Sharif Digital Repository

Quadrotors have limitation in performing fast descending maneuver due to entering a dangerous region in velocity space called Vortex Ring State(VRS) which cause considerable loss of lift that may lead to fall. The purpose of this project is to reduce time of fast descending maneuver without entering VRS region by using model predictive control. First, appropriate nonlinear dynamic model of quadrotor obtained by considering important aerodynamics effect. Then by considering velocity constraints in order to avoid entering the VRS region, a nonlinear model predictive controller(NMPC) has designed. This controller successfully reduce the time of the maneuver up to than ordinary descending.... 

The interaction of the vortices and helicopter’s blades is the most significant source of generating the main rotor’s noise, especially in landing and rising. The kind of this noise is broadband and it is produced because of establishing the vortices behind the forward blade, their interaction with backward blade, changing their structures and creating turbulency loading. The known method for modeling of the interaction of the vortices structures and solid surfaces is the simple compounding the rod and airfoil. In this project, by using this method, the noise of the interaction of vortex and blade is modeled in the Reynolds number of 48000 with LES and FW-H Analogy as a hibrid method in... 

The physics of the flow around the wing in the oscillatory motion of the wing has various patterns, including the leading edge vortex, the tip vortex and trailing edge vortex escape. The interaction of these vortices forms a complex flow structure around the wing, which will have a direct effect on the aerodynamic performance of the wing. Therefore, controlling these structures can be considered as a way to improve the performance of the flapping wing. One of the passive flow control techniques that has recently been taken into consideration is a leading-edge protuberance method. Another type of passive flow control is geometric correction, using corrugation method. According to studies, two... 

When a submarine pipeline is laid, pipeline spans may exist due to the unevenness of the seabed. Such pipeline spans when exposed to currents, may undergo vortex-induced vibration (VIV). As the flow velocity increases, the cross-flow oscillation begins to occur and vortex shedding frequency may approach the natural frequency of the pipe span. Amplified response due to resonance between the vortex shedding frequency of the free span may cause fatigue damage. Thus, the determination of the critical allowable span length under the various environments becomes the most challenging areas in the offshore industry. A substantial research effort related to various aspects of VIV and free span... 

Transverse Vortex-Induced Vibration(VIV) and Flow-Induced Vibration(FIV) of a rigid, circular, elastically mounted single degree of freedom cylinder is scrutinized in range 3×104≤ Re ≤12×104 by means of Computational Fluid Dynamics(CFD). ANSYS CFX, based on FV, is employed to simulate VIV and FIV in different system parameters such as stiffness and mass ratio. Firstly, the minimum structured, hexahedron computational cell required to simulate VIV and FIV by means of Finite Volume (FV) method is provided. Then, the appropriate size for computational domain to simulate VIV and FIV accurately is discussed concisely. Approximated amplitude and frequency ratio are compared to experimental data... 

Using flapping motions, birds produce aerodynamic forces required to fly. Flapping wing aerodynamics is one of the most complicated problems in unsteady aerodynamic field. Experimental study on the structure, the way of production and growth and the feasibility of controlling the leading edge vortex are the main goals of this research in order to improve the aerodynamic performance of flapping wings. Since leading edge vortex structure appears in both upstroke and downstroke of a complete storke in flapping motion, controlling the structure in one of the half-storkes can provide the possibility of having a better aerodynamic performance. In this research, the wing is designed and... 

The ever-increasing need for energy compels researchers to investigate different manners of energy saving. Aَs we know well, one of the effective ways to save energy is to improve the energy consumption of turbomachinery devices such as axial flow fans. Fans are the most applicable of turbomachinery which supply needed airflow, which can be used in ventilation systems, Cooling systems for cars and machines, smoke extraction, and supply required air for firing. Therefore, improving their efficiency can play a significant role in reducing energy consumption and enhancing energy storage. One of the most critical factors in reducing the efficiency of the fans is the stall phenomenon, which can... 

A global rotating atmosphere with polar vortex and super rotation is simulated using a simplified Venus General Circulation Model (GCM). The polar vortex is maintained strongly by meridional circulation develops in the form of a simple Hadley cell, extending from the equator to the pole in both hemispheres and rotation terrestrial planet. Venus Total Polar Vortex Parameter (PVP) was defined like globally integrated super-rotation but at polar skullcaps (60-90 deg). Venus Polar Vortex Parameter is PVP=1.5 that’s qualitatively similar to observed polar vortex. Then with study in terrestrial planet and Titan modeling, we appointed the Polar Vortex Parameter and critical limit of Polar Vortex... 

Heat transfer to combustion chamber walls is an unwanted phenomenon. Reduction of this heat transfer by using bidirectional swirl flow, as proved in past studies, can result in time and cost saving methods in design and fabrication of combustion chambers. Bidirectional Swirl flow, existing in vortex engine, maintains the chamber wall cool. This characteristic of such flow field, among with many other advantages for it, makes researchers more interested in studying the propulsion of it as well as its other applications. Bidirectional swirl flow in liquid fuel ramjet engines has the proven advantage of keeping the combustion chamber walls cool and in solid fuel ramjet engines in increasing the... 

