Mass Velocity Profiles Research Articles

Effects of Different Boundary Conditions at the Surfaces of the Extended Computational Domain in Computing the Natural Convection Flow in an Open Cavity

Effects of different boundary conditions at the surfaces of the extended computational domain on buoyancy driven natural convection flow in a three dimensional open cavity are studied numerically. This study is carried out for turbulent flow where Rayleigh number is greater than 108. Air is used as working fluid having properties at 25°C temperature and 1atm pressure. To capture the turbulent nature of the flow k - ? model is used. ANSYS CFX software is used to solve the governing equations subject to the corresponding boundary conditions. The methodology is verified through a satisfactory comparison with some published results. Average mass flow, temperature, stream line, contour velocity and velocity profile are studied at different height. An extended computational domain around the physical domain of the cavity at different surrounding conditions is considered to investigate the effect of its existence on the computation. Effects of different surrounding boundary conditions on the physical domain of the cavity are studied and reported.A relation among non-dimensional parameters such as Nusselt number, Rayleigh number, Prandlt number and Reynolds number is also reported.Dhaka Univ. J. Sci. 64(1): 31-37, 2016 (January)

Enhancement of natural ventilation of a novel roof solar chimney with perforated absorber plate for building energy conservation

A roof solar chimney equipped with perforated absorber plate was proposed to improve the ventilation performance of buildings. The optimized roof solar chimney consists of a glazed panel, a gap, a perforated absorber plate, and an absorber wall. Three-dimensional numerical simulations were conducted to investigate the heat transfer and fluid flow in the roof solar chimney at different gap widths and inclination angles. Mass flow rate, velocity profile, and temperature distribution in the roof solar chimney were presented. In the meantime, the comparisons of ventilation performance between the optimized configuration and the traditional roof solar chimney were performed. The perforated absorber plate, which divided the roof solar chimney into two channels and heated the air in the gap, increases the pressure head and the temperature difference between the ambient and inside air to augment the natural ventilation. The mass flow rate of the proposed optimized roof solar chimney is higher than that of the traditional roof solar chimney. It is also found that the optimization techniques using perforated absorber plate fitted in the air gap are more effective for the roof solar chimney with larger inclination angle and gap width.

Stabilization of unstable detonation waves in mixtures of nitromethane with inert diluents

Mass velocity profiles of detonation waves in mixtures of nitromethane with acetone and methanol with added diethylenetriamine sensitizer were measured using a VISAR laser interferometer. It was found that even small, about 1%, concentrations of acetone and methanol, inert diluents, led to instability of the one-dimensional detonation front in nitromethane. The results of the experiment show that the use of the sensitizer is an effective method of flow stabilization and if the concentration of the inert diluent does not exceed 10%, the detonation front becomes stable with the addition of 1% diethylenetriamine. At a higher diluent concentration, the sensitizer does not suppress the instability but decreases the oscillation amplitude by several times. The addition of diethylenetriamine to the mixture has been found to increase the detonation velocity.

Assessment of Cavitation Models for Flows in Diesel Injectors With Single- and Two-Fluid Approaches

Several recent cavitation models for the analysis of two-phase flows in diesel injectors with single- and two-fluid modeling approaches have been evaluated, including the Saha–Abu-Ramandan–Li (SAL), Schnerr–Sauer (SS), and Zwart–Gerber–Belamri (ZGB) models. The SAL model is a single-fluid model, while the other two models have been implemented with both single- and two-fluid approaches. Numerical predictions are compared with experimental results available in literature, qualitatively with experimental images of two-phase flow in an optically accessible nozzle, and quantitatively with measured mass flow rates and velocity profiles. It is found that at low injection pressure differentials there can be considerable discrepancy in the predictions of the vapor distribution from the three models considered. This discrepancy is reduced as the injection pressure differential is increased. Implementation of the SS and ZGB models with single- and two- fluid approaches yields noticeable differences in the results because of the relative velocity between the two phases, with two-fluid approach providing better agreement with experimental results. The performance of the SS and ZGB models implemented with the two-fluid approach is comparable with the SAL single-fluid model, but with significantly more computational time. Overall, the SAL single-fluid model performs comparatively better with respect to the other two models.

Nonequilibrium processes in condensed media: Part 1. Experimental studies in light of nonlocal transport theory

Based on experimental research in shock loading of solid-state materials it is shown that among the important dynamic characteristics of the process, like spatial-temporal mass velocity profiles of shock waves, are the mass velocity variation, velocity defect, and structural instability threshold recorded in real time. Analysis of these characteristics depending on the strain rate, target thickness, and structural state of material demonstrates that conventional approaches of continuum mechanics fail to provide their adequate interpretation and simulation of shock wave processes. A new concept of shock wave processes in condensed media is proposed. The concept, being based on nonlocal nonequilibrium transport theory, allows describing the transition from elastic to hydrodynamic response of a medium depending on the loading rate and time. A nonstationary elastoplastic wave model is proposed for describing the relaxation of an elastic precursor and formation of a retarded plastic front during the wave propagation in a medium with regard to structural evolution. Analysis of the experimental data shows that the division of stresses and strains into elastic and plastic components is incorrect for shock loading.

