Abdul Wahab

Numerical study of a thin film flow of fourth grade fluid

Purpose – The purpose of this paper is to study the thin film flow of a fourth grade fluid subject to slip conditions in order to understand its velocity profile. Design/methodology/approach – An exact expression for flow velocity is derived in terms of hyperbolic sine functions. The practical usage of the exact flow velocity is restrictive as it involves very complicated integrals. Therefore, an approximate solution is also derived using a Galerkin finite element method and numerical error analysis is performed. Findings – The behavior of fluid velocity with respect to various flow parameters is discussed. The results are not restrictive to small values of flow parameters unlike those obtained earlier using homotopy analysis method and homotopy perturbation method. Originality/value – An approximate solution based on finite element technique is derived.

Acoustic Scattering in Flexible Waveguide Involving Step Discontinuity

In this paper, the propagation and scattering of acoustic waves in a flexible wave-guide involving step discontinuity at an interface is considered. The emerging boundary value problem is non-Sturm-Liouville and is solved by employing a hybrid mode-matching technique. The physical scattering process and attenuation of duct modes versus frequency regime and change of height is studied. Moreover, the mode-matching solution is validated through a series of numerical experiments by testifying the power conservation identity and matching interface conditions.

Image de-noising with subband replacement and fusion process using bayes estimators, ☆ ☆☆

Abstract A hybrid image de-noising framework with an automatic parameter selection scheme is proposed to handle substantially high noise with an unknown variance. The impetus of the framework is to preserve the latent detail information of the noisy image while removing the noise with an appropriate smoothing and feasible sharpening. The proposed method is executed in two steps. First, the sub-band replacement and fusion process based on accelerated version of the Bayesian non local means method are implemented to enhance the weak edges that often result in low gradient magnitude and fade out during the de-noising process. Then, a truncated beta-Bernoulli process is employed to infer an appropriate dictionary of the edge enhanced data to obtain de-noising results precisely. Numerical simulations are performed to substantiate the restoration of the weak edges through sub-band replacement and fusion process. The proposed de-noising scheme is validated through visual and quantitative results using well established metrics.

A note on elastic noise source localization

The problem of reconstructing the spatial support of ambient noise sources from elastic wavefield boundary measurements using cross-correlation techniques is dealt with. It is demystified that the cross-correlation-based standard source localization functional in elastic media does not provide optimal refocusing due to different pressure and shear wave speeds. Then, a weighted functional is proposed to rectify the coupling artifacts. A numerical experiment is presented to substantiate the appositeness of the proposed functional.

Numerical study of two dimensional unsteady flow of an anomalous Maxwell fluid

Purpose – The purpose of this paper is to undertake an unsteady flow problem of an anomalous Maxwell fluid. The flow takes place between two side walls over a plate perpendicular to them and is driven by a sudden pressure force with constant gradient. Design/methodology/approach – A finite element method is invoked and is blended with a finite difference method for left Caputo fractional time derivatives in order to study the anomalous dynamics of the fluid. Findings – A numerical scheme for the constitutive equations of the prescribed flow in order to approximate the velocity field is designed. The graphical results to draw different physical conclusions on the flow problem are also presented. Originality/value – A rigorous mathematical exposition of the numerical scheme is provided and the results are valid for large values of the parameters.

Precision in 2D atom localization via coherent manipulation of the Raman gain process

A four-level N-type atom–field system exhibiting the Raman gain process is revisited for precision in position information of a single atom in a two-dimensional (2D) xy-plane. The atom interacts with two standing-wave fields where each standing-wave field is obtained via superposition of two orthogonal standing-wave fields, i.e., along the x and y directions, whereas a probe field is applied to observe the Raman gain process. The role of probe field detuning and phase shifts associated with the standing-wave fields in precision position information of a single atom in a 2D plane is investigated. The suggested scheme leads to enhancement in the spatial resolution in measuring the position of the atom, and a single atom localization peak in the xy-plane having a spatial resolution of the order of λ/10 is noticed.

An intermediate range solution to a diffraction problem with impedance conditions

An intermediate range solution for the problem of plane wave diffraction by a finite plate with impedance boundaries is presented. Initially, the problem is expressed in terms of two Wiener–Hopf equations with the help of Fourier transform and the boundary conditions in the transformed domain. The consideration of the intermediate range approximation in terms of source position renders integrals that are generally elusive to tackle because of the presence of branch points. These integrals are evaluated by invoking a modified stationary phase method, thereby a field valid over an intermediate range is calculated. The graphical analysis is preformed for various parameters of physical interest for both intermediate and far-field solutions.

Electromagnetic Time Reversal Algorithms and Source Localization in Lossy Dielectric Media

The problem of reconstructing the spatial support of an extended radiating electric current source density in a lossy dielectric medium from transient boundary measurements of the electric fields is studied. A time reversal algorithm is proposed to localize a source density from loss-less wave-held measurements. Further, in order to recover source densities in a lossy medium, we first build attenuation operators thereby relating loss-less waves with lossy ones. Then based on asymptotic expansions of attenuation operators with respect to attenuation parameter, we propose two time reversal strategies for localization. The losses in electromagnetic wave propagation are incorporated using the Debyes complex permittivity, which is well-adopted for low frequencies (radio and microwave) associated with polarization in dielectrics.

Beyond Born-Rytov limit for super-resolution optical diffraction tomography

Optical diffraction tomography (ODT) using Born or Rytov approximation suffers from severe distortions in reconstructed refractive index (RI) tomograms when multiple scattering occurs or the scattering signals are strong. These effects are usually seen as a significant impediment to the application of ODT because multiple scattering is directly linked to an unknown object itself rather than a surrounding medium, and a strong scatter invalidates the underlying assumptions of the Born and Rytov approximations. The focus of this article is to demonstrate for the first time that multiple scattering and high material contrast, if handled aptly, can significantly improve the image quality of the ODT thanks to multiple scattering inside a sample. Experimental verification using various phantom and biological cells substantiates that we not only revealed the structures that were not observable using the conventional approaches but also resolved the long-standing problem of missing cones in the ODT.

Resonance fluorescence based two- and three-dimensional atom localization

Two- and three-dimensional atom localization in a two-level atom–field system via resonance fluorescence is suggested. For the two-dimensional localization, the atom interacts with two orthogonal standing-wave fields, whereas for the three-dimensional atom localization, the atom interacts with three orthogonal standing-wave fields. The effect of the detuning and phase shifts associated with the corresponding standing-wave fields is investigated. A precision enhancement in position measurement of the single atom can be noticed via the control of the detuning and phase shifts.