Precession frequencies and helical mode transition in an axial double-stage counter-swirl flow: Effects of inner and outer swirl intensities

This paper primarily employs the Large Eddy Simulation method to investigate the isothermal flow characteristics within an axial double-stage swirling flow field. The swirl number (Sn) ratio between the inner and outer stages (RS, ranging from 0.29 to 4.12) is introduced as a key variable, while also focusing on the effects of both the inner and outer swirl intensities. Statistical average reveals that increasing the outer swirl intensity leads to a larger central recirculation zone, while the inner swirl intensity has a relatively weaker effect. Furthermore, the absolute value of the maximum recirculation velocity exhibits a trend of initially decreasing and then increasing as the swirl intensities of either the inner or outer stage increases, highlighting the competitive interaction between the two swirls. The fast Fourier transform analysis shows that an increase in the inner swirl intensity promotes the formation of low-frequency precession, whereas an increase in the outer swirl intensity influences the generation of high-frequency precession and shear layer instability frequencies, and the flow field structure becomes more fragmented, leading to an unstable quasi-periodic vortex fragmentation pattern. Modal decomposition methods, including proper orthogonal decomposition and spectral proper orthogonal decomposition, reveal that, with increasing RS, the frequency associated with the precession characteristic evolves from a single to a double helix and eventually to multiple helical structures near the swirler outlet. The modal energy content at the outlet plane initially decreases and then increases, also reflecting the competitive interaction between the inner and outer swirls.