Self-excited oscillations in variable density counter-current round jets

Abstract

The paper investigates the global instability phenomenon in variable-density counter-current round jets. The analysis is performed using a large eddy simulation and the computations are carried out employing a high-order numerical code. The configuration of two co-axial nozzles, where suction applied in an annular nozzle is a driving force for the counterflow, is investigated. In order to simulate conditions conducive to the emergence of global instability a wide range of velocity and density ratios is considered. Depending on these parameters two different types of modes are observed, i.e., Mode I — shear layer mode, and Mode II — jet column mode. The former is recognized by a high level of perturbations in the shear layer which vanish away from the shear layer reaching a low level at the jet centerline. The latter is characterized by strong disturbances throughout the entire jet region. The results obtained are in remarkable agreement with the theoretical basis reported in the literature. They reveal that the critical velocity ratio for which global instability emerges decreases along with the density ratio. Additionally, the transition from Mode II to Mode I is found for sufficiently strong suction. In general, the lower the density ratio, the weaker suction is required to induce Mode I. Consistently to the literature, Mode II is characterized by a lower frequency of oscillations compared to Mode I for which an increase in density ratio and/or velocity ratio shifts the peaks of the velocity spectra toward higher frequencies.