Experimental investigation on the three-dimensional organization of the flow structures in precessing jets by tomographic PIV

A self-excited precessing jet PJ generated by a 5:1 expansion of a circular jet issuing at Red = 42,500 into a short coaxial cylindrical chamber has been investigated in a water facility by means of tomographic Particle Image Velocimetry. Two inflow conditions, using either simply a short-pipe nozzle, i.e. jet without grid JWG or placing a regular grid RG in correspondence of the short pipe exit, have been considered. A statistical analysis has been conducted for both configurations revealing that the entrainment region extends along the entire length of the cylindrical chamber. A modal analysis using proper orthogonal decomposition, conducted in the sudden expansion region SER at the basis of the cylindrical chamber, indicates the dominance of the large-scale precessing motion for both configurations. It is found that the entrainment process influences the instantaneous organization of the large-scale coherent structures in SER during the precessing motion. Under the influence of both the entrainment and the induced swirling motion, the helical coherent structures, detected within the first three diameters from the nozzle exit, undergo an asymmetric reduction of their convective velocity, causing in turn the bending of the jet axis. At the middle height of the cylindrical chamber, the crosstalk between the entrainment and the jet column produces vortex filaments with zero convective velocity observed around the jet column itself. At the exit of the cylindrical chamber, the entrainment process from one side is sustained by the fluid coming from the external ambient and from the other by the interaction of the emerging jet with the quiescent fluid, leading to the formation of a recirculation region.