Study on the Breakdown of Compressible Jet and the Mechanism of its Aeroacoustic Emission

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In this study both numerical simulation and experiments regarding high-speed jets were performed. In the experiments mini-jets were used as an actuator to enhance jet mixing, the results of which are summarized as follows : (1) The flapping mode of actuation was most effective in mixing, where the axial length of the potential core was reduced by 35%, compared with that of the baseline. (2) As the frequency of actuation becomes higher, the jet breaks down more upstream. (3) As under-expanded jets are intrinsically stable, it does not change so easily by external disturbances, compared with the case of optimum jet. (4) As for noise, only steady actuation can decrease the noise level. On the other hand, regarding numerical simulations, the following results were obtained. (1) In axisymmetric calculation of jet, periodical roll-up of the jet shear layer was observed. These vortices merge with each other, and show the motion of acceleration, which leads to sound emission. (2) In 3D simulation, some instability produced upstream cannot maintain the state of being axisymmetric in the downstream, and three-dimensional unstable modes appear. As a result, the degree of jet breakdown is weakened, which reduces Mach wave-like sound emissions. (3) The velocity distribution along the jet axis shows good agreement between 3D numerical simulation and experimental data. However, there is a slight difference between them in the downstream. (4) In under-expanded jets, vortices in the jet shear layer interact with the shock cell, which makes a kind of noise.