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Research Papers: Flows in Complex Systems

Dynamic Stall Flow Structure and Forces on Symmetrical Airfoils at High Angles of Attack and Rotation Rates

[+] Author and Article Information
R. R. Leknys, M. Arjomandi, R. M. Kelso, C. H. Birzer

The School of Mechanical Engineering,
The University of Adelaide,
Adelaide 5006, Australia

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received January 11, 2018; final manuscript received September 7, 2018; published online November 13, 2018. Assoc. Editor: Pierre E. Sullivan.

J. Fluids Eng 141(5), 051104 (Nov 13, 2018) (15 pages) Paper No: FE-18-1022; doi: 10.1115/1.4041523 History: Received January 11, 2018; Revised September 07, 2018

This article describes a direct comparison between two symmetrical airfoils undergoing dynamic stall at high, unsteady reduced frequencies under otherwise identical conditions. Particle image velocimetry (PIV) was performed to distinguish the differences in flow structure between a NACA 0021 and a NACA 0012 airfoil undergoing dynamic stall. In addition, surface pressure measurements were performed to evaluate aerodynamic load and investigate the effect of laminar separation bubbles and vortex structures on the pressure fields surrounding the airfoils. Airfoil geometry is shown to have a significant effect on flow structure development and boundary layer separation, with separation occurring earlier for thinner airfoil sections undergoing constant pitch-rate motion. Inertial forces were identified to have a considerable impact on the overall force generation with increasing rotation rate. Force oscillation was observed to correlate with multiple vortex structures shedding at the trailing-edge during high rotation rates. The presence of laminar separation bubbles on the upper and lower surfaces was shown to dramatically influence the steady-state lift of both airfoils. Poststall characteristics are shown to be independent of airfoil geometry such that periodic vortex shedding was observed for all cases. However, the onset of deep stall is delayed with increased nondimensional pitch rate due to the delay in initial dynamic-stall vortex.

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Figures

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Fig. 1

Airfoil angular displacement with respect to nondimensional time for reduced frequencies of κ=0.05, κ=0.1, and κ=0.2

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Fig. 2

Schematic diagram of the water channel test facility used to conduct the constant pitch-rate experiments, showing the orientation of the digital camera, laser sheet and dynamic stall test apparatus

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Fig. 3

Wind tunnel and motion controller arrangement (a) and details of the NACA 0012 wing body showing the orientation and locations of the imbedded differential surface pressure sensors

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Fig. 4

Initiation of the leading-edge vortex showing its delay in formation to higher angles of attack as the nondimensional pitch rate is increased from κ=0.05−0.2 for a NACA 0012 airfoil. Also observed is the detached shear layer and additional stagnation points on the upper surface of the airfoil surface, near the leading and trailing-edge, during the pitch-up motion.

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Fig. 5

Normalized vorticity contours and streamlines developed during dynamic stall of a NACA 0012 (left) and NACA 0021 (right) airfoil. κ=0.05.

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Fig. 6

Normalized vorticity contours and streamlines of a NACA 0012 (left) and NACA 0021 (right) airfoil undergoing dynamic stall where κ=0.1

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Fig. 7

Normalized vorticity contours and streamline development of a NACA 0012 (left) and NACA 0021 (right) airfoil undergoing dynamic stall where κ=0.2

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Fig. 8

NACA 0012 (a) and NACA 0021 (b) lift coefficient comparing the dynamic stall and steady-state test cases. Initial lift delay is observed for the steady-state case, while inertial and vortex lift is evident in dynamic stall conditions. Error bars indicate maximum and minimum fluctuation in force coefficient evaluated, while an error of 1.2% was established about the mean to achieve a 95% confidence level.

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Fig. 9

Pressure drag coefficient of both the NACA 0012 and NACA 0021 airfoil for steady and dynamic stall conditions indicating increased drag coefficient with leading-edge vortex formation and increased nondimensional pitch rate. Error bars indicate maximum and minimum drag force coefficient values recorded, while the error of 1.2% was established about the mean to achieve a 95% confidence level.

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Fig. 10

NACA 0021 surface pressure distribution highlighting the increased suction on the lower surface resulting from the formation of a separation bubble at low angles of attack, and over the rear of the airfoil at low Reynolds numbers

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Fig. 11

Moment coefficient for the NACA 0012 (a) and NACA 0021 (b) airfoils indicating an decrease in airfoil moment with increased rotation rate, while vortex separation leads to an increase in negative pitching moment of both airfoils. The reduced frequency is κ=0.05, κ=0.1, and κ=0.2, while Re=20,000.

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Fig. 12

Vortex patterns observed, using the hydrogen-bubble wire technique, at the trailing-edge of the NACA 0012 airfoil resulting in force oscillation during ramp-up dynamic stall [65]

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Fig. 13

Surface pressure distribution around the NACA 0012 airfoil experiencing constant-pitch-rate motion to αmax=90deg and for multiple unsteady reduced frequencies showing effects of the leading-edge vortex and airfoil rotation rate on both the upper and lower pressure distribution

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