Sharif Digital Repository

Upwind methods for forward time marching integration of compressible flow equations, suffer low accuracy and convergence rate for very low mach numbers. Here we have used a preconditioning scheme to address this challenge. A preconditioner matrix is multiplied in the Euler flow equations. First we investigate subsonic flow around an airfoil to validate the numerical scheme used here, and to show grid independence of our solutions. Then very low mach numbers between 0.1 and 0.001 is solved and increase in accuracy and convergence rate is demonstrated. The proposed algorithm has flow parameters, which are studied here to find their effect on accuracy and convergence rate. The found results are... 

Up to now many different mechanisms have been proposed for a swimming micro robot, but most of the previously designed micro robots are not able to perform three- dimensional maneuver and the motion of the proposed swimmers are limited along a straight line or in a plane. Some important issues that must be ruled in designing micro robots include: reduced number of actuators, high maneuverability, high motion controllability and elimination of lateral drift. In this study the aim is to design a micro swimmer with the aforementioned capabilities as much as possible. In this regard, many different ideas based on the quadrotor motion, flagellated bacteria and spiral flagellated bacteria, Fish... 

Today, the use of controllable swimmer micro-robots has expanded from micro to macro-scale dimensions due to their many applications. These robots, as a result of their small size and dimensions, have the ability to access limited and complex environments. Considering the various applications of such swimmers, it can be said that in addition to the necessity of well designing and implementation, their maneuverability and controllability are among the most important research areas on these swimmers. Regarding the dimensions and technologies needed for these robots, various methods for designing and implementing them are suggested, which are mostly inspired by the nature of microorganisms. In... 

Recently, a lot of attention has been devoted to the use of Darrieus wind turbines because of high power density and environment aspect in urban areas. Turbine airflows don’t effect on the power coefficient of Darrieus turbine. This turbine has an acceptable power coefficient depend on the other wind turbine in the small scale turbine. Aerodynamic performance of a Darrieus turbine is very complex due to phenomena such as dynamic stall and changing forces on the turbine caused by changing horizontal angles. So study airflows around turbine must be preformed. In this thesis the aerodynamics of an H-rotor vertical axis wind turbine (VAWT) has been studied using computational fluid dynamics in... 

The design, optimization, and construction of systems that are capable of swimming in the low Reynolds conditions are urgently needed in the light of the recent human, medical, industrial and scientific needs. Therefore, it is necessary to inspire the movement of microorganisms to design and construct artificial micro swimmer as well as to achieve the best geometry for swimming. In order to achieve the optimum microstructure geometry, it is necessary to know the behavior and hydrodynamic interactions of the micro swimmer with the fluid and its outer channel. In this study, we numerically simulate the movement of a microorganism with a spiral tail in a circular channel using Computational... 

Microfluidics is a field of science that studies micron-scale characteristics of fluids. In this scale fluids show fascinating behavior far beyond our expectation compared to their large-scale counterparts. One of such peculiar behavior is the mixing of two different liquids flowing inside a microchannel. Due to low Reynolds number, the flow inside such a channel would be dominated by laminar behavior. In this regime mixing of the fluids is only mediated by molecular diffusion which is a rather slow process. In the current thesis, we utilized the rotating ability of optical tweezers to construct an active micromixer for Microfluidics. In order to do this we used birefringent microbeads. The... 

In this study, two micro swimmers have been modeled using computational fluid dynamics and the motion specifications have been presented. The mechanisms can be used for different applications like drug delivery, monitoring, doing tasks in dangerous environments like robots and etc… . These moving mechanisms are inspired from a type of plankton named “cladoceran Podon Intermedius”, accordingly we have compared our results with data obtained by particle image velocimetry. For CFD simulation, an unstructured 2-dimensional mesh has been generated. Dynamic meshing has also been utilized in a meshing zone around the moving parts. Unsteady simulations were run for axisymmetric and plane symmetry... 

In this study, at first modeling of a self-propelled microrobot and its hydrodynamic effects on flow field are investigated. Then, a cascade controller has been designed for the fully actuated microrobot in order to track the desired time trajectories. Next, in order to design an optimal controller to decrease energy consumption, regardless of tracking the desired orientation trajectory compared to the desired position trajectory, the fully actuated microrobot would change to over actuated microrobot. Thus, it would be able to orient freely itself according to the flow field so that reduce the energy consumption. By applying the optimal controller on the over actuated microrobot, it was... 

Nowadays, regarding the wide achievements in micro scale technology, it is possible to manufacture a micro robot. Micro swimmer have extensive applications in different fields especially in medical sciences. One of the most important problems of the micro robot is their propulsion system. The locomotion of microorganisms in fluids is ubiquitous and plays an important role in numerous biological processes. The physics of swimming governing life under the microscope is very different from the one we experience in the macroscopic world. This project is aimed at designing a propulsion system for a micro swimmer based on the swimming mechanism of the eukaryotic Spermatozoa. Simplicity to build is... 

