Mechanical Engineering

An analysis for sound scattering by fluid-filled spherical and cylindrical viscoelastic shells immersed in viscous fluids is outlined. The dynamic viscoelastic properties of the scatterer and the viscosity of the surrounding and core fluids are rigorously taken into account in the solution of the acoustic scattering problem. The novel features of Havriliak-Negami model for viscoelastic material dynamic behaviour description along with the appropriate wave-harmonic field expansions and the pertinent boundary conditions are employed to develop a closed-form solution in form of infinite series. Subsequently, the associated acoustic field quantities such as the scattered far-field pressure directivity pattern, form function amplitude, transmitted intensity ratio, and acoustic force magnitude are evaluated for given sets of medium physical properties. Numerical results clearly indicate that in addition to the traditional fluid viscosity-related mechanisms, the dynamic viscoelastic properties of the shell material as well as its thickness can be of major significance in sound scattering. Limiting cases are examined and fair agreements with well-known solutions are established.

Purpose -The purpose of this paper is to empirically explore market integration among five selected Association of Southeast Asian Nations (ASEAN) emerging markets (Malaysia, Thailand, Indonesia, the Philippines and Singapore) during the pre-and post-1997 financial crisis periods. Design/methodology/approach -Employs two-step estimation, cointegration and generalized method of moments (GMM). Findings -The study finds that the stock markets in the ASEAN region are cointegrated both during the pre-and post-1997 financial crisis. However, the markets are moving towards a greater integration, particularly during the post-1997 financial crisis. Finally, as measured by the error correction terms, except the emerging market of Indonesia, all other ASEAN markets appear to be the important bearers of short-run adjustment to a shock in the long-run equilibrium relationships in the region both during the pre-and post-crisis periods.

The present study uses a combination of a state-dependent Riccati equation (SDRE) controller and SDRE observer to control a class of nonlinear non-affine control systems. This type of nonlinear system features control nonlinearities that change the formulation of the SDRE and its solution techniques. Control law is obtained using the exact solution method for low-order and uncomplicated systems; numerical approaches are available for complex and high order systems. Both solution methods must use integral control formulation or solve the system equation using control law simultaneously to treat control nonlinearities. The closed loop feedback of the system is partially available, providing an observer for estimation of full-state feedback. The efficacy, advantages and compatibility of the controller with the observer guides the choices to the SDRE observer. The stability proof of the SDRE controller in the non-affine structure is presented and four case studies are simulated and discussed. Two were selected from previous studies to verify the proposed algorithm. The third, more complicated model, shows the ability of the controller to solve complex and high order systems. A brief discussion of the suboptimal cheap control problem and an example are also presented.

The purpose of this paper is to present the application of the state-dependent Riccati equation (SDRE) method for controlling a class of non-affine nonlinear systems. By mentioning a class of non-affine system, a system with nonlinear states and control inputs is meant. The non-affine in control systems have been secluded in contrast to affine in control systems, especially in the SDRE field. The SDRE
method is one of the approaches for solving the Hamilton–Jacobi–Bellman (HJB) equation as a closed loop nonlinear optimal control method. In this work, the SDRE is solved with exact solution (ES) method and online control update (OCU) formulation. The proposed algorithms are verified by three examples which are selected from previous works. The effectiveness of the SDRE is demonstrated through numerical simulations and comparisons with previous works which the SDRE brilliantly outperforms the other techniques in presence of external disturbance.

This work proposes a nonlinear suboptimal diving control scheme for autonomous underwater vehicles (AUVs) in presence of hydrodynamic parameter uncertainties and disturbance. The controller is designed via the state-dependent Riccati equation (SDRE) method that regulates the system’s states by minimization of the related quadratic cost function. To compensate the system’s restoring force (gravity/buoyancy), the common SDRE control law is supplemented with a discontinuous control law analogous to sliding surface notion. We applied the proposed controller to a well-known AUV model (REMUS). Simulation results demonstrate the effectiveness of the proposed controller under system’s parameter uncertainties and disturbance with appropriate regulation and tracking performance.

The goal of VUDJOOD (existence) algebra is to provide the capability of visual intelligence for intelligent machines that are like humans. These machines should visually recognize surrounding objects and machines and finally decide which manufacturing disciplines, to be applied on the object. The present work is an effort to accelerate achieving this purpose. In this paper we focused on the 3D recognition process using VUDJOOD algebra theory. 3D Rotary Raster Photogrammetric Scanner (3D RRP Scanner) is designed and built at the Machine Tools Control lab of IUST. This scanner is used to generate the points-cloud of objects. Then these points-clouds are checked in the VUDJOOD equations. Finally the object which has the most overlapping with the points-cloud is recognized as the desired object. Objects identifying Algorithm has been studied for a few objects with simple geometry using the experimental tests. Dimensional accuracy of detection results is acceptable.