Investigation the effects of injection pressure and compressibility and nozzle entry in diesel injector nozzle’s flow

Investigating nozzle’s orifice flow is challenging both experimentally and theoretically. This paper focuses on simulating flow inside diesel injector nozzle via Ansys fluent v15. Validation is performed with experimental results from Winkhofler et al (2001). Several important parameters such as mass flow rate, velocity profiles and pressure profiles are used for this validation. Results include the effects of contraction inside nozzle’s orifice, effect of compressibility; effect of injection pressures and several orifice entries are also simulated in this study. To consider the effect of compressibility, a user defined function used in this simulation. The Cavitation model which is used in this simulation is Singhal et al. (2002) cavitation model. Presto discretization method is used for Pressure equation and second upwind discretization method is used for Momentum equation. Converging Singhal et al. cavitation model is very challenging and it needs several efforts and simulations.

Mass flow and velocity profiles in Neurospora hyphae: partial plug flow dominates intra-hyphal transport

Movement of nuclei, mitochondria and vacuoles through hyphal trunks of Neurospora crassa were vector-mapped using fluorescent markers and green fluorescent protein tags. The vectorial movements of all three were strongly correlated, indicating the central role of mass (bulk) flow in cytoplasm movements in N. crassa. Profiles of velocity versus distance from the hyphal wall did not match the parabolic shape predicted by the ideal Hagen-Poiseuille model of flow at low Reynolds number. Instead, the profiles were flat, consistent with a model of partial plug flow due to the high concentration of organelles in the flowing cytosol. The intra-hyphal pressure gradients were manipulated by localized external osmotic treatments to demonstrate the dependence of velocity (and direction) on pressure gradients within the hyphae. The data support the concept that mass transport, driven by pressure gradients, dominates intra-hyphal transport. The transport occurs by partial plug flow due to the organelles in the cytosol.

Centrifugal Reactive-Molecular Distillation from High-Boiling-Point Petroleum Fractions. 1. Mathematical Modeling, Screening Variables, and Experimental Validation

The modeling and simulation of the reactive-molecular distillation process (centrifugal type-CRMD) to upgrade high-boiling-point petroleum fractions is presented in this work. A case study is presented for an atmospheric petroleum residue (673.15 K+) of “W” crude oil. The use of the Plackett–Burman and fractional factorial designs (2IV4–1) permitted the evaluation of the effects of the independent variables (evaporator temperature, feed flow rate, percent weight of catalyst, feed temperature, condenser temperature, rotor speed, and system pressure) on the dependent variables: film thickness, surface evaporation rate, distillate mass flow rate, concentration profiles, and velocity profiles. The results showed that the independent variables, the evaporator temperature and the percent weight of catalyst, were relevant operational conditions for the performance of the CRMD process reaching a conversion of the feedstock about 65% at 3 wt % of catalyst and 64% at 5 wt % of catalyst in the distillate stream and ...

Structure of the detonation wave front in a mixture of nitromethane with acetone

It is shown that the leading front of an inhomogeneous detonation wave is a shock wave in which wave structures of the type of triple shock configurations are moving. It was experimentally found that the reaction in these inhomogeneities occurs in oblique shock waves. The reaction sites at the wave front are ring-shaped. In a 75: 25 mixture of nitromethane with acetone, up to 70% of the front surface is occupied by the reaction at the sites in the wave front. Measurements of the mass velocity profile indicate that afterburning takes place in the unloading area behind the Jouguet plane. Calculations of the heat release in the reaction mixture with a decrease in the mass velocity indicate that the material that have not reacted in the inhomogeneities can be ignited in the induction zone. It is suggested that the adiabatic flashes are a mechanism that generates inhomogeneities in the detonation wave front.

Detonation characteristics of diluted liquid explosives: Mixtures of tetranitromethane with methanol

Detonation in mixtures of nitromethane with methanol as an inert (nonexplosive) diluent is studied. Ignition experiments with mixtures in steel tubes of various diameters provided information on the effect of the degree of dilution on detonability. Mass velocity profiles with a chemical spike characteristic of detonation waves were recorded at the unsteady detonation front in all mixtures studied. This made it possible to distinguish the Chapman-Jouguet state and obtain a fairly complete set of detonation parameters. The dependence of the pressure in the detonation products on the methanol concentration is determined, which is required, in particular, to find the true (absolute) limit of detonation propagation for the concentration of diluted liquid explosives using the method proposed and validated by A. N. Dremin. Some results were found to be inconsistent with one-dimensional detonation theory.

