Searcharxiv⌕ Search

arXiv subjects

Praveen Kumar

Publications and source records attributed to Praveen Kumar.

At least 55 records · Page 3Linked to original sources
arXiv2022

Chemically symmetric and asymmetric self-driven rigid dumbbells in 2D polymer gel

We employ computer simulations to unveil the translational and rotational dynamics of the self-driven chemically symmetric and asymmetric rigid dumbbells in two-dimensional polymer gel. Our results show that activity or the self-propulsion always enhances the dynamics of the dumbbells. Making the self-propelled dumbbell chemically asymmetric leads to further enhancement in dynamics. Additionally, the direction of self-propulsion is a key factor for the chemically asymmetric dumbbells, where self-propulsion towards the non-sticky half of the dumbbell results in faster translational and rotational dynamics compare to the case with the self-propulsion towards the sticky half of the dumbbell. Our analyses show that both the symmetric and asymmetric passive rigid dumbbells get trapped inside the mesh of the polymer gel, but the chemical asymmetry always facilitates mesh to mesh motion of the dumbbell and it is even more pronounced when the dumbbell is self-propelled. This results multiple peaks in the van Hove function with increasing self-propulsion. In a nutshell, we believe that our in silico study can guide the researchers design efficient artificial microswimmers possessing potential applications in site-specific delivery.

cond-mat.soft↗
arXiv2021

A method to determine wall shear stress from mean profiles in turbulent boundary layers

The direct measurement of wall shear stress in turbulent boundary layers (TBL) is challenging, therefore requiring it to be indirectly determined from mean profile measurements. Most popular methods assume the mean streamwise velocity to satisfy either a logarithmic law in the inner layer or a composite velocity profile with many tuned constants for the entire TBL, both of which require reliable data from the inner layer. The presence of roughness and pressure gradient brings additional complications where most existing methods either fail or require significant modification. A novel method is proposed to determine the wall shear stress in zero pressure gradient TBL from measured mean profiles, without requiring near-wall data. The method is based on the stress model of Kumar and Mahesh [Phys. Rev. Fluids 6, 024603 (2021)], who developed accurate models for mean stress and wall-normal velocity in zero pressure gradient TBL. The proposed method requires a single point measurement of mean streamwise velocity and mean shear stress in the outer layer, preferably between $20$ to $50 \%$ of the TBL, and an estimate of boundary layer thickness and shape factor. The method can handle wall roughness without modification and is shown to predict friction velocities to within $3 \%$ over a range of Reynolds number for both smooth and rough wall zero pressure gradient TBL. In order to include the pressure gradients effects, the work of Kumar and Mahesh [Phys. Rev. Fluids 6, 024603 (2021)] is revisited to derive a novel model for both mean stress and wall-normal velocity in pressure gradient TBL, which is then used to formulate a method to obtain the wall shear stress from the profile data. Overall, the proposed method is shown to be robust and accurate for a variety of pressure gradient TBL.

physics.flu-dyn↗
arXiv2020

Bundled Causal History Interaction

Complex system arises as a result of the nonlinear interactions between components. In particular, the evolutionary dynamics of a multivariate system encodes the ways in which different variables interact with each other individually or in groups. One fundamental question that remains unanswered is: how do two non-overlapping multivariate subsets of variables interact to causally determine the outcome of a specific variable? Here we provide an information based approach to address this problem. We delineate the temporal interactions between the bundles in a probabilistic graphical model. The strength of the interactions, captured by partial information decomposition, then exposes complex behavior of dependencies and memory within the system. The proposed approach successfully illustrates complex dependence between cations and anions as determinants of \textit{pH} in an observed stream chemistry system. This example demonstrates the potentially broad applicability of the approach, establishing the foundation to study the interaction between groups of variables in a range of complex systems.

cs.IT↗
arXiv2020

(2+1) dimensional cosmological models in f(R; T) gravity with $Λ$(R; T)

We intend to study a new class of cosmological models in $f(R, T)$ modified theories of gravity, hence define the cosmological constant $Λ$ as a function of the trace of the stress energy-momentum-tensor $T$ and the Ricci scalar $R$, and name such a model "$Λ(R, T)$ gravity" where we have specified a certain form of $Λ(R, T)$. $Λ(R, T)$ is also defined in the perfect fluid and dust case. Some physical and geometric properties of the model are also discussed. The pressure, density and energy conditions are studied both when $Λ$ is a positive constant and when $Λ=Λ(t)$, i.e a function of cosmological time, t. We study the behaviour of some cosmological quantities such as Hubble and deceleration parameters. The model is innovative in the sense that it has been described in terms of both $R$ and $T$ and display a better understanding of the cosmological observations.

