-951
-
Journal article (peer-reviewed)abstract
- The effect of turbulence on the mass transfer between a fluid and embedded small heavy inertial particles that experience surface reactions is studied. For simplicity, the surface reaction, which takes place when a gas phase reactant is converted to a gas phase product at the external surface of the particles, is unimolar and isothermal. Two effects are identified. The first effect is due to the relative velocity between the fluid and the particles, and a model for the relative velocity is presented. The second effect is due to the clustering of particles, where the mass transfer rate is inhibited due to the rapid depletion of the consumed species inside the dense particle clusters. This last effect is relevant for large Damkohler numbers, where the Damkohler number is defined as the ratio of the turbulent and chemical time scales, and it may totally control the mass transfer rate for Damkohler numbers larger than unity. A model that describes how this effect should be incorporated into existing simulation tools that utilize the Reynolds averaged Navier-Stokes approach is presented.
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| 35. |
- Jabbari, Sarah, et al.
(author)
-
BIPOLAR MAGNETIC SPOTS FROM DYNAMOS IN STRATIFIED SPHERICAL SHELL TURBULENCE
- 2015
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 805:2
-
Journal article (peer-reviewed)abstract
- Recent work by Mitra et al. (2014) has shown that in strongly stratified forced two-layer turbulence with helicity and corresponding large-scale dynamo action in the lower layer, and nonhelical turbulence in the upper, a magnetic field occurs in the upper layer in the form of sharply bounded bipolar magnetic spots. Here we extend this model to spherical wedge geometry covering the northern hemisphere up to 75 degrees latitude and an azimuthal extent of 180 degrees. The kinetic helicity and therefore also the large-scale magnetic field are strongest at low latitudes. For moderately strong stratification, several bipolar spots form that eventually fill the full longitudinal extent. At early times, the polarity of spots reflects the orientation of the underlying azimuthal field, as expected from Parker's Omega-shaped flux loops. At late times their tilt changes such that there is a radial field of opposite orientation at different latitudes separated by about 10 degrees. Our model demonstrates the spontaneous formation of spots of sizes much larger than the pressure scale height. Their tendency to produce filling factors close to unity is argued to be reminiscent of highly active stars. We confirm that strong stratification and strong scale separation are essential ingredients behind magnetic spot formation, which appears to be associated with downflows at larger depths.
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| 36. |
- Jabbari, Sarah, et al.
(author)
-
Surface flux concentrations in a spherical alpha 2 dynamo
- 2013
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556
-
Journal article (peer-reviewed)abstract
- Context. In the presence of strong density stratification, turbulence can lead to the large-scale instability of a horizontal magnetic field if its strength is in a suitable range (around a few percent of the turbulent equipartition value). This instability is related to a suppression of the turbulent pressure so that the turbulent contribution to the mean magnetic pressure becomes negative. This results in the excitation of a negative effective magnetic pressure instability (NEMPI). This instability has so far only been studied for an imposed magnetic field. Aims. We want to know how NEMPI works when the mean magnetic field is generated self-consistently by an alpha(2) dynamo, whether it is affected by global spherical geometry, and whether it can influence the properties of the dynamo itself. Methods. We adopt the mean-field approach, which has previously been shown to provide a realistic description of NEMPI in direct numerical simulations. We assume axisymmetry and solve the mean-field equations with the Pencil Code for an adiabatic stratification at a total density contrast in the radial direction of approximate to 4 orders of magnitude. Results. NEMPI is found to work when the dynamo-generated field is about 4% of the equipartition value, which is achieved through strong alpha quenching. This instability is excited in the top 5% of the outer radius, provided the density contrast across this top layer is at least 10. NEMPI is found to occur at lower latitudes when the mean magnetic field is stronger. For weaker fields, NEMPI can make the dynamo oscillatory with poleward migration. Conclusions. NEMPI is a viable mechanism for producing magnetic flux concentrations in a strongly stratified spherical shell in which a magnetic field is generated by a strongly quenched alpha effect dynamo.
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| 37. |
- Jabbari, Sarah, et al.
(author)
-
Turbulent reconnection of magnetic bipoles in stratified turbulence
- 2016
-
In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 459:4, s. 4046-4056
-
Journal article (peer-reviewed)abstract
- We consider strongly stratified forced turbulence in a plane-parallel layer with helicity and corresponding large-scale dynamo action in the lower part and non-helical turbulence in the upper. The magnetic field is found to develop strongly concentrated bipolar structures near the surface. They form elongated bands with a sharp interface between opposite polarities. Unlike earlier experiments with imposed magnetic field, the inclusion of rotation does not strongly suppress the formation of these structures. We perform a systematic numerical study of this phenomenon by varying magnetic Reynolds number, scale-separation ratio, and Coriolis number. We focus on the formation of a current sheet between bipolar regions where reconnection of oppositely oriented field lines occurs. We determine the reconnection rate by measuring either the inflow velocity in the vicinity of the current sheet or by measuring the electric field in the reconnection region. We demonstrate that for large Lundquist numbers, S > 10(3), the reconnection rate is nearly independent of S in agreement with results of recent numerical simulations performed by other groups in simpler settings.