In this paper a performance-oriented Parallel Distributed Compensation (PDC) controller for Stabilizing the Linear Single Inverted Pendulum system is presented as a classical challenging benchmark problem in control engineering. The main idea of the original PDC method is to partition the dynamics of a nonlinear system into a number of linear subsystems, design a number of state feedback gains for each linear subsystem, and finally generate the overall state feedback gain by fuzzy blending of such gains. In Performance-Oriented PDC algorithm the state feedback gains are not considered constant through the linear subsystems, rather, based on some prescribed performance criteria, several feedback gains are associated to every subsystem, and the final gain for every subsystem is obtained by fuzzy blending of such gains. The model-based design methodology for mechatronic systems is a key factor for innovative and operative excellence in the design process. It was shown through this method of designing and simulation studies that application of the Performance-Oriented PDC method to such a challenging problem is robust against the uncertainties, while the control effort is still kept at an acceptable level.

This work addresses an autonomous underwater vehicle (AUV) for applying nonlinear control which is capable of disturbance rejection via intelligent estimation of uncertainties. Adaptive radial basis function neural network (RBF NN) controller is proposed to approximate unknown nonlinear dynamics. The problem of designing an adaptive RBF NN controller was augmented with sliding mode robust term to improve trajectory tracking and regulation in presence of uncertainties. Moreover, stability proof of proposed control scheme was shown with Lyapunov theory. Furthermore, the control, design and simulation results are provided without any simplification of the entire system. Although the design approach of this paper is implemented on REMUS this point of view can be applied on any AUV using the same technique.

This work addresses an autonomous underwater vehicle (AUV) for applying nonlinear control which is capable of disturbance rejection via intelligent estimation of uncertainties. Adaptive radial basis function neural network (RBF NN) controller is proposed to approximate unknown nonlinear dynamics. The problem of designing an adaptive RBF NN controller was augmented with sliding mode robust term to improve trajectory tracking and regulation in presence of uncertainties. Moreover, stability proof of proposed control scheme was shown with Lyapunov theory. Furthermore, the control, design and simulation results are provided without any simplification of the entire system. Although the design approach of this paper is implemented on REMUS this point of view can be applied on any AUV using the same technique.

The present study uses a combination of a state-dependent Riccati equation (SDRE) controller and SDRE observer to control a class of nonlinear non-affine control systems. This type of nonlinear system features control nonlinearities that change the formulation of the SDRE and its solution techniques. Control law is obtained using the exact solution method for low-order and uncomplicated systems; numerical approaches are available for complex and high order systems. Both solution methods must use integral control formulation or solve the system equation using control law simultaneously to treat control nonlinearities. The closed loop feedback of the system is partially available, providing an observer for estimation of full-state feedback. The efficacy, advantages and compatibility of the controller with the observer guides the choices to the SDRE observer. The stability proof of the SDRE controller in the non-affine structure is presented and four case studies are simulated and disc...

Abstract: In order to facilitate the characterization of materials at high strain rate and high temperature, this paper introduces a very simple technique for using the traditional Split Hopkinson Pressure Bar (SHPB) system at elevated temperatures, with a different ...

Machining and rolling processes are used to produce threads. The thread rolling process is more considered by the producers because of necessity to threads with high quality and strength in different industries. Other advantages of this process are producing without chips and wasting of material with high rate of manufacturing. Because of indicated geometry and small sizes of the threads studying on them is difficultly. As for increasing demands to accuracy, speed and optimization of manufacturing process the regard to the finite elements simulations of processes has been increased by the investigators. At this article making threads on St37 steel pipes has been done. For studying microhardness, texture of material and strength of threads made by thread rolling some tests were design and done. Then the results compared with process simulation results in ABAQUS commercial code and the results got by machining threading process. The results showed that the hardness and strength is sig...

In order to facilitate the characterization of materials at high strain rate and high temperature, this paper introduces a very simple technique for using the traditional Split Hopkinson Pressure Bar (SHPB) system at elevated temperatures, with a different geometry for the specimen. This particular specimen is used to avoid a complicated SHPB system at an elevated temperature, and to keep things as simple as possible. The limitations of the compression SHPB in use at high temperatures, the need for extra computations and the requirement for additional equipment may be reasons why there is such a scarcity of reliable flow stress data for various materials at high temperatures in the literature. In the layout of the high temperature test set up at our laboratory, in addition to the conventional compression SHPB, an induction coil heater is used as the heating system and a simple holder is used to hold the specimen at the correct position during the test. The thermal behaviour of the n...

