  Axis switching of a jet ejected from a rectangular nozzle affects flow mixing
characteristics. To elucidate such a mixing mechanism, the axis switching and
vortex structure deformation should be investigated in detail. This study
performed a numerical analysis of the axis switching of a pulsating jet ejected
from a rectangular nozzle at a low Reynolds number. At all aspect ratios, a
rectangular vortex ring similar to the shape of the nozzle cross-section is
periodically shed downstream, and the side of the vortex ring deforms into a
hairpin shape downstream. A vortex pair is generated inside the vortex ring
downstream of the nozzle corner. When the aspect ratio is AR=1.0, the vortex
pair consists of symmetrical vortices, while as AR increases, the asymmetry of
the vortex pair enlarges. At AR=1.0, regeneration of a vortex ring occurs
downstream. For AR=2.0, alternately on the long and short sides of the nozzle,
an upstream vortex ring overtakes a downstream vortex ring. Regardless of AR,
downstream near the nozzle, as the vortex pair existing inside the vortex ring
distorts the vortex ring, the positions of the side and corner of the vortex
ring exchange, resulting in a 45-degree axis switching. For AR>1.0, further
downstream, the hairpin part of the vortex ring on the long side develops away
from the jet center compared to the short side, causing a 90-degree axis
switching. As a result, high turbulence occurs over a wide area, strengthening
the mixing action. As AR increases, intensive interference between the vortex
rings on the upstream and downstream sides diffuses the vortices downstream.
Then, as turbulence by the diffused vortices widely occurs, the mixing effect
is further strengthened.
