Development of an innovative double-swept rotor blade tip for the rotor test facility Goettingen

Innovative double-swept rotor blade tip planforms show significant potential to reduce noise and vibrations while improving the overall performance. Current test results for a non-rotating double-swept innovative rotor blade tip model, inspired by the DLR patent Erato, were obtained in a conventional wind tunnel (DNW TWG, Transonic Wind Tunnel Goettingen) under forced pitching motions. In the next step, the influence of rotation and the environment of an entire rotor head system should be considered. It is of great interest to investigate the influence of rotation on the aeroelastic behaviour and the unsteady aerodynamics in a dedicated further experiment. The Rotor Test Facility Goettingen (RTG) was built for research on rotating rotor blades under optimized boundary conditions and will be used for the planned experiments. Typical flow phenomena associated with rotor blades will be investigated, such dynamic stall, compression shocks as well as aeroelastic stability. For that purpose new innovative double-swept rotor blades for the RTG have been developed at the DLR Institute of Aeroelasticity in Goettingen. The design of the planform is partially based on the existing wind tunnel model. Further profile sections and a rotor head attachment were added. Design load cases were determined from two dimensional numerical flow simulations regarding several rotor blade sections. A finite element analysis, including fatigue analysis, was carried out as well. In order to extract the dominant flow phenomena the rotor blades are instrumented with unsteady pressure transducers, temperature transmitters and strain gauges. The acquired data will enable subsequent evaluation concerning aeroelastic stability of the entire rotor head system and the rotor blade tip geometry. Furthermore the influence of rotation on the dynamic stall phenomenon can be assessed

Experimentelle Untersuchung des dynamischen Strömungsabrisses an einem Rotor mit axialer Zuströmung

