Sharif Digital Repository

This paper uses a fourth-order compact implicit operator scheme for solving 2D/3D steady incompressible flows using the artificial compressibility method. To stabilize the numerical solution, numerical dissipation terms and/or filters are used. Results obtained for test cases are in good agreement with the available numerical and experimental results. A sensitivity study is also conducted to evaluate the effects of grid resolution and pseudocompressibility parameter on accuracy and convergence rate of the solution. The effects of filtering and numerical dissipation on the solution are also investigated  

As the most rigid cytoskeletal filaments, tubulin–labeled microtubules bear compressive forces in living cells, balancing the tensile forces within the cytoskeleton to maintain the cell shape. The current structure is often under several environmental conditions as well as various dynamic or static loads that can decrease the stability of the viscoelastic tubulin–labeled microtubules. For this issue, the dynamic stability analysis of size-dependent viscoelastic tubulin–labeled microtubules using modified strain gradient theory by considering the exact three-length scale parameter. Viscoelastic properties are modeled using Kelvin-Voight model to study the time-dependent tubulin–labeled... 

We have developed a fourth order compact finite volume method for the solution of low Mach number compressible flow equations on arbitrary nonuniform grids. The formulation presented here uses collocated grid that preserves fourth order accuracy on nonuniform meshes. This was achieved by introduction of a new fourth order method for calculation of cell and face averaged metrics. A special treatment of nonlinear terms is used to guarantee the stability of the fourth order compact method. Moreover an approach for applying this method to multi-block domains is presented for complicated geometries and parallel processing applications. Several test cases including the flow in a lid-driven cavity,... 

A set of transforms (SVD transforms) were introduced in (Shahshahani and Tavakoli Targhi) for understanding images. These transforms have been applied to some problems in computer vision including segmentation, detection of objects in a texture environment, classification of textures, detection of cracks or other imperfections, etc. This technique is shown to be applicable to determination of the location of eyes in a facial image. This method makes no use of color cues, prior geometric knowledge or other assumptions and does not require training. It is also insensitive to local perturbations in lighting, change of orientation and pose, scaling, and complexity of the background including... 

This paper addresses the problem of frame detection in orthogonal frequency-division multiplexing (OFDM) systems. Using fourth-order statistics, a novel approach is presented for detection of a preamble composed of two identical parts in the time domain. First, it is demonstrated that sufficient statistics for detection of a periodic preamble do not exist, and conventional methods are not optimal. Next, looking at the detection of a preamble from the viewpoints of hypothesis testing and classification, a new method is presented based on the idea that fourth-order statistics can increase class separability (between-class distance) and consequently improve detection performance. It is proven... 

An exact planar solution for the determination of electroelastic responses are presented for functionally graded piezoelectricmaterials (FGPMs). The electro-mechanical properties are assumed to vary exponentially. Exploiting the potential functions for stress and induction, the governing equations reduce to systems of fourth order inhomogeneous partial differential equations (PDEs), which are solved in a closed form manner. Validity of the obtained solution is checked by other existing results in the literatures. Several examples are provided under distinctive thermo-electro-mechanical loadings. Finally, the effect of the graded indices is examined upon the electroelastic response of the FGP... 

One of the drawbacks to post-process and to interpret ultrasound medical images is speckle noise. In this paper we used fourth-order partial differential equation method proposed by Lysaker et al. for speckle reduction of ultrasound images. We used two groups of images first was the synthesized noisy image and second is real ultrasonic images. A comparison between our results and to other methods showed that PDE has better SNR and PSNR in most levels of speckle  

In this paper, we propose a Jacobi-like Deflationary ICA algorithm, named JDICA. More particularly, while a projection-based deflation scheme inspired by Delfosse and Loubaton's ICA technique (DelLR) is used, a Jacobi-like optimization strategy is proposed in order to maximize a fourth order cumulant-based contrast built from whitened observations. Experimental results obtained from simulated epileptic EEG data mixed with a real muscular activity and from the comparison in terms of performance and numerical complexity with the FastICA, RobustICA and DelLR algorithms, show that the proposed algorithm offers the best trade-off between performance and numerical complexity when a low number (∼... 

In this paper, the problem of coarse timing synchronization in orthogonal frequency-division multiplexing (OFDM) systems is investigated, and two new timing metrics with better performance are presented. The new metrics take advantage of two novel differential normalization functions that are based on the fourth-order statistics and are designed depending on the value of carrier frequency offset (CFO). The proposed timing metrics are theoretically evaluated using two different class separability criteria. It is shown that the new schemes considerably increase the difference between the correct and wrong timing points in comparison with previous methods. The computational complexity of the... 

The Chirikov diffusion in the sphere–ellipsoid binary asteroids through the spin–orbit resonance model is analytically and numerically studied. The primary and the secondary of the binary system are considered sphere and ellipsoid, respectively. The gravitational potential function is expanded up to the fourth-order. The geography of the first- and second-order resonances is derived and studied for different values of asphericity and dynamical flattening of the secondary asteroid and the semimajor axis and eccentricity of the mutual orbit. The Chirikov diffusion due to overlapping of the first- and second-order resonances is examined. For this end, the previously derived criterion for... 

