Abstract
The paper presents experimental investigations of the low-density air-helium jets. The paper is aimed at the analysis of the flow conditions promoting the local absolute instability leading to global flow oscillations. A number of the test cases are analysed with a wide range of the shear layer thickness showing conditions favorable for the global modes and also mixing intensity triggered by such a regime. It is shown that high mixing intensity is determined not only by the global regime but also by the vortex pairing process. The results are compared with a recently proposed universal scaling law for an onset into the global mode. The results turn out to be far from this scaling law and the reasons for such discrepancies are discussed. The measurements show also that if the shear layer at the nozzle exit is thin enough the global modes are suppressed. The mechanism leading to the global mode suppression under such conditions is carefully analysed.
Highlights
A concept of absolute instability was first proposed by Landau [1] as a perturbation growing in time in contrast to the convective one in which the perturbation grows in space being swept away from its source by a convective stream
The main goal of the research was to investigate the dynamics of the low-density air-helium jets aimed at an identification of the global flow oscillations stemming from the absolute instability
The global modes were identified looking at the fluctuations intensity, fluctuation growth rate and spectral distribution of the flow oscillations
Summary
A concept of absolute instability was first proposed by Landau [1] as a perturbation growing in time in contrast to the convective one in which the perturbation grows in space being swept away from its source by a convective stream Such a phenomenon was first observed in plasma physics [2]. The main outcomes of the linear stability theory were confirmed experimentally in cardinal papers for the case with changing density by heated jet by Monkewitz et al [7] and using helium-air mixture by Sreenivasan et al [8] and Kyle and Sreenivasan [9], respectively For both types of density changing significant oscillations were visible for the low-density cases.
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