The three-dimensional swirling flow past a sudden expansion

The jet flow past an abrupt expansion under some conditions switches intermittently between two states: quasi-axisymmetric expansion and gyroscopic-like precessing motion. In this work an experimental analysis is carried out by means of Tomographic Particle Image Velocimetry (Tomo-PIV).The self-excited flow precession generated by a 5:1 expansion of a round jet in a coaxial cylindrical chamber is investigated. The experiments are performed at a Reynolds number equal to 150,000. Proper Orthogonal Decomposition (POD) is applied to extract information on the organization of the larger structures of the precessing motion. This technique highlights the dominance of three modes: the first two are associated to the jet precession; the third one is representative of the axial motion. Furthermore, an approach based on the modal energy for the evaluation of the precession probability is presented. The application of a stability criterion shows that the self-induced swirl flow results to be unstable within the jet shear layer. The instability is responsible for the generation of helical-shaped vortices in the near field, instead of the expected phenomenology of the ring-vortices generation of circular jets