Sharif Digital Repository

The dynamo effect is a class of macroscopic phenomena responsible for generating and maintaining magnetic fields in astrophysical bodies. It hinges on the hydrodynamic three-dimensional motion of conducting gases and plasmas that achieve high hydrodynamic and/or magnetic Reynolds numbers due to the large length scales involved. The existing laboratory experiments modeling dynamos are challenging and involve large apparatuses containing conducting fluids subject to fast helical flows. Here we propose that electronic solid-state materials - in particular, hydrodynamic metals - may serve as an alternative platform to observe some aspects of the dynamo effect. Motivated by recent experimental... 

In this study, the numerical simulation of the fluid flow and acoustic field of a supersonic jet is performed by using high-order discretization and the vorticity confinement (VC) method on coarse grids. The three-dimensional Navier-Stokes equations are considered in the generalized curvilinear coordinate system and the high-order compact finite-difference scheme is applied for the space discretization, and the time integration is performed by the fourth-order Runge-Kutta scheme. A low-pass high-order filter is applied to stabilize the numerical solution. The non-reflecting boundary conditions are adopted for all the free boundaries, and the Kirchhoff surface integration is utilized to... 

Cavity flameholder is known as an efficient technique for providing the ignition zone. In this research, computational fluid dynamic is applied to study the influence of the various shapes of cavity as flameholder on the mixing efficiency inside the scramjet. To evaluate different shapes of cavity flame holder, the Reynolds-averaged Navier–Stokes equations with (SST) turbulence model are solved to reveal the effect of significant parameters. The influence of trapezoidal, circle and rectangular cavity on fuel distribution is expansively analyzed. Moreover, the influence of various Mach numbers (M = 1.2, 2 and 3) on mixing rate and flow feature inside the cavity is examined. The comprehensive... 

The current study investigates the conjugate heat transfer characteristics for laminar flow in backward facing step channel. All of the channel walls are insulated except the lower thick wall under a constant temperature. The upper wall includes a insulated obstacle perpendicular to flow direction. The effect of obstacle height and location on the fluid flow and heat transfer are numerically explored for the Reynolds number in the range of 10 ≤ Re ≤ 300. Incompressible Navier-Stokes and thermal energy equations are solved simultaneously in fluid region by the upwind compact finite difference scheme based on flux-difference splitting in conjunction with artificial compressibility method. In... 

The purpose of this paper is to analyze the aerodynamic and stability characteristics of airfoils in extreme ground effect, where gradients of variables are intense. Camber line of NACA4412 is modified to become S-shaped, named M4412, which turns the passage shape between the ground and the lower surface of NACA4412 from convergent to convergent–divergent. Reynolds-averaged Navier–Stokes equations with Spalart–Allmaras turbulence model have been solved by applying the finite volume approach. Lift of M4412 is less than that of NACA4412. Divergent effect normally creates a negative lift zone. Reducing the height when angle of attack is lower than a specific angle strengthens the divergent... 

The network modeling approach is applied to provide a new insight into the onset of non-Darcy flow through porous media. The analytical solutions of one-dimensional Navier-Stokes equation in sinusoidal and conical converging/diverging throats are used to calculate the pressure drop/flow rate responses in the capillaries of the network. The analysis of flow in a single pore revealed that there are two different regions for the flow coefficient ratio as a function of the aspect ratio. It is found that the critical Reynolds number strongly depends on the pore geometrical properties including throat length, average aspect ratio, and average coordination number of the porous media, and an... 

The effect of a multiphase fluid, including an arbitrary number of liquid phases, and a functionally graded density fluid on the stability of rotating partially-filled cylindrical shells is investigated. The first-order shear shell theory is used for modeling the structural dynamics of the shell and a 2D model is introduced based on the Navier–Stokes equations, for fluid motion. The multiphase and the functionally graded density fluids are arranged according to the mass density in a steady state condition due to centrifugal forces. Using the boundary conditions between liquid phases and the boundary conditions of the fluid on the cylinder wall, the coupled fluid-structure system model is... 

Inlet performance has an important role in the operation of air-breathing propulsion systems. In this study, performance of a supersonic axisymmetric mixedcompression inlet in the supercritical operating condition is numerically studied. The effects of free-stream Mach number and engine-face pressure on performance parameters, including mass flow ratio, drag coefficient, total pressure recovery, and flow distortion, are investigated. For this sake, a multi-block density-based finite volume CFD code is developed, and Reynolds-averaged Navier-Stokes equations with Spalart-Allmaras oneequation turbulence model are employed. The code is validated by comparing numerical results against other... 

