Sharif Digital Repository

In this paper Time Splitting Projection Method (TSPM) has been implemented to model transient flow in a deformable pipe. In addition a quasi-steady model and the Brunone unsteady model have been incorporated into the algorithm to consider the effect of static and dynamic friction on the shape and attenuation of pressure waves. The deformation of the cross section of the pipe due to the change of pressure has been considered using Hooke's Law of elasticity. Numerical results of the model have been compared with a simple analytical test and results of experimental measurements for water hammer problem with different Reynolds numbers in a simple pipeline system; besides, numerical stability... 

A series of experiments were carried out to investigate unsteady behavior of the flow field as well as the boundary layer of an airfoil oscillating in plunging-type motion in a subsonic wind tunnel. The measurements involved surface-mounted hot films complimented with surface pressure. In addition, wind tunnel wall pressure distribution was acquired to furnish a baseline for the wall interference corrections. The airfoil is the section of a 660-kW wind turbine blade. The experiments were conducted at a Reynolds number of 0.42 million, and over two reduced frequencies of k = 0.06 and 0.085, at prestall, nearstall, and poststall regions. The unsteady aerodynamic loads were calculated from the... 

Numerical investigation on the application of nanofluids in Micro-Pin-Fin Heat Sinks (MPFHSs) has been presented in this paper. To investigate flow and heat transfer behavior in MPFHS the three-dimensional steady Navier-Stokes and energy equations were discretized using a finite volume approach and have been solved iteratively, using the SIMPLE algorithm. DI-water is used as a base coolant fluid while the nanoparticles used in the present study are CuO nanoparticles with mean diameters of 28.6 and 29 nm and Al 2O 3 nanoparticles with mean diameters of 38.4 and 47 nm. The results show that (i) a significant enhancement of heat transfer in the MPFHS due to suspension of CuO orAl 2O 3... 

In this article, NO production mechanisms for CH 4-H 2 combustion under MILD (moderate or intense low-oxygen dilution) conditions are studied using CDF and also zero-dimensional well stirred reactor (WSR) analysis. A H 2/CH 4 jet into a heated and diluted coflow is modeled in CFD analysis. The RANS equations with modified k equations are solved in an axisymmetric 2D computational domain. The GRI2.11 full mechanism is considered to represent the chemical reactions. The effects of oxidizer oxygen concentration, fuel hydrogen content, and fuel jet Reynolds number are studied on NO formation reactions. Results show that the measurements are predicted with an acceptable accuracy. The NNH and N 2O... 

Effect of using Al 2O 3-water nanofluids with different volume fractions and particle diameters on generated entropy, hydrodynamic performance and heat transfer characteristics of a tangential micro-heat sink (TMHS) was numerically investigated in this research. Results indicated that considerable heat transfer enhancement is possible when using Al 2O 3-water nanofluids as coolant and clearly the enhancement improves with increasing particles concentration and decreasing particles size. However, using nanofluid has also induced drastic effects on the pumping power that increases with particles volume fraction and Reynolds number. Finally, it was found that generated total entropy decreases... 

We investigate the stability of the pressure-driven, low-Reynolds-number flow of Brownian suspensions with spherical particles in microchannels. We find two general families of stable/unstable modes: (i) degenerate modes with symmetric and antisymmetric patterns; (ii) single modes that are either symmetric or antisymmetric. The concentration profiles of degenerate modes have strong peaks near the channel walls, while single modes diminish there. Once excited, both families would be detectable through high-speed imaging. We find that unstable modes occur in concentrated suspensions whose velocity profiles are sufficiently flattened near the channel centreline. The patterns of growing unstable... 

Particle transport and deposition in a channel flow with elliptic obstruction is studied. Numerical simulation of fluid flow is performed using two-dimensional lattice Boltzmann method, while one-way coupling Lagrangian method for particle tracking is used. Standard particles are injected in the inlet of the channel. Gravity, Drag force, Brownian forces, and the Saffman lift are considered in equation of particle motion. The influence of geometrical parameter, ellipse aspect ratio, is studied on dispersion and deposition of particles as well as the flow parameters, such as Reynolds number. In addition, the effect of particles size -particles of 0.01-10μm in diameter- on dispersion and... 

We use the detrended fluctuation analysis (DFA), the detrended cross correlation analysis (DCCA) and the magnitude and sign decomposition analysis to study the fluctuations in the turbulent time series and to probe long-term nonlinear levels of complexity in weakly and high turbulent flow. The DFA analysis indicate that there is a time scaling region in the fluctuation function, segregating regimes with different scaling exponents. We discuss that this time scaling region is related to inertial range in turbulent flows. The DCCA exponent implies the presence of power-law cross correlations. In addition, we conclude its multifractality for high Reynold's number in inertial range. Further, we... 

This work simulates the turbulent boundary layer of an incompressible viscous swirling flow through a conical chamber. To model the pressure gradient normal to the wall, the radial and tangential velocity components across the boundary layer have been calculated by both the integral and numerical methods. The numerical solution is accomplished by finite difference, based on the finite volume method. The results show that the radial and tangential boundary layer thicknesses depend on the velocity ratios, Reynolds number and nozzle angle. The peak of radial and tangential boundary layer thicknesses are located at zL≈0.2 and zL≈0.8 from the nozzle inlet, respectively. Due to the short length of... 

