Stall flutter of NACA 0012 airfoil at low Reynolds numbers

Bhat, Shantanu S and Govardhan, Raghuraman N (2013) Stall flutter of NACA 0012 airfoil at low Reynolds numbers. In: Journal of Fluids and Structures, 41 . pp. 166-174.

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Abstract

In the present work, we experimentally study and demarcate the stall flutter boundaries of a NACA 0012 airfoil at low Reynolds numbers (Re similar to 10(4)) by measuring the forces and flow fields around the airfoil when it is forced to oscillate. The airfoil is placed at large mean angle of attack (alpha(m)), and is forced to undergo small amplitude pitch oscillations, the amplitude (Delta alpha) and frequency (f) of which are systematically varied. The unsteady loads on the oscillating airfoil are directly measured, and are used to calculate the energy transfer to the airfoil from the flow. These measurements indicate that for large mean angles of attack of the airfoil (alpha(m)), there is positive energy transfer to the airfoil over a range of reduced frequencies (k=pi fc/U), indicating that there is a possibility of airfoil excitation or stall flutter even at these low Re (c=chord length). Outside this range of reduced frequencies, the energy transfer is negative and under these conditions the oscillations would be damped. Particle Image Velocimetry (PIV) measurements of the flow around the oscillating airfoil show that the shear layer separates from the leading edge and forms a leading edge vortex, although it is not very clear and distinct due to the low oscillation amplitudes. On the other hand, the shear layer formed after separation is found to clearly move periodically away from the airfoil suction surface and towards it with a phase lag to the airfoil oscillations. The phase of the shear layer motion with respect to the airfoil motions shows a clear difference between the exciting and the damping case.

Item Type:	Journal Article
Publication:	Journal of Fluids and Structures
Publisher:	Elsevier Science
Additional Information:	Copyright of this article belongs to Elsevier Science.
Keywords:	Stall Flutter; Oscillating Airfoil; Energy Transfer; Leading Edge Separation; Shear Layer
Department/Centre:	Division of Mechanical Sciences > Mechanical Engineering
Date Deposited:	22 Oct 2013 12:06
Last Modified:	22 Oct 2013 12:06
URI:	http://eprints.iisc.ac.in/id/eprint/47547

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