The internal flow characteristics of a fluidic oscillator were investigated experimentally. Particle image velocimetry and time-resolved pressure measurements were employed in water to visualize and quantify the internal flow patterns. The method of proper orthogonal decomposition was applied to random flow field snap shots for phase reconstruction of one oscillation cycle. The resulting phase-averaged information provides detailed insight into the oscillation mechanism as well as into the interaction between the main chamber of the oscillator and its feedback channels. A growing recirculation bubble between the main jet and the attachment wall is identified as an underlying mechanism that causes the main jet to oscillate. The flow field measurements are complemented by time-resolved pressure measurements at various internal locations which yield additional comprehension of the switching behavior and accompanying timescales. Geometrical features, in particular at the inlet and outlet of the mixing chamber, are found to have a crucial impact on important flow characteristics such as oscillation frequency and jet deflection.Peer ReviewedPostprint (published version

the results infer that the jet's oscillation pattern is approximately sinusoidal with comparable residence and switching times.

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Experiments aimed at delaying flow separation through discrete jets pointing in the direction of streaming and sweeping side to side along the span were conducted on a V-22 airfoil with and without deflected trailing-edge flaps. The results indicated substantial drag reduction and lift increase at moderately low inputs of mass and momentum. Additional experiments were carried out on a semispan V-22 wing/nacelle combination, and they too provided an increase in lift-to-drag ratio L=D of approximately 60% (although active flow control was applied to the wing only). The effectiveness of the sweeping jets on reducing the download force acting on a V-22 full-span powered model in hover was also examined. A 29% reduction in download was realized using the embedded sweeping jets, corresponding approximately to a 2000 lb increase in hover lift. NomenclatureA slot = total area of the nozzle exits A wing = area of the wing= jet momentum L=D = lift-to-drag ratio Re = Reynolds number r = air density U jet = jet velocity U 1 = freestream velocity = angle of attack

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