Dynamics of transitional jets emanating from a non-circular nozzle

The paper presents the results of an experimental study comparing free jets issuing from either a non-circular nozzle (square, hexagonal or triangular) or a circular one included as reference case. The measurements are performed at two Reynolds numbers equal to Re =5 000 and 10000. The experimental set-up is characterized by an extremely low level of fluctuation intensity (Tu & LE; 0.1%) and a thin shear layer at the nozzle exit. These conditions induce the development of large scale vortices clearly visible both in the velocity measurements and in smoke visualizations. The dynamics of the jets issuing from the circular, hexagonal and square nozzles is found to be quite similar. In these cases the influence of the nozzle shape is quite weak. The results obtained for the triangular jet turned out to be qualitatively different. Only weak vortices form in the vicinity of the nozzle, which are found to break up immediately downstream. Compared to the other three jets this implies a much faster spreading rate and faster axial velocity drop along the centreline. An interesting phenomenon was observed at Re = 10000 for all nozzle shapes, namely the occurrence of two distinct peaks in the velocity spectra, not present at Re = 5000. This suggests that in this case, the Kelvin-Helmholtz instability coexists with other instability mechanism leading to slightly different frequencies of the flow oscillations.