physics.gen-ph↗
arXiv2020

Critical Evaluation of Relative Importance of Stress and Stress Gradient in Whisker Growth in Tin Coatings

Here, we study the role of stress state and stress gradient in whisker growth in Sn coatings electrodeposited on brass. The bulk stress in Sn coatings was measured using a laser-optics based curvature setup, whereas glancing angle X-ray diffraction was employed to quantify the surface stress; this also allowed studying role of out-of-plane stress gradient in whisker growth. Both bulk stress and surface stress in Sn coating evolved with time, wherein both were compressive immediately after the deposition, and thereafter while the bulk stress monotonically became more compressive and subsequently saturated with aging at room temperature, the stress near the surface of the Sn coating continually became more tensile with aging. These opposing evolutionary behaviors of bulk and surface stresses readily established a negative out-of-plane stress gradient, required for spontaneous growth of whiskers. The importance of out-of-plane stress gradient was also validated by externally imposing widely different stress states and stress gradients in Sn coatings using a 3-point bending apparatus. It was consistently observed that whisker growth was more in the coatings under external tensile stress, however, with higher negative out-of-plane stress gradient. The results conclusively indicate the critical role of negative out-of-plane stress gradient on whisker growth, as compared to only the nature (i.e., sign and magnitude) of stress.

physics.app-ph↗
arXiv2020

Sunspot Drawings at Kodaikanal Observatory: A Representative Results on Hemispheric Sunspot Numbers and Area Measurements

The importance of the periodicity in sunspot appearance was well recognized by the mid of 19th century. Several observatories around the globe have made the record of sunspots in the form of drawings and preserved them safely for posterity. At the Kodaikanal Observatory (KO), the sunspot observations have begun in 1905. In those times observations were recorded on photographic plates and after the development of those plates in the laboratory, the drawings of the same were made on the Stonyhurst grids. In these drawings, called {\em sun charts}, different features on the sun's disk, e.g., sunspots, plages, filaments, prominences, etc. were clearly identified and visually marked with different colors. We have collected 111 years of sunspot drawing spanning over 10 solar cycles. These sunspot drawings were carefully stitched to make bound volumes, each for every 6-months. The drawings are kept at the Kodaikanal library for scientific use and analysis. In this article, we describe briefly the process of drawing, methods of counting sunspot numbers and measurement of sunspot area using square grids. We have collected the data for the Northern and Southern hemispheres separately. From the collected data, we compute the sunspot number and area and compare it with Royal Greenwich Observatory (RGO) and Sunspot Index and Long-term Solar Observations (SILSO) data. The results show that the measurement of the sunspot number is underestimated by about 40\%. The KO monthly averaged sunspot number data of both hemispheres is normalized with the RGO monthly averaged total sunspot number data.

astro-ph.SR↗
arXiv2019

Transport of probe particles in polymer network: effects of probe size, network rigidity and probe-polymer interaction

Fundamental understanding of the effect of microscopic parameters on the dynamics of probe particles in different complex environments has wide implications. Examples include diffusion of proteins in the biological hydrogels, porous media, polymer matrix, etc. Here, we use extensive molecular dynamics simulations to investigate the dynamics of the probe particle in a polymer network on a diamond lattice which provides substantial crowding to mimic the cellular environment. Our simulations show that the dynamics of the probe increasingly becomes restricted, non-Gaussian and subdiffusive on increasing the network rigidity, binding affinity and the probe size. In addition, the velocity autocorrelation functions show negative dips owing to the viscoelasticity and caging due to surrounding network. These observations go with general experimental findings. Surprisingly for a probe particle of size comparable to the mesh size, unrestricted motion engulfing large length scales has been observed. This happens with the more flexible polymer network, which is easily pushed by the bigger probe. Our study gives a general qualitative picture of transport of probes in gel like medium, as encountered in different contexts.

cond-mat.soft↗
arXiv2019

Scalable Verification of Probabilistic Networks

This paper presents McNetKAT, a scalable tool for verifying probabilistic network programs. McNetKAT is based on a new semantics for the guarded and history-free fragment of Probabilistic NetKAT in terms of finite-state, absorbing Markov chains. This view allows the semantics of all programs to be computed exactly, enabling construction of an automatic verification tool. Domain-specific optimizations and a parallelizing backend enable McNetKAT to analyze networks with thousands of nodes, automatically reasoning about general properties such as probabilistic program equivalence and refinement, as well as networking properties such as resilience to failures. We evaluate McNetKAT's scalability using real-world topologies, compare its performance against state-of-the-art tools, and develop an extended case study on a recently proposed data center network design.

