Airfoil in a high amplitude oscillating stream

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.A combined theoretical and experimental investigation was carried out with the objective of evaluating theoretical predictions relating to a two-dimensional airfoil subjected to high amplitude harmonic oscillation of the free stream at constant angle of attack. Current theoretical approaches were reviewed and extended for the purposes of quantifying the bound, unsteady vortex sheet strength along the airfoil chord. This resulted in a closed form solution that is valid for arbitrary reduced frequencies and amplitudes. In the experiments, the bound, unsteady vortex strength of a symmetric 18 % thick airfoil at low angles of attack was measured in a dedicated unsteady wind tunnel at maximum reduced frequencies of 0.1 and at velocity oscillations less than or equal to 50 %. With the boundary layer tripped near the leading edge and mid-chord, the phase and amplitude variations of the lift coefficient corresponded reasonably well with the theory. Near the maximum lift coefficient overshoot, the data exhibited an additional high-frequency oscillation. Comparisons of the measured and predicted vortex sheet indicated the existence of a recirculation bubble upstream of the trailing edge which sheds into the wake and modifies the Kutta condition. Without boundary layer tripping, a mid-chord bubble is present that strengthens during flow deceleration and its shedding produces a dramatically different effect. Instead of a lift coefficient overshoot, as per the theory, the data exhibit a significant undershoot. This undershoot is also accompanied by high-frequency oscillations that are characterized by the bubble shedding. In summary, the location of bubble and its subsequent shedding play decisive roles in the resulting temporal aerodynamic loads

Dynamische Strömungsablösung an Rotorblättern von Windkraftanlagen und deren Kontrolle

