The present study has investigated the effect of passive control devices on the flow characteristics of a rectangular jet with an aspect ratio of 2, and the relation between the three-dimensional developing vortex structures near the nozzle exit and the axis-switching phenomenon in the downstream. The serrated tabs at the exit of the rectangular nozzle, and the convergent deflectors and the convergent tapered triangular tubes in the rectangular nozzle were installed as the passive flow control devices. These additional devices can change the velocity distribution and the turbulent intensity of air flow issuing from a rectangular nozzle. The rectangular nozzle had a height H of 30 mm and a width B of 60 mm. The Reynolds number Re (= U0H/ν; where U0 is the mean bulk velocity at the nozzle exit, and ν is the kinematic viscosity of air) of the jet was 9,000. The mean velocity, the turbulent intensity, the vorticity norm, and the vorticity vector in the three-dimensional flow field were calculated by the measured data from standard and mirror-image X-type hot-wire probes. It is found that the jet spread to the y and z-directions is significantly affected by the passive control devices, and the deformation of the vortex ring relates to the distribution and the intensity of the vorticity. The axis-switching phenomenon for the rectangular jet with serrated tabs moves to the downstream in comparison with the other jets because the vorticity on the long side of the rectangular nozzle becomes weaker than that on the short side. In the case of the rectangular jet with the convergent tapered triangular tubes, at x/H = 4, there is little difference in vorticity between the long and short sides of the rectangular nozzle. As a result, the shape of the jet remains rectangular to the downstream.
Kajitani et al. (Thu,) studied this question.