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| 38. |
- K. Manikandan, Sreekanth, et al.
(author)
-
Quantitative analysis of non-equilibrium systems from short-time experimental data
- 2021
-
In: Communications Physics. - : Springer Nature. - 2399-3650. ; 4:1
-
Journal article (peer-reviewed)abstract
- Estimating entropy production directly from experimental trajectories is of great current interest but often requires a large amount of data or knowledge of the underlying dynamics. In this paper, we propose a minimal strategy using the short-time Thermodynamic Uncertainty Relation (TUR) by means of which we can simultaneously and quantitatively infer the thermodynamic force field acting on the system and the (potentially exact) rate of entropy production from experimental short-time trajectory data. We benchmark this scheme first for an experimental study of a colloidal particle system where exact analytical results are known, prior to studying the case of a colloidal particle in a hydrodynamical flow field, where neither analytical nor numerical results are available. In the latter case, we build an effective model of the system based on our results. In both cases, we also demonstrate that our results match with those obtained from another recently introduced scheme. Thermal fluctuations play a crucial role in non-equilibrium phenomena at microscopic length scales, making it challenging to analyse and interpret experimental data. Here, the authors demonstrate that the short-time thermodynamic uncertainty relation inference scheme can estimate the entropy production rate for a colloidal particle in time-varying potentials and with background flows determined by the presence of a microbubble.
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| 39. |
- Kemel, Koen, et al.
(author)
-
Active Region Formation through the Negative Effective Magnetic Pressure Instability
- 2013
-
In: Solar Physics. - : Springer Science and Business Media LLC. - 0038-0938 .- 1573-093X. ; 287:1-2, s. 293-313
-
Journal article (peer-reviewed)abstract
- The negative effective magnetic-pressure instability operates on scales encompassing many turbulent eddies, which correspond to convection cells in the Sun. This instability is discussed here in connection with the formation of active regions near the surface layers of the Sun. This instability is related to the negative contribution of turbulence to the mean magnetic pressure that causes the formation of large-scale magnetic structures. For an isothermal layer, direct numerical simulations and mean-field simulations of this phenomenon are shown to agree in many details, for example the onset of the instability occurs at the same depth. This depth increases with increasing field strength, such that the growth rate of this instability is independent of the field strength, provided the magnetic structures are fully contained within the domain. A linear stability analysis is shown to support this finding. The instability also leads to a redistribution of turbulent intensity and gas pressure that could provide direct observational signatures.
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| 40. |
- Kemel, Koen, et al.
(author)
-
Spontaneous formation of magnetic flux concentrations in stratified turbulence
- 2012
-
In: Solar Physics. - : Springer Science and Business Media LLC. - 0038-0938 .- 1573-093X. ; 280:2, s. 321-333
-
Journal article (peer-reviewed)abstract
- The negative effective magnetic pressure instability discovered recently in direct numerical simulations (DNSs) may play a crucial role in the formation of sunspots and active regions in the Sun and stars. This instability is caused by a negative contribution of turbulence to the effective mean Lorentz force (the sum of turbulent and non-turbulent contributions) and results in the formation of large-scale inhomogeneous magnetic structures from an initially uniform magnetic field. Earlier investigations of this instability in DNSs of stably stratified, externally forced, isothermal hydromagnetic turbulence in the regime of large plasma β are now extended into the regime of larger scale separation ratios where the number of turbulent eddies in the computational domain is about 30. Strong spontaneous formation of large-scale magnetic structures is seen even without performing any spatial averaging. These structures encompass many turbulent eddies. The characteristic time of the instability is comparable to the turbulent diffusion time, L2/ηt, where ηt is the turbulent diffusivity and L is the scale of the domain. DNSs are used to confirm that the effective magnetic pressure does indeed become negative for magnetic field strengths below the equipartition field. The dependence of the effective magnetic pressure on the field strength is characterized by fit parameters that seem to show convergence for larger values of the magnetic Reynolds number
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| 41. |
- Kruger, Jonas, et al.
