arXiv subjects

Taosif Ahsan

Publications and source records attributed to Taosif Ahsan.

3 recordsLinked to original sources

arXiv2026

Simply Connected Topology in Perturbed Vortices and Field-Reversed Configurations

Zero-helicity vortices, such as Hill's vortex and field-reversed configurations (FRCs), have long been assumed to be toroidal in topology. This paper proves this assumption false: under arbitrarily small odd-parity (with respect to the symmetry axis) transverse field perturbations, interior flux surfaces become simply connected. The previous topological categorization--open and closed field lines separated by an ellipsoid separatrix--is updated to three distinct categories: open field lines in the outermost region, closed field lines on torus flux surfaces in an intermediate region, and closed field lines on simply connected flux surfaces in the innermost region. In addition to a shifted ellipsoid outer separatrix separating closed and open field lines, a new crescent-shaped inner separatrix separates the torus and simply connected surfaces. The simply connected region is significant even for small perturbations; e.g., in a spherical vortex with a perturbation 10% of the background field strength, it occupies 40% of the outer separatrix. The analysis also proves the conjecture regarding field line closure under odd-parity perturbation in the full three-dimensional context. Preliminary numerical simulations of charged particle trajectories in FRC magnetic confinement under odd-parity perturbation were also conducted; crescent-like simply connected volumes were observed even when gyro-radii were small compared to the system size. Since FRCs are sustained by a rotating magnetic field with odd parity, these results motivate a revision of FRC-related fusion confinement physics. Given the mathematical equivalence to Hill's vortex, this also updates our topological understanding of fluid flow in a wide array of phenomena.

math-ph↗

arXiv2026

Retraction Dynamics of a Highly Viscous Liquid Sheet

We study the one-dimensional capillary-driven retraction of a finite, planar liquid sheet in the asymptotic regime where both the Ohnesorge number $\mathrm{Oh}$ and the initial length-to-thickness ratio $l_0/h_0$ are large. In this regime, the fluid domain decomposes into two regions: a thin-film region governed by one-dimensional mass and momentum equations, and a small tip region near the free edge described by a self-similar Stokes flow. Asymptotic matching between these regions yields an effective boundary condition for the thin-film region, representing a balance between viscous and capillary forces at the free edge. Surface tension drives the thin-film flow only through this boundary condition, while the local momentum balance is dominated by viscous and inertial stresses. We show that the thin-film flow possesses a conserved quantity, reducing the equation of thickness to heat equation with time-dependent boundary conditions. The reduced problem depends on a single dimensionless parameter $\mathcal{L} = l_0 / (4 h_0 \mathrm{Oh})$. Numerical solutions of the reduced model agree well with previous studies and reveal that the sheet undergoes distinct retraction regimes depending on $\mathcal{L}$ and a dimensionless time after rupture $T$. We derive asymptotic approximations for the thickness profile, velocity profile, and retraction speed during the early and late stages of retraction. At early times, the retraction speed grows as $T^{1/2}$, while at late times it decays as $1/T^2$. An intermediate regime arises for very long sheets ($\mathcal{L} \gg 1$). During this phase, the retraction speed approaches the Taylor-Culick value. When $T \approx \mathcal{L}$, the speed undergoes fast deceleration from the Taylor-Culick speed to late-time asymptotics.

physics.flu-dyn↗

arXiv2022

Analysis and mitigation of pulse-pile-up tail artifacts in warm-plasma pulse-height X-ray spectra

Pulse pile-up in pulse-height energy analyzers increases when the incident rate of pulses increases relative to the inverse of the dead time per pulse of the detection system. Changes in the observed energy distributions with incident rate and detector-electronics-formed pulse shape then occur. We focus on weak high energy tails in X-ray spectra, important for measurements on partially ionized, warm, pure-hydrogen plasma. A first-principles two-photon pulse-pile-up model is derived specific to trapezoidal-shaped pulses; quantitative agreement is found between the measurements and the model predictions. The modeling is then used to diagnose pulse-pile-up tail artifacts and mitigate them in relatively low count-rate spectra.

physics.plasm-ph↗