Sharif Digital Repository

This study aims to assess the impact of the spacing between cylinders on their dynamic behaviour. Observations during this experiment have helped to identify the effect of spacing on excitation mechanism of each cylinder as well as establishing the relationship between cylinders response and the spacing between them. Arrays of cylindrical Structures in close proximity are common in variety of engineering structures, particularly in the offshore industry. If the cylinders are flexible, and are subject to excitation from fluid flow, then the dynamics is influenced by complex interactions between the cylinders, dependent on a variety of parameters. Two cylinders were flexibly mounted in tandem... 

In this paper we will present some approaches on Reynolds stress production by vortex transport phenomena and nonlinear voracity generation in momentum equation. First of all we represent a history of recent works to describe how fluid particle motions can be associated with Reynolds stress through either displacement or acceleration terms. In the next section we will describe how vortex stretching causes the Reynolds stress production and what is the dominant effect near and far from the boundary where viscous effects have to be considered. On the other hand, some vortex considered methodologies such as those synthesize boundary layer, as a collection of vortical objects seem to be... 

In this paper we will present some approaches on Reynolds stress production by vortex transport phenomena and nonlinear vorticity generation in momentum equation. First of all we represent a history of recent works to describe how fluid particle motions can be associated with Reynolds stress through either displacement or acceleration terms. In the next section we will describe how vortex stretching causes the Reynolds stress production and what is the dominant effect near and far from the boundary where viscous effects have to be considered. On the other hand, some vortex considered methodologies such as those synthesize boundary layer, as a collection of vortical objects seem to be... 

Experimental hydrodynamics imaging of four Pangasius sanitwongsei were considered. A quantitative characterization of caudal fin is presented in this article. Steady swimming of four P. sanitwongsei with different total length was studied experimentally and taped by high-speed digital video, and undulatory movement of each fish at different velocity was revealed. The pattern of body undulatory movement of the fish was drawn from the video images. Three main factors that determine the fish swimming behavior are Reynolds number, Strouhal number and shape. In this study, Lf/L was chosen as a characteristic of shape, where Lf was the distance from the start of the head to the end of the head.... 

We design and simulate the motion of a swimmer, the Quadroar, with three-dimensional translation and reorientation capabilities in low-Reynolds-number conditions. The Quadroar is composed of an I-shaped frame whose body link is a simple linear actuator and four disks that can rotate about the axes of flange links. The time symmetry is broken by a combination of disk rotations and the one-dimensional expansion or contraction of the body link. The Quadroar propels on forward and transverse straight lines and performs full three-dimensional reorientation maneuvers, which enable it to swim along arbitrary trajectories. We find continuous operation modes that propel the swimmer on planar and... 

This research deals with the analysis of approximate similitude between the dynamic similar models and the full-scale prototype of an aircraft. Due to physical and technical constraints, a full dynamic similarity is not practically possible and previous works have all neglected one or two similarity criteria like Mach or Reynolds numbers for the sake of Froude number similarity. In this work, it is shown that Mach number has an important effect on aerodynamic characteristics and dynamic response of an aircraft and that neglecting it makes the generalization of the scale-model test data invalid for the full-scale prototype. In order to address this problem, a measurable quantity named... 

Mixing has become a challenge in micro-fluidic systems because of the low Reynolds number in micro-channels. The method which is implemented in this paper is to use freely-swimming bacteria to enhance the mixing process. Accordingly, the Serratia marcescens bacteria were used for this matter. The mixing performance of the system is quantified by measuring the diffusion rate of Rhodamine B in a particular section of a channel connected to a chamber with varying Rhodamine B concentration. The concentration of Rhodamine B was measured using the Laser Induced Fluorescence (LIF) technique. The channel is in the form of a pipe and is closed on the extending side. In this paper, it is demonstrated... 

The unsteady preconditioned characteristic boundary conditions (UPCBCs) based on the artificial compressibility (AC) method are formulated and applied at artificial boundaries for the direct numerical simulation (DNS) of incompressible flows. The compatibility equations including the unsteady terms are mathematically derived in the generalized curvilinear coordinates and then incorporated as boundary conditions (BCs) in a high-order accurate incompressible flowsolver. The spatial derivative terms of the systemof equations are discretized using the fourth-order compact finite difference (FD) scheme, consistent with the high-order accuracy required for the DNS. The time integration is carried... 

The aim of this study is to investigate the uniform cooling of a hot isothermal heated target surface, using four turbulent impinging air jets. Eight parameters including the width of jets, the space between the inner jets, the space between inner and outer jets, the distance of jets from the plate, the impingement angle of jets, and the overall volumetric flow rate of the cooling air per unit depth of the nozzle are considered as design variables. The normalized standard deviation of the local Nusselt number from the desired Nusselt number is considered as the objective function. An optimization algorithm based on pattern search method is utilized to obtain the optimum array of the jets.... 

In this work, a solution is applied to investigate the heat transfer characteristics in a pipe with turbulent decaying swirling flow by using the boundary layer integral scheme. The governing equation is solved using the forth-order Runge-Kutta scheme resulting in thermal boundary-layer thickness and dimensionless heat transfer coefficient, namely, the Nusselt number. Both forced- and free-vortex profiles are considered for the tangential velocity component. A comparison of the results obtained for the Nusselt number with available experimental data shows that this scheme has good capability in predicting the heat transfer parameters of swirling flow especially in the entrance region of a... 

