Searcharxiv⌕ Search

arXiv subjects

Rashmi R. Nayak

Publications and source records attributed to Rashmi R. Nayak.

At least 19 recordsLinked to original sources

A Gauge-Theoretic Action Principle for Viscous Incompressible Fluids

We propose a novel action principle for two dimensional incompressible fluid dynamics that naturally incorporates both vorticity and viscous dissipation via gauge field couplings. The action features a Chern Simons like term, $ε^{μνρ} A_μ\partial_νA_ρ$, capturing the topological structure of vorticity, alongside a quadratic term $(ε^{μνρ} \partial_νA_ρ)^2$ representing viscous damping. Incompressibility is enforced through a Lagrange multiplier, while coupling to an external potential allows applications in geophysical flows. We derive the equations of motion, recovering the vorticity formulation of the two-dimensional incompressible Navier Stokes equations and explicitly identifying the kinematic viscosity. This gauge theoretic framework leads to a Helmholtz type equation for vorticity linking topological and dissipative phenomena in viscous incompressible fluids. Analysis of Noether symmetries reveals conserved charges arising from gauge invariance and spatial translations, while viscosity explicitly breaks time reversal symmetry within this topological setting. Furthermore, the velocity vorticity gauge correspondence naturally suggests a Lindblad operator structure, providing a pathway toward a quantum description of viscous dissipation and allowing quantization of dissipative hydrodynamics. This framework also highlights how vorticity emerges as a natural Lindblad operator, capturing the transition from coherent rotational motion to thermal disorder.

physics.flu-dyn↗

Investigating (Non)-Integrability and Pulsating String in D3-Brane Background

This work explores the (non)-integrability and chaotic dynamics of classical strings in the background of a D3-brane with a non-commutative parameter, within the framework of the AdS/CFT correspondence. Using the Polyakov action, we derive the equations of motion and constraints for pulsating strings and analyze their stability through perturbation theory. In the high-energy limit, the first-order perturbed equation simplifies to the Pöschl-Teller equation, solvable via associated Legendre or hypergeometric functions, while numerical methods are employed for generic energy values. We demonstrate that the non-commutative parameter enhances chaotic behavior, as evidenced by the Largest Lyapunov Exponent (LLE). Furthermore, we investigate the integrability of geodesic motion and identify two distinct string modes: captured at and escape to infinity. Finally, we study pulsating strings in the deformed $(AdS_{3} \times S^{2})_{\varkappa}$ background, deriving dispersion relations for both short and long strings.

hep-th↗

Circular orbits and chaos bound in slow-rotating curved acoustic black holes

Acoustic black holes, analogs of gravitational black holes created in fluid systems, have recently been embedded within Schwarzschild spacetime using the Gross-Pitaevskii theory, leading to configurations with both event and acoustic horizons. This study examines the motion of vortices, modeled as unit-mass relativistic test particles, around a slow-rotating curved acoustic black hole. We analyse the stability of circular orbits, identifying the innermost stable circular orbit (ISCO), and investigate the chaotic dynamics of vortices perturbed from unstable circular orbits near the acoustic horizon. Using the Lyapunov exponent to quantify this chaos, we assess whether it satisfies the Maldacena-Shenker-Stanford bound $(λ\le 2 πT_H)$, a limit established for gravitational black holes in general relativity. Our results show that, in non-extremal cases $(ξ> 4)$, the Lyapunov exponent respects the bound near the horizon, while in extremal cases $(ξ= 4)$, it is violated due to vanishing surface gravity. These findings highlight similarities between acoustic and gravitational black holes, advancing the analogy in the context of chaos and orbital dynamics.

hep-th↗

Neumann-Rosochatius system for rotating strings in $AdS_3 \times S^3\times S^3\times S^1$ with flux

Strings on $AdS_3 \times S^3\times S^3\times S^1$ with mixed flux exhibit exact integrability. We wish to construct an integrable Neumann-Rosochatius (NR) model of strings starting with the type IIB supergravity action in $AdS_3 \times S^3\times S^3\times S^1$ with pure NSNS flux. We observe that the forms of the Lagrangian and the Uhlenbeck integrals of motion of the considered system are NR-like with some suitable deformations which eventually appear due to the presence of flux. We utilize the integrable framework of the deformed NR model to analyze rigidly rotating spiky strings moving only in $S^3\times S^1$. We further present some mathematical speculations on the rounding-off nature of the spike in the presence of non-zero angular momentum $J$ in $S^1$.

