فصلنامه مکانیک هوافضا
سال دوم شماره 2 (پیاپی 5، آذر 1385)

The propellants in a liquid fuel rocket engine are guided into the combustion chamber by means of a back pressure provided by a pressurizing system. There are two common ways to optimize the mass of a pressurizing system, namely minimizing the pressure of the system and cutting off the inlet gas flow to the propellant tank at the right moment. In the former, the minimum pressure required by the propellant pumps depends on accurate calculation of the inlet pressure of the pumps. In the later however, the cut-off time of the gas flow to the propellant tank is a function of the rocket acceleration. The above parameters have direct impact on the total mass of the pressurizing system. In this work, the system pressure minimization and the inlet gas flow cut-off have been analytically studied. Our results have been compared with the existing experimental data, showing relatively close agreements.

This work has been carried out towards low emission high heat efficiency burners which do not operate with free flames. In such burners, the entire combustion takes place in a porous matrix. A one-dimensional model is used to solve the governing equations for the porous medium and for the gas flow. The combustion in a porous medium is modeled as a spatially dependent heat generation zone. In this work, the coupled energy equations for the gas and for the porous medium, based on two–flux radiation model, are numerically solved in order to analyze the transient thermal characteristics of porous burners. This way, the effects of various factors on the performance of porous radiant burners are determined. Comparison between the present results and the benchmark data shows close agreements.

In most engineering structures, specially moving ones and in general in the ones under dynamic and static loads, absorbing energy systems are used for preventing or reducing damages due to impacts. This paper examines the effective absorbing capability of the extruded single thin walled structures as absorbing energy systems. In this work, about 90 tests were carried out, wherein the effects of geometrical parameters, such as the thickness and the height of structures, mechanical parameters, such as yielding stresses, and the change in boundary conditions in the value of folding force in out of plane loading were investigated. Finally, a numerical study, which was based on finite element method, was conducted for comparison purposes. The accuracy of the numerical model was found to be about 95%.

Manufacturing of metal bellows with high ratio of crown to root diameter is very sensitive to design parameters, because of bursting possibilities. In this work, a process is introduced in which metal bellows are manufactured from tubes using bulging and folding processes. After the folding process, internal pressure, axial force, and annular plate dies are removed and the length of the bellows is increased with spring back. The quality of the final product depends on spring back, crown diameter, and thickness distribution. The main design parameters, which affect the quality of the products,are internal pressure, die course, and axial feeding. In this work, a parametric study was performed, using a commercial explicit finite element code and the effects of the main design parameters on the quality of the product is studied. Then, the design parameters leading to a product with acceptable quality were obtained numerically. Finally, an experiment was conducted to produce the metal bellows. The numerical and the experimental result showed very close agreements.

In this work, a numerical analysis of heat transfer in a thermal storage system has been performed using enthalpy method. In this method, the solution is based on a fixed grid and the governing equations are modified, such that they are valid for both phases. The heat transfer in the thermal storage system is the conjugate problem: phase change material and the transient forced convection between the fluid heat transfer and the wall. The system consists of two concentric cylinders, whose working fluid flows through inner pipe and its outer cylinder has been filled with the phase change material. The system works periodically and the governing heat transfer equations for the working fluid and for the phase change material are solved numerically. The differential flow equations and heat transfer have been discretized using the finite volume approach which, are solved using an iterative procedure. Temperature, enthalpy, and heat transfer coefficient inside the thermal storage system have been obtained and the results of the two cases have been compared, which are in relatively close agrements.

Variational iteration method is employed to investigate the flow over a flat plate. General Lagrange multipliers are introduced in this method to construct correction functionals for problems. The multipliers in the functionals can be identified optimally via variational theory. Comparison with Adomian decomposition method and Howarth’s numerical solution reveals that the approximate solutions obtained by the proposed method are of high accuracy.

Laminated composite shells are increasingly being used in various engineering applications, including aerospace, mechanical, marine, and automotive. In this paper, sound transmission through an infinite laminated composite cylindrical shell is studied in the context of the transmission of airborne sound into aircraft interior. The shell is immersed into an external fluid medium and contains internal fluid, while the airflow in external fluid medium is moving with a constant velocity. Modal impedance method, along with the first-order shear deformation theory (FSDT), is used to calculate the transmission loss (TL), considering three directions of the shell. The TL obtained in this study is compared with that of thin laminated composite obtained by others. The effects of structural properties and flight conditions on TL are studied for a range of values, especially, Mach number, aircraft flight altitude, shell thickness, and warp angle. Comparisons of the transmission loss are made among classical shell theory (CST) and FSDT for laminated composite and isotropic shells, which show close agreements.

A physical model, based on modeling of the near wall velocity fluctuations, is used for prediction of transition in an attached boundary layer. The near wall velocity fluctuations are assumed to develop into turbulent spots when their amplitudes, exceed a threshold value. In this work, the relevant physical correlations are developed and incorporated in a conventional boundary layer computer code for prediction of transitional flows. Test cases include transitional flat plate boundary layer flows under zero and non-zero pressure gradients with various freestream turbulence intensities. The results show close agreements with available experimental data.

In this work, the transient process of surge has been investigated numerically in a gas turbine engine. A one-dimensional stage-by-stage mathematical model has been developed which can describe the system behavior during aerodynamic instabilities. It is demonstrated that, these instabilities can be stabilized by the use of active control strategies, such as air bleeding and air injection. Both steady and unsteady active control systems were considered. In the steady case, mass is removed at a fixed rate from the diffuser, or mass is injected at a fixed rate into the first stage of the compressor. In unsteady control, the rate of bleeding or injection is linked with the amplitude and the frequency of the upstream pressure disturbances. Results show that both steady and unsteady strategies eliminate surge disturbances and suppress the instabilities. Therefore, they extend the stable operating range of compressor. It is also shown that smaller amount of compressed air needs to be removed in the unsteady control case. Also, a variable area diffuser is shown to be able of suppressing surge instabilities. Active control of instabilities, caused by sinusoidal perturbations of inlet total pressure, was also investigated, which showed to destabilize the stable operating condition the design point.