cs.PL↗
arXiv2019

Energy Spectra and Fluxes in Dissipation Range of Turbulent and Laminar Flows

Two well-known turbulence models that describe the energy spectrum in the inertial and dissipative ranges simultaneously are by Pao~(1965) and Pope~(2000). In this paper, we compute the energy spectrum $E(k)$ and energy flux $Π(k)$ using direct numerical simulations on grids up to $4096^3$, and show consistency between the numerical results and the predictions by the aforementioned models. We also construct a model for laminar flows that predicts $E(k)\sim k^{-1} \exp(-k)$ and $Π(k)\sim k \exp(-k)$. Our model predictions match with the numerical results. We emphasize differences on the energy transfers in the two flows---they are {\em local} in the turbulent flows, and {\em nonlocal} in laminar flows.

physics.flu-dyn↗
arXiv2018

Cantor meets Scott: Semantic Foundations for Probabilistic Networks

ProbNetKAT is a probabilistic extension of NetKAT with a denotational semantics based on Markov kernels. The language is expressive enough to generate continuous distributions, which raises the question of how to compute effectively in the language. This paper gives an new characterization of ProbNetKAT's semantics using domain theory, which provides the foundation needed to build a practical implementation. We show how to use the semantics to approximate the behavior of arbitrary ProbNetKAT programs using distributions with finite support. We develop a prototype implementation and show how to use it to solve a variety of problems including characterizing the expected congestion induced by different routing schemes and reasoning probabilistically about reachability in a network.

cs.PL↗
arXiv2018

Bifurcation of the Kirkendall marker plane and the role of Ni and other impurities on the growth of Kirkendall voids in the Cu Sn system

The presence of bifurcation of the Kirkendall marker plane, a very special phenomenon discovered recently, is found in a technologically important Cu Sn system. It was predicted based on estimated diffusion coefficients; however, could not be detected following the conventional inert marker experiments. As reported in this study, we could detect the locations of these planes based on the microstructural features examined in SEM and TEM. This strengthens the concept of the physicochemical approach that relates microstructural evolution with the diffusion rates of components and imparts finer understanding of the growth mechanism of phases. The estimated diffusion coefficients at the Kirkendall marker planes indicates that the reason for the growth of the Kirkendall voids is the nonconsumption of excess vacancies which are generated due to unequal diffusion rate of components. Systematic experiments using different purity of Cu in this study indicates the importance of the presence of impurities on the growth of voids, which increases drastically for greather than 0.1 wt percent impurity. The growth of voids increases drastically for electroplated Cu, commercially pure Cu and Cu(0.5 at percent Ni) indicating the adverse role of both inorganic and organic impurities. Void size and number distribution analysis indicates the nucleation of new voids along with the growth of existing voids with the increase in annealing time. The newly found location of the Kirkendall marker plane in the Cu3Sn phase indicates that voids grow on both the sides of this plane which was not considered earlier for developing theoretical models.

cond-mat.mtrl-sci↗
arXiv2018

Probabilistic Program Equivalence for NetKAT

We tackle the problem of deciding whether two probabilistic programs are equivalent in Probabilistic NetKAT, a formal language for specifying and reasoning about the behavior of packet-switched networks. We show that the problem is decidable for the history-free fragment of the language by developing an effective decision procedure based on stochastic matrices. The main challenge lies in reasoning about iteration, which we address by designing an encoding of the program semantics as a finite-state absorbing Markov chain, whose limiting distribution can be computed exactly. In an extended case study on a real-world data center network, we automatically verify various quantitative properties of interest, including resilience in the presence of failures, by analyzing the Markov chain semantics.

cs.PL↗
arXiv2018

Analysis of axisymmetric boundary layers

Axisymmetric boundary layers are studied using integral analysis of the governing equations for axial flow over a circular cylinder. The analysis includes the effect of pressure gradient and focuses on the effect of transverse curvature on boundary layer parameters such as shape factor ($H$) and skin-friction coefficient ($C_f$), defined as $H = δ^*/θ$ and $C_f = τ_w/(0.5ρU_e^2)$ respectively, where $δ^*$ is displacement thickness, $θ$ is momentum thickness, $τ_w$ is the shear stress at the wall, $ρ$ is density and $U_e$ is the streamwise velocity at the edge of the boundary layer. Relations are obtained relating the mean wall-normal velocity at the edge of the boundary layer ($V_e$) and $C_f$ to the boundary layer and pressure gradient parameters. The analytical relations reduce to established results for planar boundary layers in the limit of infinite radius of curvature. The relations are used to obtain $C_f$ which shows good agreement with the data reported in the literature. The analytical results are used to discuss different flow regimes of axisymmetric boundary layers in the presence of pressure gradients.

