Coherent structures of free acoustically stimulated jet

Abstract

This paper presents an experimental stability analysis of a free round jet performed with the use of acoustic excitation. The experiment was performed for the case of a thin laminar shear layer at the nozzle exit and the ratio of jet diameter d to exit shear layer momentum θ was constrained within the range d/θ = 70–180. The Re number based on jet diameter and exit velocity was varied in the range 5 × 10³–10 × 10⁴. The results of experiments were compared with stability calculations performed for the 2D shear layer by Michalke and with an analysis made for the cylindrical discontinuity surface by Batchelor and Gill. The results obtained revealed the sensitivity of the jet to three ranges of acoustic frequencies, which correspond to three different forms of organized structures. The two low-frequency structures could be interpreted as a result of jet column instability, and the most amplified mode revealed the alternate pairing described theoretically by Lesieur as well as by Urbin and Metais. The high-frequency mode of instability could be related to the loss of stability of the 2D free shear layer. All three modes exist in the free unforced jet alternatively as was revealed by the visualization experiment shown in the attached movies. The results obtained during the experiment suggest that the round free jet is characterized by switching between three modes of instability.