مجله مکانیک سیالات و آیرودینامیک
سال یکم شماره 2 (زمستان 1391)

In this paper, static performance characteristics for a finite lenght two-lobe journal bearing lubricated with micropolar fluid is studied. Using finite element method, steady-state film pressure is obtained by solving modified Reynolds equation, based on micropolar lubrication theory. With the help of film pressure in bearing, the static performance characteristics in terms of load carrying capacity, attitude angle, friction coefficient, and rate of flow leakage of a two-lobe journal bearing are obtained for both Newtonian and micropolar lubricant. Finally, the effect of micropolar fluid characteristics parameters such as coupling number and non-dimensional characteristic length on the bearing static performance are presented. The computed results show that, compared with Newtonian fluids, the micropolar fluids exhibit an increase in load carrying capacity, also increase in attitude angle as well as increase in friction coefficient, while there will be decrease in side leakage flow for a specified external load.

In this paper, the interactions of flow over two circular or rectangular cylinders, passing by each other, have been investigated, using overset grids method. Two-dimensional incompressible governing fluid flow equations are solved using finite volume method. In this research, several interpolation methods have been tested to transfer data between grids. One of them called Mass flux base interpolation method has been presented. In addition, it is concluded that the overset grid method is very powerful to simulate fluid flow over moving bodies. The results are compared for lift, drag, and pressure coefficients with credential other numerical results. It is observed that in a short time interval, when two cylinders are passing by, a repulsion and then attraction force applies to each cylinder. In addition, drag force is reduced and then increased.

The interaction between wakes of bluff body and airfoil has profound influences on system performance in many industrial applications, e.g., turbo-machinery and cooling fan. The present work investigates the effect of configuration including airfoil’s angle of attack and transverse spacing of the models on frequency behavior of the cylinder’s near-wake. The experiments were carried out under subcritical flow regime, using hot-wire anemometry (HWA). The relationship between the Strouhal numbers, mean drag coefficient, and arrangements provide an insight into the global physical processes of wake interaction and vortex shedding. Present results show three basic types of flow pattern of wake interference, namely (1) Extended-Body Pattern, (2) Critical Pattern, and (3) Co-Shedding Pattern.

This paper presents a numerical simulation of developing flow and heat transfer of a viscoelastic fluid in a rectangular duct. In fully developed flow of a viscoelastic fluid in a non-circular duct, secondary flows normal to the flow direction are expected to enhance the rate of heat and mass transfer. On the other hand, properties such as viscosity, thermal conductivity, specific heat and relaxation time of the fluid are a function of temperature, which this dependency is also considered in this study. The rheological constitutive equation of the fluid is a common form of the Phan-Thien Tanner (PTT) model, which embodies both influences of elasticity and shear thinning in viscosity. The governing equations are discretized, using the FTCS finite difference method on a staggered mesh. The markerand- cell method is also employed to allocate the parameters on the staggered mesh and static pressure is calculated, using the artificial compressibility approach during the numerical simulation. In addition to report the results of flow and heat transfer in the developing region, the effect of some dimensionless parameters on flow and heat transfer has also been investigated. The results are in a good agreements with the results reported by others in this field.

In this paper, the accuracy of k-ε and k-ω turbulence models in super sonic boundary layer capturing of smooth/rough flat plates have been comprehensively investigated. Among these investigations, some sorts of sensitivity analysis, including change in turbulence model, generated-grid density, near wall affects, cμ magnitude (for boundary layers generated on smooth flat plates), and change in CKs, and also K+ s (for boundary layers generated on rough flat plates) have been performed. Also, the effects of compressibility on the Re laminarization of the turbulent boundary layer have been investigated in details. Finally, the accuracy of these turbulence models in capturing Cf/Cf0 for Mach number range lower than 5 have been numerically investigated. To confirm the validity of the results, the numerical results have been compared with some published references.

In this research, the calculation method of aerodynamic loads on an airplane in all flight situations is presented. In order to find an calculation equation for aerodynamic loads on an airplane in all flight conditions, the F-5 aircraft is selected as a prototype is divided into one hundred small parts longitudinally. Therefore, the numerical solutions are taken place in different conditions of Mach number, angle of attack, control surfaces changes, etc., which are the effective parameters. aerodynamic loads on each part is calvulated. Using least squares method, an equation for calculating applied aerodynamic loads on each part is determined. In order to find the variables used in mentioned equations above, an aircraft flight simulation is implemented. The results of flight simulation are validated by flight tests information. Finally, loop maneuver is perform using this simulator and the variables used in aerodynamic loads are recorded, while this maneuver is performed. Then, the aerodynamic loads applied on different parts during loop maneuver are calculated.The results show that the present method is suitable for airplane aerodynamic loads calculations.

In this study, the effects of strength and direction of a magnetic field on lid-driven cavity flow in a wide range of magnetic field strength variations (0 to 10 Tesla) are studied. The governing equations, including conservative equations of momentum, continuity and magnetic field were solved numerically. Momentum and continuity were solved by the finite volume method, using SIMPLE algorithm where the finite difference method was used for solving of magnetic field equation. In this research, the effects of magnetic field on the velocity distribution, streamlines, and vortex flow shows that the vortices of the cavity shrink up to 0.01 Tesla, where the uniform and vertical magnetic field was applied from the bottom surface. For the magnetic field of 0.01 Tesla, the vortices disappear. Higher values of magnetic field create larger vortices.

One of the most common methods for controlling and increasing maneuverability of projectiles, especially the ones which move at high altitude and in low density air, is using lateral or reaction jets. The resultant force of the reaction jet is related to the jet and flow parameters. For finding their relation experimental, numerical, and analytical methods have been used. In order to change the pendulous path of a hypersonic projectile, when is returning to the atmosphere (free flight), we compared common lateral jet method with the common ones, using a lateral nozzle (or a blunt body). For this purpose, a projectile with known geometry and minimum angular acceleration for 20 degrees circulation is assumed. Hence, we can decide about parameters of a suitable reaction jet and lateral nozzle for changing the path of a hypersonic projectile. Aerodynamic forces and moments from jet and lateral nozzle (or a bluntbody), which are the result of angular acceleration of the projectile with jet and lateral nozzle, are obtained by producing a three-dimentional mesh on a model projectile and solving the Navier-Stocks equations (assuming ideal gas and an appropriate flow regime with respect to height). Results show that in order to change the pendulous path of a hypersonic projectile, more than one nozzle reaction jet should be used and also a combination of a reaction jet and a bluff body (like a sphere) can be used.