Iranian Journal of science and Technology (B: Engineering)
Volume:31 Issue: 1, February 2007

Several approaches have been employed for grid adaptation. The most widely used ones utilize adaptive-grid redistribution and adaptive-grid embedding. However, the combination of these two main methods (r-h) is also possible. This work compares redistribution and embedding with a combination of these two methods for steady transonic airfoil flows. The Euler equations are integrated into a steady state by an explicit, finite volume, Ni’s Lax-Wendroff scheme. Comparison with other numerical solutions is employed in order to evaluate the accuracy and efficiency of the techniques. The combination of adaptive-grid redistribution and embedding is somewhat more complex than the adaptive embedding method, but the results indicate that for two-dimensional invisid flows, when high accuracy is required, their combination is more efficient.

An engineering method has been developed for the prediction of aerodynamic heating of hypersonic bodies. This method is capable of rapidly predicting the heat flux in the leeward region. This is achieved through the determination of the streamlines in the leeward region. The modified form of Maslen’s second order relation is employed, which calculates the pressure in the shock layer explicitly. The inviscid outer flow within the shock layer is solved first. The calculated solution is then used to determine the flow properties at the boundary layer edge and the orientation of the surface streamlines. Boundary layer equations, written in the streamline coordinates, are integrated along the surface to obtain the rate of heat transferred to the body surface. The present method is an inverse method in which the body shape is obtained according to the shape of the shock. In general, inviscid-boundary layer engineering methods accurately calculate the orientation of streamlines in the windward region only, and therefore they are not usually applicable in the leeward region. In the present study, a new method is proposed to determine the orientation of the surface streamlines in the leeward region. Using the present technique, three-dimensional hypersonic flow is solved fast and easy all around a cone. The obtained results show that the corrections presented in this study excellently extend the application of the method to the leeward region.

The wake flow and the onset of Karman-vortex shedding corresponding to periodic forces of a heated circular cylinder are investigated numerically with a SIMPLEC finite volume code. This study is performed for Reynolds numbers ranging from 20 to 200 and a Prandtl number of 0.7.
The first bifurcation of flow over bluff bodies, which is strictly a Hopf type, is modelled using the Stuart-Landau equation. In this study, it is shown that the flow behind a circular cylinder is steady up to Re=49 and beyond this critical value, the flow develops to a periodic state, corresponding to the vortex shedding and force oscillation on the body. The wake structure is studied close to the oscillation threshold and also at the higher Reynolds numbers using streamlines, pressure, temperature, Nusselt numbers and wall vorticity profiles and iso-lines. Some useful physical quantities, such as time mean separation angle, Nusselt number, the dominating wake frequency (Strouhal and Roshko numbers), mean drag and the RMS value of lift are computed and compared with numerical and experimental results. Based on the results of this study, the empirical relations for the Reynolds dependence of the mean Nusselt number, mean separation angle, Strouhal and Roshko numbers are obtained.

Aspects of pitching and plunging motions on unsteady aerodynamic behavior of an aircraft model were studied. Extensive wind tunnel tests were performed on a standard dynamics model, SDM, oscillating in both pitch and plunge modes. Up to now, there has been little or no result on the plunging behavior of an aircraft or missile as a whole and the present experiments can be considered as one of the first attempts to study the compressible flow field over a model undergoing both pitching and plunging motions. The experiments involved measuring normal force and pitching moment of the model at Mach numbers of 0.4, 0.6 and 1.5 and oscillation frequencies of 1.25, 2.77 and 6.00 Hz. The longitudinal dynamic derivatives were then calculated from the measured data. The pitching results have been compared with the available data on the same model and good agreement has been achieved. However no plunging data on the same or a similar model was available in the literature to be compared with the present findings.

Ground Vibration Test (GVT) is one of the standard tests used for designing new aircraft. In this paper, GVT was carried out on an aircraft structure model by the phase separation method. In order to obtain more accurate results, the structure was excited by hammer, random and sine excitations. The results were then used to extract the modal parameters, i.e. natural frequencies, modal damping factors and mode shapes. Extraction algorithms were based on multi-degree of freedom (MDoF) methods where several FRFs are analyzed simultaneously. Correlation between different test techniques was also investigated. Next, a finite element model of the structure was constructed and the theoretical modal analysis results have been compared with experimental ones. The test results obtained from the phase separation technique were also compared with a similar test carried out on the same structure by the phase resonance method. Finally, a cylindrical mass was added to the right tail wing of the model. A new set of modal tests was carried out to investigate the effect of unsymmetry on the modal properties of the structure.

Stress distribution in short and long fibers and their surrounding matrix bays of a finite width hybrid composite lamina is examined. The lamina is subjected to a tensile load of magnitude "P" at infinity, while its matrix is assumed to take only shear (shear-lag theory). The bay adjacent to the first intact filament is allowed to experience a plastic zone of size 2a, due to excessive shear load owned by cracks formed by double cuts along each filament. The plastic zone is assumed to behave as elastic–perfectly plastic. The short fibers are simulated by assuming two successive breaks along each filament. The effect of the plastic zone on short fiber load bearing capability, as well as stress concentration in the first intact filament is fully investigated. The effect of hybridization, in the presence of the plastic zone, is also examined on short fiber load bearing behavior.

M. BEHZAD** A ND M. AKBARI