Mechanism of axis switching in low aspect-ratio rectangular jets

In this work we systematically study one square jet (AR=1) and four rectangular jets with an aspect ratio of width over height AR=1.5,2,2.5, and 3 respectively using the lattice Boltzmann method for direct numerical simulation. Focuses are on various flow properties on transverse planes downstream to investigate the correlation between the downstream velocity and secondary flow. Three distinct regions of jet development are identified in all the five jets. As the length of the PC (potential core) region maintains about the same, that of the CD (characteristic decay) region strongly depends on the jet aspect-ratio (AR) and Reynolds number (Re). The 45° and 90° axis-switching occur in the CD region, with the former followed by the latter at the early and late stages of the CD region respectively. The half-width streamwise velocity contour reveals that 45° axis-switching is mainly contributed by the corner effect, whereas the aspect-ratio (elliptic) feature affects the shape of the jet when 45° axis-switching occurs. The close examinations of flow pattern and vorticity contour, as well as the correlation between streamwise velocity and vorticity, indicate that 90° axis-switching results from the boundary effect. Specific flow patterns for 45° and 90° axis-switching are identified to reveal the mechanism of the axis-switchings respectively.