On the coherent modes of high Reynolds number, strongly swirling jets discharging in compact enclosures. Part A: Mean flow structure and coherent mode processing description

An experimental study on the spatially and temporally coherent, large scale flow structure generated by a high Reynolds number, strongly swirling jet that discharges in a squared cross-sectional duct is presented. Both the jet swirl number and the ratio of the duct side length to jet diameter are of the order of unity, whereas the Reynolds number based on the jet diameter and averaged discharge velocity is O(105)O(105). The swirling jet incorporates an energetic, shear layer in its core. The selected configuration and non-dimensional parameters are similar to those characterizing the flow in lean premixed combustors presently being considered for large aeroengine applications. The swirl and Reynolds numbers of the discharging jet are large enough to produce turbulent vortex breakdown phenomena that support a closed mean flow recirculation region. In this first part of a two article study, the mean flow structure is analyzed using laser Doppler velocimetry (LDV) characterizations. In addition, hot-wire based spectral and correlation measurements are used to detect the presence of large scale modes that account for a significant percentage of the total fluctuating flow kinetic energy. It is found that some modes exhibit a tight spectral separation, and that they can coexist in time. Both aspects and the large Reynolds number flow character limit the use of relatively low sampling rate, laser based diagnostics to characterize the structure of the different coherent modes. Using a number of space fixed, hot-wire probes a post-processing method is proposed to accurately resolve the details of the large scale structure. Applied to this complex, high Reynolds number flow, the post-processing procedure allows performing mode conditional measurements using low sampling rate, high resolution, non-intrusive techniques such as LDV or particle image velocimetry (PIV). In a following article, the post-processing method is used to unveil the detailed structure of the different coherent modes.