In this work, coupled Cahn–Hilliard phase field and Navier–Stokes equations were solved using finite element method to address the effects of micro-fracture and its characterizations on water-oil displacements in a heterogeneous porous medium. Sensitivity studies at a wide range of viscosity ratios (M) and capillary numbers (Ca), and the resultant log Ca–log M stability phase diagram, revealed that in both media, with/without fracture, the three regimes of viscous fingering, capillary fingering and stable displacement similarly occur. However, presence of the fracture caused water channeling phenomenon which resulted in reduction of the number of active fingers and hence the final oil... 

In the present study, the preconditioned incompressible Navier-Stokes equations with the artificial compressibility method formulated in the generalized curvilinear coordinates are numerically solved by using a high-order compact finite-difference scheme for accurately and efficiently computing the incompressible flows in a wide range of Reynolds numbers. A fourth-order compact finite-difference scheme is utilized to accurately discretize the spatial derivative terms of the governing equations, and the time integration is carried out based on the dual time-stepping method. The capability of the proposed solution methodology for the computations of the steady and unsteady incompressible... 

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... 

This study investigates GH2/LOX coaxial jet flame at trans- and supercritical conditions using the Reynolds averaged Navier-Stokes approach. Four two-equation-turbulence models, three real equation of states, two chemical mechanisms, and three different chamber pressures are examined. Predictions show good agreement with measurements qualitatively and quantitatively. Based on the results, the predictions of the Soave-Redlich-Kwong equation of state (EOS) are closer to the experiment, while the Aungier-Redlich-Kwong EOS has more deviation than the others. Moreover, the k-ω shear stress transport model has better performance than the other turbulence models. It is also found that the flow... 

The accuracy of Reynolds Averaged Navier-Stokes (RANS) turbulence models to predict the behavior of 2-D density currents has been examined. In this work, a steady density current is simulated by the k - ε, k - ε RNG, two-layer k - ε and modified v̄2 - f model, all of which are compared with the experimental data. Density currents, with a uniform velocity and concentration, enter a channel via a sluice gate into a lighter ambient fluid and move forward down-slope. The eddy-viscosity concept cannot accurately simulate this flow because of two stress production structures found within it. Results show that all isotropic models have a weak outcome on this current, but by improving the ability of... 

This paper presents the homotopy series solution of the Navier-Stokes and energy equations for non-Newtonian flows. Three different problems, Couette flow, Poiseuille flow and Couette-Poiseuille flow have been investigated. For all three cases, the nonlinear momentum and energy equations have been solved using the homotopy method and analytical approximations for the velocity and the temperature distribution have been obtained. The current results agree well with those obtained by the homotopy perturbation method derived by Siddiqui et al (2008 Chaos Solitons Fractals36 182-92). In addition to providing analytical solutions, this paper draws attention to interesting physical phenomena... 

We numerically solve the Navier-Stokes equations to study the rarefied gas flow in short micro-and nanoscale channels. The inlet boundary conditions play a critical role in the structure of flow in short channels. Contrary to the classical inlet boundary conditions, which apply uniform velocity and temperature profiles right at the real channel inlet, we apply the same inlet boundary conditions, but at a fictitious position far upstream of the real channel inlet. A constant wall temperature incorporated with suitable temperature jump is applied at the channel walls. Our solutions for both the classical and extended inlet boundary conditions are compared with the results of other available... 

The 3D simulation of the hydrodynamic behavior of a rotating disc contactor (RDC) by means of computational fluid dynamics (CFD) was investigated for the n-butanol-succinic acid-water (BSW) system. For the two-phase liquid-liquid flow, the velocity distribution of the continuous phase and drop size distributions were determined using the k-ω turbulence model in conjunction with the Eulerian-Eulerian approach and MUSIG model. In this system in which the holdup of the dispersed phase is low, the continuous phase velocity was computed by simultaneously solving the Navier-Stokes equations beside the different models of turbulence. The motions of the dispersed phase was calculated while... 

This paper deals with the determination of free vibration characteristics and instability conditions of flexible spinning cylinders partially filled with fluid. Using the linearized Navier-Stokes equations for the incompressible, inviscid flow, a 2D model is developed for fluid motion at each section of the cylinder. The forces exerted on the cylinder wall as a result of the fluid motion are calculated as functions of lateral acceleration of the cylinder axis in the Laplace domain. Applying the Hamilton principle, the governing equations of flexural motion of the cylinder are derived and then combined with the equations describing the fluid forces to obtain the coupled field equations of the...