On the coherent modes of high Reynolds number, strongly swirling jets discharging in compact enclosures. Part B: Coherent flow structure

In this second part of a two article study, the coherent flow structure of a high Reynolds number, strongly swirling jet that includes in its core an energetic shear layer and that discharges in a square shaped cross-section duct is experimentally investigated. Selecting the jet diameter and average jet discharge velocity as characteristic scales, the swirl number, the duct cross section size, and the embedded shear layer circulation are all order one, whereas the flow Reynolds number is O(105)O(105). The selected flow configuration and non-dimensional parameters are similar to those characterizing the flow in compact, lean premixed combustors presently being considered for aeroengine applications. Two space fixed, hot-wire probes are used as coherent mode detection and phase determination sensors to conduct detailed mode conditioned, LDV measurements. Spectral analysis of the hot-wire velocity signals shows the presence of three spatially and temporally coherent flow modes. The structure of the different modes is investigated using conditional post-processing specifically designed to analyze this high Reynolds number, complex flow field. The lowest frequency mode is seen to be linked to a precession instability of the swirling jet vortex breakdown region. The two other, more higher frequency modes are connected to the Kelvin–Helmholtz instability of the shear layer that develops in the swirling jet core. Symmetry arguments are used to partially reconstruct the vorticity structure linked to the shear layer instability modes. A close relation between the precession and the shear layer instability modes is found to occur.