arXiv subjects

Kamal Kumar

Publications and source records attributed to Kamal Kumar.

9 recordsLinked to original sources

arXiv2026

A comparison of Fraunhofer-type diffraction from an atomic single-slit and a molecular double-slit

We measured the Q-value and the scattering angle distributions for non-dissociative state selective single electron capture in collisions of 7.5 keV H$^+$ and 15 keV H$_2^+$ with He. The experimental data are compared with semiclassical close-coupling calculations and predictions from the classical trajectory Monte Carlo simulations. By analogy with Fraunhofer diffraction, we also developed a toy model to reconstruct an imaginary screen that reflects the reaction impact-parameter dependence, in channels where the magnetic quantum number remains unchanged. It is well established that H$_2^+$ acts as a molecular double-slit in scattering processes. By demodulating the Young's double-slit-type interference pattern, we extracted the individual slit diffraction pattern of H$_2^+$ and compared it with that of the H$^+$ atomic single-slit. For ground state electron capture, we found that the single and the double slit diffraction patterns have equal fringe width, whereas for excited state electron capture, diffraction patterns are quite different.

physics.atom-ph↗

arXiv2025

Good Representation, Better Explanation: Role of Convolutional Neural Networks in Transformer-Based Remote Sensing Image Captioning

Remote Sensing Image Captioning (RSIC) is the process of generating meaningful descriptions from remote sensing images. Recently, it has gained significant attention, with encoder-decoder models serving as the backbone for generating meaningful captions. The encoder extracts essential visual features from the input image, transforming them into a compact representation, while the decoder utilizes this representation to generate coherent textual descriptions. Recently, transformer-based models have gained significant popularity due to their ability to capture long-range dependencies and contextual information. The decoder has been well explored for text generation, whereas the encoder remains relatively unexplored. However, optimizing the encoder is crucial as it directly influences the richness of extracted features, which in turn affects the quality of generated captions. To address this gap, we systematically evaluate twelve different convolutional neural network (CNN) architectures within a transformer-based encoder framework to assess their effectiveness in RSIC. The evaluation consists of two stages: first, a numerical analysis categorizes CNNs into different clusters, based on their performance. The best performing CNNs are then subjected to human evaluation from a human-centric perspective by a human annotator. Additionally, we analyze the impact of different search strategies, namely greedy search and beam search, to ensure the best caption. The results highlight the critical role of encoder selection in improving captioning performance, demonstrating that specific CNN architectures significantly enhance the quality of generated descriptions for remote sensing images. By providing a detailed comparison of multiple encoders, this study offers valuable insights to guide advances in transformer-based image captioning models.

cs.CV↗

arXiv2024

Electronic, optical, and transport properties of alkali metal oxides (Cs2O): A DFT study

The electronic, structural, optical, and thermoelectric properties of the Cs2O cubic structure have been investigated using density functional theory (DFT). The calculations utilize a full relativistic version of the full-potential augmented plane-wave plus local orbitals method, which is based on density functional theory, employing both the GGA and LDA approximations. Additionally, we employed the GGA proposed by Trans-Blaha (GGA-mBJ) for band structure computations, revealing the indirect band gap nature of Cs2O. The optical properties are also addressed by computing the refractive index, extinction coefficient, and complex dielectric tensor. The electrical conductivity, Seebeck coefficient, and thermal conductivity exhibit temperature-dependent variations, indicating the formation of a thermoelectric material. Our findings indicate that the compound under investigation is categorized as a p-type semiconductor, with the majority of charge carriers responsible for conduction being holes rather than electrons.

cond-mat.mtrl-sci↗

arXiv2024

DFT study of structural, electronic and optical properties of 2D MgO monolayer under bi-axial mechanical strain

The structural, electronic, and dielectric (optical) properties of graphene-like 2D MgO monolayer have been explored through first-principles calculations under bi-axial tensile and compressive mechanical strain within a range of -10% to +10%. Our findings revealed that the pristine MgO monolayer is an indirect band gap semiconducting material and the semiconducting mature of MgO monolayer remains consistent under both compressive and tensile mechanical strain. This nature of MgO is confirmed through partial density of states (PDOS) as well as electronic band structure. PDOS exhibits the contribution of different atomic orbitals in bond formation and nature of bond, while band structure provides insight into electron transitions between energy levels of valance and conduction bands. All optical parameters (dielectric function, reflectivity, energy loss, refractive index, extinction coefficient and absorption) are plotted in an energy range 0-15 eV. Within this energy interval, MgO possesses the highest value of the refractive index (2.13) at 3.12 eV energy. Also, a detailed analysis of changes in the geometrical structure of MgO monolayer is provided.

cond-mat.mtrl-sci↗

arXiv2017

Secondary instabilities in the flow past a cylinder: insights from a local stability analysis