Transient natural convection in a vertical microchannel heated at uniform heat flux

Miniaturization of devices has received a rapid expansion in the recent years and a great volume of research activities have been dedicated to micro-flow due to its new applications of micro-fluidic systems and components. In the present paper, an investigation of transient natural convection in parallel-plate vertical microchannels is carried out numerically. The vertical microchannel is considered asymmetrically or symmetrically heated at uniform heat flux. The first-order model for slip velocity and jump temperature is assumed in micro-scale conditions. The analysis is performed under laminar boundary layer assumption for different values of Knudsen number, Rayleigh number and the ratio of wall heat flux in order to evaluate their effects on wall temperatures, mass flow rate, velocity profiles and Nusselt number. Wall temperature overshoots are detected for the different conditions. These values increase with the increasing Knudsen number, Kn, at high Rayleigh number, Ra, whereas for lower Ra the lowest wall temperature is obtained at Kn=0.05. Mass flow rate increases with increasing Kn whereas Nusselt number decreases with increasing Kn. A composite correlation to evaluate the average Nusselt number in terms of Rayleigh and Knudsen numbers is proposed in the ranges 0≤Kn≤0.1 and 10−3<Ra<108.

Influence of Dense Inert Additives (W and Pb) on Detonation Conditions and Regime of Condensed Explosives

Results of experimental and theoretical studies of the unusual detonation properties of mixtures of high explosives (HEs) with high-density inert additives W and Pb were analyzed and systematized. Typical examples of the nonideal detonation of composite explosives for which the measured detonation pressure is substantially lower and the detonation velocity is higher than the values calculated within the framework of the hydrodynamic model, with the specific heat ratio for the detonation products of ∼6–8, are presented. Mechanisms of formation of anomalous pressure and mass velocity profiles, which explain the correlation between the Chapman-Jouguet pressure for HE–W and HE–Pb mixtures, the velocity of the free surface of duralumin target, and the depth of the dent imprinted in steel witness plates, are described.

Construction of the mass velocity profile for a rarefied molecular gas entrained by a rotating sphere taking into account thermophysical parameters of the gas

The problem of a sphere rotating in a molecular gas is solved in the isothermal approximation. The expression for the velocity of a rarefied molecular gas entrained by a sphere rotating in it is derived for sliding flow conditions taking into account the second-order correction in the Knudsen number. A generalization of the Boltzmann kinetic equation in the BGK model to the case of rotational degrees of freedom of gas molecules is used as the basic equation. The diffusive reflection model is employed as the microscopic boundary condition on the surface of the sphere. It is shown that this approach makes it possible to take into account the dependence of the gas velocity on the Prandtl number and gas temperature.

Specifics of the detonation of mixtures of powerful high explosives with W and Pb high-density inert additives

Results of experimental and theoretical studies of the unusual detonation properties of mixtures of high explosives (HEs) with high-density inert additives (W and Pb) were analyzed and systematized. Typical examples of the nonideal detonation of composite explosives for which the measured detonation pressure is substantially lower while the detonation velocity is higher than the values calculated within the framework of the hydrodynamic model, with the specific heat ratio for the detonation products being ∼6–8, are presented. Mathematical models capable of qualitatively and quantitatively describing the specificity of the propagation of detonation in HE-W mixtures. Mechanisms of formation of anomalous pressure and mass velocity profiles, which explain why the correlation between the Chapman-Jouguet pressure for HE-W and HE-Pb mixtures, the velocity of the free surface of duralumin target, and the depth of the dent imprinted in steel witness plates, are described.

Structure of detonation waves in tetranitromethane and its mixtures with methanol

A VISAR laser interferometer was used to measure mass velocity profiles in steady-state detonation waves in tetranitromethane and its mixtures with methanol. In the experiments with tetranitromethane, the chemical-spike pressure was found to be 1.7 times higher than the Chapman-Jouguet pressure. In mixtures with nearly stoichiometric methanol concentrations, the detonation front remained stable, but the chemical-spike amplitude increased suddenly and the shock broadened, probably due to the decomposition of the explosive at the front. A 50% increase in methanol concentration led to instability of the detonation front manifested in oscillations in the mass velocity profiles.

