| 1. |
- Afonso, Marco Martins, et al.
(författare)
-
Kazantsev dynamo in turbulent compressible flows
- 2019
-
Ingår i: Proceedings of the Royal Society. Mathematical, Physical and Engineering Sciences. - : ROYAL SOC. - 1364-5021 .- 1471-2946. ; 475:2223
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 2. |
- Agrawal, Vipin, 1994-, et al.
(författare)
-
Active buckling of pressurized spherical shells : Monte Carlo simulation
- 2023
-
Ingår i: Physical review. E. - : American Physical Society (APS). - 2470-0045 .- 2470-0053. ; 108:3
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 3. |
- Agrawal, Vipin, 1994-, et al.
(författare)
-
Chaos and irreversibility of a flexible filament in periodically driven Stokes flow
- 2022
-
Ingår i: Physical review. E. - : American Physical Society (APS). - 2470-0045 .- 2470-0053. ; 106:2
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 4. |
- Agrawal, Vipin, 1994-, et al.
(författare)
-
Flexible filament in time-periodic viscous flow: shape chaos and period three
-
Annan publikation (övrigt vetenskapligt/konstnärligt)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.
|
|
| 5. |
- Agrawal, Vipin, 1994-, et al.
(författare)
-
MeMC : A package for Monte Carlo simulations of spherical shells
- 2022
-
Ingår i: Journal of Open Source Software. - : The Open Journal. - 2475-9066. ; 7:74
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 6. |
- Agrawal, Vipin, 1994-
(författare)
-
Shells and filament in flows
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
|
|
| 7. |
- Bagheri, Faranggis, et al.
(författare)
-
Statistics of polymer extensions in turbulent channel flow
- 2012
-
Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 86:5, s. 056314-
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 8. |
- Bhatnagar, Akshay, et al.
(författare)
-
Deviation-angle and trajectory statistics for inertial particles in turbulence
- 2016
-
Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1539-3755 .- 1550-2376. ; 94:6
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 9. |
- Bhatnagar, Akshay, et al.
(författare)
-
Heavy inertial particles in turbulent flows gain energy slowly but lose it rapidly
- 2018
-
Ingår i: Physical review. E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 97:3
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 10. |
- Bhatnagar, Akshay, et al.
(författare)
-
How long do particles spend in vortical regions in turbulent flows?
- 2016
-
Ingår i: Physical Review E. - 2470-0045. ; 94:5
-
Tidskriftsartikel (refereegranskat)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.
|
|
