Eliminating noise from supersonic jet plumes is important to the success of high speed transport. There exist predictions and experimental observations that swirling motion may lead to elimination of noise emission from supersonic jets [1,2]. However, the observation was qualitative [1] and the prediction was based on a linear, inviscid and nearly isentropic theory [2]. The possible elimination of screech tones from swirling supersonic jets was attributed to either the occurrence of jet flow recirculation due to swirl [1,3] or the elimination of the shock cell structure which is necessary for screech tones [2].
Swirl has been known to shorten the flame length, an indication of enhanced mixing [4,5], and improve flame stabilization in subsonic combustors [6,7]. These are also desirable characteristics in the supersonic combustion in high speed transport. It was believed that enhanced mixing due to swirl was responsible for eliminating shock cells in supersonic jets [2]. Swirl can then be a mechanism to achieve multiple purposes in the applications of supersonic flow and high speed transport.
However, experimental data of shock structures and screech tone noise of swirling supersonic jets have been limited. An experimental effort was therefore made to reveal the screech tone characteristics of underexpanded swirling jets. These characteristics are compared with their counterparts in non-swirling supersonic jets.
Key results from previous studies on non-swirling circular jets are described. For the details, a recent article [8] can be consulted. There are three major components of the noise of a supersonic jet which, in increasing order of frequency, are turbulent mixing noise, screech tones and broadband shock noise. Turbulent mixing noise is known to exist in both subsonic and supersonic flows. The broadband shock noise arises as a result of interaction of turbulent eddies with shock waves. The screech tones are special cases of the broadband noise; namely, the results of the interaction between large scale turbulence structure or the instability waves originating at the nozzle lip with the quasi-periodical shock cell structure. The interaction occurs at the edge of the jet in the region of the fourth and fifth shock cells. The screech tone noise propagates outside the jet and upstream toward the nozzle lip, and further excites the instability waves there, thus completing an acoustic feedback loop. Their sound pressure levels (SPL) are significantly higher than the other two components when measured from the upstream direction. The quasi-periodical shock cell structures are critical for the screech to occur in narrow bands, as experimentally observed. Two modes of screech tones (axisymmetric/toroidal and helical/flapping) are known to exist, although not occurring simultaneously. Helical modes become increasingly more dominant at high pressure ratios. This phenomenon is