Abstract
Most synthetic-jet-actuator flow-control applications utilize relatively large amplitude forcing. Under these conditions the asymmetrical inflow and outflow characteristics lead to a net momentum transfer while the net mass flux is zero. Hot-wire measurements are presented which demonstrate that the response is linear and symmetrical for sufficiently small actuator amplitudes. The base flow is a Blasius boundary layer with an exceptionally small background disturbance level. For small actuator amplitudes the disturbance has the form of three-dimensional TS (Tollmien-Schlichting) waves which conform with the results of computations using the Parabolized Stability Equations (PSE). For larger actuator amplitudes, other short-wavelength instabilities develop and grow with streamwise development and they ultimately breakdown to form a turbulent wedge. The evidence suggests that the instabilities have the form of an absolute Rayleigh-type instability associated with locally inflectional velocity profiles. There is an actuator amplitude threshold below which these short-wavelength instabilities do not form, and a larger threshold below which the instabilities do not grow with streamwise development. Characteristics of the wedge are considered in some detail.
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