نشریه مهندسی مکانیک ایران
سال بیست و یکم شماره 1 (پیاپی 54، بهار 1398)

In this paper, applying the general form of the viscoelastic model, vibration characteristics of viscoelastic fluid conveying pipes are investigated. By considering various viscoelastic constitutive equations, the equation governing the motion of the fluid conveying pipes is obtained, and by introducing a new analytical method, the exact mode shapes of the system are derived. Then, the effect of various system parameters on the complex eigenvalues and the instability of kind divergence and flutter are studied. The results show that for a pipe with viscoelastic behavior, the natural frequencies decrease and at the higher modes, the divergence critical fluid velocity increases dramatically. As a new result, it is observed the viscoelastic fluid conveying pipes with certain values of viscoelastic parameters, can experience the flutter instability before the divergence one. In addition, because the viscoelastic behavior does not affect all the vibration mode shapes in the same manner, therefore, the coupled-mode flutter does not take place.

Journal bearings are vastly used in major industries such as power generation, oil and gas production, refineries, and the like. Thus, much investigation and study has taken place for more effective maintenance and fault detection of these bearings. An effective method in maintenance for these bearings is Root Cause Analysis (RCA) that tools like FMEA and FTA are used in it. In this paper, initially meanwhile investigation of journal bearing failures prioritizing of them have been down by using FMEA and FTA. For this purpose, expert opinion and field data collection were used in six gas power stations of country and by used them RPN are calculated for faults. Finally considering to RPN result and the importance of wear in journal bearing, this issue is simulated in COMSOL software and using Navier-Stokes equations and load capacity is investigated. At least some suggestions about geometry of journal bearing were presented to improve its performance

In this paper, an advisory driver assistance system is designed and implemented in real traffic flow. This system gives environmental information to the drivers by visual and audio signals to avoidance form possible collisions. The performance of this system is based on estimation of instantaneous delay of Driver-Vehicle Unit (DVU). This estimation helps us to specify the consciousness of drivers. To reach this aim, some sensors must be mounted on the vehicle, then information fusion of these sensors obtains conditions of environmental hazards. To investigate the performance of the designed system, the subsystems are implemented on a Peugeot 206 passenger vehicle and some scenarios are tested in real traffic conditions. The results show the high accuracy and good performance of this system to reach control aims and reduction of the tracking errors. This system is useful for safety improvement for vehicles in longitudinal maneuvers and decrease of reaction time of DVU.

This study investigates the free vibration of a thick FG circular plate in contact with an inviscid and incompressible fluid. Analysis of the plate is based on First-order Shear Deformation Plate Theory (FSDT) with consideration of rotational inertial effects and transverse shear stresses. Dynamic transverse displacements of the plate are approximated by set of admissible Chebyshev functions which is required to satisfy the geometric boundary conditions. Potential theory together Bernouli’s equation are utilized to obtain the fluid pressure on the free surface of the plate. The governing equation of the oscillatory behavior of the fluid is obtained by solving Laplace equation and satisfying its boundary conditions. The natural frequencies and mode shapes of the plate are determined using Rayleith-Ritz method based on minimizing the Rayleith quotient. The effects of the geometrical parameters such as height of the fluid, plate thickness to its radius ratio, fluid density, vessel height and volume fraction index on natural frequencies and mode shapes are investigated. Comparison of analytically outcome of this study is made with results of the experimental modal test for homogeneous Aluminum plate.

In this paper a neural observer is deigned to control a Quadrotor Drone. Quadrotor is a type of flying robot which can fly vertically and has a simple structure. This robot is one of the best models of flying robot that is considered by many researchers recently. Because of nonlinear dynamics of the system, Stability of the control process has an important role in this robot. In this study first, a PD controller is designed to track the desired state and stabilizing the quadrotor based on particle swarm optimization algorithm. Nonlinear observer is then synthesized in order to estimate the unmeasured states. Then a neural network observer is designed and trained based on data that extract of nonlinear observer. After that, simulation results are also provided in order to illustrate the performances of the proposed controller and observer. Finally In addition to simulation we have practically implemented these control and observer methods on a quadrotor test bench. Practical implementation results demonstrate the effectiveness of the presented method.

