Sharif Digital Repository

A modified weakly compressible smoothed particle hydrodynamics (WCSPH) is presented, which utilizes consistent discretization schemes for spatial derivatives in the flow equations. Here, each SPH particle is considered as a computational point that represents a specific part of the fluid. To overcome non-physical oscillations that usually arise in standard WCSPH, we modified the mass conservation equation by using a numerical filter. This modification is based on the difference between two discretization schemes used for the term ∇{dot operator}∇Pρ. Furthermore, a new implementation of wall boundary condition in SPH is introduced. This condition is imposed on the pressure of wall boundary... 

Several schemes for discretization of first and second derivatives are available in Smoothed Particle Hydrodynamics (SPH). Here, four schemes for approximation of the first derivative and three schemes for the second derivative are examined using a theoretical analysis based on Taylor series expansion both for regular and irregular particle distributions. Estimation of terms in the truncation errors shows that only the renormalized (the first-order consistent) scheme has acceptable convergence properties to approximate the first derivative. None of the second derivative schemes has the first-order consistency. Therefore, they converge only when the particle spacing decreases much faster than... 

Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) can lead to non-physical oscillations in the pressure and density fields when simulating incompressible flow problems. This in turn may result in tensile instability and sometimes divergence. In this paper, it is shown that this difficulty originates from the specific form of spatial discretization used for the pressure term when solving the mass conservation equation. After describing the pressure-velocity decoupling problem associated with the so-called colocated grid methods, a modified approach is presented that overcomes this problem using a different discretization scheme for the second derivative of pressure. The modified... 

Purpose - The purpose of this paper is to present a novel approach in aeroservoelastic analysis and robust control of a wing section with two control surfaces in leading-edge and trailing-edge. The method demonstrates how the number of model uncertainties can affect the flutter margin. Design/methodology/approach - The proposed method effectively incorporates the structural model of a wing section with two degrees of freedom of pitch and plunge with two control surfaces on trailing and leading edges. A quasi-steady aerodynamics assumption is made for the aerodynamic modeling. Basically, perturbations are considered for the dynamic pressure models and uncertainty parameters are associated... 

Various methods are devised to capture renewable energy or waste heat from different sectors, where among all, waste heat capturing from the generator of a wind turbine through the cooling process for freshwater and cooling production is paid less attention in Iran, in spite of the fact that many wind farms in Iran are in hot and humid regions and the residents nearby the farms desperately need freshwater and cooling load. To surmount this problem, waste heat extraction from a wind turbine (Enercon 70 Model) for freshwater and cooling production is proposed in this study. Instead of dissipating this thermal energy of the wind turbine into the environment, it can be used for freshwater and... 

Occurrence of drought, as an inevitable natural climate feature, cannot be ceased while happening. However, costs of the consequences could be alleviated using mature scientific integrated approaches. To reduce the amount of damage, it is required to provide “Contingency” and “Mitigation” action plans. For this reason, development of efficient operating instructions for various regions based on weather conditions and field studies is needed as well as having a sophisticated understanding of socioeconomic situations. This paper describes an approach to provide the first national agricultural drought risk management plan for a river basin in Iran country as a pilot. The study lasted for... 

In this paper we use tunability of FBG to introduce a novel device for optically reading, writing or modifying the low-speed label(s). We utilize tunable FBG arrays as concurrent label modulating (multiplexing) elements. This second-level intensity modulation is used to send signaling messages between nodes of WDM networks. The fiber optic small-signal modulator is formulated, and its behavior is compared with the published reports  

Tunability of fiber Bragg grating (FBG) in transition region is used to implement wavelength-selective optical intensity modulator, which superimposes a secondary low-speed data on the transit high-speed payload optical signal. Theoretical model of the device is developed and verified by measurements in the linear and nonlinear slopes of the FBG. Experiments with strong and relatively weak gratings confirm the wavelength-selectivity and stability of modulation. The fiber-based modulator is employed for optically tagging or labeling individual wavelength channels using baseband and amplitude-shift keying (ASK) modulated signals. The wavelength-selective channel labeling scheme is useful for... 

