مجله مکانیک سازه ها و شاره ها
سال سیزدهم شماره 5 (آذر و دی 1402)

Studying the effects of the model scale on wind tunnel tests of aerodynamic vehicles and their components to generalize it to the real scale is a very important because of the direct impact on the performance of the flight system. The aim of the present research is to present a methodology for applying geometric scaling effects of NACA 0012 airfoil the on airfoil aerodynamic performance. AnsysFluent2019R3 software has been used to simulate and investigate the effects of geometric scale on the airfoil aerodynamic performance. Sixteen scale scenarios include changing the Reynolds and the angles of attack assuming Mach constant is 0.256. The airfoil chord length of 50cm (Reynolds 3 million) is considered as the base model in the simulations. The rate of deviation of the validated results for the drag and lift coefficients are 11 and 1 percent, respectively. The results showed that at angle of attack of 10 doubling the airfoil length leads to a decrease of 7.92% in the drag, an increase of 1.25% in the lift and decrease of 18.30% in the pitch moment coefficients. Halving the length of the airfoil at an angle of attack of 15 leads to an increase of 16.29, a decrease of 3.49 and an increase of 9.22% of drag, lift and pitch moment coefficients. One of the important achievements of the present study is the presentation of a methodology for applying the geometric scaling effects in the form of correlations for aerodynamic performance parameters of drag, lift and pitch moment coefficients.

This paper investigates the effect of a Helmholtz resonator with one or two pairs of deep cavities on the mixing chamber of a subsonic ejector to determine its impact on the ejector's entrainment ratio. The study utilizes numerical analysis, where the depth, location, and number of cavities were varied while keeping the width of the resonator constant. The Fluent software solved the Unsteady Reynolds-Averaged Navier-Stokes equations with the k- εturbulence model. The presence of the cavity at the beginning, middle, and end of the mixing chamber reduced the entrainment ratio by 0.6%, 3.8%, and 6.6%, respectively, and the presence of the second cavity at the positions of 20, 40, and 60 mm with respect to the first cavity reduced the entrainment ratio by 9.9%, 10.1%, and 10.2% respectively. The depth effect was studied at a distance of 20 mm for a pair of cavities at depths of 75, 100, and 125 mm causing a reduction of 4.9%, 9.9%, and 13.1% in entrainment ratio. The study also observed longitudinal and pulsating oscillations inside the mixing chamber due to the simultaneous filling and emptying of opposite cavities. Furthermore, the amplitudes of the pressure oscillations in the first pair were weaker than those in the second pair. In the final part of the research, the effect of primary flow fluctuation was also investigated, the results of which showed that the primary flow fluctuation increases the entrainment ratio by 5.2% due to the increase of effective mixing between the primary and secondary flow.

Plasma actuator is an active flow control tool, which has been evaluated by the aerodynamic researchers since last decade due to its simple structure, light weight, low energy consumption, and high time response. In this paper effects of plasma on aerodynamic behavior of a rocket at different flight conditions is numerically investigated. Results of plasma effects or variation of attack angle, Mach number, and flight altitude on the drag and lift coefficients are evaluated. Applying plasma increases the vertical velocity under the rocket canards which leads to higher pressures and therefore higher pressure difference and forces are applied on the rocket canards which improves their functionality. Drag and lift coefficients are both increased due to the plasma, but the aerodynamic efficiency (lift to drag ratio) is increased by increasing potential difference. Results shows that plasma effect is reduced with increasing of angle of attack and increased with the flight altitude and aerodynamic efficiency is changed between %3 and %60 by applying plasma.

Nowadays, micro-turbojet engines are widely used in various fields, from recreational devices to drones and missiles in the military industry. Given their remarkable performance, numerous companies have entered this area and produced a variety of products. This widespread use and diverse range of products have led to an increased importance of review studies in this subject. In this research, an extensive study has been conducted on micro-turbojet engines with a thrust force below 1000 Newtons. These engines have a diameter less than 300 mm, compressor pressure ratio less than 5 and fuel consumption lower than 2500 gr/min. Statistical analysis of the engines within this thrust range has yielded valuable information regarding the structural and performance specifications of this category of engines, which is presented in the form of tables and graphs. In this study, the main components of these engines, including the compressor, combustion chamber, turbine, and auxiliary systems, as well as their interconnections, are described. Although the overall structure of these engines is similar to large-scale aircraft engines, there are significant differences in design philosophy, types of main components, and details. At the end of the paper, specifications of over one hundred micro-turbojet engines available worldwide including thrust range, dimensions, engine rotor speed, air flow rate, fuel consumption, and turbine temperature are presented.

In the present study, fluid flow and combined natural and force convection heat transfer of a nanofluid in the horizontal concentric / eccentric cylindrical with different uniform wall temperatures is numerically investigated. The force flow is induced by the cold rotating outer cylinder at slow constant angular velocity, with its axis at the center of the annulus. Moreover, in calculating the buoyancy force caused by temperature difference between annulus, used of the Boussinesq approximation. the results are presented for non-dimensional group number (Reynolds and Rayleigh (.An increase in the Rayleigh number causes non-uniformity of the flow and isothermal lines and increases the heat transfer in the walls. The highest heat transfer related to the two-phase state of non-concentric cylinders was obtained. An increase in the Rayleigh number causes non-uniformity of the flow and isothermal lines and increases the heat transfer in the walls. The highest heat transfer related to the two-phase state of non-concentric cylinders was obtained.