Rotorblätter kommerzieller Windkraftanlagen sind intensiven Fluktuationen der Strömungsgeschwindigkeit und des Anstellwinkels ausgesetzt, die zu einer dynamischen Ablösung der Grenzschicht führen können. Dieses durch die Konvektion eines starken Längswirbels gekennzeichnete instationäre Strömungsphänomen ruft plötzliche Schwankungen der aerodynamischen Kräfte hervor. Um trotz der daraus resultierenden mechanischen Belastung eine lange Lebensdauer der Rotorblätter zu gewährleisten, ist eine stabile Konstruktion notwendig, was zu erhöhten Herstellungskosten führt. Ziel dieser Arbeit ist es, zu einem besseren Verständnis der dynamischen Grenzschichtablösung beizutragen und darauf aufbauend eine wirksame Methode zur Verminderung der daraus resultierenden Auftriebsfluktuationen zu erforschen. Die vorliegende Arbeit beschreibt die Ergebnisse von experimentellen Untersuchungen, die an einem speziell für die Erzeugung starker Geschwindigkeitsfluktuationen entwickelten Windkanal durchgeführt wurden. Im ersten Abschnitt wird die dynamische Ablösung unter für Rotorblätter von Windkraftanlagen charakteristischen Bedingungen untersucht. Obwohl dieses Phänomen seit mehreren Jahrzehnten Gegenstand intensiver Forschung ist, wurde bisher wenig über den Einfluss der Geschwindigkeitsfluktuationen bekannt. Gleichzeitige Messungen des Strömungsfeldes und der Verteilungen des Oberflächendrucks zeigen, dass sich beim hier untersuchten für vertikale Windkraftanlagen typischen NACA 0018 Flügelprofil zusätzlich zum bekannten Dynamic Stall Vortex ein bisher nicht im Detail beschriebener Wirbel nahe der Hinterkante ausbildet, der eine frühe Verringerung des Kippmoments hervorruft. Die zeitliche Veränderung der Relativgeschwindigkeit führt zu einer Variation der dimensionslosen Frequenz, was einen direkten Einfluss auf die Intensität der dynamischen Effekte hat. Das „Matched Pitch Rate Concept“, welches die Ähnlichkeit der instationären Ablösung unter unterschiedlichen Anströmbedingungen vorhersagt, wurde für den Fall gleichzeitiger Variationen des Anstellwinkels und der Strömungsgeschwindigkeit validiert. Im zweiten Abschnitt wird ein neuartiges Konzept zur aktiven Strömungskontrolle untersucht, das die Verminderung der dynamischen Lastfluktuationen zum Ziel hat. Das Ausblasen von Luft tangential zur Flügeloberfläche kann dazu genutzt werden, die Grenzschicht zu destabilisieren und deren Ablösung herbeizuführen, was eine deutliche Verringerung des Auftriebs verursacht. Durch eine Erhöhung des Impulsstroms wird der gegenteilige Effekt erzielt. In diesem Fall führt die Verhinderung der Ablösung zu einer signifikanten Auftriebserhöhung. Verschiedene für Rotorblätter von Windkraftanlagen typische Strömungsszenarien wurden durch gleichzeitige harmonische Variationen des Anstellwinkels und der Windkanalgeschwindigkeit simuliert. Die experimentellen Ergebnisse belegen, dass durch dynamische Anpassung des Impulsstroms die Lastfluktuationen um ein Vielfaches reduziert werden und ein nahezu konstanter phasengemittelter Auftrieb erzielt werden kann.Dynamic stall is an unsteady flow phenomenon that occurs when an airfoil is rapidly pitched beyond the static stall angle. The transient boundary layer separation process, characterized by the shedding of a strong vortex across the suction surface, produces severe aerodynamic load fluctuations that lead to fatigue damage. This is a major problem on wind turbine rotor blades, which are exposed to highly unsteady inflow conditions. Successful control of this phenomenon could potentially yield significant reductions in the cost of energy. Even though dynamic stall has been extensively studied before, the associated flow speed variations have rarely been considered. In the present study, experiments were conducted in a unique wind tunnel facility that allows to reproduce the unsteady inflow characteristic of wind turbine rotor blades by simultaneously varying the angle of attack and the wind tunnel speed at short time scales. All experiments were performed on a NACA 0018 airfoil, typically found on vertical axis machines, that exhibits trailing-edge type stall. Flow field measurements were made using Particle Image Velocimetry and the unsteady surface pressure distributions were recorded with arrays of piezo-resistive transducers. Phase locked measurements revealed the formation of a second dynamic stall vortex across the rear half of the airfoil. This structure termed “aft dynamic stall vortex” produces a drop of the pitching moment prior to the shedding of the leading-edge dynamic stall vortex. The impact of the dynamic flow speed variation on the reduced frequency was investigated in detail for both light and deep dynamic stall. The experimental results clearly show that the reduced frequency effectively varies as a function of the freestream velocity, leading to significant differences in the transient variations of the aerodynamic coefficients. The matched pitch rate concept was extended to the case of synchronous incidence oscillations and flow speed variations, providing a framework that permits the prediction of unsteady aerodynamic loads from data obtained at constant flow speeds. A novel flow control concept termed “adaptive blowing” was successfully tested. While the working principle is in part based on classical steady blowing, the fundamental differences lie in the dynamic variation of the control jet momentum for the purpose of controlling unsteady aerodynamic loads and the use of low-momentum blowing to temporarily reduce lift. Initially, steady blowing was investigated to characterize the effect of control. A comparison of blowing at different chordwise positions revealed that the leading-edge slot provides a far larger control authority, allowing for significant changes in lift over a wide range of angles of attack. Slot blowing at moderate and high momentum coefficients produced a substantial increase in lift and fully suppressed the formation of the dynamic stall vortex, thereby eliminating the associated rapid load excursions. In contrast, low momentum blowing was found to induce boundary layer separation at relatively small angles of attack, yielding a significant lift reduction. Once the effect of steady blowing had been established, the momentum coefficient was varied dynamically to compensate for transient changes of the inflow. An iterative control approach was implemented, which successfully identified the time profiles of the control jet momentum flux required to minimize the lift excursions. This strategy provided an unprecedented control authority during various periodic inflow oscillations, producing virtually constant phase averaged lift

Pitching airfoil subjected to high amplitude free stream oscillations

An experimental investigation was carried out to quantify the aerodynamic lift acting on a NACA 0018 airfoil subjected to combined pitching and surging and to compare the results to established theories. A dedicated unsteady wind tunnel was employed that produces large surge amplitudes, and airfoil loads were estimated by means of unsteady surface mounted pressure measurements. In-phase and out-of-phase pre-stall pitching and surging cases were considered for different velocity amplitudes. When the flow was fully attached, satisfactory correspondence was observed between experiments and theory. However, differences were observed when trailing-edge separation was present; in particular the shedding of a trailing-edge vortex corresponded with discrepancies between the experiments and theory