hep-th↗

$N$-spike string in $AdS_3 \times S^1$ with mixed flux

Sigma model in $AdS_3\times S^3$ background supported by both NS-NS and R-R fluxes is one of the most distinguished integrable models. We study a class of classical string solutions for $N$-spike strings moving in $AdS_3 \times S^1$ with angular momentum $J$ in $S^1 \subset S^5$ in the presence of mixed flux. We observe that the addition of angular momentum $J$ or winding number $m$ results in the spikes getting rounded off and not end in cusp. The presence of flux shows no alteration to the rounding-off nature of the spikes. We also consider the large $N$-limit of $N$-spike string in $AdS_3 \times S^1$ in the presence of flux and show that the so-called Energy-Spin dispersion relation is analogous to the solution we get for the periodic-spike in $AdS_3-pp-$wave $\times S^1$ background with flux.

hep-th↗

On finite-size spiky strings in $AdS_3 \times S^3\times T^4$ with mixed fluxes

We discuss finite-size corrections to the spiky strings in $AdS$ space which is dual to the long $\mathcal{N}=4$ SYM operators of the form Tr($Δ_+ ^{J_1}ϕ_1Δ_+ ^{J_2}ϕ_2...Δ_+ ^{J_n}ϕ_n$). We express the finite-size dispersion relation in terms of Lambert $\mathbf{W}$-function. We further establish the finite-size scaling relation between energy and angular momentum of the spiky string in presence of mixed fluxes in terms of $\mathbf{W}$-function. We comment on the solution in pure NS-NS background as well.

hep-th↗

D1 string dynamics in curved backgrounds with fluxes

We study various rotating and oscillating D-string configurations in some general backgrounds with fluxes. In particular, we look for solutions to the equations of motion of various rigidly rotating D-strings in $AdS_3$ background with mixed flux, and in the intersecting D-brane geometries. We find out relations among various conserved charges corresponding to the breathing and rotating D-string configurations.

hep-th↗

Note on I-brane Near Horizon PP-wave Background

We find out a PP-wave spacetime with constant Neveu-Schwarz (NS) three form flux by taking a Penrose limit on the 1+1 dimensional intersection of two orthogonal stacks of fivebranes in type IIB string theory. We further find out an intersecting (D1-D5)-brane solution in this background and analyze its supersymmetry properties by solving the dilatino and gravitino variations explicitly.

hep-th↗

Fundamental String (Membrane) Orbiting D5(M5)-branes

We study fundamental string (F-string) dynamics near D5-brane in some limits. We find that when the angular momentum of the probe is proportional to string length (l_s) and square root of string coupling constant {g_s}, the F-string lies in its metastable orbit at a finite distance from D5-branes. We further study the metastable orbits of a M2-brane in M5-brane background.

hep-th↗

On the giant magnon and spike solutions for strings on AdS$_3\times$ S$^3$

We study solutions for the rotating strings on the sphere with a background NS-NS field and on the Anti-de-Sitter spacetime. We show the existence of magnon and single spike solutions on R$\times$S$^2$ in the presence of constant magnetic field as two limiting cases. We also study the solution for strings on AdS$_3\times$ S$^3$ with Melvin deformation. The dispersion relations among various conserved charges are shown to receive finite corrections due to the deformation parameter. We further study the rotating string on AdS$_3 \times$ S$^3$ geometry with two conserved angular momenta on S$^3$ and one spin along the AdS$_3$. We show that there exists two kinds of solutions: a circular string solution and a helical string. We find out the dispersion relation among various charges and give physical interpretation of these solutions.

hep-th↗

Giant Magnon in NS5-brane Background

We study the giant magnon solutions in the near horizon geometry of the Neveu-Schwarz (NS) 5-brane background. In conformal gauge, we find magnon dispersion relation in the large angular momentum (J) limit. We further show that the giant magnon poses uniform distribution of the angular momentum along the string world-sheet as in case of AdS_5\times S^5 spacetime.

