Oscillations of cylinders in waves and currents

This thesis contains the results of two research investigations conducted by the author, the first an exploratory investigation into wave induced vibrations, conducted at the British Hydrodynamics Research Association (BHRA), and the second a comprehensive investigation into the forces on cylinders oscillated in still water and inline with various currents, conducted at the River and Harbour Laboratory (VHL), Trondheim, Norway. Questions posed by the results of the first investigation were used in the formulation of the second, and results from the second led to a better understanding of the first

Particle Image Velocimetry Near the Leading Edge of a Sikorsky SSC-A09 Wing During Dynamic Stall

Dynamic stall has proven to be a complex problem in helicopter aerodynamics because it limits the helicopter flight regime. Dynamic stall is characterized by drastic increases in lift and a delay of stall due to rapid pitching motions of aerodynamic surfaces. Prediction and control of dynamic stall requires an understanding of the leading edge flow structure. An investigation was conducted of dynamic stall near the leading edge of a large-scale Sikorsky SSC-A09 airfoil, dynamically pitching about its quarter chord, under realistic helicopter flight conditions (M_infinity = 0.1, k = 0.1, Re_c = 1.0 x 10^6). A testing model with a chord of 0.46 m and a span of 2.13 m was designed and constructed for experimentation in the Dynamic Stall Facility at Texas A&M University. Particle image velocimetry data were recorded for the first 15% of the airfoil chord. Mean velocities, Reynolds stresses, and vorticity were computed. Analyses revealed that during the upstroke, stall onset is delayed in the leading edge region and the first indications of separation are observed at 18 degree angle of attack. The edge of the boundary layer has been characterized for alpha = 18 degrees. The roles of the Reynolds stresses and vorticity are examined

Experimental Investigation of a Passively Deforming Airfoil Under Dynamic Flow Conditions

A rigid and a passively deforming airfoil, designed to alleviate fatigue causing load fluctuations that appear during normal operation of wind turbines, is investigated under unsteady conditions in two dimensional wind tunnel experiments. In a first series of experiments, a vertical gust encounter is generated by means of an active grid. This approximates the wind turbine blade’s passage through the atmospheric boundary layer and corresponds to the theoretical formulation of the Sears problem. In a second experiment the airfoil is oscillated in a steady free stream, which approximates the bending and twisting motion of a wind turbine blade and corresponds to the theoretical formulation of the Theodorsen problem. The frequency dependent dynamic lift response of the rigid airfoil under attached flow conditions is compared to the Theodorsen and Sears function. If the airfoil is oscillated around its zero lift angle, experimental results and theoretical prediction agree. Contrary to theoretic assumptions, a substantial dependence on the mean angle of attack is observed: If the airfoil is oscillated around higher mean angles of attack, the dependence on the reduced frequency is is inverted. The deforming airfoil shows good performance in terms of gust load alleviation over a wide range of operating conditions: At small mean angles of attack, up to 60% of the fluctuating loads are alleviated. Under high mean angles of attack, leading edge vortices are efficiently suppressed, yielding up to 30% less load fluctuations

Experimental Investigation of the Effects of Synthetic Jet Actuation on the Kármán Vortex Formation and Shedding in the Turbulent Wake of a Cantilevered Triangular Prism

The effects of synthetic jet actuation on the Kármán vortex formation in the three-dimensional turbulent wake of a cantilevered triangular prism of aspect ratio 4 protruding a thin turbulent boundary layer is investigated experimentally at a Reynolds number of 12000. Results are reported for surface pressure measurement on the leeward face and wake of the obstacle and particle image velocimetry for selected planes in the wake. Synthetic jet is used to excite the flow through two slits spanning the obstacle height along the edges of the leeward face. The mean-field characteristics for the baseflow and actuated flows are characterized and the coherent structures of the flow are identified. It is found that the actuation generally increases the total kinetic energy of turbulent fluctuations (TKE) and the amplitude of the fluctuations due to Kármán shedding. It is shown that the process of vortex formation for the baseflow and actuated flows is qualitatively similar and the cause of shedding frequency shift due to actuation is explained. A self-regulating mechanism is identified in the wake which links the circulation flux in the wake to the streamline curvature and the base pressure. Flow recirculation is shown to have a key role in explaining the wake response to the actuation. It accounts for the increased transfer rate of circulation downstream of the bluff body and affects the base pressure. Forming vortices are observed to reach a saturation level before the shedding in a behavior similar to formation process of vortex rings and vortex lines. A non-dimensional parameter including the effective formation parameters at the moment of shedding is introduced. Similarities between the results observed for the wake of the triangular prism and other geometries suggest a broader relevance of the argument

