Sharif Digital Repository

Electroosmotic flow of power-law fluids in the presence of pressure gradient through a slit is analyzed. After numerically solving the Poisson-Boltzmann equation, the momentum equation with electroosmotic body force is solved through an iterative numerical procedure for both favorable and adverse pressure gradients. The results reveal that, in case of pressure assisted flow, shear-thinning fluids reach higher velocity magnitudes compared with shear-thickening fluids, whereas the opposite is true when an adverse pressure gradient is applied. The Poiseuille number is found to be an increasing function of the dimensionless Debye-Hückel parameter, the wall zeta potential, and the flow behavior... 

In this article, 3D modeling and simulation of bubbling fluidized beds has been conducted using various drag models, and the model predictions were validated against reported experimental data and 2D simulation results. In this regard, different drag models reported in the literature including Gidaspow, Syamlal-O'Brien, Hill-Koch-Ladd, and Wen-Yu were applied. A standard Two-Fluid Model (TFM) closed by the Kinetic Theory of Granular Flows (KTGF) was used to simulate bubbling gas-solid fluidized beds. Excellent agreements between the simulation results and experimental data, concerning bed expansion ratio, gas volume fraction, and time-averaged particles velocity, were found over a wide range... 

High bed-shear stress resulting from secondary flows and velocity increase at a channel junction cause local erosion and deposition in natural rivers and earth-lined channels. Herein, the scour patterns at the junction of two loose bed channels were studied under clear-water conditions. The main dimensionless variables are the angle between the two approach flow branches, the discharge and width ratios of the tributary to the downstream channel branches, and the ratio of the mean downstream velocity to the threshold velocity. The results indicate that the position of the maximum scour depth temporally moves to the outer wall and upstream to the main channel, as affected by the dimensionless... 

The effect of an intermediate standing baffle on the flow structure in a rectangular open channel has been investigated by a three-dimensional acoustic Doppler velocimeter. Investigation of time-averaged velocity profiles at different streamwise positions reveals that the approach flow is fully developed upstream of the baffle. By analysing the space-averaged power spectra of streamwise velocity, a peak structure was observed in the upstream baffle region. Downstream of the baffle this peak structure has been alleviated by the baffle. The same analysis for the vertical component indicates the existence of a peak structure both up- and downstream of the baffle. Consequently, a baffle affects... 

In this study, the fundamental problem of unsteady blood flow in a tube with multi-stenosis is studied. An appropriate shape of the time-dependent multi-stenosis which is overlapping in the realm of formation of arterial narrowing is constructed mathematically. Blood is considered as a viscoelastic fluid characterized by the Oldroyd-B model. For the numerical solution of the problem, which is described by a coupled, non-linear system of partial differential equations (PDEs), with appropriate boundary conditions, the finite difference scheme is adopted. The solution is obtained by the development of an efficient numerical methodology based on the predictor-corrector method. The effects of... 

In this communication, a hybrid model based on Least Square Support Vector Machine (LSSVM) was constructed to predict CO2 corrosion rate. The input parameters of the model are temperature, CO2 partial pressure, flow velocity and pH. The data used for training and testing of the developed model are 612 and 109 data, respectively. In order to benefit LSSVM from Kernel learning, we compared three kernel functions to select the most efficient one. Furthermore, Coupled Simulated Annealing (CSA) optimization technique was adapted to choose the best optimal values of the model parameters. The results elucidate that Gaussian Kernel functions is the desired function which can afford high accuracy for... 

Experimental results of detailed flow measurements using an Acoustic-Doppler Velocimeter (ADV) around a complex bridge pier (CBP) are presented. The pier consists of a column, a pile cap (PC) and a 2×4 pile group. The time-averaged velocities, turbulence intensities, and Reynolds stresses are studied and presented at different horizontal and vertical planes. Streamlines obtained from the velocity fields are used to show the complexity of the flow around the pier. It is shown that the main feature of the flow responsible for the entrainment of the bed sediments is a contracted (pressurized) flow below the PC toward the piles. A deflected flow around the PC and a strong down-flow along its... 

In this paper, the dynamic instability of a nonlinear system has been studied using the stochastic vibration analysis and employing statistical properties of the system response. In this method neither the time domain analysis nor limit cycle oscillations were used. A two degrees-of-freedom airfoil subjected to an aerodynamic quasi-steady flow with a nonlinear torsional spring was considered as the case study. The spring nonlinearity was examined in hardening and softening states. A random force in the form of the white noise with Gaussian function was added to the aerodynamic lift force. The statistical linearization and random vibration analysis were applied to the nonlinear system to... 

In this article, the behavior of gas bubbles in tapered fluidized beds is investigated with the use of a two-fluid model incorporating kinetic theory of granular flow. The effects of various parameters such as apex angle, particle size, and particle density on the size distribution and the rise velocity of gas bubbles were examined. In addition, the simulation results for the bubble fraction and axial velocity of gas bubbles were compared with experimental data reported in the literature and good agreement was observed. As the apex angle was increased, the fraction of gas bubbles with large sizes increased and the fraction of bubbles with small sizes decreased. As the particle size... 

An efficient two-dimensional reduced-order hydroelastic model for studying supercavitation phenomenonwith zero cavitation number is proposed. In order to compute fluid eigenmodes, unsteady hydrodynamic model is derived using the finite-element method along with the potential flow assumption. This model takes advantage of a new real time, direct algorithm to compute the pressure distribution around the hydrofoil, which avoids any iterative scheme to find cavity extent as like as conventional method. The present approach starts by specifying the steady cavitation domain for the zero cavitation number, then, it is assumed that unsteady cavitation flow around the steady-state leads to small... 