(author)
-
The effect of turbulent clustering on particle reactivity
- 2017
-
In: Proceedings of the Combustion Institute. - : ELSEVIER SCIENCE INC. - 1540-7489 .- 1873-2704. ; 36:2, s. 2333-2340
-
Journal article (peer-reviewed)abstract
- The effect of turbulence on the heterogeneous (solid-fluid) reactions of solid particles is studied numerically with Direct Numerical Simulations (DNS). A simplified reaction system is used, where the solid-fluid reaction is represented by a single isothermal reaction step. It is found that, due to the clustering of particles by the isotropic turbulence, the overall reaction rate is entirely controlled by the turbulence for large Damkohler numbers. The particle clustering significantly slows down the reaction rate for increasing Damkohler numbers which reaches an asymptotic limit that can be analytically derived. This implies that the effect of turbulence on heterogeneously reacting particles should be included in models that are used in CFD simulations of e. g. char burnout in combustors or gasifiers. Such a model, based on the chemical and turbulent time scales, is here proposed for the heterogeneous reaction rate in the presence of turbulence.
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| 42. |
- Mandal, Tithi, et al.
(author)
-
Mechano-regulation by clathrin pit-formation and passive cholesterol-dependent tubules during de-adhesion
- 2024
-
In: Cellular and Molecular Life Sciences (CMLS). - : Springer Nature. - 1420-682X .- 1420-9071. ; 81:1
-
Journal article (peer-reviewed)abstract
- Adherent cells ensure membrane homeostasis during de-adhesion by various mechanisms, including endocytosis. Although mechano-chemical feedbacks involved in this process have been studied, the step-by-step build-up and resolution of the mechanical changes by endocytosis are poorly understood. To investigate this, we studied the de-adhesion of HeLa cells using a combination of interference reflection microscopy, optical trapping and fluorescence experiments. We found that de-adhesion enhanced membrane height fluctuations of the basal membrane in the presence of an intact cortex. A reduction in the tether force was also noted at the apical side. However, membrane fluctuations reveal phases of an initial drop in effective tension followed by saturation. The area fractions of early (Rab5-labelled) and recycling (Rab4-labelled) endosomes, as well as transferrin-labelled pits close to the basal plasma membrane, also transiently increased. On blocking dynamin-dependent scission of endocytic pits, the regulation of fluctuations was not blocked, but knocking down AP2-dependent pit formation stopped the tension recovery. Interestingly, the regulation could not be suppressed by ATP or cholesterol depletion individually but was arrested by depleting both. The data strongly supports Clathrin and AP2-dependent pit-formation to be central to the reduction in fluctuations confirmed by super-resolution microscopy. Furthermore, we propose that cholesterol-dependent pits spontaneously regulate tension under ATP-depleted conditions.
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| 43. |
- Manikandan, Sreekanth K., et al.
(author)
-
Estimate of entropy production rate can spatiotemporally resolve the active nature of cell flickering
- 2024
-
In: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 6:2
-
Journal article (peer-reviewed)abstract
- We use the short-time inference scheme [Manikandan, Phys. Rev. Lett. 124, 120603 (2020)0031-900710.1103/PhysRevLett.124.120603], obtained within the framework of stochastic thermodynamics, to infer a lower bound to entropy production rate from flickering data generated by interference reflection microscopy of HeLa cells. We can clearly distinguish active cell membranes from their adenosine-triphosphate-depleted selves and even spatiotemporally resolve activity down to the scale of about 1 μm. Our estimate of activity is model independent.
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| 44. |
- Mehendale, N., et al.
(author)
-
A microfluidic device to measure the shear elastic modulus of single red blood cells
- 2020
-
In: MicroTAS 2020 - 24th International Conference on Miniaturized Systems for Chemistry and Life Sciences. - : Chemical and Biological Microsystems Society. ; , s. 821-822
-
Conference paper (peer-reviewed)abstract
- Red blood cells become stiffer in response to environmental and physiological cues. We developed a microfluidic device to measure the shear elastic modulus of single red blood cells (RBCs) from their tracks. The device has a straight channel opening into a funnel. A single semi-circular pillar, positioned at the mouth of the funnel, deflects each RBC from its path as it approaches the pillar. The extent of deflection depends on the RBC stiffness. Using a simple numerical model and a knowledge of RBC tracks, we could calculate the effective shear elastic modulus of healthy and chemically stiffened RBCs.
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| 45. |
- Meibohm, Jan, et al.
(author)
-
Paths to caustic formation in turbulent aerosols
- 2021
-
In: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 6:6
-
Journal article (peer-reviewed)abstract
- The dynamics of small, yet heavy, identical particles in turbulence exhibits singularities, called caustics, that lead to large fluctuations in the spatial particle-number density, and in collision velocities. For large particle inertia, the fluid velocity at the particle position is essentially a white-noise signal and caustic formation is analogous to Kramers escape. Here we show that caustic formation at small particle inertia is different. Caustics tend to form in the vicinity of particle trajectories that experience a specific history of fluid-velocity gradients, characterized by low vorticity and a violent strain exceeding a large threshold. We develop a theory that explains our findings in terms of an optimal path to caustic formation that is approached in the small inertia limit.
