Influence of ZNMF jet flow control on the spatio-temporal flow structure over a NACA-0015 airfoil
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RESEARCH ARTICLE
Influence of ZNMF jet flow control on the spatio-temporal flow structure over a NACA-0015 airfoil N. A. Buchmann • C. Atkinson • J. Soria
Received: 27 June 2012 / Revised: 22 January 2013 / Accepted: 12 February 2013 / Published online: 28 February 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract The spatio-temporal flow structure associated with zero-net-mass-flux (ZNMF) jet forcing at the leading edge of a NACA-0015 airfoil (Re = 3 9 104) is investigated using high-repetition rate particle image velocimetry. Measurements are performed at an angle of attack of 18°, where in the absence of forcing, flow separation occurs at the leading edge. Forcing is applied at a frequency of f? = 1.3 and a momentum coefficient cl = 0.0014 for which previous force measurements indicated a 45 % increase in lift over the unforced case. The structure and dynamics associated with both the forced and unforced case are considered. The dominant frequencies associated with separation in the unforced case are identified with the first harmonic of the bluff body shedding fwake closely corresponding to the forcing frequency of f? = 1.3. A triple-decomposition of the velocity field is performed to identify the spatio-temporal perturbations produced by the ZNMF jet forcing. This forcing results in a reattachment of the flow, which is caused by the generation of large-scale vortices that entrain high-momentum fluid from the freestream. Forcing at 2fwake produces a series of vortices that advect parallel to the airfoil surface at a speed lower than
This article is part of the collection Topics in Flow Control. Guest Editors J. P. Bonnet and L. Cattafesta. N. A. Buchmann (&) C. Atkinson J. Soria Laboratory for Turbulence Research in Aerospace and Combustion Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, VIC 3800, Australia e-mail: [email protected] J. Soria Department of Aeronautical Engineering, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia
the freestream velocity. Potential mechanisms by which these vortices affect flow reattachment are discussed.
1 Introduction One of the primary aims of flow control is to prevent boundary layer separation and as such extend the effective operational range of engineering devices such as diffusers, compressors, turbines and airfoils. Flow control can be implemented by either the addition of fixed geometric features such as vortex generators or flaps, (referred to as passive control), or via the addition of kinetic energy in order to modify the flow (i.e., active control, Donovan et al. 1998). Active control can take various forms, such as steady or oscillatory blowing and/or suction, vibrating surfaces or acoustic excitation (see Greenblatt and Wygnanski 2000) and unlike passive methods can be deactivated or adjusted to cater to a wider range of operating conditions. In the case of flow over airfoils, active flow control via periodic excitation has been shown to delay separation and hence increase lift at angles of attac
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