Scientia Iranica
Volume:21 Issue: 5, 2014

This paper describes the optimization of combustion chamber geometry and injection timing of new generation of EF7 engine that CNG is directly injected to the combustion chamber, with the aim of providing the best mixture at low and high speeds. The multi-objective genetic algorithm (MOGA) is coupled with the KIVA computational fluid dynamics (CFD) code, with grid generation in order to maximize the flammable mass of mixture. This would result in better combustion and improved fuel economy. The optimization variables related to the combustion chamber are seven geometry variables and injection timing. Through the present optimization, a great improvement in the mixture distribution is achieved. The optimization results show that early injection with the shallow bowl in shape can be advantageous at high speeds while late injection would result in better results at the low speed.

The effects of driving voltage waveform and frequencies on the performance of piezoelectrically actuated micropump are investigated in details. A full three-dimensional piezoelectric micropump was modeled numerically and tri electro-mechanical-fluidic coupling effects have been taken into account on its interface boundaries. Standard excitation waveforms including sinusoidal, triangle, sawtooth and square shapes were implemented and the results were compared with each other. The real time pump flow behavior was studied in each case for different membrane positions. The analysis predicts that the more sharp jump in the wave form, the more pump flow can be attained at the outlet. Square shape excitation with the sharpest instantaneous slope has the most notable overall flow rate compared to the other types in the examined range of frequencies. The behavior can be explained by considering the generated vortices in the flow. Due to sudden jump in membrane position, the flow forms strong vortices which magnify the diodicty of valves.

Natural convection of a nano uid consisted of water and SiO2 in a square enclosure cavity have been studied numerically. The sidewalls are maintained at dierent constant temperatures and the other walls are thermally insulated. Two-phase mixture model has been used to investigate the hydrodynamic and thermal behaviors of the nano uid for various inclination angles of enclosure ranging from  = -60 to  = 60, for Rayleigh numbers varying from 103 to 07 and for volume fraction  = 0 - 6%. The governing equations in a two-dimensional space are solved numerically using the nitevolume approach. The results show that the average Nusselt number increases with growth of both Rayleigh number and the volume fraction of the nanoparticles. Moreover, inclination angle can be a control parameter for nano uid lled enclosure, and the angle, in which the maximum average Nusselt number occurs for various values of Rayleigh numbers and volume fraction, is obtained.

The density jump on an inclined surface is analyzed using integral method by applying mass and momentum conservation equations. The jump occurs in a two-layered fluid flow, in which the upper layer is stagnant and very deep. A relation is derived which gives the conjugate depth ratio as a function of inlet densimetricFroude number, inlet concentration ratio, bed slope, and entrainment. A set of experiments are performed to verify the relation. The theory and the measurements are in good agreement. The analysis reveals that increasing the surface inclination results in a decrease in the conjugate depth ratio. This analysis also shows that the densimetric Froude number just after the jump is a function of the inlet densimetric Froude number and surface inclination and not inlet concentration. The model predicts a critical Froude number of 1.12 for horizontal internal hydraulic jumps in salt-water density flows. It also reveals that the critical Froude number for internal hydraulic jumps in salt-water density flows increases with surface inclination and decreases with inlet concentration of the flow.

Determination of mode shapes of vibrating system is an important step in vibration analysis. A procedure to determine the mode shapes of linear continuous systems based on the concept of normal equation is presented. This procedure is very effective especially in implementing by computers. This method can be applied easily to multi-step structures such as stepped beams and bars. To demonstrate the application of the proposed method, some examples are solved.

This paper investigates the forward kinematic problem of three 4-DOF parallel mechanisms performing three translations and one rotation motion referred to as Schönfliesmotion. The kinematic arrangements of the mechanisms under study in this paper are such that two of them are classified as 4-PRUR and one of them is a 4-PUU. They are respectively special cases of 4-PR′R′R″R″, 4-PR″R″R′R'' and 4-PR″R′R′R″ parallel mechanisms that have originated from thetype synthesis of 4-DOF parallel mechanisms with identical limb structures. The forward kinematic problem is studied in three-dimensional Euclidean space and a univariate expression describing the forward kinematic problem is obtained for each of the latter parallel mechanism by resultant method. The results obtained from this method show that a set of univariate expressions of degree (72, 64, 64, 82, 4, 28) describe the forward kinematic problem of 4-PRUR1 and 4-PRUR2 parallel mechanisms. As well as a quadratic univariate expression represent the forward kinematic problem of 4-PUU parallel mechanisms.In addition, the system of equations corresponding to the forward kinematic problem is solved upon resorting to homotopy continuation approach which clarifies that the forward kinematic problem of a 4-PRUR1, 4-PRUR2 and 4-PUU parallel mechanisms admit up to 236, 236 and 2 finite solutions, real and complex.

