Unsteady loads and associated flow fields on wings exposed to high rotation-rate dynamic stall

Aerodynamics is a facet of engineering that has progressed rapidly since the discovery of flight from as early as the mid-19th century. In recent years, high manoeuvrability aircraft, high-speed helicopters, unmanned-aerial vehicles, micro-aerial vehicles and natural flyers have attracted significant interest due to their potential for military, surveillance and rescue applications. Due to economic and global demand to limit greenhouse gas emissions, the awareness of clean energy resources, such as horizontal-axis and verticalaxis wind turbines, has resulted in the rapid growth of research focusing on improving the performance and operational efficiency of such machines. Although these machines are designed for dissimilar applications, they all suffer from a common problem; the process of dynamic stall. Dynamic stall is the unsteady aerodynamic phenomenon that occurs on pitching and plunging wings due to transient fluctuations in the operating angle of attack. During the process of dynamic stall, flow separation is delayed to elevated angles of attack. Increasing the angle of attack results in growth of a vortex structure originating at the leading edge. This vortex results in increased lift, drag and moment on the wing. Increased forces and moments continue until the vortex detaches from the wing and convects into the wake. The wing proceeds into deep-stall until the incidence angle is reduced to angles permitting reattachment of the boundary layer. Dynamic stall results in increased material fatigue, cost and maintenance, and an overall decrease in performance of machine components. In contrast, natural flyers such as birds and insects have evolved to exploit the unsteady phenomenon for sustained flight. While dynamic stall has been extensively studied for helicopter applications, recent work has focused on the operation of wind turbines. Helicopter rotor blades are exposed to sinusoidal changes in the angle of attack throughout each blade rotation. Whereas, wind turbines blades are subject to multiple variations in angle of attack. In addition, stalled rotor conditions may even be used beneficially to control power output during high wind load conditions. The purpose of this thesis is to investigate the effects of dynamic stall on wings typically associated with wind turbines, helicopter and micro-aerial vehicle applications. More specifically, the thesis will focus on the study of pitching airfoils. Under the unsteady operating conditions, unsteady aerodynamic forces and flow structure development will be investigated during both pitch-up and post-stall phases of the airfoil motion. This is achieved by replicating unsteady operating conditions in both water-channel and windtunnel facilities. Particle image velocimetry and surface pressure measurements were utilised to identify key flow structure events, and the associated forces generated on wings during unsteady motion. Constant-pitch-rate motion at a Reynolds number of 20,000 was applied to similar airfoils of different thicknesses, and includes a NACA 0012 and a NACA 0021. The aim of the investigation was to determine the flow structure variation between both thick and thin airfoil profiles during dynamic stall. Separation was shown to occur at earlier stages of the dynamic stall process for the thinner airfoil section when exposed to low rotation-rate dynamic stall. Increasing the rotation rate resulted in higher inertial loads, which in turn led to delayed stall and increased force generation at higher angles of attack. Fluctuations in forces were correlated with periodic vortex shedding at the trailing edge during airfoil ramp-up. Under steady-state conditions, the presence of separation bubbles on both surfaces of the airfoil resulted in a negative lift-curve slope prior to the collapse of both bubbles and subsequent recovery of lift. Deep stall was delayed with an increased rotation rate due to the initial delay in formation of the leading-edge vortex. However, once separation of the vortex occurred, post-stall characteristics were not influenced by airfoil geometry, with both airfoils exhibiting bluff-body separated-flow characteristics. For post-stall conditions following dynamic stall, increasing the reduced frequency delayed separation in some instances up to an angle of attack of 60°. Low surface pressure on the upper surface of the airfoil was linked to vortex structure developed during dynamic stall and in post-stall conditions. The centre of pressure was shown to shift with the development of the leading-edge vortex, and move aft of the quarter-chord location during fully-separated flow conditions. The change in centre of pressure leads to increased moment, which is transferred and linked to increases in torsional loading and fatigue of rotor blades and power transmission components or rotary machines. For investigation of a boundary layer control method, a simplified leading-edge trip wire was implemented on two airfoils experiencing dynamic stall conditions. NACA 0012 and NACA 0021 airfoils were fitted with leading-edge trip wires of varying diameters, located at a fixed displacement from the airfoil leading edge. The Reynolds number was 20,000. The trip wires were shown to decrease the maximum lift, although the stall angle of attack was not observed to change with variations in the trip wire diameter. Geometric superposition was observed between the trip wire and the airfoil body when the diameter of the wire exceeded 1.6% of the airfoil chord length. This led to increases in lift and drag during the pitch-up motion. Constant-pitch-rate rotation was utilised to investigate the effects of half-saddle movement and vortex formation on the aerodynamic characteristics of a pitching flat plate. A combination of round, square and triangular leading-edge and trailing-edge extensions were alternated during testing on a flat plate with a thickness-to-chord ratio of 0.1. The Reynolds number was 20,000. The half-saddle point, located on the upper surface, was linked to leading-edge vortex attachment. Detachment of the leading-edge vortex resulted once the position of the half-saddle point reached the trailing edge of the flat plate. Similarly, the rate of aft motion of the half-saddle point was shown to increase as a function of airfoil chord length, rotation rate and free-stream velocity. No benefit to overall force generation was observed once a critical angle of attack was reached. Maximum aerodynamic efficiency was shown to occur at angles of attack significantly below the angle of attack where maximum lift force was observed. The research in the current dissertation enhances knowledge of the dynamic-stall process, and provides information that can improve methods of boundary layer control on wings exposed to dynamic stall. Moreover, research reported herein provides critical information on the deep-stall process, which occurs after the event of dynamic stall. With the information acquired in this thesis, increased awareness of dynamic stall and deepstall characteristics can be achieved and utilised for the development of blades which are lighter, perform more efficiently and require lower costs to develop and maintain.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Mechanical Engineering, 201