Super alloys, are metal with high toughness and low thermal conductivity that they have low machinability. Sever tool wear and unsuitable surface roughness in machining of these materials are the main challenges for machining process. In this study, the effect of the cutting fluid types under minimum quantity coolinglubrication method on work piece surface roughness and tool wear in machining of A286 super alloy were studied and their results were compared with wet and dry methods. For this purpose, vegetable based lubricant and nanoparticles in minimum quantity lubrication along with cryogenic cooling were used. The results of surface roughness survey under Ra criterion showed that using of vegetable-based lubricant with nano silicon oxide under minimum quantity lubrication and cooling with liquid nitrogen are caused the surface roughness decreased 30-75% compare to dry condition and 15-64% decreased compare to wet lubrication. Also due to very low surface roughness variations comp...

The aim of this experiment is the study of the process parameters on wires produced by a novel friction stir extrusion approach. Friction stir extrusion relies on the intrinsic frictional heating and extensive plastic deformation generated by process to stir, consolidate, and extrude the powders, chips directly into useable product forms in one-step. In this work it was attempted to find effects of process parameters such as traverse speed rate and die rotational speed on physical and micro structural properties. It has been observed that hot crack and cold crack defects were existed on wires produced using too high and too low die rotational speed, respectively. Furthermore, the dimension and amount of defects such as voids in the wires generally increases with the increase of die transverse speed.

This paper deals with a behaviour analysis of a steel starting from the dynamic SHPB test. The standard analysis is given with the corresponding analytical analysis of the strain waves measurements in the elastic bars as well as the different relations that are needed in order to deduce flow stress data, particularly for large plastic deformation. Using an identification method based on the analytical computation of the Von-Mises stress values we obtain the rheological parameters corresponding to different formulations of the constitutive equation. The principal problems are linked to the feasibility of the classical analysis because several approximations are used, as for example, the homogeneity of the strain and stress in the specimen or the influence of the friction between the specimen and the bars. It is the reason why this work proposes to use a finite element simulation. The constitutive model, obtained by classical analysis, is introduced into a numerical simulation of the experimental test developed with the implicit package FORGEZ A comparison is made between the experimental and the computed values of the strain waves. To identify more correctly the constitutive model, an inverse analysis is proposed.

In order to facilitate characterization of materials at high strain rate and high temperature, this paper introduces a technique for using the traditional Split Hopkinson Pressure Bar (SHPB) system at elevated temperature, with a different geometry for the specimen. This particular specimen is used to avoid a complicated SHPB system at an elevated temperature, and to keep things as simple as possible. The limitations of the compression SHPB in use at high temperatures, the need for extra computations and the requirement for additional equipment may be reasons why there is such a scarcity of reliable flow stress data for various materials at high temperatures in the literature. In order to identify the constitutive equation, an inverse analysis method is used from a reduced finite element simulation of the specimen compression. This simple, easy, practical experimental system and inverse analysis technique, have been used to study a AA5083 aluminium alloy at high temperature and high strain rates.

At great depths of earth, fracture in rock masses occurs under the influence of confining pressure. However, most of the previous rock fracture studies deal only with ambient conditions and ignore the effects of multiaxial stresses induced because of applying confining pressure. In this paper, the influence of confining pressure on the mixed mode stress intensity factors (KI and KII) is investigated for a test specimen. A large number of 3D finite element models were analyzed for investigating the effect of various confining pressure levels on the fracture behaviour of Brazilian disc specimen. In the finite element models a uniformly distributed confining pressure in the range of 0 MPa to 20 MPa is applied to all sides of the test specimen. It is shown that the confining pressure has a significant effect on the mode I stress intensity factor, but the mode II stress intensity factor is completely independent of the applied confining pressure. A comparison between the confined and unconfined conditions shows that the effective mixed mode stress intensity factor increases by increasing the value of confining pressure for all mode mixities.

In this paper, the Al2O3-water nanofluid flow in laminar and turbulent flows inside tubes fitted with diamond-shaped turbulators is numerically modeled. The nanofluid flow is modeled by employing a two-phase mixture method and applying the constant heat flux boundary condition at tube walls. In the results, the effects of different parameters such as the geometry of turbulators, volume fraction and diameter of nanoparticles, etc. on the flow field in the tubes have been investigated. The obtained results indicate that, with the reduction of tail length ratio (TR) and increase of vertex angle of turbulators (θ), the heat transfer coefficient as well as the wall shear stress increase. Similarly, with the reduction of TR and increase of θ, the amount of secondary flows, vortices and the turbulent kinetic energy increase. Moreover, the increase in the volume fraction of nanoparticles and the reduction of nanoparticles diameter lead to the increase of the heat transfer coefficient and wa...