A large-eddy simulation study of the swirl-switching mechanism in toroidal pipes

Large-eddy simulation was conducted on turbulent flow in toroidal pipes of various lengths to study the impact of streamwise fluctuation structures on the secondary motion and explore the origin of the swirl-switching phenomenon. Proper orthogonal decomposition analysis was carried out to identify the primary coherent structures in the flow field and determine the oscillation frequencies of the swirl switching. The results show that high-frequency switching (St > 0.05) is closely related to the spatial scale of very-large-scale motion (VLSM), and that its streamwise extension is approximately the characteristic length corresponding to the switching frequency. Conversely, the characteristic length of low-frequency switching (St ≤ 0.05) is significantly larger than the physical model, which is not attributed to the spatial scale of streamwise fluctuations but rather to the temporal oscillations of the VLSM. Due to the sensitivity of the VLSM structure, the alignment mechanism of VLSM changes under vertical excitation, leading to a reduction in the streamwise characteristic length and an increase in the frequency of swirl switching. The spatiotemporal variations of the VLSM feedback into the frequency of swirl switching, thus confirming the significant effect of streamwise fluctuation. Additionally, the wall shear stress exhibits oscillation frequencies associated with swirl switching and VLSM, indicating a correlation between flow field fluctuations and wall shear stress, which could potentially contribute to material erosion and local resonance.