Journal of Aerospace Mechanics
Year:2011 | Volume:7 | Issue:3 (25)|Papers Count:8

In contrast to the conventional aerodynamic models, the model of wavelet transform and high order Taylor series of aerodynamic functions are proposed. Using different mother wavelet transforms, different aerodynamic functions are constructed. Results show that wavelet aerodynamic model and also high order aerodynamic model of aircraft have more accuracy than the conventional aerodynamic models. The suitable wavelet aerodynamics for identification and simulation of the aircraft response is proposed.

Using suitable coordinate systems in CFD have been of interest for many researchers. The objective of this study is numerical modeling of supersonic flows, using a new coordinate system, i.e. Unified coordinate system (UCS) and iterative Riemann problem solution. Unified coordinate system has advantage over traditional coordinate systems (Eulerian/Lagrangian) in supersonic flows, especially in discontinuous regions (shocks and expansions). Moreover, most of the difficulties of the traditional coordinate system may be removed using UCS. For 2-D gas dynamics calculations in UCS, it is required to approximate fluxes on cell faces. This is normally modeled by Riemann problem solution. It is important to mention that in using UCS, there is no need to generate a body-fitted mesh prior to computing flow past a body, the grid is automatically generated by the flow. The UCS has the advantages of both Eulerian and Lagrangian systems. These can be seen in our results, which also show the high speed in our computations.

Drag reduction has made super cavitation very popular. One of the most important issues in super cavitation is its stability and control. It would be unsuitable if cavity closure occurs on a body resulting in some instability. Current work focuses on the nature of cavity instability and thus it was required to apply a dynamical model for such flow simulation. Using source and sink model attributes implemented in this study had some modifications. Semi-Hill vortex, as an impulse disturbance, was inserted into the Navier-Stokes equations to investigate the cavity instability. Finally, image processing technique was used to study the instability effects on cavity boundary and was found that a convective instability exists on the boundary of the cavity. Instability analysis on cavities of different lengths revealed that longer cavities are more sensitive to the disturbances. Hence, existing natural disturbances in a cavitating flow makes a long cavity boundary to fluctuate and finally collapse. This is the main reason that long cavities are not seen in nature.

In this paper the results of simulation of a 3-D turbulent normal jet-into-cross flow at Reynolds number of 4700, based on the diameter and velocity of the jet, using hybrid LES/URANS approach, are presented. The computational methodology is based on finite volume and the grid is non-uniform and staggered. The results show that the hybrid LES/URANS approach can well predict complex flows. Furthermore, the flows upstream and downstream of the jet were affected by variation of the velocity ratio.

In lubrication theories, the bearing surfaces are considered smooth, which is not really true. It has been shown in cases where the roughness is in the same order as the lubricant film that there would be a major effect in bearing performance. The surface geometry is so irregular that it is sometimes necessary to use statistical models. Usually, when we are faced with roughness effects, the analytical solution is very difficult. In this paper, finite element method is used to examine the roughness effects on the performance parameters of step slider bearings. The problem was solved for two different surface profiles and the results were compared showing relatively close agreements.

Due to vast use of gas lubricated bearings in various industries and their advantages in specific applications, they have been investigated by many researchers. However, analytical treatments of gas lubrication are tedious due to high non-linearity of the pressure equation as the consequence of lubricant compressibility. Hence, in this paper a feed-forward neural network is employed to investigate the performance of three-lobe gas lubricated bearings .It is believed that neural network can easily compete with theoretical model in predicting the solution of lubrication problems. The performance parameters considered are stability margin, power loss, and attitude angle for various values of bearing compressibility numbers, mount and tilt angles. The results of the neural network are compared to theoretical model (FEM) and it is observed that they are in good agreements. The results also indicate that the above parameters can influence the performance of the bearings.

Nowadays, compressible flow transmission pipelines are used in many industrial processes. Pipelines in chemical processes, gas turbine power plants, natural gas transmission lines, and high pressure steam transmission pipelines are some industrial examples. Therefore, learning the manner of treatment of a transient compressible fluid flow in pipelines with junctions and branches is important. In this paper, the attempt is to derive the governing equations in compressible fluid flow in a fixed area pipeline in the presence of friction. To achieve this, a numerical method of analysis (McCormack) is used. Also the method of characteristics is added to solve the problem in the junctions. Finally, a theoretical case and an industrial one are investigated and some common transient processes are simulated. In addition, the effects of boundary conditions on the flow characteristics, such as pressure and flow rates, are presented.

In this research, numerical simulation of the interaction between a large bubble and a boundary layer, considering various parameters in a low Reynolds number flow, have been studied. This work can be a start for more complicated tasks with higher Reynolds numbers. The considered flow has been solved using the well-known front tracking method. The results indicate that the wall shear stresses considerably depend on the bubble location. Also, when bubbles are inside the boundary layer, the overall shear stresses and the skin friction drag increase.