Sinusoidal-gust generation with a pitching and plunging airfoil

The generation of uniform, periodic gust disturbances in an experimental context is demonstrated using a single oscillating airfoil. A pitching and heaving symmetric airfoil is suggested as a simpler alternative to existing gust-generation methods. The Theodorsen theory of unsteady aerodynamics is used as an analytical tool to dictate the kinematics necessary to produce well-defined sinusoidal gusts downstream of the airfoil. These analytic predictions improve the symmetry of fluctuations in the vertical velocity induced by the airfoil, as well as minimize the influence of vorticity shed by the oscillating airfoil. The apparatus is shown to produce smooth, repeatable gusts with high amplitudes and reduced frequencies compared to other gust-generation mechanisms in the literature. Furthermore, the control of downstream flow properties by airfoil motion kinematics has applications in experimental aerodynamics, the design of rotorcraft and light aerial vehicles, and biological propulsion.Comment: Under revie

Die Berechnung dünnwandiger Ringträger mit geschlossenem Querschnitt auf der Grundlage des halbmomentenfreien Schalenmodells nach Wlassow, Teil ll

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Insights into leading edge vortex formation and detachment on a pitching and plunging flat plate

The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite the parameters influencing the development of the LEV topology must be fully understood and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities using water and air as working media by making use of a common baseline case. The motivation behind this comparison is that with two facilities the overall parameter range can be significantly expanded. This comparison includes an overview of the respective parameter ranges, control of the airfoil kinematics and careful scrutiny of how post-processing procedures of velocity data from time-resolved particle image velocimetry (PIV) influence the integral properties and topological features used to characterise the LEV development. Second, and based on results coming from both facilities, the appearance of secondary structures and their effect on LEV detachment over an extended parameter range is studied. A Lagrangian flow field analysis based on finite-time Lyapunov Exponent (FTLE) ridges allows precise identification of secondary structures and reveals that their emergence is closely correlated to a vortex Reynolds number threshold computed from the LEV circulation. This threshold is used to model the temporal onset of secondary structures. Further analysis indicates that the emergence of secondary structures causes the LEV to stop accumulating circulation if the shear layer angle at the leading edge of the flat plate has ceased to increase. This information is of particular importance for advanced flow control applications, since efforts to strengthen and/or prolong LEV growth rely on precise knowledge about where and when to apply flow control measures

Insights into Leading Edge Vortex Formation and Detachment on a Pitching and Plunging Flat Plate

The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite, the parameters influencing the development of the LEV topology must be fully understood, and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities using water and air as working media by making use of a common baseline case. The motivation behind this comparison is that with two facilities the overall parameter range can be greatly expanded. This comparison includes an overview of the respective parameter ranges, control of the airfoil kinematics and careful scrutiny of how post-processing procedures of velocity data from time-resolved particle image velocimetry (PIV) influence the integral properties and topological features used to characterise the LEV development. Second, and based on results coming from both facilities, the appearance of secondary structures and their effect on LEV detachment over an extended parameter range is studied. A Lagrangian flow field analysis, based on finite-time Lyapunov Exponent (FTLE) ridges, allows precise identification of secondary structures and reveals that their emergence is closely correlated to a vortex Reynolds number threshold computed from the LEV circulation. This threshold is used to model the temporal onset of secondary structures. Further analysis indicates that the emergence of secondary structures causes the LEV to stop accumulating circulation if the shear layer angle at the leading edge of the flat plate has ceased to increase

Investigation and Control of Unsteady Flow Conditions on Airfoils

In this thesis, efficiency enhancement approaches for two unsteady aerodynamic scenarios are considered to provide the basis for future improvements. The first scenario addresses transient inflow conditions on a stationary airfoil, representative of gust loads on wind turbine blades or bridge decks. It aims at the experimental validation of aerodynamic transfer functions, known as the Sears and Atassi formulations, which allow for a prediction of unsteady loads. Both functions are found to be capable of load prediction if inflow assumptions are carefully reproduced. It is also shown that no fundamental difference between both functions exists, if they are normalized appropriately. In order to simplify the generation of periodic inflow conditions in a wind tunnel, a gust generation approach utilizing a single pitching and plunging airfoil is derived and experimentally validated. It is demonstrated that high frequency and amplitude gusts can be generated with the aid of optimized airfoil kinematics, derived from the Theodorsen theory, using a single airfoil. The second scenario considers steady inflow conditions on a pitching and plunging airfoil in the deep dynamic stall regime where a leading edge vortex (LEV) occurs, representative for beating wings of Micro Air Vehicles (MAVs). The detachment process of the vortex is investigated to provide the basis for consecutive flow control efforts. A model that allows for the prediction of the occurrence of secondary structures, which can initiate the vortex detachment, is derived and validated using facilities with complementary parameter spaces due to different working media. In order to prolong the LEV growth phase on the airfoil and attain higher overall lift, a DBD plasma actuator is used to manipulate the flow field at topologically critical locations on different airfoils. The growth phase of the vortex is prolonged, which indicates an enhancement of the induced lift. Considerations regarding the control authority of the actuator are used to derive and test the minimum effective actuation period, which is sought to demonstrate the potential for future enhancements of the control concept