Aerodynamic and structural investigation of an active back-flow flap for dynamic stall control

Opitz, Steffen; Gardner, Anthony Donald; Kaufmann, Kurt

doi:10.1007/s13272-014-0106-3

Cited by 2 publications

(8 citation statements)

References 11 publications

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“…Figure 3a shows the reduction in maximum lift caused by different lengths and hinge positions of the flap, showing that with increasing flap length, the peak lift is reduced. Flap positions near the airfoil leading edge suffered from both low control authority and (as measured in [37]) large forces on the flap during attached flow which tended to open it. Flaps near the trailing edge did not affect the formation of the leading edge vortex.…”

Section: Design Of the Experiments 21 Auxiliary Cfd Airfoil Computationsmentioning

confidence: 99%

“…Unfortunately this differs from the real flap due to this gap, Fig. 1 Back-flow flap geometry [37] and the computation used a simplified prescribed flap movement rather than modeling the dynamics of the flap, meaning that the simulations cannot be considered to be highly accurate. The test case used was a deep dynamic stall case equivalent to a ¼ 22 AE 8 at k ¼ 0:06, and the Spalart-Allmaras turbulence model was used.…”

Section: Design Of the Experiments 21 Auxiliary Cfd Airfoil Computationsmentioning

confidence: 99%

“…Initial investigations using CFD indicated that a reduction of the pitching moment peak of the order of 25-35% could be achieved [36]. Further investigations showed that actuating the flap passively might not work as hoped [37], and using this background, a glove for an existing wind-tunnel model was designed with a solid-state hinge, a magnetic actuator system and straps to limit the flap opening angle to 30 [37,38]. The variant as investigated can be actuated either passively or actively and does not increase the drag for attached flow.…”

Section: Introductionmentioning

confidence: 99%

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Reduction of dynamic stall using a back-flow flap

Gardner¹,

Opitz²,

Wolf³

et al. 2017

CEAS Aeronaut J

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A back-flow flap attached to the suction side of an airfoil is investigated in both passively and actively actuated modes for the control of dynamic stall. This method of dynamic stall control has low power requirements and no parasitic drag when not actuated. Experiments in a low-speed wind tunnel at 50 m/s were used to characterize the reduction in dynamic stall hysteresis using pressure measurements on the midline airfoil section. It was found that the pitching moment peak is reduced by an average of 25% for all deep stall test cases for active actuation of the flap, while for passive actuation the pitching moment peak is reduced by 19%. In each case the maximum lift remained the same, while the peak drag increased by an average of 2.5% for the active flap, and by 0.9% for the passive flap. With the flap closed at low angles of attack, the reference values of the airfoil are retained. KeywordsHelicopter blade Á Wind tunnel Á Flow control Á Dynamic stall Á Back-flow flap Á Experiment List of symbols a; a max Angle of attack; maximum ( ) c Airfoil model chord (m) C D ; C D max Drag coefficient; peak C L ; C L max Lift coefficient; peak C M ; C M min Pitching moment coefficient; peak C P Pressure coefficient f Frequency of pitching (Hz) M Mach number k Reduced frequency: k ¼ pfc=v 1 Re Reynolds number based on c t Time (s) T Period (s) u, v Velocity: in x direction, in z direction (m/s) v 1Freestream velocity (m/s) x, zCoordinates: chord, upward (m)This paper is based on a presentation at the AHS 72nd Annual Forum,

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Section: Design Of the Experiments 21 Auxiliary Cfd Airfoil Computationsmentioning

confidence: 99%

Section: Design Of the Experiments 21 Auxiliary Cfd Airfoil Computationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Reduction of dynamic stall using a back-flow flap

Gardner¹,

Opitz²,

Wolf³

et al. 2017

CEAS Aeronaut J

Self Cite

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“…In both cases, a retention system limits the maximum opening angle to about 30 • . A detailed description of the flap design and further details, for example the determination of the flap angle using Hall-effect sensors, is given by Opitz et al [17,18]. The model can be mounted into a test rig using a shaft protruding from both sides of the airfoil.…”

Section: Model Geometry and Test Conditionsmentioning

confidence: 99%

“…Therefore, the back-flow flap assembly and an active electromagnetic actuation system was built as a "retro-fit" kit for an existing wind tunnel model which uses the OA209 helicopter main rotorblade airfoil. The design of the flap setup is outlined by Opitz et al [17,18], including a detailed description of the flap's kinematics and its flexure hinge. For dynamic stall conditions, the active back-flow flap showed very promising results as seen by surface-pressure measurements [19].…”

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confidence: 99%

Influence of a back-flow flap on the dynamic stall flow topology

et al. 2017

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Dynamic stall is a major concern for highly loaded helicopter rotors in fast forward flight. The potential of a back-flow flap for dynamic stall reduction is investigated. The flap assembly is mounted on the suction side of a helicopter main rotor-blade airfoil undergoing deep-stall pitch oscillations. Wind-tunnel experiments using high-speed particle image velocimetry were conducted to identify the flow topology and to investigate the flap's method of operation. A phase-averaged proper orthogonal decomposition (POD) is used to identify relevant flow events and to compare test cases with and without flap. The evolution of the large-scale dynamic stall vortex in the initial phases of flow separation is analyzed in detail. The back-flow flap splits the vortex into two smaller vortices and thereby reduces the pitching moment peak. This effect can be described through the eigenmode coefficients of the POD. The study closes with an analysis of different pitching frequencies, which do not affect the flap's method of operation.

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Aerodynamic and structural investigation of an active back-flow flap for dynamic stall control

Cited by 2 publications

References 11 publications

Reduction of dynamic stall using a back-flow flap

Reduction of dynamic stall using a back-flow flap

Influence of a back-flow flap on the dynamic stall flow topology

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