Journal Of Applied Fluid Mechanics
Volume:5 Issue: 1, Jan-Feb 2012

A computation of three-dimensional flow past an isolated NACA0012 airfoil using Reynolds-averaged Navier -stokes equations (RANS) method is conducted. Tow closure models k-ε and SST are applied to the numerical prediction of the turbulent flow. A two-dimensional numerical simulation of oscillatory movement of the profile, based on solving URANS equations is then studied for the dynamic stall prediction. FlUENT''s software is used for the numerical solution. Flow methodology for modelling 2-D unsteady viscous flow is presented and based on the incidence angle variation taken as a pulse using Gaussian function and oscillation forms. Results compared with experiment, show a very satisfactory agreement and highlight the need to take into account the unsteady flow and loads induced by movement of the structure.

A finite difference / front tracking method is used to examine the lateral migration of a three-dimensional deformable drop in plane Poiseuille flow at a finite-Reynolds-number. The computations are based on an improved implementation of the front tracking method at finite Reynolds numbers that include convective terms. The elliptic pressure equation is solved by a multigrid method. Both neutrally buoyant and non-neutrally buoyant drop are studied. The computation is performed within a unit cell which is periodic in the direction along the channel. A neutrally buoyant drop lags the fluid slightly, and the wall effect balances the effect of the curvature of the velocity profile, giving rise to an equilibrium lateral position about halfway between the wall and the centerline (the Segre-Silberberg effect). Results are presented over a range of density ratios. In the non-neutrally buoyant case, the gravity force is imposed along the flow direction. Non-neutrally buoyant drops have more complicated patterns of migration, depending upon the magnitude of the buoyancy force. When the density difference is small, the equilibrium position is either near the wall or near the centerline, depending on whether the drop leads or lags the local fluid. When the density difference is large enough, the equilibrium position shifts towards the centerline, irrespective of whether the drop is lighter or heavier than the fluid. The effect of Reynolds number and capillary number on the non-neutrally buoyant drops is investigated. The accuracy of the method is assessed by comparison with the other simulations and experiments.

The results of three-dimensional numerical simulations of drop collisions without the effect of a surrounding environment are presented. The numerical model is based on an Eulerian, finite-difference, Volume-of-Fluid method. Surface tension is included using the Continuum Surface Force method. Head-on collisions using equal size drops with three different fluid properties of water, mercury and tetradecane are presented. Various drop diameters ranging from 200 μm to 5 mm are considered. A separation criterion based upon deformation data is found. The lower critical Weber numbers are found for mercury and water drops while tetradecane drops did not result in separation for the range of Weber numbers considered. The effect of Reynolds number is investigated and regions of permanent coalescence and separation are plotted in the Weber-Reynolds number plane. The role of viscosity and its effect on dissipation is also investigated. Finally, the validity of the assumptions made in some of the collision models is assessed.

A numerical study is performed for a sonic jet issuing from a blunted cone to provide possible directional control in supersonic crossflow by solving the unsteady Reynolds-averaged Navier-Stokes (RANS) equations with the two-equation turbulence model. Results are presented in the form of static aerodynamic coefficients, computed at a free stream Mach number 4.0, with varying pressure ratios, incidence angle and keeping zero yaw and roll angles. The morphology and flow structure for the jet exhausting in crossflow at various pressure ratios is described in detail. The Flight control of the projectile can be accomplished by taking advantage of a complex shock-boundary layer interaction produced by jet interacting with the oncoming crossflow by altering pressure distribution in vicinity of the jet, a net increase in the net force can be utilized for maneuvering of vehicle and possible flight control. Computed static aerodynamic coefficients and pressure distribution using CFD analyses is with an accuracy of  5% in the supersonic range.

The effect of different forms of basic temperature gradients on the criterion for the onset of convection in a layer of an incompressible couple-stress fluid-saturated porous medium is investigated. It is shown that the principle of exchange of stability is valid, and the eigenvalue problem is solved numerically using the Galerkin technique. The parabolic and inverted parabolic basic temperature profiles have the same effect on the onset of convection and similar is the case between piecewise linear temperature profiles heating from below and above. Amongst the various basic temperature profiles, the linear temperature profile is found to be more stabilizing on the onset of instability. In addition, the influence of thermal depth on the criterion for the onset of convection is assessed in the case of piecewise linear temperature profiles. Moreover, an increase in the value of couple-stress parameter is found to delay the onset of convection and to increase the width of convection cells. It is also noted that the critical wave numbers are slightly affected by the nature of basic temperature profiles.

