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The Onset of Dynamic Stall Revisited

Mulleners, Karen and Raffel, Markus (2012) The Onset of Dynamic Stall Revisited. Experiments in Fluids, 52 (3), pp. 779-793. DOI: 10.1007/s00348-011-1118-y. ISSN 0723-4864.

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Official URL: http://www.springerlink.com/content/0723-4864/52/3/


Dynamic stall on a helicopter rotor blade comprises a series of complex aerodynamic phenomena in response to the unsteady change of the blade’s angle of attack. It is accompanied by a lift overshoot and delayed massive flow separation with respect to static stall. The classical hallmark of the dynamic stall phenomenon is the dynamic stall vortex. The flow over an oscillating OA209 airfoil under dynamic stall conditions was investigated by means of unsteady surface pressure measurements and time–resolved particle image velocimetry. The characteristic features of the unsteady flow field were identified and analysed utilising different coherent structure identification methods. An Eulerian and a Lagrangian procedure were adopted to locate the axes of vortices and the edges of Lagrangian coherent structures, respectively; a proper orthogonal decomposition of the velocity field revealed the energetically dominant coherent flow patterns and their temporal evolution. Based on the complementary information obtained by these methods the dynamics and interaction of vortical structures were analysed within a single dynamic stall life cycle leading to a classification of the unsteady flow development into five successive stages: the attached flow stage; the stall development stage; stall onset; the stalled stage; and flow reattachment. The onset of dynamic stall was specified here based on a characteristic mode of the proper orthogonal decomposition of the velocity field. Variations in the flow field topology that accompany the stall onset were verified by the Lagrangian coherent structure analysis. Furthermore, the orientation of the trajectories of vortices that originated at the very leading edge shortly before and after stall onset was observed to be altered. The mechanism that results in the detachment of the dynamic stall vortex from the airfoil was identified as vortex induced separation caused by strong viscous interactions. Finally, a revised criterion to discern between light and deep dynamic stall was formulated.

Document Type:Article
Additional Information:Published online: 24. May 2011
Title:The Onset of Dynamic Stall Revisited
AuthorsInstitution or Email of Authors
Mulleners, Karenkaren.mulleners@dlr.de
Raffel, Markusmarkus.raffel@dlr.de
Journal or Publication Title:Experiments in Fluids
Refereed publication:Yes
In ISI Web of Science:Yes
Page Range:pp. 779-793
Keywords:dynamic stall, time–resolved PIV, coherent structure analysis, unsteady separation
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Aeronautics
HGF - Program Themes:Rotorcraft (old)
DLR - Research area:Aeronautics
DLR - Program:L RR - Rotorcraft Research
DLR - Research theme (Project):L - The Virtual Aerodynamic Rotorcraft (old)
Location: Göttingen
Institutes and Institutions:Institute of Aerodynamics and Flow Technology > Helicopters
Deposited By: Karen Mulleners
Deposited On:01 Mar 2012 15:16
Last Modified:07 Dec 2015 02:23

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