Sharif Digital Repository

Compressible turbulent flow through the abrupt enlargement in pipes is studied numerically by means of Advection Upstream Splitting Method (AUSM+-up). In low Mach numbers, a pressure correction equation of elliptic type is derived. This equation is compatible with the nature of governing equations and retrieves hyperbolic characteristic at higher Mach numbers. It is shown that the proposed numerical algorithm is computationally more efficient than the preconditioned density-based methods. The flow parameters such as reattachment length, pressure loss coefficient and wall shear stress are predicted. It is found that the loss coefficient of the compressible flow rises drastically with... 

Several semi-analytical models are considered for a double-gyre problem in a turbulent flow regime for which a reference fully numerical eddy-resolving solution is obtained. The semi-analytical models correspond to solving the depth-averaged Navier–Stokes equations using the spectral Galerkin approach. The robustness of the linear and Smagorinsky eddy-viscosity models for turbulent diffusion approximation is investigated. To capture essential properties of the double-gyre configuration, such as the integral kinetic energy, the integral angular momentum, and the jet mean-flow distribution, an improved semi-analytical model is suggested that is inspired by the idea of scale decomposition... 

Results are presented from a numerical study examining the flow of a viscous, incompressible fluid through a random packing of non-overlapping spheres at moderate Reynolds numbers, spanning a wide range of flow conditions for porous media. By using a laminar model including inertial terms and assuming rough walls, numerical solutions of the Navier-Stokes equations in three-dimensional porous packed beds resulted in dimensionless pressure drops in excellent agreement with those reported in a previous study. This observation suggests that no transition to turbulence could occur in the range of the Reynolds number studied. For flows in the Forchheimer regime, numerical results are presented of... 

The present effort is the development of a multiscale modeling, simulation methodology for investigating complex phenomena arising from flowing fiber suspensions. Specific consideration was given to dynamic simulations of viscoelastic fibers in which the fluid flow is predicted by a method that is a hybrid between Direct Numerical Simulations (DNS) and Large Eddy Simulation techniques (LES), and fluid fibrous structure interactions (FSI) will be taken into account. Numerical results are presented for which focus is on fiber floc deformation by hydrodynamic forces in turbulent flows. Dynamics of simple fiber networks in a shearing flow of water in a channel flow illustrate that the... 

A mathematically rigorous, multiscale modeling methodology capable of coupling behaviors from the Kolmogorov turbulence scale through the full scale system in which a fiber suspension is flowing is presented. Here the key aspect is adaptive hierarchical modeling. Numerical results are presented focus of which are on fiber floe formation and destruction by hydrodynamic forces in turbulent flows. Specific consideration was given to molecular-dynamics simulations of viscoelastic fibers in which the fluid flow is predicted by a method which is a hybrid between direct numerical simulations and large eddy simulation techniques, and fluid fibrous structure interactions were taken into account. The... 

An appropriate combination of the thin-layer Navier-Stokes (TLNS) and parabolized Navier-Stokes (PNS) solvers is used to accurately and efficiently compute hypersonic transitional/turbulent flowfields of perfect gas and equilibrium air around blunt-body configurations. The TLNS equations are solved in the nose region to provide the initial data plane needed for the solution of the PNS equations. Then the PNS equations are employed to efficiently compute the flowfield for the afterbody region by using a space marching technique. Both the TLNS and the PNS equations are numerically solved by using the implicit non-iterative finite-difference algorithm of Beam and Warming. A shock fitting... 

Low-speed wind tunnel experiments were conducted to examine the effects of jet exit behavior on the near-field characteristics of jets in crossflow. To better understand this problem, a row of six square jets were perpendicularly injected into the main turbulent flow. The jet-to-crossflow velocity ratios examined were 0.25, 0.5 and 1.0, while the jet spacing to jet diameter was 3.0. No significant temperature differences between the jet and the crossflow were introduced. The analysis of the vertical structure of the transverse jets, including focusing on the jet shear layer and the vorticity dynamics of the exiting jets, is complicated. The vorticity around the circumference of the jets was... 