In the present paper, an experimental and numerical investigation of fluid flow and heat transfer in the case of wall injection besides main flow through a circular sudden enlargement are studied. The injected flow is achieved through an annular slot placed around the inner side wall of the step. The static pressure variation along the sudden enlargement length is measured and calculated at different values of injection ratio () and injection flow angles. The average heat transfer with Reynolds number (ReJ) of injected flow at different values of the inlet flow angle is obtained. The velocity, turbulent kinetic energy and temperature contours are presented in this study. Reynolds number of injected flow is varied between 320 and 840, Reynolds number of main flow is between 5895 and 8450 and the injection flow angles are 0o, 15o, 30o, 45o and 60o. In the injection case, the results indicate that, the pressure recovery coefficient increases by decreasing the injection ratio and increasing the flow angle. The average heat transfer coefficient increases as both injection Reynolds number and the injection flow angle increase. The numerical results showed that two recirculation zones generate behind the step between the injection flow and the main flow. The size of these recirculation zones decreases by increasing the injection flow rate. The turbulent kinetic energy increases within region between the recirculation zones and main zone also, it decays by injecting flow in the recirculation zone. The length for higher value of flow temperature decreases by injecting flow in the recirculation zone, and that length increases as the injection flow rate increases. The comparison between the experimental results and the numerical results gives good agreement using the k-ε model with Leschziner and Rodi correction.

Turbulent flow past circular cylinder at moderate to high Reynolds number has been analysed employing an second-order time accurate pressure-based finite volume method solving two-dimensional Unsteady Reynolds Averaged Navier Stokes (URANS) equations for incompressible flow, coupled to eddy-viscosity based turbulence models. The major focus of the paper is to test the capabilities and limitations of the present turbulence model-based 2D URANS procedure to predict the phenomenon of Drag Crisis, usually manifested in reliable measurement data, as a sharp drop in the mean drag coefficient around a critical Reynolds number. The computation results are compared to corresponding measurement data for instantaneous aerodynamic coefficients and mean surface pressure and skin friction coefficients. Turbulence model-based URANS computations are in general found to be inadequate for correct prediction of the mean drag coefficients, the Strouhal number and also the coefficients of maximum fluctuating lift over the range of flow Reynolds number varying from 104 to 107.

The paper examines transient MHD Couette flow of an electrically conducting fluid in the presence of an applied transverse magnetic field and thermal radiation through a porous medium. The dimensionless governing equations of the flow are coupled non-linear partial differential equations and are solved by an efficient and unconditionally stable finite difference scheme of Crank–Nicolson type. The influence of the medium permeability is also assessed. The velocity and temperature profiles for the flow are studied for various interesting parameters of Prandtl number, Nahme number and Hartmann number, and are presented graphically. The results show that the thermal radiation has appreciable influence on the flow.

The aim of this work is to control the flow around ground vehicles by active or/and passive strategies. The active control is achieved by steady, pulsed or closed-loop jets located at the backof the simplified car model. The passive control is performed using porous layers between the solid body and the fluid in order to modify the shear forces. The two previous control methods can be coupled to improve the drag reduction.

Gasification is a process that converts carbonaceous materials (coal, biomass, organic waste) into carbon monoxide and hydrogen by reacting the raw material at high temperatures with a controlled amount of oxygen and/or steam. The resulting gas mixture: syngas, can be used in energy production process. Syngas may be burned directly in internal combustion engines, used to produce methanol and hydrogen, or converted via the Fischer-Tropsch process into synthetic fuel. In addition, the high-temperature combustion refines out corrosive ash elements (chloride, potassium) allowing clean gas production after flying ash removal. Therefore, the main issue to syngas use in internal combustion engine is the removal of tarry product, ash and corrosive gaseous compounds. This paper consists in a state of the art on the composition of gaz from gazeifier considering the gasification operating conditions and the gazeified waste composition. The literature survey considers processes available to remove tarry products and particles from syngas produced in small scale gazeifier and to purify gas from unwanted gazeous compouds.

The one phase Stefan problem is discussed using the Goodman HBI method and an explicit numerical method including modified boundary immobilization scheme. The main advantage of the HBI method lie in the remarkable association of simplicity, flexibility and acceptable accuracy which an error less than 2.5% in predicting the moving front location for Stefan number less than unity which covers most usual isothermal phase change material. An accurate explicit numerical model to track the moving front in Stefan-like problems is provided. The scheme is obtained using the variable space step method based on variable domain. The method is developed using central difference approximations to replace spatial and temporal derivatives. Furthermore, iterative procedure, in numerical calculation, is avoided by introducing simple assumptions. The numerical results show that the accuracy of the method has been considerably improved without additional computational cost.

A numerical program is been developed to simulate the natural convection in a rectangular cavity in presence of a magnetic field. The cavity in filled with mercury with a Prandtl number equal to 0.024. The flow is induced by a vertical temperature gradient. This type of configuration concerns the crystal growth using the Bridgman vertical method. The mass, momentum and energy equations, adopting the Boussinesq approximation, are solved numerically using the finite-volume method in conjunction with the SIMPLER algorithm the flow under consideration is steady, laminar and two-dimensional. The temperature gradients are assumed to be weak. The results show that the dynamic and temperature fields are strongly affected by variations of the magnetic field intensity and the angle of inclination. Numerical simulations have been carried out considering different combinations of Grashof and Hartmann numbers to study their effects on the streamlines, the isotherms and the Nusselt number.