arXiv subjects

Iraj Gholami

Publications and source records attributed to Iraj Gholami.

4 recordsLinked to original sources

arXiv2026

Spontaneous flows and interfacial instabilities in oxygen-sensitive living active matter

Active fluids generate motion and stress internally, but in living systems their activity is often regulated by environmental fields that organisms consume or produce. How such fields localise active stresses and create flow-generating interfaces remains unclear. Here we show that oxygen organises suspensions of the flagellated microswimmer \textit{Euglena gracilis} into an annular living interface. In circular chambers with an air-exposed periphery, a labyrinthine bioconvective pattern and an annular cellular accumulation emerge nearly simultaneously, whereas sealing the periphery suppresses the annulus. The accumulation then sharpens, develops finite-wavelength protrusions and forms a long-lived, collectively rotating corona. An oxygen-coupled polar active-fluid model qualitatively recapitulates this progression: oxygen transport, cellular consumption and oxygen-regulated swimming and reorientation assemble and position the annulus, whereas dipolar active stresses destabilise it and generate collective flow. These results show how a metabolically shaped chemical field can create and activate a living interface, linking taxis, bioconvection and active interfacial hydrodynamics.

cond-mat.soft↗

arXiv2011

A decision between Bayesian and Frequentist upper limit in analyzing continuous Gravitational Waves

Given the sensitivity of current ground-based Gravitational Wave (GW) detectors, any continuous-wave signal we can realistically expect will be at a level or below the background noise. Hence, any data analysis of detector data will need to rely on statistical techniques to separate the signal from the noise. While with the current sensitivity of our detectors we do not expect to detect any true GW signals in our data, we can still set upper limits (UL) on their amplitude. These upper limits, in fact, tell us how weak a signal strength we would detect. In setting upper limit using two popular method, Bayesian and Frequentist, there is always the question of a realistic results. In this paper, we try to give an estimate of how realistically we can set the upper limit using the above mentioned methods. And if any, which one is preferred for our future data analysis work.

gr-qc↗

arXiv2011

Slow dynamics and precursors of the glass transition in granular fluids

We use event driven simulations to analyze glassy dynamics as a function of density and energy dissipation in a two-dimensional bidisperse granular fluid under stationary conditions. Clear signatures of a glass transition are identified, such as an increase of relaxation times over several orders of magnitude. As the inelasticity is increased, the glass transition is shifted to higher densities and the precursors of the transition become less and less pronounced -- in agreement with a recent mode-coupling theory. We analyze the long-time tails of the velocity autocorrelation and discuss its consequences for the nonexistence of the diffusion constant in two dimensions.

cond-mat.stat-mech↗

arXiv2005

Improved Stack-Slide Searches for Gravitational-Wave Pulsars

We formulate and optimize a computational search strategy for detecting gravitational waves from isolated, previously-unknown neutron stars (that is, neutron stars with unknown sky positions, spin frequencies, and spin-down parameters). It is well known that fully coherent searches over the relevant parameter-space volumes are not computationally feasible, and so more computationally efficient methods are called for. The first step in this direction was taken by Brady & Creighton (2000), who proposed and optimized a two-stage, stack-slide search algorithm. We generalize and otherwise improve upon the Brady-Creighton scheme in several ways. Like Brady & Creighton, we consider a stack-slide scheme, but here with an arbitrary number of semi-coherent stages and with a coherent follow-up stage at the end. We find that searches with three semi-coherent stages are significantly more efficient than two-stage searches (requiring about 2-5 times less computational power for the same sensitivity) and are only slightly less efficient than searches with four or more stages. We calculate the signal-to-noise ratio required for detection, as a function of computing power and neutron star spin-down-age, using our optimized searches.

gr-qc↗