This paper deals with miss distance analysis of single-lag optimal guidance law (OGL) using linearized equations of motion in normalized form. The radome refraction error, acceleration limit, constant target acceleration, and arbitrary-order binomial guidance and control system are considered in the formulation. In addition, body rate feedback is utilized in the OGL formulation as a well-known classical compensation method of radome effect for proportional navigation guidance. The numerical solution of normalized equations produces normalized miss distance curves, which are useful tools for guidance designer for analysis and design of guidance parameters for an allowable miss distance and acceleration limit. Moreover, a modified first-order guidance scheme, based on an analytical stability analysis and normalized miss distance curves, is presented for reducing the achievable miss distance.

In this study, the homotopy perturbation transform method (HPTM) is performed to give approximate and analytical solutions of the first order linear and nonlinear system of time fractional partial differential equation. The HPTM is a combined form of the Laplace transform, the homotopy perturbation method, and He’s polynomials. The nonlinear terms can be easily handled by the use of He’s polynomials. The proposed scheme finds the solutions without any discretization or restrictive assumptions and is free from round-off errors and therefore, reduces the numerical computations to a great extent. The speed of convergence of the method is based on a rapidly convergent series with easily computable components. The fractional derivatives are described here in the Caputo sense. Numerical results show that the HPTM is easy to implement and accurate when applied to time- fractional system of partial differential equations.

In supersonic water vapor flows of low pressure turbines, the nucleation phenomena and consequent condensation is commonly observed. Internal heat transfer which is caused by phase change is strongly irreversible and has unwanted effects on turbine efficiency. Also, the strike of formed droplets on the surfaces results in large amounts of mechanical damages. Condensation heat release to supersonic flow is named condensation shock and leads to a considerable pressure rise which in turn reduces the outlet velocity and occasionally causes severe oscillations and makes the flow supercritical. The authors have presented a novel analytical approach for the reduction of these unwanted results in Laval nozzles by volumetric heating of the convergent section. In this paper and in continuation of the series of papers by the authors, one dimensional, supersonic and two-phase flow is modeled analytically and the simultaneous effects of volumetric heat transfer and friction in the convergent nozzle are investigated. It is concluded that the simultaneous use of friction and volumetric heatingcan be an appropriate and useful technique for the control of two-phase flow conditions and keeping them in the desired range.

Under the influence of intense expansion and supersonic acceleration of the steam flow in the divergent channel, the instability of the flow intensifies. In the lack of external surfaces, this non-equilibrium state causes nucleation and consequent growth of the formed nuclei. Due to the release of latent heat from condensation to the supersonic flow at the location of nucleation, an increase in pressure is developed in this small region which is known as the condensation shock. In this research, the effects of this shock on the boundary layer parameters are investigated. First, the water vapor flow which has the capability of nucleation is modeled analytically as adiabatic, inviscid and one dimensional. Then, using the mathematical equations of laminar and turbulent boundary layer parameters, and using the Inviscid-Viscous Interaction method. The results of these analytical modeling show that although the influence of the boundary layer on the expansion flow is limited, it still causes approximately 3% increase in the diameter of the water droplets. However, the effects of two-phase flow on the boundary layer parameters at the location of the condensation shock is considerable. The major novelty of this research is determining the quantification and qualification of these effects.

In this paper the optimal control framework is formed to control rotor-active magnetic bearing (AMB) systems. The multi-input–multi-output non-affine model of AMBs is well established in the literature and represents a challenging problem for control design, where the design requirement is to keep the rotor at the bearing centre in the presence of external disturbances. To satisfy the constraints on the states and the control inputs of the AMB nonlinear dynamics, a nonlinear optimal controller is formed to minimizethe tracking error between the current and desired position of rotor. To solve the resulted nonlinear constrained optimal control problem, the Gauss pseudospectral collocation method (GPCM) is used to transcribe the optimal control problem into a nonlinear programming problem (NLP) by discretization of states and controls. The resulted NLP is then solved by a well-developed algorithm known as SNOPT. The procedure for modeling, compilation and solving of resulted optimal control problem is done in Matlab optimal control software known as PROPT. The results illustrate the effectiveness of the proposed approach to deal with the control of AMBs.

The pheromone update phase in Ant Colony Optimization (ACO) has been addressed by various researchers in the context of scheduling problems. Various artificial intelligence (AI) techniques have also been used to investigate and improve the pheromone trail in worker assignment issue at the workshop floor level. This paper proposes a novel way of investigating and analyzing the issue of pheromone assignment through Neural Augmented Ant Colony Optimization (NaACO) technique. The technique thus developed has its roots in combining the strengths of Artificial Neural Networks (ANN) and the extra ordinary convergence capabilities of Ant Colony Optimization (ACO) thus formulating NaACO (Neural Augmented ACO). A set of hundred problems has been taken and an extensive methodology has been formulated to address the issue of pheromone updates in worker assignment on these problems. The results have been formulated and areas of future research have also been indicated.