Journal of Aerospace Mechanics
Year:2006 | Volume:2 | Issue:1|Papers Count:10

In this work, a three-dimensional incompressible viscous flow inside a spiral channel is numerically investigated, using the finite volume method on a collocated unstructured grid. Tetrahedral elements are used to discredit the governing equations. First, the accuracy of the code developed was tested by comparing the numerical results obtained for flow inside a squared channel with the available experimental data. Then, the flow inside a spiral channel was simulated. The results obtained indicate large gradients of flow quantities near the tongue of the volute, where the stagnation point develops. These results may be used in studying the flow field in volutes of fans and pumps as well.

In this work, the amount of energy delivered to a flat circular metal plate by an underwater explosion of a concentrated charge, placed at some standoff distance from the plate surface, was determined. Then, the strain energy of a conical shell was calculated. Analytical modeling was performed regarding assumptions such as, consistency of plate thickness during deformation, non-work hardening of materials, and linear variation of effective strain. Finally, the required mass of explosives for forming of the cone was obtained by equating the delivered energy and the strain energy. Some experiments were designed and carried out for comparison with our theoretical results. Our analytical results were also compared with another set of available experimental result. These companions show relatively close agreements.

In this work, early fracture' of the fan blades of a national power plant has been investigated both from mechanical and metallurgical standpoints. Recently, in that center, only after 12 hours of periodical system check, the fracture of blades occurred. Using fractography, the fatigue in high cycles was known to be the cause of these events. The initiation of cracks in the internal concave down and external concave up surfaces and also the shallow half elliptical shape cracks are due to the excess stresses applied to the blades during fatigue and fracture. The results based on fracture mechanics indicate that, these stresses are about four times greater than those of the normal situation. Also, the results of investigating the age of the fatigue cracks under the applied stresses using the France3D software show that, the required time for improvement of an initial small crack having a critical size is about 5.5 hours, which is decreased to 0.5, if we include resonance frequency. Based on fractography, stress analysis, and aging simulation, the probability of system excess vibration, increase of bending stresses on blades, crack initiation, rapid fatigue propagation, and early fracture of the blades during resonance are quite high.

In this work, the vibration characteristics of rotating Timoshinko blades were studied by finite element method. To develop the governing equations, 3 types of elements, i.e. 4, 5, and 8 degrees of freedom (D.O.F) elements were used and their results were compared. Elements with 5 and 8 D.O.F. had been proposed for non-rotating blades. In this study, for the first time, they are also developed for rotating blades. Also, in this article, the effects of rotational speed, settling angle, hub radius, blade thickness, rotating inertia, and shear deformation on natural frequencies of the rotating blades were studied.  

Integral transformation is a powerful technique to solve the partial differential equation (PDE) of heat transfer in the initial binding process of rapid prototyping methods. Applying Hankel and Laplace transformations to the related PDE and using the related boundary and initial conditions during initial binding process, a second order ordinary differential equation (ODE) has been obtained. After solving this ODE using inverse Laplacetind Hankel transformations, temperature distribution is obtained as an analytical function. Then, the temperature distribution is plotted at the end of exerting concentrated plasma thermal energy step. In addition, the analytical solutions were evaluated by the experimental results that have been obtained using the K-type thermocouples. The results of this study show that, at some special regions of the powder-bed, which is subjected to receiving plasma Arc thermal energy, increasing the temperature provides the required condition for binding of powder particles.  

In this work, an observer backstepping approach has been proposed for the feedback control of an aircraft longitudinal control. Backstepping controller is used to stabilize all state variables simultaneously based on Lyapunov theory. The control objectives tracking the flight-path angle reference signal the controller uses two internally generated pitch rate and angle of attack signals, derived from backstepping algorithm. The fact that, there is no measurement for the angle of attack makes this work different from previous ones in this area. An observer is implemented on the system to estimate the angle of attack. In order to compensate for the errors the observers introduced and imposes onto the backstepping algorithm, the design is modified utilizing non-linear damping. The lyapunov stability analysis, as well as simulation results, show high performance of the proposed controller. Uncertainties and disturbances in the aerodynamic coefficients and saturation limits in actuators and state variables are examined and the effectiveness of the proposed approach in the presence of these effects is demonstrated by simulation results.

A two-dimensional guidance law, using Lyapunov stability theory, is presented in this paper. The non-linear dynamic equations of pursuit are used so that not only the maneuvering of target, but also the tangentional acceleration of the missile are considered. The proportional navigation guidance (PNG) law is one of the most efficient and practical one, which is widely used. It is argued that, the main drawback of PNG law, against a maneuvering target, is that a large acceleration command is required tit the final stage of engagement for the missile. Simulation results have shown that, the required acceleration command of the missile in the final stage in the proposed guidance law is less than that of the PNG law. Additionally, the required acceleration of the total flight and the time of flight to capture the target is less than that of the PNG. Finally, the appropriate implementation of the parallel navigation geometry, using the proposed guidance law, is examined and verified.

In this work, based on first order shear deformation theory, the equations of motion for a viscoelastic rectangular plate have been derived, which consider the effects of shear deformation and rotary inertia. For the viscoelastic material, the Kelvin-Voigt model was used. Then, using the Galerkin method, expanding the invariant manifold theory for plate problems, and using the multiple scale method, the closed form relations for amplitude, non-linear, natural frequencies, internal resonances, and non-linear mode shapes have been obtained. Finally, as an example, a plate with simply supported boundary condition has been considered and using the obtained relations, the non-linear natural frequency and mode shape have been calculated. Then, the effects of different system parameters, such as thickness, Poisson's ratio, and damping coefficient on the response of the system have been investigated.

In the present work, an explicit 4th order Runge-Kutta (RK) scheme for time marching in conjunction with a 4th order compact scheme for discretizaion of spatial derivatives, using a cell averaged approach, have been used to simulate a turbulent channel flow. Using Large Eddy Simulation (LES) approach and different subgrid-scale (SGS) models, the effects of explicit filtering on the convergence rate of the pressure correction equation and also on the near wall turbulence structures have been investigated. The results show that, when explicit filtering is employed, the convergence rate is improved by a factor of 1.75. The only adjustable parameter inherent in the explicit filtering model is the ratio of test filter to grid filter widths, which was shown to have a considerable influence on convergence rate. Our investigations show that the appropriate value for this ratio is 2. In addition, comparing the three-dimensional iso-surfaces of the stream wise velocity with and without explicit filtering reveals that the results with explicit filtering have no small spurious turbulent fluctuations. Also, it is evident from the results that the influence of the explicit filtering has been limited to 15 <y+<80 for turbulent channel flows.