فهرست مطالب saeid irani

In aircraft with flexible high aspect ratio wings, it is possible to couple flight dynamics modes and aeroelasticity. This coupling can lead to the body-freedom flutter (BFF) phenomenon, which is the subject of this article. In this study, the planar flight dynamics assumption and the model of a complete aircraft with flexible wings have been used. In this type of aircraft, due to the high aspect ratio of the wings, the effects of nonlinear terms on the dynamic response of the aircraft cannot be ignored. Therefore, for modeling the structure, the generalized nonlinear flexible Euler-Bernoulli beam model with bending-bending-torsion degrees of freedom and for modeling the aerodynamics, the Wagner function with the static stall model has been used. Using the developed model, the nonlinear behavior of the wing and the aircraft due to the occurrence of the BFF and wing flutter (WF) is studied. Also, a sensitivity analysis for the BFF is done and post-instability limit cycle oscillations and subcritical behaviors are investigated.

Nonlinear phenomena widely exist in engineering applications. A common example of this phenomenon in aerospace structures is the creation of fatigue cracks that open and close continuously under dynamic loads during the vibration cycle, causing a bilinear behavior in the structure. Late detection of such cracks can lead to a catastrophic failure. Therefore, identifying the nonlinear behavior of the cracked structure is very important for the prevention of structural failures. In this study, the nonlinear vibration behavior of a cantilever beam with a transverse breathing crack with bilinear behavior has been studied. For this purpose, we first estimate the polynomial function equivalent to the bilinear function of the beam stiffness. Then, using the well-known perturbation method, the method of multiple scales, the nonlinear equation of the beam is solved, and the frequency response equations for both harmonic and superharmonic resonances are extracted. Then, the sensitivity of the responses to the crack parameters such as depth, location, excitation force, and damping coefficient are investigated. The beam frequency response curves in the main resonance have become highly nonlinear due to the increase of the crack parameters and causes softening of the curves. Also, according to the results of the beam vibrations in superharmonic resonance, it was observed that the behavior of the beam in superharmonic resonance has a higher sensitivity to the existence of the breathing crack.

In this paper the flutter phenomenon in turbomachinary is introduced. The importance and characteristics of the flutter as a dynamic aeroelastic instability is presented. Conventional methods for the blade flutter test and different approaches in flutter analysis of blade are described. Among the existing analysis methods, one approach which only examines the stabilizing effect of fluid is used in order to analyze the flutter in this paper. Firstly, its equations are described and a criterion for the determination of the stability based on the analysis results is presented. According to the criterion the local and global stability can be concluded. Numerical analysis has been performed by ANSYS CFX. Mesh independence and two different turbulence models have been examined and results have been validated by test results. Numerical analysis has been carried out for two steady and unsteady states. In unsteady state the response of fluid to blade vibration in three modes has been calculated. In order to assess the total response two methods have been used and the results have been compared. Eventually local instability has been calculated and the results presented in figures which illustrate the contribution of adjacent blades in instability of specific blade. The evaluation of global instability for three modes has been presented and the obtained results are in excellent agreement with experiment.

In this study random vibration of a cantilever tapered beam under distributed stationary stochastic excitation with Gaussian probability density function is investigated. early free vibration analysis is performed to obtain the mode shapes of beam in form of Bessel functions, then the response is described in summation of mode shapes, and auto correlation of response is shaped by considering the mode shapes of tapered beam, also spectral density matrix of excitation is derived with cooperation of mode shapes and two dummy variables. in next step by means of frequency response and taking Fourier integral of autocorrelation of response, spectral density of displacement is computed and by using spectral density of displacement, variance of random displacements for various positions along the beam are achieved. Finally elasticity equation is applied to derive random strain and stress of beam. Comparing the variance of random stress with yield stress of beam leads to obtain probability of beam failure.

Abstract: In this work the equations of motion of a Solid State Wave Gyroscope (SWG) with rotary thin cylindrical shell resonator is analyzed using the shell & plates elasticity theory. The gyroscope conversion factor found in this analytical study corresponds with the experimental results obtained and listed in the References. The function of the SWG to measure the angular velocity or the rotating angle depends on the type of resonator’s excitation. This paper analyzes two types of excitations, a positional and a parametric excitation. The former has a rate gyroscope whereas the latter has a rate integrating gyroscope. The equations of motion are solved using Bobnov-Galerkin method, where the forces produced from excitation electrodes are assumed to be the external non-conservative forces. Finally, the relationship between the input and output of the gyroscope are derived to demonstrate that the type of the excitation, determines the type of gyroscope.