In the present study, dynamic stability of a viscoelastic cantilevered pipe conveying fluid which fluctuates harmonically about a mean flow velocity is considered; while the fluid flow is exhausted through an inclined end nozzle. The Euler-Bernoulli beam theory is used to model the pipe and fluid flow effects are modelled as a distributed load along the pipe which contains the inertia, Coriolis, centrifugal and induced pulsating fluid flow forces. Moreover, the end nozzle is modelled as a follower force which couples bending vibrations with torsional ones. The extended Hamilton's principle and the Galerkin method are used to derive the bending-torsional equations of motion. The coupled... 

By employing the nonlocal continuum field theory of Eringen and Von Karman nonlinear strains, this paper presents an analytical model for linear and nonlinear dynamics analysis of single-walled carbon nanotubes (SWNTs) conveying fluid with different boundary conditions. In the linear analysis the natural frequencies and critical flow velocities of SWNTs are computed. However, in the nonlinear analysis the effect of nonlocal parameter on nonlinear dynamics of cantilevered SWNTs conveying fluid is investigated by using bifurcation diagram, phase plane and Poincare map. Numerical results confirm existence of chaos as well as a period-doubling transition to chaos. Copyright © 2018 Techno-Press,... 

In this article, the behavior of gas bubbles in tapered fluidized beds is investigated with the use of a two-fluid model incorporating kinetic theory of granular flow. The effects of various parameters such as apex angle, particle size, and particle density on the size distribution and the rise velocity of gas bubbles were examined. In addition, the simulation results for the bubble fraction and axial velocity of gas bubbles were compared with experimental data reported in the literature and good agreement was observed. As the apex angle was increased, the fraction of gas bubbles with large sizes increased and the fraction of bubbles with small sizes decreased. As the particle size... 

The Eulerian methods are susceptible to generate the nonphysical spurious currents in the multiphase flow simulations near the interfaces. This paper presents a new Eulerian method to accurately simulate the velocity fields, especially near the multiphase flow interfaces and prevents the numerical results from generating the nonphysical currents. A Eulerian central difference finite-volume scheme equipped with the suitable numerical dissipation terms is used to simulate incompressible multiphase flows. The interface is captured by Flux Corrected Transport-Volume of Fluid method (FCT-VOF). Increasing the accuracy near the sharp gradients, such as interface, the conservative form of... 

Dense underflows are continuous currents that move downslope due to their density being heavier than that of the ambient water. In this work, a steady density current with a uniform velocity and concentration from a narrow sluice gate enters into a wide channel of lighter ambient fluid and moves forward downslope. Experiments varying inlet velocity and concentration and hence inlet Richardson numbers were conducted. Numerical simulations were also performed with a low-Reynolds number k-ε model. The results of numerical simulation agree well with the experimental data. © 2009 ASCE  

In the present study, shock-disturbances interaction in high-speed compressible inviscid flow is simulated utilizing the weighted essentially non-oscillatory (WENO) scheme by employing the shock-capturing technique. For this aim, the two-dimensional Euler equations in strong conservative form are discretized by using the explicit third-order TVD Runge–Kutta scheme in time and the fifth-order WENO finite difference scheme in space. The main advantage of using the WENO scheme is its capability for properly solving the discontinuities in the domain without needing any artificial viscosity, limiter function or filter. Hence, this scheme is stable, and thus, it is suitable for simulating very... 

Ignition process in a premixed methane-air swirl configuration is studied using a large eddy simulation method with Smagorinsky sub-grid scale model. A developed thickened flame combustion approach with two-step methane-air mechanism is used. Non-reacting mean and RMS axial, tangential and radial velocity profiles are validated against the experimental results. It is shown that the flow field consists of four zones: Inner Recirculation Zone, Inner Shear Layer, Outer Shear Layer and Corner Recirculation Zone. The mean and RMS of velocities and temperature in reacting flow are then validated against the experimental data. Large eddy simulation is used to investigate the ignition sequence by... 

Studying the dynamic behavior of droplets is of great importance in the electrowetting phenomena. However, despite the widespread use of non-Newtonian fluids in industry and daily life including medicine, food, petroleum, environmental biomass, and lab on a chip, most studies have focused on Newtonian fluids. In this study, a power-law fluid is considered as a typical example of non-Newtonian fluids and its dynamic behavior is investigated within a microchannel, and the results are compared with those of the Newtonian fluids. Both the grooved and non-grooved substrates are considered. For this purpose, the governing equations for the two phase fluid flow are solved using the finite element... 

Precipitation (P), plant water use, and evaporation from the soil surface control the travel time of streamflow (Q) and evapotranspiration (ET) in a complex way. However, the impact of soil moisture and energy availability on the travel time distribution (TTD) of evaporated and transpired waters are yet less understood. In this study, we investigate how the seasonal variability of P and ET in terms of phase shift and rate influences the temporal dynamics of TTDs. To this end, we choose four contrasting climate types described as in-phase P and ET, out-of-phase P and ET, year-round constant P with seasonal ET, and year-round constant ET with seasonal P. We use a physically-based hydrological... 

This paper investigates the nonlinear dynamic response of a viscoelastic pipe conveying fluid subjected to a uniform external cross flow based on the Euler-Bernoulli theory. The main objective of this work is to find the proper viscoelastic coefficients to mitigate the dynamic response of a marine riser. A nonlinear oscillator is utilized to simulate the mean drag force and the vortex-induced lift force. Also, the pipe material is assumed to be viscoelastic and consisted of the Kelvin-Voigt type. The extended Hamilton's principle along with the Galerkin discretization are employed to construct the nonlinear model of the coupled fluid-structure system. Moreover, the assumed mode method along...