A comparison of non-Newtonian models for lattice Boltzmann blood flow simulations

In the present paper, three non-Newtonian models for blood are used in a lattice Boltzmann flow solver to simulate non-Newtonian blood flows. Exact analytical solutions for two of these models have been derived and presented for a fully developed 2D channel flow. Original results for the use of the K–L model in addition to the Casson and Carreau–Yasuda models are reported for non-Newtonian flow simulations using a lattice Boltzmann (LB) flow solver. Numerical simulations of non-Newtonian flow in a 2D channel show that these models predict different mass flux and velocity profiles even for the same channel geometry and flow boundary conditions. Which in turn, suggests a more careful model selection for more realistic blood flow simulations. The agreement between predicted velocity profiles and those of exact solutions is excellent, indicating the capability of the LB flow solver for such complex fluid flows.

An exact solution of the boundary value problem on thermal creep for a quantum Fermi gas

We solve analytically the boundary value problem on the thermal creep of a quantum Fermi-gas along a plane surface in a half-space. The solution uses the τ model Boltzmann equation. The mass velocity profile and the distribution functions on the half-space and on the wall are constructed. The first problem for the τ -model Boltzmann equation that was successfully solved analytically was Kramers’ problem about isothermal creep of a gas [1]. In [1], Cercignani initiated the study of exact solutions to boundary value problems in kinetic theory. Then, Loyalka and Cippola [2] found a solution to the problem about thermal creep of a gas. Latyshev and Yushkanov [3] solved Kramers’ problem for quantum Fermi gases. In the present paper, we continue the study and solve the thermal creep problem for quantum Fermi gases.

Computational simulation of intracoronary flow based on real coronary geometry.

To assess the feasibility of computationally simulating intracoronary blood flow based on real coronary artery geometry and to graphically depict various mechanical characteristics of this flow. Explanted fresh pig hearts were fixed using a continuous perfusion of 4% formaldehyde at physiological pressures. Omnipaque dye added to lead rubber solution was titrated to an optimum proportion of 1:25, to cast the coronary arterial tree. The heart was stabilized in a phantom model so as to suspend the base and the apex without causing external deformation. High resolution computerized tomography scans of this model were utilized to reconstruct the three-dimensional coronary artery geometry, which in turn was used to generate several volumetric tetrahedral meshes of sufficient density needed for numerical accuracy. The transient equations of momentum and mass conservation were numerically solved by employing methods of computational fluid dynamics under realistic pulsatile inflow boundary conditions. The simulations have yielded graphic distributions of intracoronary flow stream lines, static pressure drop, wall shear stress, bifurcation mass flow ratios and velocity profiles. The variability of these quantities within the cardiac cycle has been investigated at a temporal resolution of 1/100th of a second and a spatial resolution of about 10 microm. The areas of amplified variations in wall shear stress, mostly evident in the neighborhoods of arterial branching, seem to correlate well with clinically observed increased atherogenesis. The intracoronary flow lines showed stasis and extreme vorticity during the phase of minimum coronary flow in contrast to streamlined undisturbed flow during the phase of maximum flow. Computational tools of this kind along with a state-of-the-art multislice computerized tomography or magnetic resonance-based non-invasive coronary imaging, could enable realistic, repetitive, non-invasive and multidimensional quantifications of the effects of stenosis on distal hemodynamics, and thus help in precise surgical/interventional planning. It could also add insights into coronary and bypass graft atherogenesis.

Dynamics of laser ablation of microparticles prior to nanoparticle generation

To better understand the process of nanoparticle formation when microspheres are ablated by a high-energy laser pulse, we investigated the Nd:YAG laser-induced breakdown of 20 μm glass microspheres using time-resolved optical shadow images and Schlieren images. Time-resolved imaging showed the location of the initial breakdown and the shockwave motion over its first 300 μm of expansion. From these measurements, we determined the shockwave velocity dependence on laser fluence. Measured shockwave velocities were in the range of 1–10 km/s. We also developed a numerical model that simulated breakdown in the glass microsphere and the propagation of this disturbance through the edge of the sphere where it could launch an air shock. Our objective was to simulate the shockwave velocity dependence on laser fluence and to generate glass density, temperature, and mass velocity profiles after breakdown. The simulation and experimental data compared favorably.

Initiation of a Porous Explosive by Overdriven Gas Detonation Products

This paper reports results of experiments on initiation and development of detonation in cylindrical charges of a porous explosive by overdriven detonation products of a gas mixture C2H2 + 2.5 O2. Explosive charges with a bulk density of about 1 g/cm3 in fragile shells were studied. For PETN and RDX charges, the critical initial pressure of the gas mixture at which detonation initiation still occurs is determined and the pressures acting immediately on the charge are given. For PETN, critical initial pressures and initiation delays were measured for the first time for charges with particles of various diameters. The obtained dependence characterizes the following abnormal property of porous charges: there is an optimum particle size for which the explosive sensitivity is maximal. Streak records of self‐luminosity for typical initiation modes are given. Mass velocity profiles in initiation waves at different depth of the charge are obtained using an electromagnetic procedure.