Journal of Aerospace Mechanics
Year:2005 | Volume:1 | Issue:2|Papers Count:9

In this work, a new mathematical modeling of servo-pneumatic system's dynamics is presented. In this new modeling, internal forces, such as friction and external forces acting on the moving rod of actuator, such as loads or disturbances, are mathematically modeled with their appropriate logic. First, pressure distribution on two sides of moving piston is assigned to cancel out the friction force with respect to the piston velocity sense and to balance the external forces according to their acting direction. The regulation of pressure continues to provide the desired acceleration in the piston. The model of pressure valves is assumed linear. Due to the entrance of non-linear terms into the equations describing dynamics of the system, mathematical modeling is based on sliding mode logic. Also, sliding mode control algorithm is used to achieve the positioning aim. Finally, a numerical simulation is done to show the performance of the control activity in the presence of the new mathematical modeling.

In the present study, a new type of air heater is developed using porous radiant burner. The air heater has three layered structure and works based on energy conversion method between gas enthalpy and thermal radiation. The main layer operates as a porous radiant burner, where the energy of combustion is converted into thermal radiation and emitted towards the two low temperature sections. The porous media in these sections are heated by absorbing the emitted radiation and the low temperature air flowing through the media is effectively heated by the reverse conversion from thermal radiation into gas enthalpy. In order to investigate the heat transfer characteristics of the heater, the coupled energy equations for the gas flows and the porous layers for each section are solved numerically by an iterative method. The Computational results show that this type of air heater has a very high efficiency, especially when the porous layers have high optical thickness.

Balance strain gauge bridges should operate with a high accuracy level and therefore the system of balance is considered as a precise device. Several signify cant matters, 'like safety, dependability, and reliability are involved in designing precise devices. Therefore, the fault tolerance property of balance systems is very important. The structure of a balance system and all types of errors affecting its operation are analyzed in this work. Then, the failure rate of balance components, such as balance structure, strain gauges, and power supply, are estimated using manufacturer technical documents or by doing field tests. Finally, the effect, of each component on the reliability of the system is investigated and the system reliability is calculated using a proposed Markov model. Simulation results show that shortening the duration time of experiments in the wind tunnel leads to system reliability improvement and experiment cost reduction.

Flight simulation is frequently used to analyze airplane performance. However, the related results must be compared with corresponding flight test data. This paper uses statistical procedures and spectral analysis in determining the validity of the mathematical model. For validation of this methodology, the simulation results have been compared with three actual airplane flight tests. This comparison shows relatively close agreements.

In this research, the turbine blade trailing edge flow ejection effects on Mach number distribution has been analyzed. For flow analysis RNG.K-ε turbulence model was used and the results as Mach number distribution on the blade surface were compared with experimental data. This comparison shows that the precision of the mentioned model for analyzing the Mach number loss at the trailing edge is due to the vortex which is generated there. The results also show the trailing edge flow ejection effects on blade surface Mach number distribution.

The main reason for the slow response of stirling engine with respect to power requirement supply is that the system energy supply is provided from wall to the working gas inside the cylinder. On contrary to most control systems, in stirling engines, actuator has the highest lag time, since energy transfer from the wall is taking place very slowly. In this article, increasing the reaction speed of stirling engine with respect to the required power is considered. Also, besides the temperature and pressure control inputs, a piston speed is also taken into account. As a result, control system is considered as a constant pressure and based on needed demand, the required parameters are calculated first from power-speed tables. At first, this speed is provided by an auxiliary electrical DC motor. Then, by comparing the output signal with the required output, the controlling commands are specified for pressure and gas temperature adjustments. In striling engine modeling, the isothermal assumption is eliminated in order to have a real behavior. The simulation of closed loop system and the designed controller show the effective increase in engine response speed. In this article, it is also shown that the control system is resistant, subjected to internal as well as external disturbances. These disturbances are in the form of changes in heat sink and system parameters. Since, torque is considered constant in a large speed interval, the engine efficiency will not be much affected.

Apple juice is a heat sensitive material. Vitamins, proteins, and other organic materials present in apple juice may be easily decomposed during heat processing. Therefore, evaporators with minimum residence time and maximum efficiency should be used for concentrating this material. Thus, long tubular type of falling film evaporator is a suitable device. In this work, the concentration of apple juice obtained from Uroumieh (North West region of Iran) up to 40°BX in a pilot-plant with single effect of falling film evaporator has been investigated. The results show that liquid film of apple juice flows down in turbulent regime. Also, variations of the Nusselt number and thickness of liquid film versus Reynolds number have been presented. Comparison of obtained results with the results of Chun-Seban, Garwin-Kelly and Mudawware-EI Masri, show that Nusselt numbers in this case are very close to their results and the film thickness is less than the results of Brauer, Feind, and Takahama. These results mean that the heat transfer coefficients of the liquid film in evaporation of apple juice are greater than the evaporation of regular water.

Carbon fibers are special materials with unique properties (i.e., high specific strength and specific modulus) used as reinforcement in different composites in various applications, e.g., aerospace and defense industries. These fibers are mainly manufactured from special polyacrylonitrile (PAN) fibers. Their fabrication method is composed of oxidative stabilization at low temperature in an air atmosphere following by carbonization at high temperature in an inert atmosphere. However, the high price of these fibers is related to raw material (special PAN fibers) and processing costs. In this study, the properties of carbon fibers fabricated from low cost commercial PAN fibers are compared to carbon fibers produced by special PAN fibers. The results reveals that, carbon fibers could be fabricated by commercial fibers successfully, but their tensile strength, elastic modulus, and densities are less than the carbon fibers fabricated by special PAN fibers. Higher diameter, linear density, and lower tensile strength of commercial PAN fibers are the main parameters which decrease the properties of carbon fibers fabricated from them.

Explosive Formed Projectiles (EFP) is applied extensively in anti-armour missiles. However, due to its complexity in analysis, numerical simulation is widely used. In particular, analysis of EFP forming and penetration can be effectively utilized in anti-armour warhead design. In this paper, the forming and penetration of two types of liner, including spherical with variable thickness and polynomial curve, have been simulated by LS-DYNA Code. For comparison and evaluation, several ballistic tests have also been carried out as experimental work. The results of numerical simulation, such as penetration depth and crater diameter, have been compared with the experimental results and good agreement has been obtained.