arXiv subjects

Amal Manoharan

Publications and source records attributed to Amal Manoharan.

5 recordsLinked to original sources

arXiv2026

Time-Symmetry of Lagrangian Coherent Structures in Active Turbulence

Active flows are central to mixing and transport across living systems. While Newtonian fluids remain laminar, diffusive and predictable at the microscale, living fluids like dense bacterial suspensions can exhibit highly chaotic flows like active turbulence, with anomalous transport capabilities. The underlying spatiotemporally persistent structures that drive mixing in active flows, however, remain uncharted. Using Lagrangian Coherent Structures, we now uncover networks of attracting and repelling hyperbolic surfaces. We study changes in the distribution and spectra of Finite-Time Lyapunov Exponent fields in response to increasing activity. Despite the dominance of vorticity in the flow, extreme forward and backward time chaotic mixing is found to originate from straining regions, emphasizing the role of saddles. Fractal dimensions of ridges reveal a morphological simplification of LCS networks with increasing activity, while retaining isotropic crossing. Throughout our work, we also probe a hitherto unasked question-Are signatures of Lagrangian irreversibility manifest in attracting and repelling LCS? To the contrary, we find there is a striking time-symmetry. Our work takes the first steps towards linking flow structures in active turbulence to invariant mixing surfaces. These findings will crucially help in designing activity modulation protocols to seed or inhibit flow structures, and thence mixing, in a bid to tame active turbulence for varied applications.

physics.flu-dyn↗

arXiv2025

The significance of two-way coupling in two-dimensional, dusty turbulence

The significance of small-scale forcing of particles on the carrier two-dimensional turbulent flow has been shown to influence the spectral scaling properties of the carrier fluid. We investigate possible consequences of such two-way coupling in a turbulent suspension of inertial particles through one- and two-point Eulerian and Lagrangian statistics. In particular, we find signatures of enhanced intermittency in the vorticity distributions. We characterize the changes in the small-scale geometry of the flow via the Okubo-Weiss parameter. Finally, we examine the scaling properties of the second-order vorticity structure functions and find a non-trivial form of scale-invariance at finite mass loading. Motivated by these observations, we propose an effective multiscale forcing framework in which particle feedback is modeled as a spatially localized small-scale forcing. This dual-scale forcing captures the emergence of modified spectral scaling and provides a minimal Eulerian description of particle-laden turbulence that reproduces key statistical signatures of the system.

physics.flu-dyn↗

arXiv2025

Topological entropy of stationary three-dimensional turbulence

Topological entropy serves as a viable candidate for quantifying mixing and complexity of a highly chaotic system. Particularly in turbulence, this is determined as the exponential stretching rate of a fluid material line that typically necessitates a Lagrangian description. We extend our recent work [A. Manoharan, S. Subramanian, and A. Joy, Phys. Rev. E 112, 015106] to three dimensions, and present an exact Eulerian framework to compute the topological entropy of stationary turbulent flows. The only prerequisite is a distribution of eigenvalues of the local strain-rate tensor and their decorrelation times. This can be easily obtained from a single wire probe at a fixed location, thereby eliminating the need for Lagrangian particle tracking which is formidable due to the chaotic nature of the flow. We believe that our results lend great utility in experiments targeting transport and mixing in many industrial and natural flows.

physics.flu-dyn↗

arXiv2024

Topological Entropy of Two Dimensional Turbulence

Deformation of material lines drives transport and dissipation in many industrial and natural flows. Here we report an exact Eulerian formula for the stretching rate of a material line, also known as the topological entropy, in a prototype two-dimensional fluid. The only requirement is a distribution of eigenvalues of the strain rate tensor and their decorrelation time. This eliminates the need for Lagrangian tracking in experimental turbulence where particle trajectories are entangled, and thus poorly resolved. Numerical simulations reveal an excellent agreement between our Eulerian estimate and the stretching rate of a Lagrangian material line, over a wide range of Reynolds number.

physics.flu-dyn↗

arXiv2023

Persistence in Active Turbulence

Active fluids such as bacterial swarms, self-propelled colloids, and cell tissues can all display complex spatio-temporal vortices that are reminiscent of inertial turbulence. This emergent behavior despite the overdamped nature of these systems is the hallmark of active turbulence. In this letter, using a generalized hydrodynamic model, we present a study of the persistence problem in active turbulence. We report that the persistence time of passive tracers inside the coherent vortices follows a Weibull probability density whose shape and scale are decided by the strength of activity -- contrary to inertial turbulence that displays power-law statistics in this region. In the turbulent background, the persistence time is exponentially distributed that is remindful of inertial turbulence. Finally we show that the driver of persistence inside the coherent vortices is the temporal decorrelation of the topological field, whereas it is the vortex turnover time in the turbulent background.

physics.flu-dyn↗