فصلنامه مکانیک هوافضا
سال پنجم شماره 1 (پیاپی 15، بهار 1388)

In this article, flow through tube banks has been numerically simulated in order to determine flow induced forces. A Cartesian-staggered grid, based on finite-volume method has been implemented. Furthermore, the ghost-cell method in conjunction with great source term technique has been employed, in order to directly enforce no-slip condition on the tube's boundaries. The accuracy of the solver was validated by simulation of flow in a cavity and also flow over a single circular cylinder. The results are completely compatible with the experiments. Eventually, the flow through two types of unconfined tube banks with different arrangements in low Reynolds numbers were simulated and analyzed. For these tube bundles, the maximum drag and lift coefficients and maximum gap velocities were numerically obtained. The results shows more lift and drag coefficients in first rows. Also, confined tube boundle produces more folw induced forces on the cylinders.

Analytical methods have always been of interest to scientists in order to solve and investigate mathematical models of many physical problems. With the advent of computers, the roles of numerical methods have been prevailed. Recently, due to the enhancement of computers in symbolic mathematics, an increased attention is paid to developing and devising analytical methods. The Adomian decomposition method (ADM) is one of the recent developments. The capabilities and precision of the method are the subject of the present study which were attended to by investigating the Blasius problem. Also, the precision of the method is improved by employing the Padé approximantion. To best of our knowledge, this is the first time that by utilizing ADM, a very precise value for the Howarth number is obtained. The results of this study are compared to those of other methods, numerical or analytical, in the existing literature. The comparison suggests the advantage and competency of ADM.

This paper is presented to compute the wave-making resistance of an ellipsoid submarine, using boundary element method (BEM), in which the body surface and free surface are discretized into hyperboloidal elements. The fundamental solution of the governing Laplace's equation was obtained using Green's function via boundary integral equation. The linearized free surface boundary condition was applied and the unknowns of the doublet on the body and source on the free surface were obtained by solving the discretized equations. Then, the numerical results of pressure, wave-making resistance, and wave elevation were determined. We presented the results for the Wigley hull and for the submerged moving ellipsoid shape of the submarine UV11. The validity of the numerical results was examined by comparing it with experimental results.

Carburetor is replaced with port fuel injection systems for reduction of fuel consumption and exhaust emissions. These engines must be optimized for less exhaust emission and fuel consumption for serious emission regulations. In this paper, the effect of injection time and injection angle on the rate of fuel evaporation and emissions in the EF-7 engine has been studied, using KIVA-3V software for improvement of mixing formation process. The results show that fuel injection delay time reduces and directs injection to hot points (such as top face of valve) increasing the rate of fuel evaporation. Injection with 57 degree angle improvs the quality of mixture and combustion resulting in reduction of HC and CO emissions and some increase in NO emission. Other methods, such as spark ignition time adjustment, must be used for reducing NO emission.

In this work, the effects of injection initial conditions on displacement and rotation of flammable region in a two-phase fuel–air cloud in open atmosphere have been studied numerically. Investigating the internal conditions of the cloud, we recognize the flammable region and study the effects of injection velocity, droplet initial size, and temperature on the behavior of this region. This study can designate the proper position for successful ignition. Validation was made by comparing the numerical and experimental results. Results show increasing injection velocity and initial size of droplet decreasing droplet initial temperature, increase the rotation of flammable region.

In this research, the continuous adjoint method is applied to optimize an airfoil in subsonic and transonic flows. An Euler flow solver is used to analyze the inviscid compressible flow over airfoils in each design cycle. Two design problems appearing in aerodynamic shape optimization, namely inverse pressure design and drag minimization were investigated. In the first part, a test case was carried out to evaluate the performance of the adjoint method in inverse design problem. The results show that we can use the adjoint method as an efficient tool in inverse aerodynamic design problems. In the second part, the constrained optimization was investigated in a drag minimization problem. The investigated samples show that a small variation of airfoil geometry has caused considerable decrease in the drag coefficient. To evaluate the performance of the adjoint method in design problems with numerous design variables and also to evaluate the effects of the adoption of the design vector on the optimization results, the constrained drag minimization was performed using two different design vectors. The results shows that the mechanism and the value of drag reduction are affected by the type of design vector. Also, computational cost of the adjoint method are independent of the number of design variables.

This paper has investigated the flow interference between three circular cylinders of equal diameter in an equilateral-triangular arrangement and also between four circular cylinders in a square arrangement when subjected to a cross-flow. Wind tunnel experiments were conducted to measure force coefficients for six spacing ratios () varying from 1.5 to 4 at subcritical Reynolds number of. The pressure distributions on the surface of the cylinders were measured, using pressure transducers. It was found that for three cylinders at, the upstream cylinder experiences lower mean drag coefficient than that of the downstream ones. Also, the minimum drag coefficient values of the downstream cylinders occur at and. Moreover, It was revealed that for four cylinders at, due to severe flow interference between cylinders, there is a difference between lift coefficients for the upstream cylinders. Also, for, the mean drag coefficients for downstream cylinders are negative. In addition, it was concluded that the variations in strongly affect the aerodynamic coefficients. Also, by decreasing, the effects of the flow interference between the cylinders increase.