Der dynamische Strömungsabriss (Dynamic Stall) am rücklaufenden Blatt des Hauptrotors schränkt die Flugenvelope von modernen Helikoptern ein. Durch großflächige Strömungsablösungen verursacht er hohe strukturelle Lasten mit starken Hystereseeffekten und beeinträchtigt die Steuerbarkeit. Mit analytischen und empirischen Modellen ist der Dynamic Stall nur schlecht vorhersagbar und wird im Hubschrauberentwurf wenig berücksichtigt. In dieser Dissertation wird ein Rotorteststand entwickelt, an dem der Dynamic Stall am Rotor vereinfacht und unter optimierten Randbedingungen untersucht werden kann. Er bietet eine gute optische Zugänglichkeit zur Anwendung berührungsloser Messverfahren. Der Dynamic Stall wird durch eine zyklische Rotorblattansteuerung erzielt. Bisher mussten Vorwärtsflugzustände am Rotor in sehr teuren Windkanalversuchen hergestellt werden, um den Dynamic Stall am Rotor zu untersuchen. Durch die Konstruktion des Teststandes werden darüber hinaus der Betrieb im Bodeneffekt und eine Rezirkulation des Rotornachlaufs vermieden. Der Rotor besteht aus zwei negativ verwundenen Rotorblättern mit einer Streckung von 7.1 und einem Radius von 0.65m. Er wird bei einer Drehfrequenz von 23.6Hz betrieben, was bei 75% Radius eine Machzahl von 0.21 und eine auf die Profiltiefe bezogene Reynoldszahl von 353000 ergibt. Am Rotorblatt wird die Umströmung anhand von Strömungsvisualisierung mit tufts, instationären Oberflächendruckmessungen, Strömungsfeldmessungen mit der Particle Image Velocimetry (PIV) sowie der Differential Infrared Thermography (DIT) zur Ablösedetektion vermessen. Weiterhin wird die Blattspitzenauslenkung erfasst. Messungen werden an unterschiedlichen radialen Schnitten auf dem Rotorblatt und erstmalig für den gesamten Rotorumlauf durchgeführt. Ein Telemetriesystem ermöglicht synchronisierte, instationäre Messungen im rotierenden System mit einer Signalbandbreite von 19kHz. Für eine zyklische Blattansteuerung mit einer Amplitude von 6.2° und einem mittleren Steuerwinkel von 16.9° bei 75% Radius zeigen PIV-, DIT-, tuft- und Oberflächendruckdaten eine Strömungsablösung für die Hälfte des Rotorumlaufes. Die stark dreidimensionale Ablösung wird bei 84% Radius initiiert und breitet sich von dort zur Blattspitze und -wurzel aus. Mehrere Ablösezellen, wie an schwingenden Profilen und Blattspitzen beobachtet, treten am Rotor nicht auf. An der Blattspitze und bei halber Spannweite tritt der Dynamic Stall schwächer und verzögert gegenüber 84% Radius auf. Während der Ablösung spielen Zentrifugalkräfte eine dominante Rolle im Rezirkulationsgebiet. Dort kommt es zu hohen spannweitigen Strömungsgeschwindigkeiten, die in Richtung Blattspitze linear zunehmen. Eine globale Einteilung des Dynamic Stalls in Light Stall oder Deep Stall kann aufgrund großer radialer Unterschiede im Ausmaß und im zeitlichen Verlauf des Dynamic Stalls nicht erfolgen. Durch die hohe Aperiodizität des Dynamic Stalls weisen Phasenmittelungen der PIV- und Druckdaten Abweichungen zu den instantanen Messwerten von bis zu 64% auf. Es werden dreidimensionale Strömungsphänomene aufgezeigt und mit dem nicht-rotierenden Fall sowie mit Ergebnissen aus numerischen Strömungssimulationen verglichen. Dies ermöglicht ein besseres Verständnis des Einflusses der Rotation auf den Dynamic Stall.Dynamic stall on the retreating main rotor blade limits the flight envelope of modern helicopters. High structural loads with hysteresis effects and handling quality issues are introduced due to extensive flow separation. It is difficult to predict dynamic stall by means of analytical and empirical methods. Therefore dynamic stall is not considered in detail during the helicopter rotor design process. In this thesis a test facility is developed to investigate dynamic stall on the rotor under improved boundary conditions. The improvements due to the design of the rotor test stand are: operation out of ground effect, the reduction of rotor wake recirculation and good optical access to apply non-intrusive measuring techniques. Dynamic stall is triggered by a cyclic variation of the blade pitch angle. Until now expensive wind tunnel test campaigns were necessary to simulate forward flight conditions in which dynamic stall occurs. The rotor consists of two negatively twisted blades with a tip radius of 0.65m and an aspect ratio of 7.1. It is operated at a rotational frequency of 23.6Hz, leading to a Mach number of 0.21 and a chord based Reynolds number of 353000, both at 75% radius. The flow around the blades is analyzed by means of tuft flow visualizations, unsteady surface pressure measurements, particle image velocimetry (PIV) for flow field measurements and differential infrared thermography (DIT) to detect flow separation. In addition the blade tip deflection is measured. Optical measurements were taken at different radii, covering the whole blade revolution for the first time. A telemetry system enabled for synchronized and unsteady measurements in the rotating frame with a bandwidth of 19kHz. For a cyclic pitch case with an amplitude of 6.2° and a mean pitch angle of 16.9° at 75% radius, PIV, DIT, tufts and surface pressure data show flow separation over one half of the blade revolution. The dynamic stall is observed to be strongly three-dimensional with the initial and strongest separation at 84% radius from whence it spreads in in- and outboard direction. Multiple stall cells, as found on pitching airfoils and blade tips, were not observed on the rotor. The separation behavior is delayed and weakened near the tip and at mid-span compared to 84% radius. Centrifugal forces dominate the recirculating flow during separation with instantaneous outward flow velocities increasing linearly towards the tip. Furthermore a global classification of the dynamic stall, namely, Light Stall or Deep Stall, is not reasonable due to radial differences. Phase-averaging of the PIV and surface pressure data is found to lead to an underestimation of instantaneous peak values up to 64% due to the strong aperiodicity of the dynamic stall. In this study differing three-dimensional flow features compared to the non-rotating pitching blade tip are revealed and the results are compared with numerical flow simulations. A deeper insight in the role of rotation in dynamic stall is given

