مجله مهندسی مکانیک امیرکبیر
سال پنجاه و ششم شماره 6 (شهریور 1403)

The semi-submerged propeller is a type of marine propeller that is used in high-speed vessels. This propeller has attracted designers attention; because it resists the destructive phenomenon of cavitation. also Semi-submerged propellers have suitable thrust and torque stability at high speeds and offer high maneuverability. Therefore, further investigation around it is important. The current research aims to examine the effect of creating a vertical groove on the surface of the blades of a semi-submerged propeller, an idea inspired by the grooved or corrugated surface on the bottom of some high-speed vessels. The propeller examined in this study is the semi-submerged propeller 841-B known as Olofsson. The 3D domain meshing and numerical simulation have been done in the Star CCM software. The specific condition considered in this numerical simulation is a submergence depth ratio of 33%, an advance coefficient of 0.8, a cavitation number of 2.3, a Froude number of 6, and a shaft angle of zero degrees (horizontal position). The results showed that the efficiency of the grooved propeller is 44.5%, which is less than the efficiency of the ungrooved propeller. In the end, it can be said that the created groove has an adverse effect on the efficiency and performance of the propeller and it is not recommended to use it in the working conditions of the investigated propeller.

In this article, the effect of stress triaxiality and Lode angle for titanium alloy has been investigated. Four samples with different geometries including dog-bone, notched and two shear samples were made. Uniaxial tensile test at ambient temperature was performed on all samples. Using a dog-bone sample, the mechanical properties of grade two titanium alloy were extracted under tensile loading. The samples were simulated in ABAQUS software and the failure behavior of the samples was analyzed until the end of the plastic zone. To match the force-displacement diagrams of the samples, Swift's combined equation and the fourth-order polynomial for the hardening of the material have been used. The results of experimental tests and simulation with ABAQUS were compared and a good agreement between the force-displacement diagrams up to the end of the plastic zone was observed. For the mentioned four samples, the stress triaxiality between 0 and 0.5 has been obtained numerically. The results show that the closer the stress triaxiality is to zero, the failure of the samples in the experimental tests has occurred in shear. In addition, by examining the results, it can be seen that the in-crease of the stress triaxiality with the change of the Lode angle is not uniform and has changed in an increasing and decreasing manner. As a result, these changes also affect the failure strain of the samples.

This research investigates the temperature difference between the surface and moist air as an environmental factor affecting heat transfer and droplet distribution. For this purpose, a test device was built to control environmental conditions. The condensation phenomenon occurred when two types of hydrophilic and hydrophobic surfaces were exposed to moist air. The relative humidity and speed of humid airflow were kept constant at 88% and 5 m/s, respectively, and the temperature difference considered was 4, 7, and 10 degrees Celsius. The varying heat transfer with time during the 60 minutes has shown that it takes time to start the condensation process; the more the temperature difference and the amount of surface energy, the shorter this time and the higher the average heat transfer. The photography of the experiments has also shown that with the increase in temperature difference and surface energy, the time required for the first drop to fall is shorter, and the hydraulic diameter of the dropped drop is bigger. The distribution of the droplets in the 20th minute after the start of each experiment, in which no droplets have left the test surfaces yet, shows that with the increase in the temperature difference, the number of larger droplets is more due to the increase in the condensation rate. The number of the smallest droplets is higher on hydrophobic surfaces than hydrophilic ones.

One of the most important parameters in the micromechanical study of material behavior is the possibility of investigating microcracks and their initiation and propagation patterns in different parts of the materials. In the present work, an attempt has been made to examine the failure of dual-phase (DP) steels under uniaxial tensile static loading using the peridynamics numerical method. The SEM images prepared from the samples show that the grain boundary voids and their growth on a micro scale can be considered the place of damage accumulation.According to the numerical analysis, the initiation and propagation mechanism of inter-grain and intra-grain microcracks in the microstructure of DP steel has been studied using the non-local peridynamic method. Analyzing this problem using the peridynamic method, considering the irregular geometry, the non-locality, reformulating the continuum mechanics in the form of integrodifferential equations, and studying progressive failure is one of the goals of this work, which have been carried out based on representative volume elements (RVE) obtained from the real microstructure. Also, due to the possibility of predicting the damage initiation and the propagation path in different parts of the material simultaneously without considering the default paths, inter-grain and intra-grain microcracks have been simulated using the peridynamics model.

The Stewart platform is primarily used for generating arbitrary motions in three-dimensional space. However, it can also be utilized for measuring the three-dimensional position of an object attached to the moving platform. In this configuration, the Stewart platform functions as a sensory system. One challenge of this application is the high computational cost associated with determining the position of the moving platform relative to the fixed reference platform using data from six length sensors. In this study, the sensory capabilities of the Stewart platform are investigated by introducing a high-performance and numerically agile approach. This approach involves developing an extended set of nonlinear algebraic equations that are well-suited for real-time applications. By applying this procedure to derive the Cartesian coordinates of three points on the moving platform and comparing the results with those obtained from computer-aided design (CAD) software, a strong correlation is observed. To further evaluate the effectiveness of the approach, its performance is analyzed when subjected to harmonic time histories from six length sensors and when the legs' base positions are arranged regular or non-regular on the fixed platform. The results demonstrate that the present method, particularly by updating initial conditions at every time increment, exhibits high computational efficiency.

The presence of vibrations and disturbances causes serious damage to the system. In this research, the agile eye mechanism is installed on a moving car and moves along with the car. Vibrations enter the base plate of the agile eye. A large part of the road disturbances is dampened by the car suspension system. The purpose of the research is to keep the operator of the agile eye robot in the desired direction. The movement of the mechanism is investigated under the influence of two types of hilly and mountainous roads. The PID controller is adjusted by trial and error method and Ziegler-Nichols method, and the results of these conditions are compared in the presence of vibration absorber and normal condition. A genetic algorithm has been used to adjust the spring damper coefficients. which improves system response, saves time and money. The coefficients obtained from the optimization algorithm play a significant role in reducing the vibrations from the vehicle to the base of the mechanism. In fact, the absorber improves the control of the system, prevents disturbances from entering the mechanism and prevents the mechanism from being damaged due to vibrations. The effective value of the error in the case of using the vibration absorber on the examined roads was reduced to the range of 0.02 to 0.04.