Hot–wire measurements reveal the evolution of three-dimensional TS (Tollmien-Schlichting) waves and other nonlinear disturbances generated by a ZNMF (Zero Net Mass-Flux) jet. The base flow consists of a highly two-dimensional Blasius boundary layer with extremely small extraneous background disturbance levels (u/U1 < 0.08 %). The response is shown to be linear and symmetrical for sufficiently small actuator amplitudes and under these conditions the TS wave motions conform with the PSE (Parabolized Stability Equations) results of Mack & Herbert (1995). The observations suggest that a small-amplitude ZNMF jet would be a suitable device for active LFC (Laminar Flow Control) applications. For larger actuator amplitudes, other short–wavelength instabilities develop and grow with streamwise development and they ultimately breakdown to form a turbulent wedge. There is an actuator amplitude threshold below which these instabilities do not form, and a larger threshold below which the instabilities do not grow with streamwise development. The characteristics of the turbulent wedge are also considered in some detail.

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