Instability of the interface between two immiscible fluids representing the so-called Kelvin-Helmholtz instability problem is studied using smoothed particle hydrodynamics method. Interfacial tension is included, and the fluids are assumed to be inviscid. The time evolution of interfaces is obtained for two low Richardson numbers Ri=0.01 and Ri=0.1 while Bond number varies between zero and infinity. This study focuses on the effect of Bond and Richardson numbers on secondary instability of a two-dimensional shear layer. A brief theoretical discussion is given concerning the linear early time regime followed by numerical investigation of the growth of secondary waves on the main billow.... 

In this paper, a pressure splitting formulation is proposed for Weakly Compressible SPH (WC-SPH) method and its capability in the suppression of the spurious oscillations is studied by conducting a stability analysis. The proposed formulation is implemented within the framework of a consistent SPH method. The predictions from the theoretical analysis are verified by the results of numerical test-cases. This method is applied to the incompressible fluid flow around periodic array of circular cylinders. The accuracy and the convergence of the results are investigated for benchmark problems. The results are also compared with those of the conventional WC-SPH method. In a similar test-case, the... 

Discretization of spatial derivatives is an important issue in meshfree methods especially when the derivative terms contain non-linear coefficients. In this paper, various methods used for discretization of second-order spatial derivatives are investigated in the context of Smoothed Particle Hydrodynamics. Three popular forms (i.e."double summation","second-order kernel derivation", and"difference scheme") are studied using one-dimensional unsteady heat conduction equation. To assess these schemes, transient response to a step function initial condition is considered. Due to parabolic nature of the heat equation, one can expect smooth and monotone solutions. It is shown, however in this... 

In this paper, a modified finite-volume Eulerian-Lagrangian localized adjoint method (FVELLAM) extended for convection-diffusion problems with a non-linear flux function is introduced. Tracking schemes are discussed using viscous Burgers' equation. It is shown that in order to have smooth results, only the new time level values should be used in tracking process. Then, the proposed method is employed to study immiscible incompressible two-phase flows in porous media. Various one- and two-dimensional test cases involving internal sources and sinks are solved and accuracy of solution and performance of the method are investigated by comparing the results obtained using FVELLAM with those of... 

The Smoothed Particle Hydrodynamics (SPH) method is extended to solve magnetostatic problems involving magnetically interacting solid bodies. In order to deal with the jump in the magnetic permeability at a fluid-solid interface, a consistent SPH scheme is utilized and a modified formulation is proposed to calculate the magnetic force density along the interface. The results of the magnetostatic solver are verified against those of the finite element method. The governing fluid flow equations are discretized using the same SPH scheme, developing an efficient method for simulating the motion of paramagnetic solid bodies in a fluid flow. The proposed algorithm is applied to a benchmark problem... 

A direct numerical simulation approach is utilized to understand the oscillatory shear rheology of a confined suspension of magnetic chains formed by paramagnetic circular cylinders under the influence of an external magnetic field. The common assumption of gap-spanning chains made in the literature is relaxed in this work, so that a fully suspended (periodic) array of magnetic chains is formed. In this sense, the effective rheological parameters are only influenced through a layer of fluid adjacent to the walls. All tests are conducted at very low but finite particle Reynolds numbers, and typical inertial effects are discussed. The main aim of the present study is to investigate the... 

Adirect numerical simulation approach is used to investigate the effective non-linear viscoelastic stress response of non-gap-spanning magnetic chains suspended in a Newtonian fluid. The suspension is confined in a channel and the suspended clusters are formed under the influence of a constant external magnetic field. Large amplitude oscillatory shear (LAOS) tests are conducted to study the non-linear rheology of the system. The effect of inertia on the intensity of non-linearities is discussed for both magnetic and non-magnetic cases. By conducting magnetic sweep tests, the intensity and quality of the non-linear stress response are studied as a function of the strength of the external... 

Adsorption ability of prepared magnetic nanoparticles supported on activated carbon (AC-MNPs) was evaluated to synthesize an efficient and a low cost adsorbent for removal of Pb(II) and Cr(VI) ions from single and binary component aqueous solutions in the presence of salinity. Magnetic adsorbent was prepared by co-precipitation over activated carbon derived from almond shell by physical activation method. AC-MNPs was modified by oxygen containing functional groups to enhance the adsorption capacity of adsorbent. XRD, XPS, BET, Boehm, TEM, FT-IR, DLS and XRF were used to characterize the AC@Fe3O4@SiO2-NH2-COOH. Characterization analyses indicated the high dispersion of Fe3O4 crystallites on...