Applied Aerodynamics

Aerodynamics, from a modern point of view, is a branch of physics that study physical laws and their applications, regarding the displacement of a body into a fluid, such concept could be applied to any body moving in a fluid at rest or any fluid moving around a body at rest. This Book covers a small part of the numerous cases of stationary and non stationary aerodynamics; wave generation and propagation; wind energy; flow control techniques and, also, sports aerodynamics. It's not an undergraduate text but is thought to be useful for those teachers and/or researchers which work in the several branches of applied aerodynamics and/or applied fluid dynamics, from experiments procedures to computational methods

Small Wind Turbine Starting Behaviour

Small wind turbines that operate in low-wind environments are prone to suffer performance degradation as they often fail to accelerate to a steady, power-producing condition. The behaviour during this process is called “starting behaviour” and it is the subject of this present work. This thesis evaluates potential benefits that can be obtained from the improvement of starting behaviour, investigates, in particular, small wind turbine starting behaviour (both horizontal- and vertical-axis), and presents aerofoil performance characteristics (both steady and unsteady) needed for the analysis. All of the investigations were conducted using a new set of aerodynamic performance data of six aerofoils (NACA0012, SG6043, SD7062, DU06-W-200, S1223, and S1223B). All of the data were obtained at flow conditions that small wind turbine blades have to operate with during the startup - low Reynolds number (from 65000 to 150000), high angle of attack (through 360◦), and high reduced frequency (from 0.05 to 0.20). In order to obtain accurate aerodynamic data at high incidences, a series of CFD simulations were undertaken to illustrate effects of wall proximity and to determine test section sizes that offer minimum proximity effects. A study was carried out on the entire horizontal-axis wind turbine generation system to understand its starting characteristics and to estimate potential benefits of improved starting. Comparisons of three different blade configurations reveal that the use of mixed-aerofoil blades leads to a significant increase in starting capability. The improved starting capability effectively reduces the time that the turbine takes to reach its power-extraction period and, hence, an increase in overall energy yield. The increase can be as high as 40%. Investigations into H-Darriues turbine self-starting capability were made through the analogy between the aerofoil in Darrieus motion and flapping-wing flow mechanisms. The investigations reveal that the unsteadiness associated with the rotor is key to predicting its starting behaviour and the accurate prediction can be made when this transient aerofoil behaviour is correctly modelled. The investigations based upon the analogy also indicate that the unsteadiness can be exploited to promote the turbine ability to self-start. Aerodynamically, this exploitation is related to the rotor geometry itself

Aeronautical engineering: A special bibliography with indexes, supplement 82, April 1977

This bibliography lists 311 reports, articles, and other documents introduced into the NASA scientific and technical information system in March 1977

Aeronautical Engineering: A continuing bibliography (supplement 138)

This bibliography lists 366 reports, articles, and other documents introduced into the NASA scientific and technical information system in July 1981