فهرست مطالب
Journal Of Applied Fluid Mechanics
Volume:6 Issue: 3, JulAug 2013
 تاریخ انتشار: 1392/05/12
 تعداد عناوین: 17


Pages 311320An overview is presented of recent advances in the filtered density function (FDF) modeling and simulation of turbulent combustion. The review is focused on the developments that have facilitated the FDF to be broadly applied in large eddy simulation (LES) of practical flows. These are primarily the development of a new Lagrangian Monte Carlo solver for the FDF, and the implementation of this solver on Eulerian domains portrayed by unstructured grids. With these developments, it is now much easier to apply FDF for predictions of reacting flows in complex geometrical configurations.

Pages 321329The objectives of the present study are to investigate the radiation effects on unsteady heat and mass transfer flow of a chemically reacting fluid past a semiinfinite vertical plate with viscous dissipation. The method of solution is applied using Finite element technique. Numerical results for the velocity, the temperature and the concentration are shown graphically for various flow parameters. The expressions for the skinfrication, Nusselt number and Sherwood number are obtained. The result shows that increased cooling (Gr>0) of the plate and the Eckert number leads to a rise in the velocity. Also, an increase in the Eckert number leads to an increase in the temperature, whereas increase in radiation lead to a decrease in the temperature distribution when the plate is being cooled.

Pages 331338The hemodynamics provides a way to predict effect of atherosclerosis by means of mathematical models. The pulsatile flow of blood through an artery with two sidetoside axisymmetric stenoses has been considered. A static transverse magnetic field to the flow is taken into account. The velocity profile, Wall Shear Stress and Wall Shear Stress Gradient to the flow have been simulated under the influence of magnetic field for various values of length and thickness of the stenosis. The upstream flow velocity in the subsequent stenosis region is significantly lower down from the velocity in the preceding stenosis region. The flow velocity decreases with the increase of Hartmann number. In the stenosis region wall shear stress (WSS) increases from unstenosed region to maximum thickness of stenosis. The wall shear stress (WSS) increases with the increase of Hartmann number and Womersley number. The WSSG have local maximum value in the vicinity of the throat of the stenoses and oscillates in the stenosed portion of the artery. The magnitude of WSSG is directly proportional to the Hartmann number. WSSG increases in magnitude on the upstream and downstream section of both the stenoses with the increase of Womersley number. Generated data are analyzed and discussed through graphs.

Pages 339350The logwake law with biquadratic boundary correction for the vertical velocity distribution which was changed from cubic boundary correction by Guo for the pipe data is applied to turbulent flow in openchannels. The biquadraticlog wake law is tested with experimental data from Coleman, Lyn, Wang and Qian and Kironoto and Graf. It shows that the biquadraticlogwake law matches well with flume data. A new mathematical model for vertical concentration distribution using the biquadraticlogwake law is proposed and tested with the existing laboratory data. This study reflect the fact that sediment suspension has significant effects on both von Karman constant and Coles’ wake strength.

Pages 351356The effect of chemical reaction, radiation on heat and mass transfer along a continuously moving surface in presence of thermophoresis has been discussed. The fluid viscosity is assumed as an inverse linear function of temperature. The system of nonlinear partial differential equations developed in the process have finally transformed into a set of ordinary differential equations with the help of similarity transformation and then solved numerically using Runga Kutta method with shooting technique. The results showing the effect of physical parameters on velocity, temperature and concentration have been computed and presented graphically to discuss them in detail. It has been observed that temperature increases with an increase in radiation parameter. Also, it is seen that the concentration decreases with the increase in chemical reaction parameter and Schmidt number.

Pages 357367Numerical solutions of, unsteady laminar free convection from an incompressible viscous fluid past a vertical cone with nonuniform surface heat flux m w q x a x varying as a power function of the distance from the apex of the cone (x 0) is presented. Here m is the exponent in power law variation of the surface heat flux. The dimensionless governing equations of the flow that are unsteady, coupled and nonlinear partial differential equations are solved by an efficient, accurate and unconditionally stable finite difference scheme of CrankNicolson type. The velocity and temperature fields have been studied for various parameters viz. Prandtl number Pr, semi vertical angle and the exponent m. The local as well as average skinfriction and Nusselt number are also presented and analyzed graphically. The present results are compared with available results in literature and are found to be in good agreement

Pages 369373A mathematical model was developed for determining the heat transfer between a moving sheet that passes through a moving fluid environment to simulate the fabrication process of sheet and fiberlike materials. Similarity transformations were introduced to reduce the governing equations to two nonlinear ordinary differential equations. For high values Prandtl number, the energy equation became much stiffer or singularly perturbed and the standard numerical methods failed to handle it. An innovative procedure combining shooting and singular perturbation technique was developed. The results show that the heat transfer depends on the relative velocity between the moving fluid and the moving sheet to a certain value after that value the relative velocity has no effect. If blowing effect is found the thermal layer becomes thinner and temperature profiles are backed together.

