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The purpose of this study is to clarify the effect of the tapered triangular tubes added to the four corners of a 2:1 rectangular nozzle on the flow characteristics and vortical structures of a rectangular jet. The angles of tapered triangular tubes were changed α = +6° and −6°. The mean bulk velocity U0 from the nozzle exit was about 4.5 m/s. The Reynolds number Re (= U0H/ν; ν, kinematic viscosity of air) of the jet was 9,000. In the case of the jet without tapered triangular tubes, the three-dimensional deformation of the vortex ring produces strong positive and negative vorticity around each axis and also increases the turbulent kinetic energy at the jet corners. The spread of the jet is larger than that of other jets. In the case of jet with divergent tapered triangular tubes with α = +6°, the local vorticity and turbulent kinetic energy increase at the corners of the jet were suppressed and the profile followed the jet shape. As a result, the jet becomes a circular jet shape until relatively downstream, and the jet spread was smaller than that of the other jets. On the other hand, in the case of the jet with convergent tapered triangular tubes with α = −6°, the jet maintains a relatively rectangular jet shape upstream due to the local increase in vorticity on the long side of the nozzle and the increase in turbulent kinetic energy along the nozzle shape. However, the overall spread of the jet is almost the same as that without the tapered triangle tubes due to the diffusion of the jet along the nozzle span length. Therefore, it is possible to change the flow characteristics and vortical structures of the rectangular jet by adding tapered triangular tubes with different angles.

The purpose of the present study was to establish a passive flow control method for a rectangular jet using two types of deflectors installed symmetrically inside a nozzle. This deflector in a rectangular nozzle generates the rectangular coaxial jets. The effect of the slant angle of the deflectors on the flow characteristics and the spread of the rectangular jet was investigated experimentally and by large-eddy simulation. The experiment and the numerical simulation were carried out at a Reynolds number of 9000. The rectangular jet with no deflectors generates a vortex ring from the nozzle exit. The vortex ring collapses in the downstream region and the outline of the jet changes from rectangular to diamond-shaped as a result of the axis-switching phenomenon. The rectangular jet with divergent and convergent deflectors shows particularly noticeable changes in the flow characteristics and vortical structures, as compared to the case with no deflectors. In the case of the rectangular jet with divergent deflectors (slant angle of α = 6˚), minor axis spread is promoted more than major axis spread, and axis switching occurs closer to the nozzle exit than that in the case of no deflectors. The outline of the jet also changes from lateral rectangular to vertical rectangular as a result of axis switching. On the other hand, in the case of a rectangular jet with convergent deflectors (α = −6˚), minor axis spread is suppressed more than major axis spread, and axis switching occurs farther from the nozzle exit than that in the case with no deflectors. The outline of the jet does not change until the downstream region. Therefore, the spread and the axis-switching location for the rectangular jet can be controlled by the deflectors inside the rectangular nozzle.