Phase-locked analysis of velocity fluctuations in a turbulent free swirling jet after vortex breakdown

Abstract

Large-scale organized vortical structures were studied experimentally in a free swirling jet of air experiencing vortex precession (PVC) at ambient conditions. Detailed measurements were performed in the region near the nozzle exit using phase-locked LDV and PIV, at a Reynolds number of Re ≈ 24,400 and a swirl parameter S ≈ 1.0. The investigation allowed reconstruction of the time-averaged flowfield, with the associated distribution of turbulent fluctuations, the phase-locked structure of the jet and the associated precessing vortex structure. An original joint analysis of power spectra and probability density functions of velocity data led to quantification of the PVC effect on turbulent fluctuations. This analysis showed that the PVC contribution can be properly separated from the background random turbulence, reproducing the results of phase-locked measurements. It is found that the background turbulence in the near field is substantially weaker if compared to the coherent fluctuations induced by vortex precession

On Impact of Helical Structures on Stabilization of Swirling Flames with Vortex Breakdown

“…Similar coherent structures were later detected in other studies of swirling jets with a vortex breakdown using pointwise LDA and planar particle image velocimetry (PIV) measurements [29][30][31]. Oberleithner et al [29] phase-averaged the velocity fields measured in a high-swirl jet with a vortex breakdown by PIV by using a snapshot method for proper orthogonal decomposition (POD) [32].…”

Study of the vortex-induced pressure excitation source in a Francis turbine draft tube by particle image velocimetry

“…8b). Martinelli et al (2012) found similarly that the contribution of the higher harmonics is negligible by fitting the power density function of the velocity components with an analytical expression. The velocity fields obtained with this method are used to compute the vortex characteristics, as described in the following section.…”

“…Different authors performed phase-locked PIV measurements by triggering the PIV acquisition directly with a reference pressure signal in order to recover the velocity field evolution over one typical period (Müller et al 2013;Martinelli et al 2012).…”

Analysis of the pressure fields in a swirling annular jet flow

“…The precessing vortex core is associated with a low-pressure region along the central axis of the jet and the maximum pressure fluctuations occur upstream of the vortex breakdown location, where the 166 Page 2 of 13 2001). These large coherent structures have been well studied for swirling circular jet flows (Panda and McLaughlin 1994;Billant et al 1998;Al-Abdeli and Masri 2004;Cala et al 2006;Oberleithner et al 2011;Martinelli et al 2012;Litvinov et al 2013;Markovich et al 2014;Oberleithner et al 2014). However, for the case of annular jet flows, much remains to be resolved, especially regarding the interaction between the instabilities and the CRZ.…”