Scientia Iranica
Volume:22 Issue: 3, 2015

A series of subsonic wind tunnel tests was conducted on a canard-wing configuration to study the interaction of canard and wing vortices. Both the canard and the wing in the present experiments had equal sweep angles. The velocity contours were measured using a total-static rake at three chordwise positions at front, middle and rear parts of the wing in a plane perpendicular to the wing surface. The experiments were performed at various combinations of angle of attack and canard deflection angle. The results show a close relationship between the strength of the vortices and the outcome of their interaction. According to the present findings, when the wing strong vortex is exposed to the strong vortex of the canard, the result of this interaction would be a weaker vortex on the wing with a smaller size than the original two vortices, while a weak vortex on the canard when interacted with a strong vortex on the wing, amplifies it and results in a stronger vortex on the wing surface. This has also been shown to be true when a strong canard vortex interacts with a weak vortex on the wing. The result would again be a stronger vortex on the wing surface.

In this paper, we investigate the existence of positive solutions for the eigenvalue problem of nonlinear fractional differential equation with p-Laplacian operator D 0+(p(D 0+u(t))) = f(u(t)), 0 1, −1 p = q, 1/p + 1/q = 1,  > 0 is a parameter, and f: (0,+1)! (0,+1) is continuous. By using the properties of the Green function and the Guo–Krasnosel’skii fixed-point theorem on cones, several new existence results of at least one or two positive solutions in terms of different eigenvalue interval are obtained. Moreover, the nonexistence of positive solution in term of the parameter  is also considered.

The present article relates to a new transom stern form for high speed planing hulls. To depress the intensive transom wave known as “rooster tail”, an innovative form of transom stern is originated. The new form of the transom stern is introduced by cutting the bottom of the hull near the transom stern. To verify the capability of the modified transom stern, a three dimensional RANS solver is used. It is demonstrated that by using the new transom stern shape, the intensive wave behind the transom will be substantially reduced in the range of the parameters under investigation. In the meantime, it is expected that the radar signature will also be decreased. To study the effect of modified transom at various Froude numbers and different trim angles, a set of numerical experiments are conducted. Three Froude numbers and four trim angles are considered and it is observed that the cutted transom stern has a significant effect on the reduction of height and length (height reduction up to 61% and length reduction up to 63%) of the rooster tail. Additionally, the accuracy of the numerical solutions is validated by comparing them against the results of empirical formula existing in the literature.

In this paper, the pre- and post-buckling behavior of beams made of functionally graded materials (FGMs), a mixture of ceramic and metal, under separately mechanical and thermal loading is studied. To this end, the finite element formulation is established based on the Euler-Bernoulli beam theory. The effects of geometrical nonlinearity and imperfection are taken into account. The arc-length algorithm is employed to obtain the secondary path beyond the bifurcation point. The influences of material index, imperfection, geometrical parameters and different boundary conditions of simply-supported, clamped-simply and clamped-clamped on the post-buckling characteristics of FGM beams are thoroughly investigated. The results generated are compared with the existing data in the literature and good agreements are achieved. The investigation undertaken here proves the essentiality of performing post-buckling analysis FGM beams especially with simply-supported end conditions.

Reducing the bottom hole differential pressure (BHDP) and so as to reduce the cuttings hold-down effect can significantly improve the rate of penetration (ROP). This paper analyzes the pressure drawdown mechanism of the jet pump bit, and then designs a novel annular jet pump bit. Using hybrid grid method, the key factors which affect the pressure drawdown capacity of the bit are discussed. The backflow below the reverse nozzle outlet is the main reason that reduces the pressure drawdown capacity of the jet pump bit. The design principle of the jet pump bit is given: the axial and radial angles are 150°-180° and 60°-75° respectively, the bit clearance should be kept below 3 mm, preferably using the vortex-jet flow combination pressure drawdown theory to reduce the bottom hole pressure, and the bit rotation speed should be as large as possible, which provide the theoretical guide for its development.

In this article, some theoretical relations are derived to predict instantaneous axial load during the splitting process of square columns on rigid pyramidal dies. For this purpose, it is assumed that kinetic energy is dissipated by four different deformation mechanisms: bending, friction, crack propagation and expansion. These mechanisms are carefully assessed. Based on the principle of energy conservation, external work of axial force is equated with total dissipated energies during the plastic deformation; and final relations are obtained to predict load-displacement diagram. Also, curl radius of square columns during the splitting process is calculated theoretically. Then, some metal tubes are tested and compressed axially between a rigid platen and a pyramidal die. Cracks propagate along four corners of the column. Experiments show that all four free end sides roll up into curls with a constant radius and applied load becomes constant after the crack propagation. This mechanism results in a large stroke and a constant load. Therefore, splitting is introduces as an energy absorber mechanism with large stroke to length ratio and high specific absorbed energy. Comparison of the theoretical predictions by derived equations with the experimental results shows a good correlation.

