Effect of delta tabs on axis switching of a coaxial square jet

The effects of delta tabs in the near field of a coaxial square jet have been studied experimentally using a laser Doppler anemometer. The tabs are added to the centers of the four sidewalls of the inner nozzle trailing edge, with the tip of each tab tilted into the inner jet, that is, ? = 45 deg, or into the outer jet, that is, ? = -45 deg. For each tab orientation, three velocity ratios have been considered: ?u = 0, 0.5, and 1.0 for ? = 45 deg and ?u = 1.0, 2.0, and ? for ? = -45 deg. The results illustrate that tabs increase the mixing between the inner jet and outer jet and may promote or suppress the phenomenon of axis switching, depending on the orientation of the tabs. Axis switching is defined as follows: As a noncircular (here square) jet spreads, its cross-section may evolve from the initial shape similar to that the jet exit, to have its axis rotated some angle at some downstream location. In the case of ? = 45 deg, the cross-sectional shape of the inner mixing layers is bifurcated into a four-finger structure, which tends to suppress the onset of axis switching. When ? = -45 deg, it exhibits an eightfold mode, and a rapid axis-switching occurs in the near-field. The effect of tabs is briefly discussed using inviscid vortex dynamics.