Three-dimensional effects associated with the low-frequency breathing motion of a turbulent separation bubble

In this article, we address the low-frequency dynamics of a turbulent separation bubble (TSB) with a mean length of Lb ≈ 0.18 m that occurs in a one-sided diffuser at an inflow velocity of U∞ = 20 m/s. Distinct low-frequency content at Strouhal numbers of O(f Lb/U∞) = 0.01 is detected through time-resolved wall pressure measurements. By applying Spectral Proper Orthogonal Decomposition to velocity fluctuations in the diffuser symmetry plane, these pressure fluctuations can be linked to a large-scale longitudinal contraction/expansion motion falling into the same frequency range. Further velocity field measurements in cross-sections of one diffuser half-plane indicate that the mean flow field of the TSB is characterized by a near-surface outward-directed velocity component and a large-scale streamwise side wall vortex. During the expanded TSB state, however, these flow features are suppressed as the streamwise velocity is temporarily increased near the side wall but reduced near the symmetry plane. The results presented in this article suggest that the low-frequency breathing motion is a highly three-dimensional phenomenon. Future studies should address the associated complex dynamics that are shown to be governed by aperiodic events of temporary TSB expansion.