Flowfield and mixing control of an underexpanded jet

Flowfield and schlieren flow visualization were used to study the growth rate and mixing characteristics of a self-excited underexpanded jet. The underexpanded jet was excited at three characteristic frequencies that correspond to the initial instability, a preferred mode, and a subpreferred mode. The spreading rate and turbulent flow characteristics resulting from the three excitation frequencies were compared with the unforced baseline case. Axial and radial mean and fluctuating velocity components in the flow along with time-averaged schlieren images indicated that the growth rate and turbulence level in the jet near field could be controlled independently by exciting the jet at the different characteristic modes of the jet. The effects of forcing on mixing and spreading rates were minimal in the jet far field. The forced underexpanded jet exhibited characteristics that are strikingly similar to those obtained earlier in subsonic jets. It was observed that forcing the underexpanded jet at its preferred mode increased the spreading rate and turbulence intensity in the jet near field. Forcing the high-speed jet at frequencies close to the initial jet frequency minimizes the spreading rate of the jet. Excitation at the subpreferred mode frequency results in an increased near-field jet growth rate when compared to the preferred mode. Both the axial and radial turbulence intensity increased by 35-40% around the jet core at a subpreferred mode forcing of Sr-D 0.09 as compared to the forcing near the initial instability frequency Sr-theta = 0.0085 and at the baseline frequency.