مجله مهندسی مکانیک
پیاپی 140 (آذر و دی 1400)

 In this paper, buckling analysis of composite sandwich cylinders with transverse flexible core is investigated using an improved higher order sandwich panel theory (IHSAPT). The face sheets of sandwich cylinder are made of laminate composite that lay up symmetric and anti-symmetric. The case study is under axial compression load and external pressure. The first order shear deformation theory and second Frostig’s model is proposed for deriving displacement field of face sheets and core, respectively. Lagrange’s principle considering initial stresses is used for extracting governing equations. The Buckling problem of the shell has been numerically solved by using the Galerkin’s method. In this research, the effects of some important parameters including core radius to cylinder thickness ratio, length to core thickness ratio, face sheet to cylinder thickness ratio and length to core radius ratio on the buckling response of composite sandwich cylinder are studied. The obtained results indicate that by decreasing face sheet to cylinder thickness ratio and length to core radius ratio, and increasing core radius to cylinder thickness ratio buckling load of composite sandwich cylinder are increased.

This paper investigates the obstacle avoidance problem of fixed-wing Unmanned Aerial Vehicles (FWUAVs) while considering moving obstacles and unexpected collisions. The routine methods in designing obstacle avoidance systems are investigated and the potential function approach is selected since it is suitable for online missions and unexpected collisions. Due to the non-holonomic dynamic of FWUAVs, several challenges including the local minima, minimum stall speed, and minimum turn radius exist in obstacle avoidance. In this research, a novel approach is introduced to resolve the above challenges. Finally, the performance of the proposed algorithm is demonstrated through the application of the proposed algorithm on the nonlinear dynamic of FWUAV in several simulation scenarios in Matlab.

The numerical modeling of particle deposition in industrial problems is essential due to domination on intricate details of this phenomenon. In the present study, suggestions for properly using the governing equations and conditions have been presented. Modeling particle deposition phenomenon is generally divided into the Lagrangian and Eulerian approaches. Eulerian approaches are used to model particles deposition when the volumetric ratio is higher than 10−3 and The Lagrangian approaches are used for the volumetric ratio of particles less than 10−3 . The Drift Flux method is more common among the Eulerian approaches than others. In Lagrangian approaches, if the particles are small (in Nano size), and the volumetric ratio of particles is less than 10−6 due to the greater Van der Waals force between the particles and deposition place, the condition of the sticky wall is invoked. Furthermore, with increasing the size of the particles, the possibility of rebounding after their collision should be investigated. Moreover, if the flow rate is high, the possibility of particles detachment from the deposited layer should be considered. Also, the temperature affects particles deposition. Increasing temperature changes the phase of the particles, and as a result, they settle when they hit the surface.

The mechanism of operation of the double skin facade with natural ventilation (NVDSF) is based on the thermal behavior and air flow in the middle cavity. Computational fluid dynamics can improve the performance of these views. The purpose of this paper is to investigate various options related to turbulence and radiation model of Fluent software and geometric features including two-dimensional or three-dimensional simulated geometric model, scaled threedimensional model and mesh dimensions, to simulate the thermal performance of the façade with Fluent software with the aim of improving the accuracy of the results and achieving the shortest computer computation time. The results show that the RNG k-ε turbulence model, the DO radiation model and the 3D geometric model will have the most accurate results. In mesh with dimensions of 8 mm and a scale of 33% of the real three-dimensional sample, the simulation results were independent of the mesh dimensions and scaled geometry. Simulations geometry with a scale of 33% of real 3D samples compared to real 3D geometry led to similar results in terms of accuracy. Finally, the reduction of computer computation time simulation of NVDSF with scaled geometry of 33% of the real sample to about one third was achieved by maintaining the accuracy of the results.

The most widely used measuring equipments for industrial gas consumers are turbine flowmeters. These types of flowmeters have many performance advantages such as good accuracy in applications close to their nominal capacity and easy maintenance. Despite these advantages, their weakness are related to the unacceptable accuracy of measuring gas flow at low consumption and the inability to measure at very low flow consumption. In this paper, in order to provide a solution to solve this problem, two stage flowmeter with using automatic control system was designed and implemented in the gas measuring station with a capacity of 1000SCMH. When the gas flow was less than minimum flow of the main meter, by automatic control system, switched to the auxiliary line and gas flow- that was not previously considered -was measured.The performance of the proposed system at the station under study, were monitored for one year. The result showed that the amount of 25,152 cubic meters of natural gas flow -that former could not be measured- was observed. Also, considering the industrial gas tariff and gas export price, the payback period was calculated less than three and a half years and about two months, respectively.

In this article the effect of the displacement rate under tensile loading on the adhesive connection in glass-epoxy and carbon-epoxy composite and metal pipes has been investigated. For this purpose, bonded pipe specimens were fabricated with different thicknesses and adhesive areas (the length of 10, 20, and 30 mm and the thickness of 0.25, 0.10, and 0.40 mm). Then, samples were exposed to tensile loading at various displacement rates (0.5, 5, and 50 mm/min) and moreover, their strengths and failure types were determined. Based on the results, for the carbon-epoxy specimens, the maximum force increased by increasing the displacement rate. The fracture area was also changed from the adhesive area to the composite due to the increase in the displacement rate. It could be claimed about the glass-epoxy samples that under tensile loading, the maximum force enhanced gradually by increasing the displacement rate. Finally, the regression analysis for the maximum force and the elongation was not properly valid based on the sensitivity analysis.

Improving the aerodynamic performance of vehicles due to the impact on fuel consumption, stability and controllability of vehicles has always been on the attention of engineers. In this study, in order to obtain a brief view of the effectiveness of methods to improve the aerodynamic performance of vehicles, a review of research conducted to reduce aerodynamic forces using the passive flow control method, which can be effective in consideration, implementation and selection of the proper method. The results show the high importance of car body design in reducing drag and lift such that implementing the channel and curvature on the sides of the car to transfer air flow from high pressure areas in front of the body to low pressure areas behind the body has the highest value of reduction in drag force. Also, using a spoiler at the end of the body to delay separation and to direct airflow upwards reduces the lifting force. The effect of these methods on reducing aerodynamic forces is more than other mechanisms, but the exact extent of the effectiveness of each method depends on the type of vehicle and the way of its implementation on the body.

A review of the general strategies used in the aerodynamic optimization of the helicopter rotor has been done. The aim is reducing the computational resources of the optimizing process of the new aerodynamic designs of the rotor. This study begins with a brief review of the concepts and sources of the unsteady aerodynamics and the fundamental challenges to addressing this fundamental problem in helicopter rotor design and optimization. Then, past and present design and optimization methods used or proposed and the development of analysis tools to evaluate rotor performance reviewed to gain a better understanding of the problem. The results showed that despite the use of classical aerodynamics, wind tunnel, model rotor experiments and aerodynamic calculations in the past, the recent development of computational fluid dynamics (CFD) now provides an opportunity to accurately predict the viscous and compressible flow field and provide predicting the new rotor designs performances’. Therefore, the use of these tools for highfidelity simulations along with the use of surrogate modeling methods with variable fidelity will cover most aspects of the aerodynamic optimization problem of the helicopter rotor with reduced computational and time resources.