Search results

The conservation element and solution element (CE/SE) method, an accurate and efficient explicit numerical method for resolving moving discontinuities in fluid mechanics problems, is used for the first time to solve phase change problems. Several isothermal phase change cases are studied and comparisons are made to existing analytical solutions. The CE/SE method is found to be accurate and robust for the numerical modeling of phase change problems.

Our recent results on stability and multiplicity of flow states for confined flows of an incompressible Newtonian fluid are surveyed. The considered laminar flows are caused by either thermal, mechanical, or electromagnetic effects and beyond the stability limit exhibit multiplicity of stable, steady or oscillatory, asymptotic states. Stability diagrams as well as examples of multiple flow states are given. It is concluded that beyond the critical value of the characteristic non‐dimensional parameter, and below the threshold to stochastic or turbulent state, multiple stable asymptotic flow states can be expected. This means that at such flow regimes, any computational (experimental) result may be strongly dependent on its initial condition and/or computational (experimental) path. Uncertainties of experimental and numerical modeling, which follow from this conclusion, are discussed. The global spectral Galerkin method using divergence free basis functions has been employed for the spatial approximation of the velocity and temperature fields. Several numerical experiments were performed comparing the present and other formulations, each of which confirmed the computational efficiency of the present approach over other classical numerical methods.

The purpose of this paper is to discuss the need to attend correctly to the accuracy and the manner in which the value of the streamfunction is determined when two or more impermeable boundaries are present. This is discussed within the context of the paper by Nandalur et al. (2019), which concerns the effect of a centrally located conducting square block on convection in a square sidewall-heated porous cavity. Detailed solutions are also presented which allow the streamfunction to take the natural value on the surface of the internal block.

Steady solutions are obtained using finite difference methods. Three different ways in which insulating boundary conditions are implemented are compared. Detailed attention is paid to the iterative convergence of the numerical scheme and to its overall accuracy. Error testing and Richardson’s extrapolation have been used to obtain very precise values of the Nusselt number.

The assumption that the streamfunction takes a zero value on the boundaries of both the cavity and the embedded block is shown to be incorrect. Application of the continuity-of-pressure requirement shows that the block and the outer boundary take different constant values.

The Darcy–Rayleigh number is restricted to values at or below 200; larger values require a finer grid.

This paper serves as a warning that one cannot assume that the streamfunction will always take a zero value on all impermeable surfaces when two or more are present. A systematic approach to accuracy is described and recommended.

In a previous paper [COMPEL, Vol. 2, No. 3, pp. 117–139], an analysis was presented of a discretization procedure for a class of elliptic problems, including the Scharfetter‐Gummel method for the continuity equations of stationary semiconductor device models. Here, the previous results are extended in two directions: firstly, to such problems in an arbitrary number of dimensions and, secondly, to include the computation of suitable moments, as opposed to point‐values, of the carrier density distributions.

This paper aims to fabricate and characterize ZnO-based multilayer varistors.

Tape casting technique was utilized for preparation of multilayer varistors based on ZnO doped with Pr, Bi, Sb, Co, Cr, Mn and Si oxides. Scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) methods were used to study the microstructure, elemental and phase compositions, respectively, of the varistors. Dielectric properties were investigated by impedance spectroscopy. Current–voltage (I–U) dependences were measured to characterize nonlinear behavior of the fabricated varistors.

XRD, SEM and EDS studies revealed dense microstructure of ceramic layers with ZnO grains sized 1-4 μm surrounded by nanometric Bi-rich films, submicrometer Zn₇Sb₂O₁₂ spinel grains and needle-shaped Pr₃SbO₇ crystallites. Praseodymium oxide was found to be very effective as an additive restricting the ZnO grain growth. I–U characteristics of the fabricated multilayer varistors were nonlinear, with the nonlinearity coefficients of 23-27 and 19-51 for the lower and higher Pr₂O₃ content, respectively. The breakdown voltages were 60-150 V, decreasing with increasing sintering temperature.

Low-temperature cofired ceramics technology enables attaining a significant progress in miniaturization of electronic passive components. Literature concerning application of this technology for multilayer varistors fabrication is limited. In the present work, the results of XRD, SEM and EDS studies along with the I–U and complex impedance dependences are analyzed to elucidate the origin of the observed varistor effect. The influence of sintering temperature and Pr₂O₃-doping level was investigated.

The purpose of this article is to investigate the propagation characteristics (such as particle motion, attenuation and phase velocity) of a Rayleigh wave in a nonlocal generalized thermoelastic media.

The bulk waves are represented with Helmholtz potentials. The stress-free insulated and isothermal plane surfaces are taken into account. Rayleigh wave dispersion relation has been established and is found to be complex. Due to the presence of radicals, the dispersion equation is continuously computed as a complicated irrational expression. The dispersion equation is then converted into a polynomial equation that can be solved numerically for precise complex roots. The extra zeros in this polynomial equation are eliminated to yield the dispersion equation’s roots. These routes are then filtered for inhomogeneous wave propagation that decays with depth. To perform numerical computations, MATLAB software is used.

In this medium, only one mode of Rayleigh wave exists at both isothermal and insulated boundaries. The thermal factors of nonlocal generalized thermoelastic materials significantly influence the particle motion, attenuation and phase velocity of the Rayleigh wave.

Numerical examples are taken to examine how the thermal characteristics of materials affect the existing Rayleigh wave’s propagation characteristics. Graphical analysis is used to evaluate the behavior of particle motion (such as elliptical) both inside and at the isothermal (or insulated) flat surface of the medium under consideration.

While there is anecdotal evidence that internationally mobile workers often form isolated nation-based communities or “expatriate bubbles,” previous academic scholarship on the expatriate communities and their subjective boundaries is limited. The primary purpose of this article is to advance the theoretical or conceptual understanding of expatriate communities as bubbles.

