Abstract
The paper focuses on a global instability phenomenon in counter-current round jets issuing from co-axial nozzles. Three different configurations that differ in a way of the counter flow generation are investigated. Besides typical configurations used in experimental and numerical research performed so far, in which suction applied in an annular nozzle is a driving force for the counterflow, a novel set-up is proposed where the annular nozzle is oriented in the opposite direction and placed above the main one. Such a configuration eliminates the suction of fluid from the main jet, which in previous research was found to have a destructive impact on the occurrence of the global flow instability. The research is performed using a large eddy simulation (LES) method and the computations are carried out applying a high-order numerical code, the accuracy of which has been proven in previous works and also in the present research through comparisons with available experimental data. The research is complemented by the linear stability analysis which supports the LES results and formulated conclusions. In agreement with a number of the previous works it has been shown that the global modes can be triggered only when the velocity ratio (I) between the main jet velocity and the velocity of the jet issuing from the annular nozzle is above a certain threshold level (I_{text {cr}}). It has been shown that in the classical configurations of the co-axial nozzles the range of Ige I_{text {cr}} for which the global instability phenomenon exists is very narrow and it disappears for larger velocity ratios. Reasons for that have been identified through detailed scrutiny of instantaneous flow pictures. In the new set-up of the nozzles the global instability persists for a significantly wider range of I. It has been shown that I_{text {cr}} depends on both the momentum thickness of the mixing layer formed between the counter-current streams and the applied configuration of the nozzles. The LES results univocally showed that the latter factor decides on the type of the instability mode (Mode I or Mode II) that emerges in the flow, as it directly influences on a length of the region where the counter-current streams are parallel allowing the growth of short or long wave disturbances characteristic for Mode I and Mode II, respectively.
Highlights
Round jets, due to their occurrence in many industrial applications, have received huge interest for decades
The paper presented the analysis of the global instability phenomenon in the counter-current round jets
We studied two configurations (CONF-1, CONF-2) of the round jet with the suction applied around the nozzle to establish the counter-current flow in the same way as in earlier experimental researches (e.g. Strykowski and Niccum 1991; Strykowski and Wilcoxon 1993; Jendoubi and Strykowski 1994; Boguslawski et al 1999)
Summary
Due to their occurrence in many industrial applications, have received huge interest for decades. LES and DNS turned out to be very useful for predictions of the global instability in low-density jets (Lesshafft et al 2007; Foysi et al 2010; Boguslawski et al 2016), annular jets (Wawrzak et al 2019), counter-current mixing layers formed in square and rectangular jets (Grinstein and DeVore 2002) and planar mixing layers (Kalghatgi et al 2011; Kalghatgi and Acharya 2012), as well as in advanced studies on dependence of jet flows on initial conditions (Tyliszczak and Geurts 2019) and their control, e.g. in research on bifurcating (Tyliszczak and Geurts 2014) and blooming jets (Tyliszczak 2015; Gohil et al 2015; Tyliszczak 2018).
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