The This research was defined with the aim of analyzing combustion and development of fire in a passenger car with Pyrosim software, and it is intended to be a complete study on the optimization of the interior design material structure of the passenger car against fire, which will reduce the amount of casualties in the event of an accident. This work was done by simulating fire development by Pyrosim software and using experimental data. A reference model that has been subjected to fire testing in real scale was modeled in Pyrosim software. After validating the existing model and then changing the insulating materials used in the wagon body and seats, new insulations of compressed polystyrene, expanded polystyrene , stone wool and glass wool were used.The results showed that glass wool and stone wool insulation had a good performance against fire, that is, they showed a lower heat emission rate and recorded a lower development rate, and the internal temperature of the wagon and the amount of smoke reached the critical range over a longer period of time. Foams had a very poor performance and in addition to increasing the rate of heat release and internal temperature, they had a higher amount of soot production. It was concluded that from the point of view of optimal control of fire development, it is better to use glass wool or stone wool insulation in the car body and to use phenolic foam insulation in the interior design of the seat for passenger comfort.

The occurrence of pipeline failures can lead to significant damage to the environment and natural resources, as well as high repair costs. In this study, the finite element simulation is employed to model the fluid-structure interaction between the fluid flow and the damaged pipe wall to investigate stress distribution and failure in damaged pipes. Given the time-consuming nature of this simulation, an artificial neural network is also used to predict the behavior of the damaged pipe. This neural network is trained using a recurrent backpropagation algorithm. To this end, the maximum stress in the damaged pipe is considered as the objective function and is calculated by the finite element method for different values of the flow velocity, size, distance, and depth of the defects. The design parameters are selected by Taguchi method to optimize the neural network structure and increase its accuracy. The results have suggested that combining the finite element and artificial neural network methods is an effective approach for failure prediction in defective pipelines.

The liver is one of the most important organs of the body, which is responsible for the metabolism of proteins and detoxification of the body. Examining tissue and studying its mechanical properties can be a platform for the early diagnosis of cancer and then the identification of treatment methods. An atomic force microscope is a very powerful tool in imaging and identifying the mechanical properties of nanoparticles in more advanced stages for the manipulation of these particles. In this research, Young's modulus of liver cancer tissue was investigated using an atomic force microscope and using three types of cantilevers with rectangular, V-shaped, and dagger geometries. Then, using the Hertz contact model, the range of Young's modulus was simulated for all three types of atomic force microscope cantilevers. The results of experimental work and theoretical simulations were compared. Finally, in order to validate, the results obtained in this study were compared with other studies. The obtained results showed that the use of a V-shaped cantilever achieves a more accurate range of Young's modulus. Also, Young's modulus for liver cancer tissue was obtained in the range of 800 to 1500 pascals.

This paper aims to investigate the influence of bearing elastic properties on nonlinear dynamics of unbalanced rotors. Accounting for the influence of asymmetric magnetic pull, the governing equations of motion associated with the rotor are obtained using the nonlinear Euler-Bernoulli beam theory. Adopting the Galerkin projection method, the reduced equations of motion are extracted and then solved analytically through the method of multiple time scales for the cases of free vibrations and primary resonances. Aside from the numerical simulations, the present findings are compared and successfully validated by those published in the previous studies. Afterward, a detailed parametric study is conducted to assess the influences of asymmetric magnetic pull, nonlinear stiffnesses of the bearings and the eccentricity on nonlinear dynamics of the system. Results reveal that accounting for the influence of asymmetric magnetic pull decreases the natural frequencies of the system. In addition, it is observed that increasing the eccentricity increases the amplitudes of vibrations and also broadens the bistable resonance zone.

Inspection of polyethylene pipe connections is very important in various industries due to its many applications in water, gas, and chemical transmission networks. Among the non-destructive inspection methods, the ultrasonic method is the most suitable method for this type of connection. Due to the polymer material of these connections and a result of the high attenuation of the wave, this type of inspection is associated with challenges. Using the phased array ultrasonic inspection method due to the focus of the wave and creating high-energy points at the joints is an alternative solution to the usual ultrasonic inspection techniques.In this paper, using the finite element method, Ultrasonic inspection is simulated by the usual method and also by the phased array method for monitoring the connections of polyethylene pipes. some kinds of possible defects have been investigated and their effect on the reflection signal has been determined. Using the numerical method based on successive simulations, the reflection signal has been analyzed in the Phased array method. The results showed that the increase in the number of piezo increased the performance of the inspection as well as an increase in the concentration of the mechanical wave up to 160% for 32 elements and 270% for 64 elements compared to the probe with 16 piezoelectric elements.

In recent years, the definition and analysis of inverse hyperelastic problems due to the wide use of these materials in various industries and also in manufacturing of artificial tissues of the body, has received more attention than before. In mechanical analysis of hyperelastic materials, both material behavior and material deformation are considered nonlinear. In this article, an inverse problem related to the failure of hyperelastic bodies is defined and two different methods are proposed to solve it. The inverse analysis of hyperelastic bodies that have failed, can be useful to prevent the recurrence of failure in these materials. In the inverse problem, it is assumed that a two-dimensional hyperelastic solid is failed and the place of its failure is known. The distribution of the load (boundary conditions) in a part of the boundary is considered unknown and is calculated by solving the inverse problem. By defining an appropriate objective function, the defined inverse problem is converted to an unconstrained optimization problem. To solve the optimization problem, a zero-order method based on the equal interval search method and a first-order method based on the steepest descent method are used. To make the problem more practical, the inverse problem input data, which are the location of failure and the critical equivalent strain, are used with some error. Finally, considering the location of the failure and the critical equivalent strain, the load causing failure is identified. It can be seen that the performance of the first-order method is better than the zero-order method.