Structure of Supersonic Twin Jets

The flow characteristics of coupled oscillatory motion of Mj =1.5 supersonic twin jets are described using the particle image velocimetry measurements. As a result of self-excitation caused by screech, sinuous oscillation of individual jets is observed. The coupling between the jets, characterized by the presence of coherent large-scale eddies, results in a symmetric mode with respect to the midplane separating the two jets. A control technique using microjets placed at the nozzle exit is used to suppress the unsteady flow. The results show that the predominant unsteady character of the twin jets is reduced considerably as a result of the elimination of large-scale organized structures with the introduction of the control. The turbulence levels show a 50% decrease in turbulent kinetic energy in the mixing region of the twinjet. Consequently, up to 4-dB reductions in overall sound pressure level of jet noise are observed. The reduction in noise and turbulence is accompanied by a decrease of about 20% in the entrainment of the ambient fluid around the jet.