physics.flu-dyn↗
arXiv2017

Final State Resolved Quantum Predissociation Dynamics of SO2(C1B2) and Its Isotopomers via a Crossing with a Singlet Repulsive State

The fragmentation dynamics of predissociative SO2(C1B2) is investigated on an accurate adiabatic potential energy surface (PES) determined from high level ab initio data. This singlet PES features non-C2v equilibrium geometries for SO2, which are separated from the SO + O dissociation limit by a barrier resulting from a conical intersection with a repulsive singlet state. The ro-vibrational state distribution of the SO fragment is determined quantum mechanically for many predissociative states of several sulfur isotopomers of SO2. Significant rotational and vibrational excitations are found in the SO fragment. It is shown that these fragment internal state distributions are strongly dependent on the predissociative vibronic states, and the excitation typically increases with the photon energy.

physics.chem-ph↗
arXiv2017

Comparing Topology of Engineered and Natural Drainage Networks

We investigated the scaling and topology of engineered urban drainage networks (UDNs) in two cities, and further examined UDN evolution over decades. UDN scaling was analyzed using two power-law characteristics widely employed for river networks: (1) Hack's law of length ($L$)-area ($A$) scaling [$L \propto A^{h}$], and (2) exceedance probability distribution of upstream contributing area $(δ)$ [$P(A\geq δ) \sim a δ^{-ε}$]. For the smallest UDNs ($<2 \>\text{km}^2$), length-area scales linearly ($h\sim 1$), but power-law scaling emerges as the UDNs grow. While $P(A\geq δ)$ plots for river networks are abruptly truncated, those for UDNs display exponential tempering [$P(A\geq δ) \>\text{=}\> a δ^{-ε}\exp(-cδ)$]. The tempering parameter $c$ decreases as the UDNs grow, implying that the distribution evolves in time to resemble those for river networks. However, the power-law exponent $ε$ for large UDNs tends to be slightly larger than the range reported for river networks. Differences in generative processes and engineering design constraints contribute to observed differences in the evolution of UDNs and river networks, including subnet heterogeneity and non-random branching.

physics.geo-ph↗
arXiv2016

Kulfi: Robust Traffic Engineering Using Semi-Oblivious Routing

Wide-area network traffic engineering enables network operators to reduce congestion and improve utilization by balancing load across multiple paths. Current approaches to traffic engineering can be modeled in terms of a routing component that computes forwarding paths, and a load balancing component that maps incoming flows onto those paths dynamically, adjusting sending rates to fit current conditions. Unfortunately, existing systems rely on simple strategies for one or both of these components, which leads to poor performance or requires making frequent updates to forwarding paths, significantly increasing management complexity. This paper explores a different approach based on semi-oblivious routing, a natural extension of oblivious routing in which the system computes a diverse set of paths independent of demands, but also dynamically adapts sending rates as conditions change. Semi-oblivious routing has a number of important advantages over competing approaches including low overhead, nearly optimal performance, and built-in protection against unexpected bursts of traffic and failures. Through in-depth simulations and a deployment on SDN hardware, we show that these benefits are robust, and hold across a wide range of topologies, demands, resource budgets, and failure scenarios.

cs.NI↗
arXiv2015

Standing Together for Reproducibility in Large-Scale Computing: Report on reproducibility@XSEDE

This is the final report on reproducibility@xsede, a one-day workshop held in conjunction with XSEDE14, the annual conference of the Extreme Science and Engineering Discovery Environment (XSEDE). The workshop's discussion-oriented agenda focused on reproducibility in large-scale computational research. Two important themes capture the spirit of the workshop submissions and discussions: (1) organizational stakeholders, especially supercomputer centers, are in a unique position to promote, enable, and support reproducible research; and (2) individual researchers should conduct each experiment as though someone will replicate that experiment. Participants documented numerous issues, questions, technologies, practices, and potentially promising initiatives emerging from the discussion, but also highlighted four areas of particular interest to XSEDE: (1) documentation and training that promotes reproducible research; (2) system-level tools that provide build- and run-time information at the level of the individual job; (3) the need to model best practices in research collaborations involving XSEDE staff; and (4) continued work on gateways and related technologies. In addition, an intriguing question emerged from the day's interactions: would there be value in establishing an annual award for excellence in reproducible research?

cs.DC↗