An effective numerical technique is presented to model turbulent motion of a standing surface wave in a tank. The equations of motion for turbulent boundary layers at the solid surfaces are coupled with the potential flow in the bulk of the fluid, and a mixed BEM-finite difference technique is used to model the wave motion and the corresponding boundary layer flow. A mixing-length theory is used for turbulence modelling. The model results are in good agreement with previous physical and numerical experiments. Although the technique is presented for a standing surface wave, it can be easily applied to other free surface problems. Copyright © 2005 John Wiley & Sons, Ltd  

Nowadays, increasing the CO2 emissions are one of the inevitable challenges in the world. In addition, in many industries, fossil fuels are the main source of energy demands which worsen the problem. Since in most applications, the performance and energy loss are highly affected by the drag force, many investigations have been proposed to improve the surface properties to moving surface against the water, and consequently, increasing drag reduction. To this end, one effective way is utilizing superhydrophobic coatings. In this study, we prepare two different superhydrophobic coatings using Al2O3 nanoparticles (NPs) modified by HMDS (1,1,1,3,3,3-Hexamethyldisilazane) and PDMS... 

Turbulent swirling decay pipe flow has been investigated numerically in a vertical straight fixed pipe. The swirling flow is created by means of a rotating honeycomb which produces the solid body rotation at the inlet of the fixed pipe. Since there are no experimental data at the inlet of the fixed pipe; different axi-symmetric approaches may be considered to model the honeycomb effects at the downstream flow. Considering the appropriate approach and using the resulting flow field properties from the exit of the modeled swirl generator which are applied as the inlet boundary condition for the fixed pipe, several high Reynolds turbulence models are used to predict this type of the swirling... 

Despite significant progress in unstructured grid generation and employment, the robust employment of body-conforming coordinate systems promotes the workers to consider it as a major alternative to treat relatively complex flow fields in irregular geometries. Contrary to the laminar flow treatment, there are many ambiguities around treating turbulent flow on body-fitted coordinate system. The ambiguities are mainly originated from the curvature on the boundaries. Considering the past taken efforts in improving the results of treating turbulent flow on curvilinear coordinate system, we have extended a new formulation on body-fitted coordinate system using physical covariant velocities as the... 

In this study, the fluid dynamics and convective heat transfer for turbulent flows through a 3-sub channel of a rod bundle, which is representative of those used in VVER-1000, are examined. The rod bundle is constructed from parallel rods in a hexagonal array. The rods are on constant pitch by spacer grids spaced axially along the rod bundle. The geometry details of the bundle and heat flux from the fuel rod are similar to that of the Iranian nuclear reactor under construction. A numerical study using Computational Fluid Dynamics (CFD) was carried out to estimate the flow field, pressure loss and heat transfer coefficients in spacer grids and rod bundles. Turbulence has been modeled using... 

Evaporation of polymeric solid fuels in backward facing step geometry subject to an inlet oxidizer flow at elevated temperatures is considered and convective heating of the fuel surface by the hot oxidizing inlet flow and subsequent mixing of the evaporated fuel with the oxidizer flow and its combustion is numerically studied. The objective of this work is to gain insight into the auto-ignition of the fuel and its controlling parameters in this configuration. The system of governing equations is solved with a finite volume approach using a structured grid in which the AUSM scheme is used to calculate the gas phase convective fluxes. The flowfield is turbulent and the SpalartAllmaras... 

In this paper, three-dimensional turbulent flow field around a complex bridge pier placed on a rough fixed bed is experimentally investigated. The complex pier foundation consists of a column, a pile cap, and a 2×4 pile group. All of the elements are exposed to the approaching flow. An acoustic-Doppler velocimeter was used to measure instantaneously the three components of the velocities at different horizontal and vertical planes. Profiles and contours of time-averaged velocity components, turbulent intensity components, turbulent kinetic energy, and Reynolds stresses, as well as velocity vectors are presented and discussed at different vertical and horizontal planes. The approaching... 

In this work, a supersonic turbulent flow over a long axisymmetric body was investigated, both experimentally and computationally. The experimental study consisted of a series of wind tunnel tests for the flow over an ogive-cylinder body at a Mach number of 1.6 and at a Reynolds number of 8 × 10 6, at angles of attack between -2 and 6 degrees. It included the surface static pressure and the boundary layer profile measurements. Further, the flow around the model was visualized using a Schlieren technique. All tests were conducted in the trisonic wind tunnel of the Qadr Research Center (QRC). Also, the same flow at zero angle of attack was computationally simulated using a multi-block grid...