Sharif Digital Repository

The characteristics of unsteady flow field around a low-aspect-ratio pitching wing with an airfoil section used in horizontal axis wind turbine are investigated at Reynolds number 2.8×105 and Mach number 0.06. The major objective of this study was to investigate the effects of reduced frequency, oscillating amplitude, mean angle of attack and 3D effects on aerodynamic coefficients. Using aerodynamic coefficients, the unsteady aerodynamics in pre-, near-, and post-static stall regions are analyzed and the results are validated with the corresponding experimental data. The two-dimensional simulation results demonstrate the significant effects of reduced frequency, mean angle of attack and... 

Supercritical airfoils are designed for transonic flow to reduce the strength of shock and its drag. These type of airfoils are used in design and construction of commercial airplanes because of their low drag in transonic regime and, as a result, a reduction in fuel consumption. Using supercritical airfoils needs accurate investigation of flow field due to several influential factors such as their specific geometry, nonlinear nature of flow phenomena, and compressibility effects. However, the availability of experimental and numerical data for supercritical airfoils is less than that for conventional airfoils. Thus, conducting experimental tests is essential for understanding of flow field... 

Stratified flows are common in natural systems such as the oceans and the atmosphere. The hydraulics of two-layer flow over an obstacle, which helps with understanding and control of these flows, has been studied by many researchers. However, a type of two-layer flow, called the Approach-controlled flow, has not yet been fully understood, and there are ambiguities about it. Having considerable nonhydrostatic pressure in the so-called supercritical leap region, this flow type cannot be explained even qualitatively without accounting for the nonhydrostatic pressure. Also, the composite Froude number which is defined based on the hydrostatic pressure assumption for two-layer flows, is not... 

Focusing on unsteady flow field over delta wings due to vacuity of researches in this branch on one hand and requiring of modern aircrafts designers to this science on the other hand, expresses necessity of accomplishing present investigation. Scrutiny of pressure distribution of low aspect ratio wings in unsteady state and determination of aerodynamic characteristics of these wings in variant conditions, can be beneficial to plan aircrafts need to fly with high maneuverability in high speeds. This issue becomes important when in supersonic regime, with respect to emerged shock waves and vortices, develop complicated pressure distributions on the wing surface in unsteady state that affect... 

Aerodynamic shape optimization has been always of great importance in aerospace design. Recent advancements in computational algorithms and hardware, has made it feasible to have advanced optimization as a standard part of desing procedure. Here we have promoted aerodynamic optimization of air propeller.
Propeller optimization process requires a propeller performance analysis program and an efficient optimization algorithm. A vortex lattice model is used for the propeller performance analysis and a computer program is developed based on it. Experiment and litritures are used for validation of VLM code. In order to optimize the propeller peformance, propeller efficiency is applied as cost... 

The aim of this research was an investigation, development, and application of low- dimensional proper orthogonal decomposition (POD) method for simulation of unsteady flow around bodies. Since analytical methods have low accuracy and limitations and also experimental methods have other problems, researchers usually use computational approaches, which mostly do not have acceptable efficiency and speeds of computations (especially in 3-D unsteady flows). On the other hand, the idea of application of reduced order modelling, which orginally comes from control theories and structural analysis, have attracted many researchers in recent years. In this research, we also used POD for reduced order... 

Application of wind energy and specially wind turbines as a green source of energy is getting more popular and as a result, the knowledge of wind energy has been improved dramatically. The purpose of this research is to determine the flow characteristics in the boundary layer profile over an airfoil which is used in a wind turbine blade. The model is sinusoidally oscillated in pitch. For the present study, the critical cross section of the wind turbine blade has been selected and the boundary layer velocity profiles over its airfoil have been investigated experimentally. Dynamic motion is produced with sinusoidal oscillator and the reduced frequency is varied in the range of 0.054 to 0.1.... 

The objective of this research is to model the wing structure of a flapping wing MAV and investigate structural dynamic behavior of its wing. in this research, to investigate and identify dynamic instabilities, modeling of the structure based on a nonlinear four-node plane element and aerodynamic modeling based on the strip theory in unsteady flow has been done, and by coupling these two models, the final model is formed. In the model of the wing structure, the displacement and changes angle inside the plane of the wing sections have been ignored and only outside the plane have been assumed for the degrees of freedom. In addition, the veins in the flapping wing have been omitted and the... 

Based on the air entrainment, the water conveyance tunnels can be categorized as pressurized, two-phase flow. When the flow is controlled by gates, sudden closure of gates would produce unsteady two-phase flow. The most important effect of such an event is the sudden change in the pressure which can be a function of channel slope, air and water inflow and their ratio. In this thesis the pressure change in unsteady two-phase in an experimental set up using rectangular tunnel is studied. About 100 tests were carried out for single and two-phase flow with various water and air discharges causing unsteady flow with different slopes. It was observed that in a defined slope the ratio of the... 

in this study, turbulent natural convection inside an air filled square cavity with several hot and cold internal bodies in the range of Rayleigh numbers between 1010 and 1012 has been investigated numerically. Vertical walls of the enclosure have been considered isothermal and horizontal walls are insulated in all cases. An appropriate arrangement of internal bodies in which the minimum heat transfer occurs is desired. For this purpose, two dimensional Reynolds-averaged Navier-Stokes equations have been solved with the finite volume discretization method in a staggered grid. In addition, two equation high-Reynolds-number model with standard wall functions is used to simulate the... 

Wake aerodynamic of plunging airfoils have received less attention than the other unsteady phenomena. An experimental investigation was carried out to study the unsteady characteristics of flow field behind a model of a plunging airfoil in various conditions. The model has 0.25cm chord and is the 16m section of a 660kW wind turbine blade. All tests were conducted in the subsonic closed circuit wind tunnel in Iran. Invariable parameters were plunging amplitude, mean angle of attack and plunging frequency. The Wakes of plunging airfoil can be characterized as drag-producing, neutral, or thrust-producing .Our preliminary results show that the structure of vortex in the wake for the plunging... 

Many human inventions are inspired by nature, such as fish swimming, bird/insect flight, etc. A basic consideration for the design of swimming machines is the design of propulsors. A creative design of propulsors can be inspired by fish locomotion. The term locomotion means that thrust is generated by undulation of fish body. Thus, there is no need to have an external propulsor. We need to understand how thrust is obtained by fish locomotion. In this study, sub-carangiform motion, which is a well known locomotion and which is practiced by most fish, is simulated numerically using Lattice Boltzmann method (LBM). To simulate the geometry of fishlike body, the profile of a flexible airfoil...