We perform a three-dimensional, short-wavelength stability analysis on the numerically simulated two-dimensional flow past a circular cylinder for Reynolds numbers in the range $50\le Re\le300$; here, $Re = U_{\infty}D/\nu$ with $U_\infty$, $D$ and $\nu$ being the free-stream velocity, the diameter of the cylinder and the kinematic viscosity of the fluid, respectively. For a given $Re$, inviscid local stability equations from the geometric optics approach are solved on three distinct closed fluid particle trajectories (denoted as orbits 1, 2 & 3) for purely transverse perturbations. The inviscid instability on orbits 1 & 2, which are symmetric counterparts of one another, is shown to undergo bifurcations at $Re\approx50$ and $Re\approx250$. Upon incorporating finite-wavenumber, finite-Reynolds number effects to compute corrected local instability growth rates, the inviscid instability on orbits 1 & 2 is shown to be suppressed for $Re\lesssim262$. Orbits 1 & 2 are thus shown to exhibit a synchronous instability for $Re\gtrsim262$, which is remarkably close to the critical Reynolds number for the mode-B secondary instability. Further evidence for the connection between the local instability on orbits 1 & 2, and the mode-B secondary instability, is provided via a comparison of the growth rate variation with span-wise wavenumber between the local and global stability approaches. In summary, our results strongly suggest that the three-dimensional short-wavelength instability on orbits 1 & 2 is a possible mechanism for the emergence of the mode B secondary instability.

physics.flu-dyn↗

arXiv2017

Autoignition of Butanol Isomers at Low to Intermediate Temperature and Elevated Pressure

Autoignition delay experiments for the isomers of butanol, including n-, sec-, tert-, and iso-butanol, have been performed using a heated rapid compression machine. For a compressed pressure of 15 bar, the compressed temperatures have been varied in the range of 725-855 K for all the stoichiometric fuel/oxidizer mixtures. Over the conditions investigated in this study, the ignition delay decreases monotonically as temperature increases and exhibits single-stage characteristics. Experimental ignition delays are also compared to simulations computed using three kinetic mechanisms available in the literature. Reasonable agreement is found for three isomers (tert-, iso-, and n-butanol).

physics.chem-ph↗

arXiv2017

Autoignition of Methyl Valerate at Low to Intermediate Temperatures and Elevated Pressures in a Rapid Compression Machine

Methyl valerate (C$_6$H$_{12}$O$_2$, methyl pentanoate) is a methyl ester and a relevant surrogate component for biodiesel. In this work, we present ignition delays of methyl valerate measured using a rapid compression machine at a range of engine-relevant temperature, pressure, and equivalence ratio conditions. The conditions we have studied include equivalence ratios from 0.25 to 2.0, temperatures between 680 K and 1050 K, and pressures of 15 bar and 30 bar. The ignition delay data demonstrate a negative temperature coefficient region in the temperature range of 720 K-800 K for both $\phi$=2.0, 15 bar and $\phi$=1.0, 30 bar, with two-stage ignition apparent over the narrower temperature ranges of 720 K-760 K for the lower pressure and 740 K-800 K at the higher pressure. In addition, the experimental ignition delay data are compared with simulations using an existing chemical kinetic model from the literature. The simulations with the literature model under-predict the data by factors between 2 and 10 over the entire range of the experimental data. To help determine the possible reasons for the discrepancy between simulations and experiments, a new chemical kinetic model is developed using the Reaction Mechanism Generator (RMG) software. The agreement between the experimental data and the RMG model is improved but still not satisfactory. Directions for future improvement of the methyl valerate model are discussed.

physics.chem-ph↗

arXiv2017

Autoignition of n-Butanol at Elevated Pressure and Low to Intermediate Temperature

Autoignition experiments for n-butanol have been performed using a heated rapid compression machine at compressed pressures of 15 and 30 bar, in the compressed temperature range of 675-925 K, and for equivalence ratios of 0.5, 1.0, and 2.0. Over the conditions studied, the ignition delay decreases monotonically as temperature increases, and the autoignition response exhibits single-stage characteristics. A non-linear fit to the experimental data is performed and the reactivity, in terms of the inverse of ignition delay, shows nearly second order dependence on the initial oxygen mole fraction and slightly greater than first order dependence on initial fuel mole fraction and compressed pressure. Experimentally measured ignition delays are also compared to simulations using several reaction mechanisms available in the literature. Agreement between simulated and experimental ignition delay is found to be unsatisfactory. Sensitivity analysis is performed on one recent mechanism and indicates that uncertainties in the rate coefficients of parent fuel decomposition reactions play a major role in causing the poor agreement. Path analysis of the fuel decomposition reactions supports this conclusion and also highlights the particular importance of certain pathways. Further experimental investigations of the fuel decomposition, including speciation measurements, are required.

physics.chem-ph↗

arXiv2014

Secure Multipath Routing Scheme using Key Pre-Distribution in Wireless Sensor Networks

Multipath routing in WSN has been a long wish in security scenario where nodes on next-hop may be targeted to compromise. Many proposals of Multipath routing has been proposed in ADHOC Networks but under constrained from keying environment most seems ignorant. In WSN where crucial data is reported by nodes in deployment area to their securely located Sink, route security has to be guaranteed. Under dynamic load and selective attacks, availability of multiple secure paths is a boon and increases the attacker efforts by many folds. We propose to build a subset of neighbors as our front towards destination node. We also identified forwarders for query by base station. The front is optimally calculated to maintain the security credential and avail multiple paths. According to our knowledge ours is first secure multipath routing protocol for WSN. We established effectiveness of our proposal with mathematical analysis

cs.NI↗