Liquid-liquid extraction is one of most important processes in chemical industries. So determination of operating condition effects is very critical. In this thesis we modeled concentration profile in the jet via mathematical equations and compare with experimental data. The major problem in concentration profile and solving equations is lack of knowledge in velocity profile within the jet was formed. Therefore, different flow models such as piston flow model, parabolic flow model, vortex flow model and a cone flow model was used and terminal concentrations inside the jet compared with laboratory values. Results shows the most accurate model to predict the flow concentration along the... 

Vortex Tube is a mechanical device without any moving parts. It separates a compressed gas stream into a low and a high temperature region (temperature or energy separation effect). Advantages of VT are existence of no moving parts, no external power (electricity or flames) required to operate, tiny and lightweight, inexpensive tube & parts material, convenience and pollution free. Heating, cooling, cleaning and drying gases, separating gas mixtures, gas liquefaction, natural gas pressure reduction and dehydration are the applications of vortex tube. The main Objectives in this research are investigation of various types of gas streams including pure gas or mixture of gases in vortex tube... 

As the leading kind of multi-rotors, quadrotors have become one of the most popular Unmanned Areal Vehicles (UAVs). These robots are very agile, but their speed is limited when performing decent maneuvers. In this project, a special quadrotor with dihedral-rotating arms is presented in order to increase the decent velocity of the vehicle. The conceptual and detailed designs of the vehicle are presented first, followed by the construction of the quadrotor. The fabricated quadrotor measures 25 cm in size, has a weight of about 1 kg, and can support up to 70 degrees of dihedral angle. Afterwards, the dynamic equations of the dihedral quadrotor are driven and simulated. The effects of dihedral... 

Energy harvesting is a process in which energies in nature are recycled on a small scale and used to power equipment such as sensors, data recorders and transmitters, IoT monitoring and applications. Recently, energy harvesting from a new model based on movements caused by vortices of wind and water flow has been considered. The fall of unstable vortices behind the body induces aero-hydrodynamic forces into the body. If the body has degrees of freedom of movement, the vortex phenomenon can cause the body to rotate or rotate, resulting in energy extraction. Previous studies show that very little activity has been done so far on energy extraction from the Stall flutter phenomenon. The main... 

One of the major concerns in the use of light flying objects is propulsion for cruise and take off from land. Creating propulsion force is important in such way that no excessive weight is introduced, especially in case of UAVs. A common trend for propulsion in light flying objects is using propellers. Propellers are commonly driven by an external motive force and due to this force and their geometry, they convert energy from this motive force into the momentum of fluid in which they are immersed as a result a lift force is generated.In the present work, trend of propeller design with optimum performance is introduced with use of vortex theory, optimum energy loss condition or Betz condition... 

Smoke must be exhausted, as the first action, in probable fire in subways, underground transportation systems and mines in order to survive people exposed to it. To achieving this purpose, the axial fan should be used to move the smoke to the exit lines. These fans should be had same performances in suction and discharge directions (main and reverse directions). This means flow rate should be the same at main and reverse directions. Therefore, fully reversible axial flow fans should be used in emergency ventilations. The fully reversible axial flow fans have same efficiencies in suction and discharge directions. The symmetric profiles should be used in manufacturing of the blades of these... 

It is well-known that helicopters descending fast may enter a region in the velocity space called Vortex Ring State where the blade’s lift differs significantly from regular regions and includes high amplitude fluctuations. These fluctuations may lead to instability and therefore, this region is avoided, typically by increasing the horizontal speed. However, this region is not fully identified for multirotors, which their blades are rigid in contrary of helicopter’s blades which have two degrees of freedom. This project researches this phenomenon in the context of small-scale quadcopters. The region corresponding to the VRS is identified by combining first-principles modeling and wind-tunnel... 

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... 

The aim of the present research is to investigate the combustion chamber cooling with vortex method in presence of kerosene and liquid oxygen as propellants. For this purpose, a part of oxidizer is entered into the cylindrical combustion chamber tangentially (from bottem of the chamber), and creates a vortex flow (outer vortex). With motion of created outer vortex along the wall and eventually hit its top wall (the head of the chamber), secondary vortex (inner vortex) occurs along the center line, with a downward direction (opposite to the primary vortex). The fuel and other part of the oxidizer injection is also carried in the head of chamber, so by mixing the fuel and oxidizer that happens... 

In present study physics film cooling with two rows fan shape’s holes with RSM turbulence model is simulated. At the first, structure of flow in film cooling with fan shape’s holes is identified, Then effective flow parameters and geometry parameters in film cooling problem with 2 rows fan shape’s holes is investigated. As the result of present study, four transverse arrangements for arrangement is leading to optimum effectiveness is introduced. The continuing effects of trench in cylindrical and fan-shaped holes are investigated. Finally, the efficiency of film cooling cylindrical and fan-shaped holes compared with the anti vortex holes . The results show fan-shaped holes in the high...