This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions. It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer. The wind tunnel measurements involved surface-mounted hot-film sensors and boundary-layer rake. The experiments were conducted at Reynolds numbers of 0.42×10 6 to 0.84 × 10 6 and the reduced frequency was varied from 0.01 to 0.11. The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases. For... 

The use of more efficient energy consuming devices, which are closely associated with reduction of environmental pollution, has gained significant interest in the recent decades. The reduction of drag coefficient also improves safety and durability of environmental structures subjected to high-velocity fluid flow, and causes the noise and vibration to decrease as well. This paper describes the efficiency improvement in energy management by means of reducing drag coefficient in a practical divergent tubular- truncated cone. Extensive numerical simulations with emphasis on the shape optimization study were performed in order to find minimum drag coefficient for both laminar and turbulent flows... 

Generally speaking, most micro-fluidic mixing systems are limited to the low Reynolds number regime in which diffusion dominates convection, and consequently the mixing process tends to be slow and it takes a relatively long time to have two fluids completely mixed. Therefore, rapid mixing is essential in micro-fluidic systems. In order to hasten the mixing process in micro scale, in this study we come up with a novel scheme for a two dimensional micro-fluidic mixer which encompasses three pairs of electrodes, one pair embedded in the mixing chamber and two pairs located in the micro-channels before and after the mixing chamber. The width of the middle pair is assumed to be twice of the... 

The nonequilibrium molecular dynamics (NEMD) simulations are performed to calculation the cross drag over a nanotube located in a uniform liquid argon flow. As is known, the behavior of fluid flows in nano-scale sizes is very different from that in microscopic and macroscopic sizes. In this work, our concern is on the flow of argon molecules over a nanotube which occurs in nanoscale sizes. We calculate the cross drag enforced the nanotube at Re<10. In this regard, we use the molecular dynamics and simulate the flow of argon molecules over (6,0), (8,0) and (10,0) nanotubes. The simulations are performed at different velocities and the cross drag coefficient is computed at different Reynolds... 

A series of experimental and numerical investigations on two tandem cylinders wake have been studied. The velocity profile and turbulence intensity have been acquired by a single one dimensional Hot Wire anemometer. The two cylinders were mounted in a tandem manner in the horizontal mid plane of the working section. The effect of the upstream cylinder diameter, Reynolds number and the distance between the cylinders on the wake profile and turbulence intensity on the downstream cylinder was investigated, while the Reynolds number ranged between1.5× (10)∧4 ∼ 3×(10)∧4. The upstream cylinder diameter (d) was 10, 20 and 25 mm, while the downstream cylinder diameter (D) was 25 mm, corresponding to... 

An explicit weakly compressible SPH method is introduced to study movement of suspended solid bodies in Oldroyd-B fluid flows. The proposed formulation does not need further stabilizing treatments and can be efficiently employed to study particulate flows with Deborah to Reynolds number ratios up to around 10. A modified boundary treatment technique is also presented which helps to deal with the movement of solid particles in the flow. The technique is computationally efficient and gives an improved evaluation of fluid-solid interaction forces.A number of test cases are solved to show performance of the proposed method in simulating particulate viscoelastic flows containing circular and... 

There are different ways in order to achieve higher velocity in underwater vehicles. One of these methods is using a body with special form. This paper presents a towing tank based experimental study on drag forces for different Reynolds Numbers of a special underwater model. This paper investigates drag force and drag coefficient in a different flow direction over the model. Obtained experimental results in towing tank are explained. Computational Fluid Dynamic (CFD) simulation also is performed using commercial CFD software package FLUENT 6.3.26. There is a significant decrease in drag coefficient of model moving with small diameter at upstream  

For the first time since 1940 and presentation of theodorson's theory, distribution of thrust, torque and efficiency along the blade of a counter rotating propeller axial fan was studied with a novel method in this research. A constant chord, constant pitch symmetric fan was investigated with Reynolds Stress Turbulence method in this project and H.E.S. method was utilized to obtain distribution profiles from C.F.D. tests outcome. C.F.D. test results were validated by estimation from Playlic's analytical method. Final results proved ability of H.E.S. method to obtain distribution profiles from C.F.D test results and demonstrated interesting facts about effects of solidity and differences... 

A series of low speed wind tunnel tests were conducted on a section of a 660 kW wind turbine blade to examine the effects of distributed surface contamination on its performance characteristics. The selected airfoil was tested with a clean surface, two types of zigzag roughness, strip tape roughness and distributed contamination roughness. The straight and zigzag leading edge roughness models simplify the contamination results in an early turbulence transition. In this study, surface contamination was simulated by applying 0.5 mm height roughness over the entire upper surface of the airfoil. The distribution density varied from the leading edge to the trailing edge of the model. Our data... 

This paper addresses the numerical simulation of thermo-fluid characteristics of triangular jets. The results of spatially developing, three dimensional jets from isosce-les and equilateral nozzles at different Reynolds numbers and distances between jets are presented. The system of governing equations, subject to the proper boundary condi-tions is solved with the finite volume method with collo-cated grid arrangement. SIMPLEC algorithm was used for the pressure-velocity coupling to discrete the governing equations of flow and energy. The turbulent stresses are approximated using k-ε model. The velocity and tempera-ture fields are presented and rates of their decay at jet cen-terline are...