In this article, velocity field and settling distribution of microparticles in a dilute suspension in low-Reynolds-number Poiseuille flow in a microchannel is experimentally investigated using microscopic image analysis. An effective technique is applied to manipulate single-particle tracking in order to determine the controlling parameters on transportation and settling of microparticles in microchannels. The results show that the velocities of dispersed phase are affected by the hydrodynamic properties, and this velocity deviation can be significant when the hydrodynamic coupling between particles and channel walls is considerable. Increasing the Reynolds number would result in decrease in... 

Transitional boundary layer over an airfoil in the combined pitch-plunge oscillating motions at low Reynolds number is experimentally investigated. The study is based on the data obtained from both surface hot-film and surface static pressure. Different oscillation zones prior to, within, and beyond the static stall angle of attack of the airfoil are considered. The hysteresis loops for the laminar separation bubble, transition, relaminarization points, as well as the airfoil lift coefficient are investigated and are compared for different types of dynamic motions. Two different types of combined pitch-plunge motions, constructive and destructive, are considered. In the constructive motion,... 

This paper presents the mathematical modeling of single-phase natural circulation of the University of Genoa's rectangular loop (LOOP#1) by a computer program and using RELAP5 system code. The mass, momentum and energy conservation equations in transient form were solved numerically using the finite difference method. One-dimensional linear stability analysis was performed for the single-phase natural circulation loop and the numerical perturbation technique was used in this analysis. The Nyquist criterion was employed to find the stability map of the LOOP#1. The obtained transient results using the first order upwind scheme of the fluid temperatures in various sectors of the LOOP#1 are... 

In this article, the transesterification of waste cooking oil (WCO) to biodiesel fuel (BDF) has been studied using KOH loaded on millimetric γ-Al2O3 particles in a novel type of two-impinging-jets reactor (TIJR). The effects of various parameters such as KOH loading (wt %), catalyst loading, and methanol-to-oil molar ratio on the BDF yield were studied. The catalyst particles with a KOH loading of 25 wt % at appropriate reaction conditions (i.e., catalyst loading = 4 wt % and methanol-to-oil molar ratio = 15:1) and at 65 °C were used in the TIJR. The influences of various operating and design parameters such as jet Reynolds number value, feed flow rate, jet diameter, and the internozzle... 

Dynamic similar models are designed to study the flight behavior of the full-scale aircraft in early design stages. Due to physical and operational constraints, full dynamic similarity between the scaled-down model and full-scale aircraft is not feasible. Thus, the scale model would be flying at different Reynolds number and Mach number. A given aircraft configuration with specific aerodynamic characteristics will have different performance if Mach number and Reynolds number are changed considerably, which results in different dynamic behavior of the scale model. To compensate for these dissimilarities, it is proposed to modify the airfoil geometry of the scale model to preserve aerodynamic... 

Underwater surface of man-made structures is quickly covered by unwanted aquatic organisms. They change the flow regime around the structure. Despite the important contributions of previous studies on the vortex induced vibration (VIV) of bluff bodies, the flow induced vibration of non-stationary cylinders covered by marine fouling appears not to have received due attentions in the literature. The current paper reports on an attempt for better understanding the marine fouling effects on the VIV of circular cylinders. A structured review of the literature related to the fluid structure interactions in marine fouled cylinders is first presented. Results of an experimental study on the VIV of... 

The current study is aimed at getting a further insight into the changes the fouling brings to the Vortex Induced Vibration (VIV) of circular cylinders. Instead of regular patterns considered in previous studies, using the Poisson Cluster Process, an aggregated spatial distribution was considered for the artificial marine fouling. This is believed to better simulate the natural settlement of the marine biofouling. Different coverage ratios and fouling shapes plus regular and aggregated distributions were considered. The towing tank VIV tests were conducted on elastically mounted rigid cylinders. The Reynolds number ranged from around 7.8 × 103 to 4.9 × 104. On the whole, the maximum... 

Two-phase flow simulations around a body were not studied before and considering these flows can play a significant role in long-term reliability and safety of industrial systems. In this paper, flow regimes, drag coefficient and void fraction around different cross-section prisms were considered. To achieve this aim, main equations of flow have been developed for investigation of drag coefficient in air-water two phase. Our numerical analyses were performed by a designed and written CFD package which is based on Eulerian-Eulerian approach. Geometries, which have been studied in this article, are: circle, rectangle and triangle, for different aspect ratio (length over width) and leading edge... 

Dense underflows are continuous currents which move down-slope due to the fact that their density is heavier than that ambient water. In this work, 2-D and 3-D density current in a channel were investigated by a set of experimental studies and the data were used to simulate the density current. The velocity components were measured using Acoustic Doppler Velocimetry (ADV). The height of density current (current's depth) was also measured. In this study, the density current with a uniform velocity and concentration enters the channel via a sluice gate into a lighter ambient fluid and moves forward down-slope. A low-Reynolds number turbulent model (Launder and Sharma, 1974) has been applied to... 

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