Nonlinear free vibration of symmetric circular fiber metal laminated (FML) hybrid plates is investigated. Considering the Von Karman geometric nonlinearity, the first order shear deformation theory (FSDT) is used to obtain the equations of motion. For the first time, five equations of motion of circular FML plates are derived in terms of plate displacements. The obtained equations are simplified for analyzing the first mode of symmetric circular plates. Using Galerkin method, five coupled nonlinear partial differential equations (PDEs) of motion are transformed to a single nonlinear ordinary differential equation (ODE) which is solved analytically by multiple time scales method, and an analytical relation is found for the nonlinear frequency of these plates. The obtained results are compared with the published results and good agreements are found. Moreover, the effects of several parameters on linear and nonlinear frequencies and the free vibration response are investigated.

The paper presents a mathematical model for the determination of spectra of equivalent slewing bearing loads of a rotating platform drive in hydraulic excavators with a backhoe attachment. The spectrum of equivalent bearing loads is defined on the basis of possible digging resistances which represent the minimal value from a set of boundary digging resistances that enable the stability of the excavator and boundary digging resistances which can be overcome by the excavator drive mechanisms. Having in mind that the same model of the excavator can have different variants of the kinematic chain which can occupy a number of different positions and working conditions, based on the general mathematical model, software was developed for a detailed analysis of the slewing bearing load of a rotating platform drive for a desired number of positions in the entire working range of the excavator for every possible variant of the kinematic chain.

This work numerically simulates thermally developing turbulent flow in a partially lled porous pipe. The pipe is made up of a clear fluid region and a centering fluidsaturated porous medium region. The pipe is subjected to a constant wall temperature. Darcy-Brinkman-Forchheimer model is used to model the momentum equations in the porous medium.

The performance of visual servoing systems can be enhanced through nonlinear controllers. In this paper, a sliding mode control is employed for such purpose. The controller design is based on the outputs of a pose estimator which is implemented on the scheme of the position-based visual servoing (PBVS) approach. Accordingly, a robust estimator based on unscented Kalman observer cascading with Kalman filter is used to estimate the position, velocity and acceleration of the target. Therefore, a PD-type sliding surface is selected as a suitable manifold. The combination of the estimator and nonlinear controller provides a robust and stable structure in PBVS approach. The stability analysis is verified through Lyapunov theory. The performance of the proposed algorithm is verified experimentally through an industrial visual servoing system.

The buoyancy-driven magneto-hydrodynamic flow is investigated using a two-dimensional numerical simulation in a filled square enclosure, which is heated by a uniform volumetric heat density. The top and bottom walls of the enclosure are adiabatic and the side walls have constant temperature. A fixed magnetic field is applied in the direction normal to the left side wall of the enclosure. The stationary dimensionless governing equations are solved numerically for the stream function, vorticity and temperature, with finite difference method, using MATLAB software. The Prandtl (Pr) number of the fluid is assumed to be 0.733; the Rayleigh (Ra) number is made to vary from 104 to 107 and the Hartmann (Ha) number between 0 and 103. The stream function equation is solved using fast Poisson''s equation solver on a rectangular grid (POICALC function in MATLAB) vorticity, and temperature equations are solved using the red-black Gauss-Seidel and bi-conjugate gradient stabilized (BiCGSTAB) methods, respectively. The proposed method is fast, and there is no need for the variables under-relaxation. It is interesting to know that convergence of this method is superior to other segregated algorithms. The ratio of the Lorentz force to the buoyancy force (Ha2Ra) is introduced as an index, to compare the contribution of natural convection with magnetic field strength on heat transfer.

A parallel mechanism was designed and developed to perform tasks with micro meter accuracy within millimeter-range workspace. The system employs two Stewart platforms; while squiggle motors were used in one of the platforms to provide larger workspace, the second platform which uses piezoelectric actuators accurately positions the tool tip in the desired point. Error model for tool tip was developed for the first platform. Considering worst case scenario and using Particle Swarm Optimization algorithm, positioning error of the first platform was evaluated numerically throughout the respective workspace, upon which the design of the fine tuning piezo driven second stage was carried out. Positioning error and workspace of the whole system was evaluated. Using a single-deck platform with squiggle motors caused 40 micrometers positioning error while application of the fine-tuning piezo-driven Stewart platform reduced the total positioning error to 10 micrometers.

Ballast Water Treatment systems which are type approved and commercially available will require improvements to meet stricter standards. Heat treatment could be a viable additional option. Considering the waste heat potential on a ship, a system harvesting the engine exhaust heat may be envisaged for which a heat exchanger will be vital. Design optimisation of a heater employing exhaust gases of the engine as utility fluid and ballast sea water as the process fluid was achieved using Lagrangian methods, keeping the annual cost as the objective function. Limiting the number of variables, optimal values were calculated with cost considerations for utility fluid and also pumping costs for utility and process fluids. In all, four optimum designs and three comparative designs were developed. Heat balance data from an operational tanker, specific fuel consumption values and fuel costs were considered for the design. The thermodynamic and geometric designs were worked out using computer based software for a comparison. Designs were compared on the basis of annual cost, optimum exit temperature of shell side fluid, optimum mass flow of tube side fluid and heat exchanger effectiveness. It is demonstrated that an optimal heat exchanger design can be obtained with simple optimisation procedures.