hep-th↗

Time-dependent supergravity solutions in null dilaton background

A class of time dependent pp-waves with NS-NS flux in type IIA string theory is considered. The background preserves 1/4 supersymmetry and may provide a toy model of Big Bang cosmology with non trivial flux. At the Big Bang singularity in early past, the string theory is strongly coupled and Matrix string model can be used to describe the dynamics. We also construct some time dependent supergravity solutions for D-branes and analyze their supersymmetry properties.

hep-th↗

D-Brane solutions in a light-like linear dilaton background

The light-like linear dilaton background presents a simple time dependent solution of type II supergravity equations of motion that preserves 1/2 supersymmetry in ten dimensions. We construct supergravity D-brane solutions in a linear dilaton background starting from the known intersecting brane solutions in string theory. By applying a Penrose limit on the intersecting (NS1-NS5-NS5')- brane solution, we find out a D5-brane in a linear dilaton background. We solve the Killing spinor equations for the brane solutions explicitly, and show that they preserve 1/4 supersymmetry. We also find a M5-brane solution in eleven dimensional supergravity.

hep-th↗

D1-brane with Overcritical Electric Field in AdS3 and S-brane

We study aspects of Dirichlet S-branes, which are defined as Dirichlet boundary condition on a time like embedding of open strings, in general backgrounds. By applying T-duality along an isometry of the unphysical dS2-branes in NS-NS supported AdS3-background, we find S0-brane. We also study the time dependent tachyon condensation on the unstable Dp-brane and interpret the singular solutions as lower dimensional S(p-1)-brane that couples to real Ramond-Ramond fields while to imaginary NS-NS modes.

hep-th↗

D-brane dynamics in a plane wave background

By using the Dirac-Born-Infeld action we study the dynamics of Dp-brane propagating in the NS5-near horizon plane wave background. We study systematically D-brane embedding in this pp-wave background, and analyze the equations of motion for various auxiliary fields. We further discuss the motion of the probe Dq-brane in the presence of source Dp-branes in this plane wave background.

hep-th↗

D-branes in a Big Bang/Big Crunch Universe: Misner Space

We study D-branes in a two-dimensional Lorentzian orbifold R^{1,1}/Γwith a discrete boost Γ. This space is known as Misner or Milne space, and includes big crunch/big bang singularity. In this space, there are D0-branes in spiral orbits and D1-branes with or without flux on them. In particular, we observe imaginary parts of partition functions, and interpret them as the rates of open string pair creation for D0-branes and emission of winding closed strings for D1-branes. These phenomena occur due to the time-dependence of the background. Open string 2 -> 2 scattering amplitude on a D1-brane is also computed and found to be less singular than closed string case.

hep-th↗

D-branes in a Big Bang/Big Crunch Universe: Nappi-Witten Gauged WZW Model

We study D-branes in the Nappi-Witten model, which is a gauged WZW model based on (SL(2,R) x SU(2)) / (U(1) x U(1)). The model describes a four dimensional space-time consisting of cosmological regions with big bang/big crunch singularities and static regions with closed time-like curves. The aim of this paper is to investigate by D-brane probes whether there are pathologies associated with the cosmological singularities and the closed time-like curves. We first classify D-branes in a group theoretical way, and then examine DBI actions for effective theories on the D-branes. In particular, we show that D-brane metric from the DBI action does not include singularities, and wave functions on the D-branes are well behaved even in the presence of closed time-like curves.

hep-th↗

Brane Solutions with/without Rotation in PP-wave Spacetime

We present two classes of brane solutions in pp-wave spacetime. The first class of branes with a rotation parameter are constructed in an exact string background with NS-NS and R-R flux. The spacetime supersymmetry is analyzed by solving the standard Killing spinor equations and is shown to preserve the same amount of supersymmetry as the case without the rotation. This class of branes do not admit regular horizon. The second class of brane solutions are constructed by applying a null Melvin twist to the brane solutions of flat spacetime supergravity. These solutions admit regular horizon. We also comment on some thermodynamic properties of this class of solutions.

hep-th↗