Shock Oscillations in a Supersonic Jet Exhibiting Antisymmetrical Screech

T HIS contribution follows a Technical Note published by the authors inAIAAJournal [1], inwhich a shock-trackingalgorithm was applied to a screeching underexpanded jet. It was shown that the shocks oscillate at the screech frequency, in a symmetrical or antisymmetrical manner, according to the screech mode. The temporal signals of twodiametrically opposed near-fieldmicrophones were also considered. At a nozzle pressure ratio (NPR) of 2.54, corresponding to a flapping mode B, both microphones showed a simultaneous modulation of the screech amplitude at a frequency of the order of 1=1000 of the screech frequency. A schlieren video was extracted from the complete recording for each one of the weak and strong screech spells. The tracking algorithm was applied on the resulting movies, and it was deduced that the shock oscillation amplitude increases with an increase in the screech level, which is in agreement with the analytical model proposed by Panda [2]. The picture of the flapping screech viewed by two diametrically opposed microphones appears, however, too simplistic, and a property of screech that remained unseen at that time is highlighted in the present work. Screech is a tonal component of shock-associated noise arising in incorrectly expanded supersonic jets. It was first studied by Powell [3], who already mentioned the existence of modes. Of particular importance for the present study is the flapping mode B, whose pressure signal in the near field is antisymmetrical about a plane. The modal behavior of screech was later extensively studied, especially by Powell et al. [4] as well as Ponton and Seiner [5], who noted that the plane of antisymmetry can slowly rotate or oscillate. This feature, referred to in the following as plane rotation, was investigated in some detail by the authors [6] using a near-field azimuthal microphone antenna comprising 18 transducers located every 20 deg. It was shown that the plane rotation was related to the simultaneous presence of two counter-rotating helices of slightly different frequencies in the screech azimuthal-mode content. It was also explained that the screech level is minimum near the plane of antisymmetry and maximum perpendicular to that direction. In light of [6], it is believed that the strong amplitude modulation in time of the microphone signals in the original study [1] was caused by the rotation of the plane of antisymmetry. The plane rotation was then interpreted as a time modulation of screech strength due to the small number of microphones used. The identification of the plane rotation in the acoustic field has suggested a possible rotation of the plane of antisymmetry of the shock motion. This question is considered in the present paper. The experimental facility is first presented. Then, results for two cases of flapping screech are discussed.