Havada asılı konumdaki çırpan kanat profilinin sayısal ve deneysel analizi. Mikro hava araçları'na uygulaması

The aerodynamics phenomena of flapping motion in hover are considered in view of the future Micro Air Vehicle applications. The aim of this work is to characterize the vortex dynamics generated by the wing in motion using direct numerical simulation and experimental analysis then to propose a simplified analytical model for prediction of the forces in order to optimize the parameters of the motion leading to maximum force. A great number of cases are investigated corresponding to different angles of attack, location of start of change of incidence, location of start of change of velocity, axis of rotation, and Re number. The airfoil used is symmetrical. The flow is assumed to be incompressible and laminar with the Reynolds numbers between 500 and 2000. The experimental results obtained by the laser sheet visualization and the Particle Image Velocimetry (PIV) techniques are used in parallel with the direct numerical simulation results for the phenomenological analysis of the flow. The model developed for the aerodynamic forces is an indicial method based on the use of the Duhamel Integral and the results obtained by this model are compared with the ones of the numerical simulations.Ph.D. - Doctoral Progra

Aeronautical engineering: A continuing bibliography with indexes (supplement 322)

This bibliography lists 719 reports, articles, and other documents introduced into the NASA scientific and technical information system in Oct. 1995. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Dynamics of Flexible Riser Systems

This thesis is concerned with the static and dynamic analyses and the model testing of deep sea flexible riser systems which are the key components associated with semi-submersible oil platforms. A numerical method based on explicit integration of Newton's second law is developed to predict the 3-dimensional dynamic behaviour of the riser due to the hydrodynamic loadings induced by wave and current motion. In this analysis the effects of waves and currents from separate directions, vessel movements, vortex-shedding and structural damping are included. The material damping for the riser is modelled by a single Kelvin system and the hydrodynamic loadings are assessed from the modified Morison equation. The effect of vortex-shedding on the riser is modelled by considering the interaction of drag-inertia and lift forces due to wave and current motion. The drag coefficient being modified ~ the vortex-shedding effects which are predicted by calculating the maximum response of the flexible riser in the lift force direction. The formfinding and static analysis of the riser when subject to structural self-weight and other static loadings is carried out by the method of Dynamic Relaxation using kinetic damping. The method is well suited to computer aided design procedures in which various shapes for the riser catenary have to be investigated together with the effects of boundary support conditions and alternative arrangements of mooring buoys. The theories described above are implemented into three computer programs. The first program deals with the formfinding of the riser and the second investigates the dynamic behaviour of the riser due to non-linear current and wave loadings. The third program is concerned with the static solution of the riser due to current loading. The latter is employed when the hydrodynamic force consists of current loading only and therefore the dynamic solution is not desired. The iterative use of the first and third programs allows potential designs to be quickly investigated. The results predicted by the numerical analyses are compared with those obtained from two series of model tests in wave flumes. The tests were scaled from prototype situations using Froud number criteria. The first set of tests used a small scale flexible chain model with negligible material damping and structural bending stiffness and no induced vortex shedding; the aim of these tests being only to validate the assessment of the hydrodynamic forces on the riser. '!he second set investigated the response of a larger scale model which induced vortex-shedding and in which the riser system had significant structural damping. The comparison of experimental and computed results showed close agreement. The developed computer programs were also validated numerically by comparing the predicted results with those obtained from the well known riser program "FLEXRISER"

Aeronautical engineering: A continuing bibliography with indexes (supplement 323)

This bibliography lists 518 reports, articles, and other documents introduced into the NASA scientific and technical information system in November 1995. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics