STREAMWISE VORTICITY AND ENTRAINMENT IN THE NEAR-FIELD OF A ROUND JET

The transition in round jets is accompanied by the transport of azimuthal to streamwise vorticity in the near field of the flow. In free shear layers, this forms Bernal-Roshko structures which are streamwise vortex pairs. Similar structures are shown to exist in round jets. The vortical ring structures form, grow, and amalgamate as they move downstream; they develop azimuthal instabilities. The resulting radial component of vorticity in the flow is tilted backward and stretched by the axial shear in the braid region of the flow. As the vortex filaments stretch backward, the vorticity increases and the induced velocity of the resulting vortex pair moves it farther outward, away from the shear layer into the stagnant fluid. The end of the pair is then left behind the braid and forms a streamerlike structure. This process is identified via flow visualization and confirmed by detailed entrainment measurements. The structures are shown to grow in the braid region. The streamwise structures have a major impact on the instantaneous entrainment field and, therefore, on the mixing process of the jet.