Experimental investigations of swirling jets

The ‘plug’ flow emerging from a long rotating tube into a large stationary reservoir was used in the experimental investigation of centrifugally unstable swirling jets. A moderate Reynolds number, Re = 1000, was studied extensively, and swirl numbers, S, the ratio of nozzle exit rotating speed to the mean mass axial velocity, were in the range 0∼1 to cover four regimes: non-swirling jets, weakly swirling jets, strongly swirling jets before vortex breakdown and vortex breakdown. Vortex breakdown is characterized by an abrupt flow transition and a stagnation point or region followed by a reverse flow.; The high turbulence level in the shear layer, the location of the fully turbulent region further upstream, fast decay of the jet and enhanced entrainment of the ambient fluid makes swirling jets very different from non-swirling ones, especially after vortex breakdown.; Particular attention was paid to the dominant role of the underlying vortical flow structures and their dynamic evolution. Kelvin-Helmholtz instability in the axial shear layer, leading to vortex ring formation, dominated non-swirling and weakly swirling jets. After the introduction of sufficient rotation, centrifugal instability dominated the azimuthal shear layer and co-rotating helical waves were produced. For strongly swirling jets before breakdown, the flow lost its axisymmetry from the spatial origin, and strong helical waves m = 2 or 3 replaced vortex rings as the dominant vortex structure, where m is the azimuthal wave number. After vortex breakdown, strong helical waves m = 1 and 2 co-existed, with m = 1 being dominant.; Based on the spectral study of flow fields in natural, forced and transient experiments, we strongly suggest that the helical wave m = 2 for strongly swirling jets before vortex breakdown and conclusively prove m = 1 after breakdown are globally unstable/self-excited, a behavior identified as a super-critical Hopf bifurcation. The onset of the unstable global mode m = 1 after vortex breakdown was related to the existence of a certain region of local absolute instability in the wake region of the breakdown structure. The wake region around the stagnation point served as the wave maker and imposed its frequency on the whole flow.