Journal of Applied and Computational Sciences in Mechanics
Year:2019 | Volume:30 | Issue:1 (19) |Papers Count:9

In this study, the instabilities of elliptical, rectangular and circular water jets were experimentally investigated. Shadowgraph technique was employed to capture the jet column status instantaneously. Jet parameters such as disintegration length and axis switching wavelength were quantitatively studied. The results showed that due to the similar characteristics of rectangular and elliptical jets, their breakup length in the second wind-induced regime is nearly equal, while the breakup length of circular jet is perceptibly shorter. Both rectangular and elliptical jets are affected by aerodynamic forces sooner and thus enter the first wind induced regime at lower velocities. Furthermore, it was revealed that the axis switching wavelength is not dependent upon the jet geometry at low Weber numbers and remains equal for both elliptical and rectangular jets. However, at high Weber numbers, the axis switching wavelength of rectangular jet become larger than the elliptical one. Based on the obtained results, a semi-empirical relation was proposed to predict the axis switching wavelength of rectangular and elliptical jets.

Updating stress in a nonlinear finite element analysis is the most important part as the precision of the stress-updating algorithm greatly affects the accuracy of the final solutions. The most important part of the analysis is the stress-updating. There are two key factors that have impact on the efficiency evaluation; those are the accuracy and time. Based on this point, investigating the accuracy of the integration methods becomes important. In this study, the von-Mises plasticity model along with the linear isotropic and kinematic hardening mechanisms is considered in the small strain realm. In solving the plasticity differential equations system through a semi-implicit method, the prevalent approach is picking up the variables from the middle of the plasticity step. In order to assess the accuracy, here, the relations are derived so that one can pick up the variables from the first quarter of the plasticity step. Finally, to determine the accuracy and comparing two aforementioned methods, the numerical tests are performed.

Biped robots with point feet demonstrate faster gaits and more natural dynamics while their gait planning is very difficult due to their underactuation. This research focuses on modelling, optimization and gait generation of two different real biped models including a telescopic springy biped model and compass gait biped with kneed swing leg. All of these models have point feet and their torso angle is constrained and they move in sagittal plane. The main difference of these models with their corresponding theoretical models is that to give realization to the gaits of these models the knee of their swing leg bends, clears the ground and straightens before touch-down. This increases degrees of freedom and divides single support phase to two sub-phases. Different phases of walking gait of each model including single support phase, touch-down event and double support phase are modeled using Lagrange equation and validity of equations are demonstrated. Then the dynamic equations of the phases are combined together to make dynamic model of a full walking gait. Afterwards, optimization parameters, objective functions and constraints are presented and successive stages of optimization are performed to find optimal gaits. The optimization diagrams are discussed and the needed motor torques for the optimal gait of each model are illustrated.

Frost formation has important effects on many fields, such as refrigeration industry, airconditioning, aviation, etc. The frost develops when a humid air contacts with a cooler surface at its freezing point. Frost formation is a complicated transient phenomenon in which a variety of heat and mass transfer mechanisms are taking place simultaneously. In this work, effect of roughness on frost formation over horizontal cylinder in force convection conditions was investigated experimentally. For creating the force flow field, a wind tunnel was used. Results indicated that the frost thickness behind the cylinder is at the maximum and in the front of cylinder is in the minimum. Further increasing of the wall roughness, result in the frost thickness increases and increasing the thermal conductivity coefficient of rough surface, increases the frost thickness too.

The importance of energy and shortage of its sources have necessitated appropriate planning for developing systems with high efficiency. Combined cooling, heating and power generation systems CCHP, are systems that used for increasing the efficiency of generated energy, while decreasing costs. Since the invention off CCHP systems, different methods have been provided for determination capacity of equipment and their optimization. In this research, at first these systems summary are presented and then three main methods namely maximum load method (MLM), maximum rectangle method (heating MRM, h and electric MRM, e ) and relative equivalent uniform annual benefit method(REUAB), for determination of capacity of prime mover have been introduced. Then for a residential convened that its prime mover is gas engine, with consideration three mentioned methods and a new proposed method, for optimization the CCHP will be explained. Finally, the results of the optimization with applying real conditions for prime mover are investigated.

In this study, the effect of heat transfers on lift and drag coefficients of two symmetric and camber airfoils with plunging motion in laminar flow is numerically investigated. The compressible Navier– Stokes equations are discretized using a finite volume method and they are solved by PIMPLE algorithm. In this simulation, fluid dynamic viscosity and thermal conductivity are considered to be variable. Also dynamic mesh method is used for oscillating motion. The results show that, by reducing the airfoil surface temperature less than the free flow temperature, lift to drag ratio increases. Moreover, influence of heat transfer on vortexes pattern at airfoils trailing edge and therefore its impact on the aerodynamic coefficients are investigated.

In this article, the transient dynamic analysis of decagonal quasicrystal (QC) is carried out using the meshless generalized finite difference (GFD) method. The transient behaviors of phonon and phason displacements in these types of QCs, when they are subjected to mechanical shock loading are studied. Also, the meshless GFD method is developed to solve the dynamic problems of QCs, considering coupling between phonon and phason displacements. The governing equations of the problem are obtained in the coupled system of PDEs by applying the coupling coefficient in the governing equations to simulate the interaction between phonon and phason displacements. A 2D rectangular domain made from decagonal (Al-Ni-Co) QCs is assumed for the problem to show the transient behaviors of phonon and phason displacements, when one side of 2D domain is excited by phonon displacement shock loading. Based on the coupling between phonon and phason displacements, any disturbance in phonon field causes some variations in phason field. To study the dynamic behaviors of phonon and phason fields, the coupled governing equations are transferred to Laplace domain. After analysis the problem by GFD method, the obtained results in Laplace domain are transferred to time domain using the Laplace inversion technique. The interactions between phonon and phason fields are studied in details. Also, the effects of coupling parameter and the phason friction coefficient on the dynamic and transient behaviors of phonon and phason displacement fields are obtained and discussed in details.

Currently there are just a few papers about fault detection of tubes and cylindrical shells. Recently, methods based on modal curvatures have gained great attention due to their sensitivity to defects. But the methods require dense number of data extraction points which limits their industrial application. In this research a new method for damage identification of cylindrical shells has been introduced. The method does not need intact structure's data and in contrast to other methods requires a few data extraction points. By transferring the modal information of the structure to the ANN, output of the network is exact position of the defect on the structure and the method does not need skilled technician to interpret the data. Performance of the network is validated by unfamiliar data for the network and 0. 97 regression is obtained.

At this paper, a new equation has been presented to calculate the electrical resistance caused by constriction and presence of surface films on the sheets in small scale resistance spot welding. Then, the simulations have been conducted using the new model and its validity has been evaluated. Experimental verification was carried out on sheets made of Hastelloy X and it was found that the experimental nugget diameters have a good agreement with the results of the simulation. The maximum nugget diameter was created in the case of thin film. Also, existence of film layer improves tensile strength. At the presence of thick film the height of the weld is maximum that can be due to expulsion.