Abstract A hydrocyclone is a centrifugal separation device with self‐induced rotating flow field, and the air core oscillation is a pivotal phenomenon profoundly impacting its efficiency and energy consumption. This study employs computational fluid dynamics (CFD) and dynamic mode decomposition (DMD) to analyze the periodic fluctuations of the air core in a hydrocyclone. The results reveal a pair of conjugate modes extracted by the DMD method closely matches the principal frequency of the quasi‐periodic flow. Notably, Modes 2 and 3 reflect the oscillating motion of the air core and the intense rotating airflow with the four‐helix vortex structure. They have intense rotation, stretching, and shear, which will disrupt the stability of the flow field of the hydrocyclone and affect its performance. These findings provide new insights into the periodic motion within the hydrocyclone, and the established methodology can be applied to the modal analysis of other rotational flow fields.
Liu et al. (Sun,) studied this question.