Journal of Aerospace Mechanics
Year:2007 | Volume:3 | Issue:1|Papers Count:10

Many parameters affect the quality of welding in the resistance upset welding (UW) process. These parameters are material and geometry of parts, welding force, thermal cycle at the weld zone, current density and welding time. The controllable parameters in this process are current density, welding time, and welding force, where current density and welding time are more significant. In this paper, the effect of welding time and current density on the temperature distribution of the welding zone is presented. For this purpose, a thermal-electrical finite element model is developed to analyze the thermal behavior of the joint produced. A transient temperature field obtained from thermal-electrical simulation of UW process is applied as nodal load on the model. The results of thermal-electrical analysis are used to predict the status of the weldment. Specifically, the quality of the weld at the seam can be evaluated. The simulation results are evaluated by performing tensile test on the welded joints. The experimental results show that by increasing in the length of the joint, the maximum tensile load applied to the joint increases.

In this article, the fundamental calculations of swirl injectors were investigated and the design method for these kinds of injector, i.e. dual-base liquid-liquid injectors, was presented based on this theory and some experimental results. Then, some special conditions related to dual based liquid-liquid injectors were studied and the corresponding results were considered in the design manipulation. The behavior of injector in various performing conditions and mutual effects on twin spray envelopes in dual base injector were investigated, and the design procedure was presented based on the obtained results. Finally, the computer code for designing this type of injector was proposed. Using this code, some injectors were designed and tested. The results were satisfactory.

Explosive compaction is one of the complementary methods in powder metallurgy process in which the densification and sintering of powder particles is performed in one stage by passing a shock wave through the powder medium. For production of tungsten parts, it is preferred to use this method. In this research, an analytical equation is obtained for choosing the necessary explosive material. To verify the theoretical relationship, several explosive experiments were designed and performed for compaction of tungsten powder and several specimens were obtained. The density and the hardness of these specimens were measured and their fragment surfaces were analyzed, using the scanning electron microscope (SEM). The results approved the analytical relationship and indicated that a suitable explosive material must be utilized for compaction of tungsten powder.

This paper reports the numerical study of the flow field arised form impinging of a supersonic moving jet on a solid wall. The flow properties, such as surface pressure distribution are compared with the experimental data for a quasistatic case. The results show that when the distance of the nozzle from the wall is not too small, the quasi-static result is very close to that obtained from an unsteady solution. Also a new combined algebraic turbulence model is proposed for the calculation of eddy viscosity near the solid wall and the shear layer. The new combined model can suitably predict the flow structures, such as the separation region behind the nozzle, the expansion waves at the nozzle exit, the recirculation flow region near the wall, the vortices near the nozzle lateral face, and the transition of regular reflection to a Mach disk at the symmetry axis. For quantities such as the position of Mach disk, the height of the stagnation bubble, and the pressure peak on the surface, the results of the combined model is also in a better agreement with experiment than those obtained from the two-or one-equation models.

The behavior of multi-jet impinging flows with fountain formation is important for the design of vertical takeoff and landing (VTOL) aircrafts near the ground. In this work, the flow field generated by the impingement of two axisymmetric turbulent jets on a flat plate was studied. The incompressible flowfield characteristics of twinjet impinging and upwash fountain formation with different jet spacing and height above the ground were considered. The SIMPLE algorithm, the k - e and k - w two-equation turbulence models and the QUICK convection discretization scheme were employed. The agreement of our calculations with the experimental data is relatively good.

In this work, the elastoplastic torsion of solid sections without work hardening with variable yield stress is examined. In similar previous works with constant yield stress assumption, exact numerical or analytical approaches were utilized. In view of the new assumptions, the coupled elastoplastic differential equations were decoupled. In variable yield stress assumption, a yield stress gradient from surface to core is assumed, which is a valid and reasonable assumption in cast and heat treated parts. Thus, in elastoplastic torsion of prismatic bars with the aforementioned yield stress gradient assumption, the above mentioned method was used. To carry out the computations, new software with coupled programming approach was used. A few examples were run both on our developed software and on accredited finite element software (ABAQUS). Comparison of the results affirms the benefit of our approach.

In this work, the results of a standard dynamic model obtained from the data acquisition system of a wind tunnel were studied with emphasis on error sources. The results show that different components contain substantial amount of noise and thus make it hard-if not possible-to obtain aerodynamic coefficients. Therefore, genetic algorithm (GA) is used to filter out this noise. Performance of this method was evaluated in a test with one Mach number with forced oscillation frequency of one and six Hz, attack angle of zero and oscillation amplitude of one and five cm. plunging compared to classic method. The resulting vertical force component of standard dynamic model is satisfactory, showing GA to be a powerful and advanced way for finding optimum results. Using this algorithm, comparison of different results is carried out faster and is more accurate. Also the cost and the repeating times were reduced using GA.

Wood consists not only of water but also of polymers such as cellulose, hemicelluloses, and lignin. Drying of wood is a complex operation involving transient transfer of heat and mass along with several processes, such as physical or chemical transformations, which in turn, may cause changes in product quality. The drying of wood is an important step in the processing of timber. Little detailed data is available on different heating modes during drying of wood. This paper presents experimental work using convectional air drying, microwave drying, and infrared drying of Guilan spruce woods. The experimental results show drying time of the microwave heating is significantly reduced, while the strength stays higher than that obtained in convectional and infrared drying.

Film cooling of a two-dimensional flat plate at different jet to cross flow velocity ratios (R) is simulated at the jet Reynolds number of 4700, using large eddy simulation (LES) approach. Our computational methodology includes the use of finite volume method, applying the unsteady SIMPLE algorithm and a multi-block and non-uniform staggered grid. The governing equations have been discretized applying the Power-Law scheme for the spatial terms and the Crank-Nicholson scheme for the temporal terms. Two solution approaches are discussed here, namely: 1-neglecting and 2-considering density ratio effects. The results showed that the density ratio has significant effects on the flow structures, namely on the penetration, the expansion, and the reattachment point of the vortical regions. Therefore, it changes the temperature distribution and the film cooling effectiveness, and thus, it should not be easily neglected.