Steady two-dimensional jet flows have attracted the interest of many investigators because of their fundamental and practical importance. Much of the currently available information has been summarized by Harsha [5.45], Rajaratnam [5.46], and Everitt and Robins [5.47]. Significant interest in unsteady-jet-flow effects first was sparked by the development of the pulse-jet engine, especially when Bertin [5.48] and Lockwood [5.49] noted the favorable effect of pulsating jet flow on secondary-flow entrainment. Lockwood [5.49] also identified the generation of ring vortices in pulsing flow, a phenomenon later verified more clearly by Curtet and Girard [5.50]. Further investigations of pulsating jet flows were performed by Johnson and Yang [5.51], Didelle et al. [5.52], Binder and Favre-Marinet [5.53], Crow and Champagne [5.13] and, most recently, Bremhorst and Harch [5.54] and Bremhorst and Watson [5.55]. A different type of unsteady jet flow is produced by time-varying jet deflection, either by mechanical oscillation of the jet nozzle or by fluidic jet actuation. The flow patterns produced by a mechanically oscillated jet exhausting into a secondary flow recently were measured in some detail by Simmons et al. [5.56], whereas fluidic jet nozzles were developed and investigated by Viets [5.57, 5.58].