A system of 36 impinging microjets was implemented on a round jet of Mach number 0.9, and the noise reduction was studied as a function of the microinjection mass flux, the number of microjets blowing, the layout of the blowing microjets, and the microjet diameter. Depending on the microinjection flux parameters, overall jet-noise reduction varied from 0 to 1.5 dB, showing some nonmonotonic behavior due to the change between subsonic to supersonic microjet regimes. The study of the microjet layout showed that the noise reduction decreased when the microjets were too close to each other and that certain configurations of microjet pairs could be favorable; this can be related to the flow structures induced by the microjets. Spectral analysis disclosed different control mechanisms, with highfrequency regeneration for high-injection flux, local control behavior at midfrequencies, and global behavior at low frequencies.
This experimental study examines the efficiency of wall pulsing control that is applied to a confined flow going through a two-dimensional diffuser continued by a duct. One-and dual-side pulsing generated by synthetic jet actuators are tested as control means. Particle image velocimetry is used to investigate instantaneous velocity fields and obtain quantitative information. One-side pulsing provokes vectoring of the primary flow in the same direction as the periodic excitation and increases its lateral expansion. Dual-side pulsing neutralizes vectoring but is more efficient for spreading. In both cases, the inner structure of the primary flow was found to be strongly modified, with the generation of highly periodic large-scale structures which kept the flow attached to the diffuser walls. The fluctuating velocity maps point out that the high-fluctuation zone greatly spreads out under control. The flow is considerably widened and the mixing is enhanced.
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