Pages 375384Computation of flow past high speed vehicles requires the use of a reliable turbulence model. Unfortunately, most of the turbulence models are developed for incompressible flows. Application of these models directly to high speed boundary layers with large density gradients can lead to significant errors in prediction of skin friction. Several compressibility corrections have been suggested in literature to predict these turbulent flows at high Mach numbers. In the present work, we have used two such corrections for the SpalartAllmaras turbulence model and studied their performance at high anglesofattack. Flow past an ogive cylinder is considered for the study.

Pages 385395In this paper, the effects of variable thermal conductivity and thermal radiation on the MHD flow and heat transfer of a nonNewtonian powerlaw liquid film at a horizontal porous sheet in the presence of viscous dissipation is studied. The governing time dependent boundary layer equations are transformed to coupled, nonlinear ordinary differential equations with powerlaw index, unsteady parameter, film thickness, magnetic parameter, injection parameter, variable thermal conductivity parameter, thermal radiation parameter, the Prandtl number and the Eckert number. These coupled nonlinear equations are solved numerically by an implicit, finite difference scheme known as the Keller box method. The obtained numerical results for velocity and temperature profiles are presented graphically. Also, the obtained results of our study for some special cases are compared with the previously published results, and the results are found to be in very good agreement. The effects of unsteady parameter on the skin friction, wall temperature gradient and the film thickness are explored for different values of the powerlaw index and the magnetic parameter. The results obtained reveal many interesting behaviors that warrant further study of the equations related to nonNewtonian fluid phenomena, especially the shearthinning phenomena.

Pages 397404This article concerns with a steady twodimensional flow of an electrically conducting incompressible dissipating fluid over an inclined semiinfinite surface with heat and mass transfer. The flow is permeated by a uniform transverse magnetic field. A scaling group of transformations is applied to the governing equations. The system remains invariant due to some relations among the parameters of the transformations. After finding three absolute invariants, a thirdorder ordinary differential equation corresponding to the momentum equation, and two secondorder ordinary differential equations corresponding to energy and diffusion equations are derived. The coupled ordinary differential equations along with the boundary conditions are solved numerically. Comparisons with previously published work are performed and the results are found to be in very good agreement. Many results are obtained and a representative set is displayed graphically to illustrate the influence of the various parameters on the dimensionless velocity, temperature and concentration profiles. It is found that the velocity increases with an increase in the thermal and solutal Grashof numbers. The velocity and concentration of the fluid decreases with an increase in the Schmidt number. The results, thus, obtained are presented graphically and discussed.

Pages 405412When a dielectric fluid is exposed to a high electric field (>1 kVmm1), electric forces are generated due to the nonuniformity of electric conductivity and dielectric constant. The electric body forces often produce complex and macroscopic flow such as convection, turbulent and chaotic flow. The secondary flow induced in high electric fields is well known as electrohydrodynamic (EHD) effects. According to previous EHD experiments and numerical simulation in DC fields, the velocity of flow has been reported to be of the order of 102 ms1 in electric fields of several kVmm1. However, on the application of high DC electric fields to some dielectric oils, a fluid jet with a velocity of about 1 ms1 can be created from the positive electrode as a bulk flow. In this study, the numerical simulation of EHD jet is carried out from the engineering aspects. The high speed jet flow is theoretically reproduced, and the obtained flow patterns are compared with the experimental results under the conditions of simple electrode allocations.

Pages 413423The Prandtl second kind of secondary current occurs in any narrow channel flow causing velocity dip in the flow velocity distribution by introducing the anisotropic turbulence into the flow. Here, a study was conducted to explain the occurrence of the secondary current in the outer region of flow velocity distribution using a universal expression. Started from the basic NavierStokes equation, the velocity profile derivation was accomplished in a universal way for both smooth and rough open channel flows. However, the outcome of the derived theoretical equation shows that the smooth and rough bed flows give different boundary conditions due to the different formation of log law for smooth and rough bed cases in the inner region of velocity distribution. Detailed comparison with a wide range of different measurement results from literatures (from smooth, rough and field measured data) evidences the capability of the proposed law to represent flow under all bed roughness conditions.