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| 46. |
- Mitra, Dhrubaditya, et al.
(author)
-
CAN PLANETESIMALS FORM BY COLLISIONAL FUSION?
- 2013
-
In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 773:2, s. 120-
-
Journal article (peer-reviewed)abstract
- As a test bed for the growth of protoplanetary bodies in a turbulent circumstellar disk, we examine the fate of a boulder using direct numerical simulations of particle seeded gas flowing around it. We provide an accurate description of the flow by imposing no-slip and non-penetrating boundary conditions on the boulder surface using the immersed boundary method pioneered by Peskin. Advected by the turbulent disk flow, the dust grains collide with the boulder and we compute the probability density function of the normal component of the collisional velocity. Through this examination of the statistics of collisional velocities, we test the recently developed concept of collisional fusion which provides a physical basis for a range of collisional velocities exhibiting perfect sticking. A boulder can then grow sufficiently rapidly to settle into a Keplerian orbit on disk evolution timescales.
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| 47. |
- Mitra, Dhrubaditya, et al.
(author)
-
Intense bipolar structures from stratified helical dynamos
- 2014
-
In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 445:1, s. 761-769
-
Journal article (peer-reviewed)abstract
- We perform direct numerical simulations of the equations of magnetohydrodynamics with external random forcing and in the presence of gravity. The domain is divided into two parts: a lower layer where the forcing is helical and an upper layer where the helicity of the forcing is zero with a smooth transition in between. At early times, a large-scale helical dynamo develops in the bottom layer. At later times the dynamo saturates, but the vertical magnetic field continues to develop and rises to form dynamic bipolar structures at the top, which later disappear and reappear. Some of the structures look similar to delta spots observed in the Sun. This is the first example of magnetic flux concentrations, owing to strong density stratification, from self-consistent dynamo simulations that generate bipolar, super-equipartition strength, magnetic structures whose energy density can exceeds the turbulent kinetic energy by even a factor of 10.
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| 48. |
- Mitra, Dhrubaditya, et al.
(author)
-
OSCILLATORY MIGRATING MAGNETIC FIELDS IN HELICAL TURBULENCE IN SPHERICAL DOMAINS
- 2010
-
In: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 719:1, s. L1-L4
-
Journal article (peer-reviewed)abstract
- We present direct numerical simulations of the equations of compressible magnetohydrodynamics in a wedge-shaped spherical shell, without shear, but with random helical forcing which has negative (positive) helicity in the northern (southern) hemisphere. We find a large-scalemagnetic field that is nearly uniform in the azimuthal direction and approximately antisymmetric about the equator. Furthermore, the large-scale field in each hemisphere oscillates on nearly dynamical timescales with reversals of polarity and equatorward migration. Corresponding mean-field models also show similar migratory oscillations with a frequency that is nearly independent of the magnetic Reynolds number. This mechanism may be relevant for understanding equatorward migration seen in the solar dynamo.
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| 49. |
- Mitra, Dhrubaditya, et al.
(author)
-
Oscillatory migratory large-scale fields in mean-field and direct simulations
- 2009
-
In: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 5, s. 197-201
-
Journal article (peer-reviewed)abstract
- We summarise recent results form direct numerical simulations of both non-rotating helically forced and rotating convection driven MHD equations in spherical wedge-shape domains. In the former, using perfect-conductor boundary conditions along the latitudinal boundaries we observe oscillations, polarity reversals and equatorward migration of the large-scale magnetic fields. In the latter we obtain angular velocity with cylindrical contours and large-scale magnetic field which shows oscillations, polarity reversals but poleward migration. The occurrence of these behviours in direct numerical simulations is clearly of interest. However the present models as they stand are not directly applicable to the solar dynamo problem. Nevertheless, they provide general insights into the operation of turbulent dynamos.
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| 50. |
- Mitra, Dhrubaditya, et al.
(author)
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Particle energization through time-periodic helical magnetic fields
- 2014
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In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 89:4
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Journal article (peer-reviewed)abstract
- We solve for the motion of charged particles in a helical time-periodic ABC (Arnold-Beltrami-Childress) magnetic field. The magnetic field lines of a stationary ABC field with coefficients A=B=C=1 are chaotic, and we show that the motion of a charged particle in such a field is also chaotic at late times with positive Lyapunov exponent. We further show that in time-periodic ABC fields, the kinetic energy of a charged particle can increase indefinitely with time. At late times the mean kinetic energy grows as a power law in time with an exponent that approaches unity. For an initial distribution of particles, whose kinetic energy is uniformly distributed within some interval, the probability density function of kinetic energy is, at late times, close to a Gaussian but with steeper tails.
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