During the past years, the development of piezoelectric energy harvesters is extensively increased for providing the required energy of sensor nodes. It has been proven that changing the beam’s cross-section along its length direction may lead to extract more power with less mass. In this article, a hybrid metaheuristic algorithm called tabu continuous ant colony system is employed to optimize the tapered piezoelectric energy harvester in a fast and course manner. The exponential and fourth-order polynomial functions are considered as shaping functions for tapering the beam’s width along its length direction. An experiment is also set up to evaluate the performance of the optimization... 

An exact thermoelasticity solution for a two-dimensional thick composite consisting of homogeneous and functionally graded layers is presented. The thermomechanical properties of functionally graded layers are assumed to vary exponentially through the thickness while the Poisson's ratio is taken to be constant. The heat transfer problem is solved under steady state condition accounting for the heat convection. Utilizing the stress function the governing equation reduces to a fourth order inhomogeneous partial differential equation which is solved exactly using Fourier series method. A comparative study is done between two sandwich structures with homogeneous and functionally graded coatings,... 

In this letter, a new notion of stability is introduced, which is called triangular stability. A system is called triangularly stable if the norm of its state vector is bounded by a decreasing linear function of time such that its intersection point with the time axis can be arbitrarily commanded by the user. Triangular stability implies prescribed-time stability, which means that the nonlinear system is converged to zero equilibrium at an arbitrary finite time. A prescribed-time controller with guaranteed triangular stability is developed for normal form nonlinear systems with uncertain input gain, which is able to reject the disturbances and unmodeled dynamics. Numerical simulations are... 

A high-order compact finite-difference lattice Boltzmann method (CFDLBM) is proposed and applied to accurately compute steady and unsteady incompressible flows. Herein, the spatial derivatives in the lattice Boltzmann equation are discretized by using the fourth-order compact FD scheme, and the temporal term is discretized with the fourth-order Runge-Kutta scheme to provide an accurate and efficient incompressible flow solver. A high-order spectral-type low-pass compact filter is used to stabilize the numerical solution. An iterative initialization procedure is presented and applied to generate consistent initial conditions for the simulation of unsteady flows. A sensitivity study is also... 

In this paper, strain gradient elasticity formulation for analysis of FG (functionally graded) micro-cylinders is presented. The material properties are assumed to obey a power law in radial direction. The governing differential equation is derived as a fourth order ODE. A power series solution for stresses and displacements in FG micro-cylinders subjected to internal and external pressures is obtained. Numerical examples are presented to study the effect of the characteristic length parameter and FG power index on the displacement field and stress distribution in FG cylinders. It is observed that the characteristic length parameter has a considerable effect on the stress distribution of FG... 

This study presents an analytical approach for proper selection of output functions to be regulated for the Vienna rectifier such that the resultant closed-loop systems are minimum phase. Specifically, two different adaptive control methodologies based on the input-output linearisation are developed and categorised. In the first category, three output functions are introduced and imposed to be zero by using three dynamic feedback laws. On the basis of states to be regulated, four different cases are studied and it is shown that only one of these cases results in a one-dimensional zero dynamics with an asymptotically stable equilibrium point. In the second category, two output functions are... 

In this paper, free vibration analysis of thin symmetrically laminated skew plates with fully clamped edges is investigated. The governing differential equation for skew plate which is a fourth order partial differential equation (PDE) is obtained by transforming the differential equation in Cartesian coordinates into skew coordinates. Based on the multi-term extended Kantorovich method (MTEKM) an efficient and accurate approximate closed-form solution is presented for the governing PDE. Application of the MTEKM reduces the governing PDE to a dual set of ordinary differential equations. These sets of equations are then solved with infinite power series solution, in an iterative manner until... 

A high-order compact finite-difference lattice Boltzmann method (CFDLBM) is extended and applied to accurately simulate two-phase liquid-vapor flows with high density ratios. Herein, the He-Shan-Doolen-type lattice Boltzmann multiphase model is used and the spatial derivatives in the resulting equations are discretized by using the fourth-order compact finite-difference scheme and the temporal term is discretized with the fourth-order Runge-Kutta scheme to provide an accurate and efficient two-phase flow solver. A high-order spectral-type low-pass compact nonlinear filter is used to regularize the numerical solution and remove spurious waves generated by flow nonlinearities in smooth regions... 

In this study, the Chebyshev collocation spectral lattice Boltzmann method (CCSLBM) is developed and assessed for the computation of low-speed flows. Both steady and unsteady flows are considered here. The discrete Boltzmann equation with the Bhatnagar-Gross-Krook approximation based on the pressure distribution function is considered and the space discretization is performed by the Chebyshev collocation spectral method to achieve a highly accurate flow solver. To provide accurate unsteady solutions, the time integration of the temporal term in the lattice Boltzmann equation is made by the fourth-order Runge-Kutta scheme. To achieve numerical stability and accuracy, physical boundary...