arXiv subjects

Aklant K. Bhowmick

Publications and source records attributed to Aklant K. Bhowmick.

24 records · Page 2Linked to original sources

arXiv2019

On the Rayleigh-Taylor unstable dynamics of 3D interfacial coherent structures with time-dependent acceleration

Rayleigh-Taylor instability (RTI) occurs in a range of industrial and natural processes. Whereas the vast majority of existing studies have considered constant acceleration, RTI is in most instances driven by variable acceleration. Here we focus on RTI driven by acceleration with a power-law time-dependence, and by applying a group theoretic method find solutions to this classical nonlinear boundary value problem. We deduce that the dynamics is dominated by the acceleration term and that the solutions depend critically on the time dependence for values of the acceleration exponent greater than $-2$. We find that in the early-time dynamics, the RTI growth-rate depends on the acceleration parameters and initial conditions. For the later-time dynamics, we link the interface dynamics with an interfacial shear function, and find a continuous family of regular asymptotic solutions and invariant properties of nonlinear RTI. The essentially interfacial and multi-scale character of the dynamics is also demonstrated. The velocity field is potential in the bulk, and vortical structures appear at the interface due to interfacial shear. The multi-scale character becomes clear from the invariance properties of the dynamics. We also achieve excellent agreement with existing observations and elaborate new benchmarks for future experimental work.

physics.flu-dyn↗

arXiv2019

On the small scale clustering of quasars: constraints from the MassiveBlack II simulation

We examine recent high-precision measurements of small-scale quasar clustering (at $z\sim0.5-2$ on scales of $\sim25~\mathrm{kpc/h}$) from the SDSS in the context of the MassiveBlackII (MBII) cosmological hydrodynamic simulation and conditional luminosity function (CLF) modeling. At these high luminosities ($g < 20.85$ quasars), the MBII simulation volume ($100~\mathrm{cMpc}/h$ comoving boxsize) has only 3 quasar pairs at distances of $1-4$ Mpc. The black-hole masses for the pairs range between $M_{bh}\sim1-3\times 10^{9}~M_{\odot}/h$ and the quasar hosts are haloes of $M_h\sim1-3\times10^{14}~M_{\odot}/h$. Such pairs show signs of recent major mergers in the MBII simulation. By modeling the central and satellite AGN CLFs as log-normal and Schechter distributions respectively (as seen in MBII AGNs), we arrive at CLF models which fit the simulation predictions and observed luminosity function and the small-scale clustering measured for the SDSS sample. The small-scale clustering of our mock quasars is well-explained by central-satellite quasar pairs that reside in $M_h>10^{14}~M_{\odot}/h$ dark matter haloes. For these pairs, satellite quasar luminosity is similar to that of central quasars. Our CLF models imply a relatively steep increase in the maximum satellite luminosity, $L^*_{\mathrm{sat}}$, in haloes of $M_h>10^{14}~M_{\odot}/h$ with associated larger values of $L^*_{\mathrm{sat}}$ at higher redshift. This leads to increase in the satellite fraction that manifests itself in an enhanced clustering signal at $\lesssim$ 1 Mpc/h. For the ongoing eBOSS-CORE sample, we predict $\sim 200-500$ quasar pairs at $z\sim1.5$ (with $M_h \gtrsim10^{13}~M_{\odot}/h$ and $M_{bh} \gtrsim10^{8}~M_{\odot}/h$) at $\sim25~\mathrm{kpc}$ scales. Such a sample would be $\gtrsim10$ times larger than current pair samples.

astro-ph.GA↗

arXiv2019

On the fundamentals of Richtmyer-Meshkov dynamics with variable acceleration

Richtmyer-Meshkov instability (RMI) plays important role in nature and technology, from supernovae and fusion to scramjets and nano-fabrication. Canonical Richtmyer-Meshkov instability is induced by a steady shock and impulsive acceleration, whereas in realistic environments the acceleration is usually variable. This work focuses on RMI induced by acceleration with a power-law time-dependence, and applies group theory to solve the classical problem. For early-time dynamics, we find the dependence of RMI growth-rate on the initial conditions and show it is free from the acceleration parameters. For late time dynamics, we find a continuous family of regular asymptotic solutions, including their curvature, velocity, Fourier amplitudes, and interfacial shear, and we study the solutions stability. For each of the solutions, the interface dynamics is directly linked to the interfacial shear, and the non-equilibrium velocity field has intense fluid motion near the interface and effectively no motion in the bulk. The quasi-invariance of the fastest stable solution suggests that nonlinear coherent dynamics in RMI is characterized by two macroscopic length-scales - the wavelength and the amplitude, in excellent agreement with observations. We elaborate new theory benchmarks for experiments and simulations, and put forward a hypothesis on the role of viscous effects in interfacial nonlinear RMI.