Todays, hydrogels have attracted many researchers due to individual response to exterior stimuli. According to the environmental stimuli, these materials absorb a huge amount of water and swell. Due to the vast application of the functionally graded pH sensitive hydrogels, investigation of their mechanical behavior during changing of pH is very important. In this regard, the energy density is decomposed additively to energy of the network stretching, energy of mixing and energy of dissociation. Considering the variation of properties along thickness, the semi analytical solution is presented under plane strain condition. Verifying the accuracy of presented method, the result of this method is compared with the finite element method including radius, radial and tangential stresses, bending curvature and bending semi angle between pH = 2 to 8. In addition, the continuity of stresses and deformation is one of the benefits of using functionally graded hydrogel instead of multi layer ones.

This research studies the effect of hydro-static pressure on the free vibration of a circular plate in contact with an inviscid and bounded fluid. The plate is in bottom of rigid cylindrical vessel with clamped boundary condition. At first, the potential energy of the plate on elastic foundation is performed by solving the governing equation on circular plate with clamped boundary condition. Next, the governing equation of the oscillatory behavior of the fluid is obtained by solving Laplace equation and satisfying its boundary conditions. The natural frequencies and mode shapes of the plate are determined by applying the Rayleigh-Ritz method on the total energy of the system. The effects of the parameters such as fluid height, fluid density, plate thickness to its radius ratio, and wave numbers on the natural frequencies and mode shapes are investigated. Comparison of analytically outcome of this study is made with results of the experimental modal test.

Since the car is working in different climatic conditions, any Changes in temperature of the climatic conditions can affect various parts of the car. So it is necessary to optimize the energy consumption & keep the passengers in a suit & comfort state considering this kind of changes. The automotive air conditioning system is one of the largest ancillary loads in the passenger cars , with considerable effects on the vehicle fuel consumption .While most of the studies have represented linearized equations & simplified steady state matrixes , In the simulations of this paper, Nonlinear dynamic equations with using a model predictive controller and advanced control theory is designed for automotive air conditioning system.At the same time, a genetic algorithm technic which is a method of generation evaluation, is finding out the best answer in the whole problem space using a fitness function to select optimized inputs for sending to the plant. Also in this system a compressor capacity & fan flow rate has considered as two inputs to achieve the main objectives like minimizing the steady state error, system fast response, reduction of noise effects, minimizing of energy consumption and comforter of passengers.

Due to the various injuries that because of exercise or stroke occur to people and require rehabilitation treatments the presence of a wearable robot is important. In this paper, assistive robot is designed that consists of a six-bar mechanism. This mechanism has one degree of freedom and is used to perform radial /ulnar movement of the wrist and moves in the form of parallel with the person's body. The design of the robot is somehow that the dimensions of the robot are proportional to the patient's hand, and also provides the natural movement of the hand and helps the patient to perform movements until his recovery .At first by doing the experimentation the direction of movement for doing the radial /ulnar motion was obtained and then the appropriate mechanism was selected and in terms of Kinematics and dynamics have been studied and the accuracy of the obtained equations is evaluated.

Carbon nanotubes are obtained from rolling a graphene sheet of a given size and at a specific direction. Due to the strong carbon-carbon covalent bonds, nanotubes have unique mechanical and electrical properties. In this paper, using finite element and molecular mechanics methods, the covalent bonds between the carbon atoms in the nanotube have been modeled using linear beam element. Carbon nanotubes with different diameter and length ranges have been analyzed. The effects of nanotube length, diameter and chirality on nanotubes Young's and shear Modului have been investigated, independently. Also, the decrease in nanotube modulus due to the number and position of vacancy defects has been determined. The results show that, nanotube diameter has a larger effect, compared to nanotube length, on elastic properties, especially Young's modulus. Comparing the results obtained for armchair, zigzag, and chiral nanotubes, vacancy defect had the most effect on chiral nanotube Young's modulus, with a chiral angle of 49.15 degrees.