CFD-Simulation zur Untersuchung von Interferenzen an einem Rotorteststand

Inhalt dieser Diplomarbeit ist die aerodynamische Analyse eines Teststandes an dem skalierte Modelle von Hubschrauberrotoren untersucht werden sollen. Hierzu wurde ein numerisches Modell für den RANS-Strömungslöser TAU erstellt. Der Versuchsaufbau besteht aus einer Düse, in deren Freistrahl ein Rotor installiert ist, dessen Achse parallel zur Anströmung ausgerichtet ist. Die Untersuchungen konzentrierten sich hauptsächlich auf Interferenzen von Düsenfreistrahl und Rotorströmung sowie den versperrenden Einfluss des Teststands an dem der Rotor installiert ist. Das Rotormodell wurde hierzu zunächst von der Düse, bzw. dem Teststand isoliert betrachtet, um anschließend im Vergleich mit dem Gesamtaufbau die jeweiligen Einflüsse zu bestimmen. Es zeigte sich, dass die angesaugte Düsenscherschicht mit zunehmender Rotorleistung deutlich stärkere Störeinflüsse verursacht, während die Rückstauung durch den Teststand an Bedeutung verliert. Abstract: The subject of this thesis is an aerodynamic analysis of a test facility for the research on scaled helicopter rotors. For this purpose, a numerical model was created for the RANS-flow-solver TAU. The test facility is composed of a rotor, which is installed in the free jet of a nozzle with its axis parallel to the incoming flow. The main focus of the research is on the interference effects of the free jet of the nozzle and the rotor flow, as well as the blocking influence of the test bench itself. For this purpose, the rotor model was isolated from the nozzle and respectively from the test bench, to determine the influences in comparison to the full test configuration. As a result it became evident that the disturbance effects from the aspirated shear flow of the nozzle strongly grow with increasing rotor power, whereas the blockade of the test bench becomes less important

The effect of vortex generators on the flow around a circular cylinder

The effect of circular low-profile vortex generators for rotorcraft application has been investigated on a circular cylinder at a high Reynolds number. These so called Leading Edge Vortex Generators (LEVoGs) have shown significant reductions on the impact of dynamic stall on a pitching helicopter airfoil. However, the principle of operation of such devices remained unclear. In order to understand the aerodynamic phenomena, a wide range of experimental techniques such as oil flow visualization, infrared thermography, surface pressure measurements and stereo particle image velocimetry were employed. In addition, several post-processing approaches such as the proper orthogonal decomposition method were selected. The passive devices were found to significantly alter the flow around the cylinder. Depending on the azimuth angle of the vortex generators, the flow conditions could be classified into three categories, each with characteristic features. Similar to previous experiments on a helicopter rotor blade section, bundling of vortex generator wakes could be found under certain conditions. The vortex generators reduced the periodic Reynolds stress components v'v'/U∞2 and u'v'/U∞2 for all cases. For the cases where bundling was present, the size and peak values of the periodic u'u'/U∞2 component was increased significantly compared to the clean case. This means that the vortex generators almost only influence the periodic motion of the flow under these conditions. Even though the principle of operation couldn't be entirely cleared, the present study sheds some light onto the various mechanisms in the flow around such devices

Dynamic Stall Experiments on a Rotor with High Cyclic Setting in Axial Inflow

Dynamic stall is one of the limiting aerodynamic phenomena that occur on modern helicopter rotors during fast forward and maneuvering flight conditions. During dynamic stall the effective angle of attack on the retreating side of the rotor exceeds the static stall angle and a lift peak as well as a negative peak in the pitching moment are observed. These loads can be crucial regarding the blades and the pitch links structural integrity and therefore limit the flight envelope of modern helicopters. Separated flows, especially in a rotating frame, are strongly three-dimensional, which is neglected when examining two-dimensional pitching airfoils where the flow is assumed to be two-dimensional in the mid-section of the model. Therefore experiments are conducted on model rotors, e.g. in hover chambers. Two major disadvantages apply to most (small) hover chambers: usually the rotor is operated in ground effect and the rotor inflow is disturbed due to recirculation of the rotor wake. The closest representation of the flow around a full-scale helicopter is achieved in experiments conducted in wind tunnels where it is possible to achieve fast forward flight conditions. However these are often very complex and expensive. During the design phase of the rotor test facility (RTG) at the DLR in Göttingen, three major aspects were considered: avoidance of an operation in ground effect, supply of a defined rotor inflow with prevention of a rotor wake recirculation and construction of an easy to use facility with good optical access to the rotor. These aspects resulted in the design

Development of a Rotor Test Facility for the Investigation of Dynamic Stall

The flight envelope of modern helicopters is limited due to unsteady flow phenomena such as dynamic stall. Most investigations regarding this topic, both experimental and numerical, were done on pitching two-dimensional airfoils. Therefore, it is of great interest to investigate the dynamic stall on rotating blades in order to get a better understanding of the unsteady aerodynamics and to improve future rotor designs. A rotor test facility is being built at the DLR in Göttingen. The test rig is integrated into an existing wind tunnel facility and the surrounding conditions were defined. Critical load cases were derived from numerical flow simulations of the rotor blade, and finite element analysis including a fatigue analysis was performed. A telemetry system is integrated into the rotor head and unsteady pressure sensors are installed in the rotor blades. The development process with its validation and the design of the rotor head and blades are described in this paper