Multimodal Instability in the Weakly Underexpanded Elliptic Jet

Particle image velocimetry and schlieren imaging are used to study the interrelationship between the hydrodynamic and acoustic fields of a screeching underexpanded elliptical jet. An elliptical jet with an aspect ratio equal to two is studied at a nozzle pressure ratio of 2.2. Two-point correlation analysis of the schlieren image sets suggests that the flow is dominated by a flapping mode about the major axis plane, however, a proper orthogonal decomposition of the transverse velocity field suggests that there is also a helical mode that becomes dominant further downstream. The flapping mode appears to grow, saturate, and decay more rapidly and is associated with the production of discrete acoustic tones in the form of jet screech, whereas no discrete tone sound production is observed from the helical mode. It is suggested that the lack of tonal generation by the helical mode is linked to its slower growth; it does not produce coherent fluctuations in shear layer vorticity in the vicinity of the jet shock cells.