As developed by Lamont and Molnár (2002), the theory of symbolic boundaries is applied and set to scrutinize the production and maintenance of insulated expatriate communities. Empirically, an ethnographic study of a community of Finnish expatriates in a Southeast Asian country is undertaken to describe how symbolic boundaries are constructed.

The main theoretical implication of the paper is the recognition that expatriates themselves are involved in creating the “bubble.” The boundaries separating the national expatriate community are not externally imposed but can be viewed as consequences of the active boundary work of the expatriates. The empirical study demonstrates how the Finnish expatriates negotiated the symbolic boundaries of their community, drawing on cultural, moral and spatial modalities in different levels of boundary work.

There need to be more systematic attempts to develop a theoretically grounded understanding of insulated expatriate communities and their boundaries. This article contributes to the sociological conceptualization of expatriate bubbles by utilizing the symbolic boundary approach, which adds perspective to the embryonic theory of the subjective boundaries of expatriate communities. The multiplicity of different types of symbolic boundaries and their modalities suggests that an expatriate bubble is rarely a finished state or structure.

The purpose of this paper is to study the propagation of inhomogeneous waves in a partially saturated poro-thermoelastic media through the examples of the free surface of such media..

The mathematical model evolved by Zhou et al. (2019) is solved through the Helmholtz decomposition theorem. The propagation velocities of bulk waves in partially saturated poro-thermoelastic media are derived by using the potential functions. The phase velocities and attenuation coefficients are expressed in terms of inhomogeneity angle. Reflection characteristics (phase shift, loci of vertical slowness, amplitude, energy) of elastic waves are investigated at the stress-free thermally insulated boundary of a considered medium. The boundary can be permeable or impermeable. The incident wave is portrayed with both attenuation and propagation directions (i.e. inhomogeneous wave). Numerical computations are executed by using MATLAB.

In this medium, the permanence of five inhomogeneous waves is found. Incidence of the inhomogeneous wave at the thermally insulated stress-free surface results in five reflected inhomogeneous waves in a partially saturated poro-thermoelastic media. The reflection coefficients and splitting of incident energy are obtained as a function of propagation direction, inhomogeneity angle, wave frequency and numerous thermophysical features of the partially saturated poro-thermoelastic media. The energy of distinct waves (incident wave, reflected waves) accompanying interference energies between distinct pairs of waves have been exhibited in the form of an energy matrix.

The sensitivity of propagation characteristics (velocity, attenuation, phase shift, loci of vertical slowness, energy) to numerous aspects of the physical model is analyzed graphically through a particular numerical example. The balance of energy is substantiated by virtue of the interaction energies at the thermally insulated stress-free surface (opened/sealed pores) of unsaturated poro-thermoelastic media through the bulk waves energy shares and interaction energy.

Using turbulators, obstacles, ribs, corrugations, baffles and different tube geometry, and also various arrangements of these components have a noticeable effect on the shell and tube heat exchangers (STHEs) thermal-hydraulic performance. This study aims to investigate non-Newtonian fluid flow characteristics and heat transfer features of water and carboxyl methyl cellulose (H₂O 99.5%:0.5% CMC)-based Al₂O₃ nanofluid inside the STHE equipped with corrugated tubes and baffles using two-phase mixture model.

Five different corrugated tubes and two baffle shapes are studied numerically using finite volume method based on SIMPLEC algorithm using ANSYS-Fluent software.

Based on the obtained results, it is shown that for low-mass flow rates, the disk baffle (DB) has more heat transfer coefficient than that of segmental baffle (SB) configuration, while for mass flow rate more than 1 kg/s, using the SB leads to more heat transfer coefficient than that of DB configuration. Using the DB leads to higher thermal-hydraulic performance evaluation criteria (THPEC) than that of SB configuration in heat exchanger. The THPEC values are between 1.32 and 1.45.

Using inner, outer or inner/outer corrugations (outer circular rib and inner circular rib [OCR+ICR]) tubes for all mass flow rates can increase the THPEC significantly. Based on the present study, STHE with DB and OCR+ICR tubes configuration filled with water/CMC/Al₂O₃ with f = 1.5% and d_np = 100 nm is the optimum configuration. The value of THPEC in referred case was 1.73, while for outer corrugations and inner smooth, this value is between 1.34 and 1.57, and for outer smooth and inner corrugations, this value is between 1.33 and 1.52.

The purpose of this paper is to present a model to analyze free vibrations in a transradially isotropic, thermoelastic hollow sphere subjected to stress free, thermally insulated or stress free, isothermal and rigidly fixed, thermally insulated or rigidly fixed, isothermal boundary conditions.

The potential functions along with spherical wave solution have been used to reduce the system of governing partial differential equations to a coupled system of ordinary differential equations in radial coordinates after employing non-dimensional quantities. Matrix Frobenius method of extended power series has been employed to obtain accurate solution of coupled differential equations in terms of radial coordinates. The mathematical model of the considered problem has been solved analytically to obtain the characteristics equations after imposing the appropriate boundary conditions at the outer and inner surfaces of the hollow sphere. The characteristic equations which govern various types of vibration modes expected to exist have been derived in the compact form. The special cases of spheroidal and toroidal modes of vibrations have been deduced from the characteristic equations and discussed.

The toroidal mode has been found to be independent of temperature change. The magnitude of lowest frequency and damping factor are significantly affected in the presence of thermal field and increase with an increase in the spherical harmonics in addition to geometry of the structure.

The matrix Frobenius method has been used to develop analytical solutions and functional iteration technique to carry out numerical simulations of such structures for the first time. The simulated results are presented graphically and compared with the available literature.