There is an importance issue for preservation and survival of ecological systems by vegetation. A study focusing on the hydrodynamics of these complex ecosystems could help to explain the filtering ability of vegetation in wetlands and estuaries. Wetlands assist in flood control, shoreline protection, and groundwater recharge.In addition, they play a purification role, removing sediments and contaminants from polluted waters, nutrient uptake, and oxygen production. Laboratory experiments have been designed to study the influence of marsh vegetation on lateral dispersion. The flow through vegetation is modeled by a circulating flow going through an array of plastic dowels in an experimental... 

Preservation and survival of ecological systems by vegetation is important. A study focusing on the hydrodynamics of these complex ecosystems could help to explain the filtering ability of vegetation in wetlands and estuaries. Wetlands assist in flood control, shoreline protection, and groundwater recharge. In addition, they play a purification role in removing sediments and contaminants from polluted waters, nutrient uptake, and oxygen production.
Laboratory experiments have been designed to study the influence of marsh vegetation on lateral dispersion in flexible vegetation that has been modelled with inclined cylinder. The flow through vegetation is modelled by a circulating flow... 

Mass transfer of a drop moving in a bulk fluid is an interesting subject which has attracted a lot of attention. Understanding effective parameters in this process would be helpful in order to increase the efficiency of the industrial processes related to mass transfer. In this research, the deformation and mas transfer of a moving drop was studied numerically. Navier-Stokes equations and concentration equation were solved separately with a VOF method in an axisymmetric domain using Fluent software. Comparision between numerical results and former experimental results proves the accuracy of the modelling in both fields of deformation and mass transfer. Moreover, effects of the physical... 

The field of microrobotics has seen tremendous advances in recent years. Potential high impact applications of untethered microrobots such as minimally invasive diagnosis and treatment inside the human body, biological studies or bioengineering, microfluidics, desktop micromanufacturing, and mobile sensor networks for environmental and health monitoring are reported. However, There are significant challenges in microrobotic technology, including energy resources, control systems, reliability and safety, design and manufacturing of micromotors and localization. In this work, overall size scale being micron scale is emphasized where mobile robots able to fit in spaces smaller than a millimeter... 

Nowadays, the lattice Boltzmann method has been widely used by scientists and engineers as an alternative to conventional numerical solvers for the Naiver-Stokes equations. The drag force decrease on the surfaces in industrial applications special in transport industries has always been of special importance. In the current research, the effects of the drag decrease has been investigated by making some rectangular grooves on the millimeter and micrometer scale on a flat surface that has been under external laminar flow of a single-phase fluid with the uniform type. Making grooves on the surfaces usually lead to change the contact area from the solid-fluid to fluid-fluid in the grooves area.... 

Drag reduction on surfaces always has been a great case of study, especially in transport industry. This project discusses the external laminar flow of a single-phase fluid over a flat plate and the effects of making rectangular tiny grooves - in micron or millimeter - on them to reduce total drag. Making grooves with appropriate geometry on surfaces causes the fluid-solid contact area on the groove's surface turn into fluid-fluid and as a result the fluid will not shed into the groove. In other words the flow instead of having a no-slip boundary condition on inner surfaces of the groove, creates vortices in them that causes a remarkable reduction in velocity gradient followed by a skin... 

In this study, the physic of an unsteady flow surrounding two oscillating airfoils in tandem configuration at low Reynolds-numbers has been simulated in a 2D manner by utilizing the Sliding-mesh approach and K-ω-SST turbulent model in Ansys-Fluent application. The first step in this study is selecting the decent numerical grid and flow solution scheme. After choosing the proper solution for this problem, next is to verify the methodology with respect to the references. In the next step, The tandem configuration and pitching motion of the airfoils is being implemented with existing tools and the flow structure, leading-edge and trailing-edge vortex shedding and blade-vortex interactions is... 

The purpose of this study is to investigate a flapping hydrofoil with semi-active kinematics for energy harvesting and evaluate the effect of different dynamic parameters and structures on its performance. The turbulent flow field with Re = 5 × 105 around a NACA 0015 hydrofoil is simulated by URANS method using commercial software and the dynamics of the oscillating hydrofoil is modeled as a mass-spring-damper set in combination with the fluid solver. The sliding mesh method is used for the pitching motion of the hydrofoil with a reduced frequency of 0.14 in combination with the dynamic mesh method for Transitional motion. Initially, the oscillating hydrofoil was considered as a single... 

Microrobots (MRs) have attracted a lot of attention during recent years due to their promising biomedical applications. Due to their small size, these robots have limited capability to carry energy sources, sensors, or actuators, so researchers try to transfer energy from outside as much as possible. For this purpose, various actuation methods such as optical, acoustic, electric, and magnetic have been proposed. Magnetic field is the most used energy source. The current study deals with the design and implementation of a system for simultaneous independent control of multiple swimming magnetic MRs. The ability to control multiple MRs simultaneously and independently could lead to higher... 

Understanding the physics of unsteady flow is very important due to various applications. Axial compressors, turbines, helicopter blades, and energy harvesters are the essential applications of this problem. An airfoil under pitching oscillation is one of the common cases in unsteady aerodynamics, and several characteristics of the flow around it have been investigated up to now. The frequency analysis of the wake flow for pitching airfoils has received less attention. In this research, experimental tests have been carried out for symmetric airfoil NACA 0012 under pitching oscillation in low Reynolds flow (6×104) and before the airfoil stall condition. The experiments were carried out in the...