physics.flu-dyn↗

arXiv2019

On the fundamentals of Rayleigh-Taylor dynamics with variable acceleration

Rayleigh-Taylor instability (RTI) has critical importance for a broad range of processes in nature and technology, from supernovae to plasma fusion. In most instances RTI is driven by variable acceleration whereas the bulk of existing studies have considered constant acceleration. This work focuses on RTI driven by acceleration with power-law time-dependence, and applies group theory to solve the classical problem. For early time dynamics, we find dependence of RTI growth-rate on acceleration parameters and initial conditions. For late time dynamics, we directly link interface dynamics to interfacial shear, find continuous family of regular asymptotic solutions, and discover invariance properties of nonlinear RTI. Our results reveal the interfacial and multi-scale character of RTI with variable acceleration. The former is exhibited in structure of flow fields with intense fluid motion near the interface and effectively no motion in the bulk; the latter follows from the invariance properties of nonlinear dynamics defined by the interplay of two macroscopic length-scales - the wavelength and the amplitude. Our theory resolves the long-standing problem of RTI nonlinear dynamics, achieves excellent agreement with observations, and elaborates diagnostic benchmarks for future experiments and simulations.

physics.plasm-ph↗

arXiv2019

Rayleigh-Taylor instability with variable acceleration

We consider the long-standing problem of Rayleigh-Taylor instability with variable acceleration, and focus on the early-time dynamics of an interface separating incompressible ideal fluids of different densities subject to an acceleration being a power-law function of time for a spatially extended threedimensional flow periodic in the plane normal to the acceleration with symmetry group p6mm. By employing group theory and scaling analysis, we discover two distinct sub-regimes of the early time dynamics depending on the exponent of the acceleration power-law. The time-scale and the early-time dynamics are set by the acceleration for exponents greater than -2, and by the initial growth-rate (due to, e.g., initial conditions) for exponents smaller than -2. At the exponent value (-2) a transition occurs from one regime to the other with varying acceleration strength. For a broad range of the acceleration parameters, the instability growth-rate is explicitly found, the dependence of the dynamics on the initial conditions is investigated, and theory benchmarks are elaborated.

physics.plasm-ph↗

arXiv2018

BlueTides simulation: establishing black hole-galaxy relations at high-redshift

The scaling relations between the mass of supermassive black holes ($M_{\bullet}$) and host galaxy properties (stellar mass, $M_{\star}$, and velocity dispersion, $σ$), provide a link between the growth of black holes (BHs) and that of their hosts. Here we investigate if and how the BH-galaxy relations are established in the high-$z$ universe using \textsc{BlueTides}, a high-resolution large volume cosmological hydrodynamic simulation. We find the $M_{\bullet}-M_{\star}$ and $M_{\bullet}-σ$ relations at $z=8$: $\log_{10}(M_{\bullet}) = 8.25 + 1.10 \ \log_{10}(M_{\star}/10^{11}M_{\odot})$ and $\log_{10}(M_{\bullet}) = 8.35 + 5.31 \ \log_{10}(σ/200kms^{-1})$ at $z=8$, both fully consistent with the local measurements. The slope of the $M_{\bullet}-σ$ relation is slightly steeper for high star formation rate and $M_{\star}$ galaxies while it remains unchanged as a function of Eddington accretion rate onto the BH. The intrinsic scatter in $M_{\bullet}-σ$ relation in all cases ($ε\sim 0.4$) is larger at these redshifts than inferred from observations and larger than in $M_{\bullet}-M_{\star}$ relation ($ε\sim 0.14$). We find the gas-to-stellar ratio $f=M_{\rm gas}/M_{\star}$ in the host (which can be very high at these redshifts) to have the most significant impact setting the intrinsic scatter of $M_{\bullet}-σ$. The scatter is significantly reduced when galaxies with high gas fractions ($ε= 0.28$ as $f<10$) are excluded (making the sample more comparable to low-$z$ galaxies); these systems have the largest star formation rates and black hole accretion rates, indicating that these fast-growing systems are still moving toward the relation at these high redshifts. Examining the evolution (from $z=10$ to 8) of high mass black holes in $M_{\bullet}-σ$ plane confirms this trend.

astro-ph.GA↗