Journal of Aerospace Mechanics
Year:2013 | Volume:9 | Issue:2 (32)|Papers Count:10

In this work, nonlinear model of a Heating, Ventilating, and Air Conditioning (HVAC) system in the cooling mode is first derived using energy and mass balance equations. All components of the system are independently modeled in the modeling process. The responses of the open loop system are then obtained and the reliability of modeling is verified by comparing the actual system records with those of the other researchers. The nonlinear model of the system is linearized using the Jacobian matrix method, and a PID-action as well as the Linear Quadratic Regulator (LQR) controller as the linear control systems are developed on the linear model. PID-action is a classic and robust controller and the LQR is a modern and optimal controller system. In the final part, Sliding Mode Controller (SMC) is designed on the nonlinear model as a robust and proper controller system. The comparison between these proposed controller systems confirms the superiority of the SMC controller, which results in a reduction of the settling time, the omission of overshoot, and an improvement in the actuator's performance via the reduction of energy consumption in the system.

subjected to follower force, where each step has a point mass with transverse and rotary inertia. A finite element code is developed to model two-dimensional Timoshenko beams using the extended Hamilton's principle. The effects of each section length and distributed and concentrated mass on the type of the instability and the critical follower force have been studied for different models of beams. It is concluded that when mass of last section is subsided, the magnitude and the type of instability changes significantly and in most cases it decrease. It is observed that change in rotary inertia parameter of Timoshenko beam and the rotary inertia of concentrated masses affect the magnitude and the type of instability. In this case, by change concentrated mass location, the critical follower force increase suddenly. Effect of joint springs for two section beam with damping and stiffness variation is analysed. Increase in joints parameters lead to more stability.

In this paper, Improve of transmission loss (TL) for simple geometry mufflers using micro-perforated panels (MPP) is studied. The TLs given by numerical method using SYSNOISE are compared with experimental and analytical results of other researchers. Different configurations have been used so as to detect the real effect of resonator absorbers based on micro-perforated panels in the expansion chambers. It is shown that applying these absorbers can effectively enhance the TL in desired frequencies if their parameters are well chosen. In addition, the reactive effects of the geometry are of high importance in compare with dissipative ones. However, their dissipative effects are negligible when they act at a reactive effect resonance. Finally, numerical modeling of micro-perforated panel in numerical software that it was impossible, in this paper a suitable solution using the equivalent impedance method for modeling in boundary element software, SYSNOISE, is presented.

This primary purpose of this work was to develop control laws for three axis stabilization of a magnetic actuated satellite with a passive gravity gradient boom. the satellite shall be three axis stabilized with its boom pointing outwards. This was achieved by classical linearized control of satellite. in addition to a theoretical treatment, the theses contain a large portion of application considerations. Stability analysis was done to find the moment of inertia of a satellite, in order to reach the passive gravity gradient control with boom. The control concept considered was that interaction between the earth’s magnetic field and a magnetic field generated by a set of coils in the satellite can be used for actuation. magnetic torquing was found attractive for generation of control torques on small satellites. Since magnetic control systems are relatively lightweight, require low power and are inexpensive. However, this principle is inherently none linear and difficult to use, classical linearized control and fuzzy controllers were used to stabilized the satellite and their performance was tested via simulation.

Modal Analysis for obtaining the shape modes and frequencies of a structure is a major step in design and analysis of marine structures. It is known that the natural frequency is reduced due to added mass effects under the water, and excitation near the natural frequencies may result in dangerous vibrations in the system. So understanding how the vibration property of a structure is changed under water is an important problem. In this paper modal analysis of a cylindrical structure under water and also in the air is investigated. The effects of water depth and water surface on the mode shapes and natural frequencies of the structure are studied via experimental and numerical methods. To this end three Aluminum cylindrical structures consist of 7 cabins with different diameters are constructed. Then modal experiments are performed in the air and also under water with 15 different depths from surface to 3- meter depth. Different test results are compared with the numerical/analytical results obtained from the ANSYS software and a semianalytical method. The results show that there is a nearly good agreement between the numerical/analytical and experimental results; however some differences between the results are considered which should be studied further in future works.

Functionally Graded cylindrical structures, because of their good mechanical ability and their high heat resistance capability, are noteworthy in structural designers view. So, possessing an efficient analysis method for vibration of such structures is so important. In this paper, free vibration of functionally graded cylindrical shell is analyzed based on Donnell and Sanders theories. Also, the first order shear deformation theory is used for displacement field. FG material properties change through the thickness direction according to a power law distribution function. Equations of motion are solved with state space method and as an advantage, numerical results are calculated for every classical boundary conditions. Numerical Results are compared with literature and 3D finite element model and good agreement are observed. Also, effects of geometry and material parameters change on natural frequencies are studied for different boundary conditions. In some cases, one of the theories has major error which is mentioned.

In this paper forced vibration of FGM Timoshenko beam with piezoelectric layers are analyzed based on the first shear deformation beam theory. The governing equations of motion are derived using Hamilton’s principle. Dynamic behaviors of beam are investigated by navier solution and the results obtained from this solution are compared with the existing theoretical results and showed relatively good agreements. Furthermore the effects of increasing the velocity of moving load in maximum deflection center of beam is studied and the critical velocity in different distribution models of FGM are investigated.

In this paper, with a focus on sliding mode, designing a quarter car active suspension system has been perfomed using model reference strategy and then it has been improved. the aim of designing this system is creating an optimal tradeoff between car body displacement and wheels stability on the road, so that during travel, it's favorably safeguarded passengers comfort and car controlability, too. in this paper after introducing sky-hook and ground-hook mathematical models, performance of single and hybrid model reference controllers has been studied and then for improving system behavior, some suggestions such as optimum rotation of sliding surfaces and using fuzzy system for regulating a basic parameter has been mentioned. Simulation results show proposed methods efficiency for improving system performance.