Abstract
A circular 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 investigation into the self-excited precessing flow generated by a 5:1 expansion of a round jet in a coaxial cylindrical chamber is carried out by means of tomographic particle image velocimetry. The experiments are performed on a jet issued from a short pipe at a Reynolds number equal to 150,000. Proper orthogonal decomposition (POD) is applied to extract information on the organization of the large coherent structures of the precessing motion. The application of this technique highlights the dominance of three modes: the most energetic two are associated with the jet precession; the third one is representative of the axial motion. An estimate of the precession probability based on the modal energy obtained from the application of POD is proposed. The precession frequency is extracted using a low-order reconstruction (LOR) of a subset of the POD modes. The reconstructed flow field topology obtained by the LOR highlights an underlying mechanism of swirl generation in proximity of the inlet nozzle; the phenomenon is closely related to the interaction between the entrainment in the far field and the recirculation regions in the near field. The application of a stability criterion shows that the self-induced swirl flow results to be unstable. The instability is responsible for the generation of helicalshaped vortices in the near field, even though the dominant feature for the unconfined jet issued from the same nozzle is the axisymmetric ring-vortices generation.