Boundary Layer Receptivity to Localized Disturbances in Freestream Caused by a Vortex Ring CollisionPages 425433The receptivity of a smooth flat plate to localized disturbances in freestream is investigated experimentally and numerically. The disturbances are generated outside a nominallyzeropressuregradient laminar boundary layer by a collision of two identical vortex rings with opposite signs. The vortex rings are generated by intermittent ejections of short duration jets from nozzles facing each other in the spanwise direction. A pair of rolled up vortex rings is given as the initial disturbances in the direct numerical simulation, and the growth of a boundary layer is simulated for arange of the Reynolds number based on the displacement thickness of boundary layer, 704 ≦ Re * ≦ 844. In the experimental results, high and lowspeed regions aligned in the streamwise direction are observed in the boundary layer before the external disturbances in the freestream reach the outeredge of the boundary layer. Although velocity fluctuations inside both regions become stronger downstream, a transition to turbulence takes place only in the highspeed region at approximately Re * = 844. In the numerical results, vortical fluctuations similar to the experiment appear near the wall immediately after the vortexringtype disturbances are added in the freestream, but it is found that the vortical fluctuations do not directly grow into strong vortical structures. On the contrary, the development of strong vortical structures that leads to transition is triggered by the external disturbances directly intruding the boundary layer.

Pages 435442In this paper, the problem of slowly rotating pervious axially symmetric body with source at its centre placed in an incompressible viscous fluid has been tackled. The method of separation of variables has been used and the general solution in terms of Legendre functions and Whittaker’s polynomial is given. As a first approximation, for n = 1, the results are in confirmation with spherical body. It is found that the effect of source at the centre is to reduce the resulting moment. Further, it has been conjectured that the results of couple for other bodies (i.e., for n 2) can also be obtained on the same ground but presently it is beyond the scope of the paper and would likely to appear in the future paper.

Pages 443451We have investigated an unsteady flow of a viscous, incompressible electrically conducting, laminar free convection boundary layer flow of a moving infinite vertical plate in a radiative and chemically reactive medium in the presence of a transverse magnetic field. The equations governing the flow are solved by Laplace transform technique. The expressions for velocity, temperature, concentration are derived and based on these quantities the expressions for skin friction; rate of heat transfer and the rate mass transfer near the plate are derived. The effects of various physical parameters on flow quantities, wise magnetic parameter, Grashof number, modified Grashof number, heat source parameter, the chemical reaction parameter, Schmidt number and radiation parameter are studied numerically and the results are discussed with the help of graphs. Some important applications of physical interest for different type motion of the plate like case (i) when the plate is moving with uniform velocity, case (ii) when the plate is moving with single acceleration and case (iii) when the plate is moving with periodic acceleration, are discussed.

Pages 453463Turbulence schemes have long been developed and examined for their accuracy and stability in a variety of environments. While many industrial flows are highly turbulent, models have rarely been tested to explore whether their accuracy withstands such augmented freestream turbulence intensity or declines to an erroneous solution. In the present study, the turbulence intensity of an air flow stream, moving parallel to a flat plate is augmented by the means of locating a grid screen at a point at which Rex=2.5×105 and the effect on the flow and the nearwall boundary is studied. At this cross section, the turbulence intensity is augmented from 0.4% to 6.6% to flow downstream. Wind tunnel measurements provide reference bases to validate the numerical results for velocity fluctuations in the main stream and at the nearwall. Numerically, four of the most popular turbulence models are examined, namely the oneequation SpalartAlmaras, the two equation Standard k , the two equation Shear Stress Transport and the anisotropy multi equation Reynolds Stress Models (RSM). The resulting solutions for the domain are compared to experimental measurements and then the results are discussed. The conclusion is made that, despite the accuracy that these turbulence models are believed to have, even for some difficult flow field, they fail to handle high intensity turbulence flows. Turbulence models provide a better approach in experiments when the turbulence intensity is at about 2% and/or when the Reynolds number is high.

Pages 465471The flow of the axisymmetric jet constitutes a subject of research from the origins of fluid dynamics; however it remains a subject of interest due to the recent findings that denote the influence of flow and geometry conditions in configurations that diverge from the theoretical “freejet” case. In the present study, the effect of a wall boundary produced by a circular disk of twice the jet diameter, which is imposed on the exit of the jet is investigated experimentally and numerically. Computational simulations are performed to predict the flow characteristics incorporating different turbulence models (kε and Reynolds Stress) and solvers (Simplec and Coupled). Supportive pressure measurements are used to evaluate the predictions within the initial region of a circular jet at two Reynolds numbers (50,000 and 65,000). The results indicate that the presence of the exit wall boundary results to the formation of recirculation zone behind the exit, which prevents the entrainment of ambient fluid. Comparing with the flow field of the free from confinement jet, it is shown that the imposition of the wall has a minor effect on the mean velocity field; it is however capable of altering the turbulent flow properties, including the normal and the Reynolds shear stresses, in the region before the establishment of the selfsimilarity zone.