Turbulent pipe flow downstream of a bend

Time-resolved stereoscopic PIV was used to investigate the curvature-induced structures downstream of a 90^∘ bend at Reynolds numbers between 20× 10³ and 115× 10³ . Data were taken at three downstream locations to investigate the evolution of the structures. Snapshot proper orthogonal decomposition (POD) analysis shows that the most energetic structure is not the well-known Dean motion but a bimodal single cell structure with alternating direction of rotation, called the ‘swirl switching’ mode. The strengths of the Dean motion and the swirl-switching structures are similar, indicating that the difference in energy is related to their duration of occurrence, where the Dean motion is associated with a comparatively rapid transition between the two states in the swirl switching mode.