Circular and noncircular subsonic jets in cross flow

Abstract

Experiments were conducted to investigate the impact of circular and noncircular injector nozzle geometries on the penetration, mixing, and structure of a jet injected perpendicular into subsonic cross flow. Circular, triangle, and elongated slot nozzles were studied including varying their orientations relative to the cross flow, to comprise a matrix of five different test cases. The cross-sectional areas of all geometries were equal. The interaction between the jets and the cross flow was strongly dependent on the different geometries and their orientations. Each of the injector geometries resulted in a distinctive region of backflow at the leeward side of the jet. The location and size of this region and the reverse flow magnitude were largely dependent on the aspect ratio and the jet cross section lateral dimension change in the axial direction. A combination of instantaneous and time-averaged particle image velocimetry images taken in the axial and lateral cross sections of the jet near the exit plane revealed several large scale turbulent structures downstream of the jets for two different blowing ratios r=3.2 and 8. Detailed flow field data are presented for the higher blowing ratio due to the better spatial resolution of the large scale structures in the near field. A counter-rotating vortex pair was identified in the near field of the jets resulting in a reverse flow region. These structures had an upright nature similar to the wake vortices with the exception that the former were attached to and constantly fed from jet fluid mixed with the cross flow. The latter was formed by the cross flow, fed from its boundary layer vorticity, and remained separate from the jet. It was attached to the surface of the flat plate at one end and to the trajectory of the bent jet at the other end. Both the shape and orientation of the nozzle geometries had a strong effect on the jet spread and penetration. Compared to the baseline circular nozzle, the slot major (slot nozzle’s major axis oriented along the cross stream) exhibited the highest penetration, whereas the slot minor penetrated the least. The triangular jets’ characteristics were significantly affected by their orientation relative to the cross flow. Several different scaling techniques were explored in order to compare the present jet trajectories with previously published data. Commonly used length scaling parameters of rd and r2d were shown to be inadequate to scale different experimental data. Newly proposed scaling that was based on the velocity profiles of the cross flow boundary layer and the jet, prior to their interaction, yielded better results.