The objective of this work is to study the basic mechanism of the active control of free shear flows and look for new concepts in actuation methodologies. The possibility of controlling the evolution of a triangular jet is discussed in Chapter 2. The piezoceramic actuators were mounted on the flat sides of the nozzle. The flow was fully sheared near the nozzle. Single mode excitation with frequency as the varying parameter was found to be ineffective for controlling the far field evolution. In contrast, excitation of the jet with non-integer and counter propagating azimuthal modes yielded marked changes in the jet evolution. A new method employing dynamic spatial modes to control the far field of a circular jet is examined in Chapter 3. The basic mechanism that governs the far field control was found to be the non-linear interaction of the instabilities of the standing waves. This observation agrees with conclusions of former investigators but with a new understanding that the non-linear interaction mechanism and the jet expansion are not to become effective until the potential core ends, after which the jet demonstrates large, directional expansion. It was shown in the experiment that this expansion could be easily predicted and controlled. These results point out a better control mechanism -- the dynamic mode control method. Three schemes were proposed: 1) the phase modulation, 2) the switching modulation, and 3) the spatial mode modulation, where the first two were implemented with great success in controlling the evolution of the jet flow. Finally, a triangular jet with a top hat initial velocity profile is examined in Chapter 4. The results of excited and unexcited jets were compared with those studies where the initial flow conditions were different. It was found that the initial flow condition affected the flow field in two ways: the axis switching and the jet expansion. The combination of an initial top-hat velocity profile and a non-symmetric nozzle geometry was proved to be the necessary condition to create axis switching. Proper combination of initial velocity profile, nozzle geometry and spatial mode could greatly enhance the jet expansion. / Ph. D.