Influence of ZNMF jet flow control on the spatio-temporal flow structure over a NACA-0015 airfoil

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 × 10 4 ) 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 c μ = 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 f wake 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 2 f wake produces a series of vortices that advect parallel to the airfoil surface at a speed lower than the freestream velocity. Potential mechanisms by which these vortices affect flow reattachment are discussed.