THE ROLE OF STREAMWISE VORTICITY IN THE NEAR-FIELD ENTRAINMENT OF ROUND JETS

The role of streamwise vortex structures in the near-field (x/d < 10) evolution of a round jet is examined. In free shear layers the streamwise vorticity develops into Bernal-Roshko structures which are streamwise vortex pairs. Similar structures are shown to exist in round jets. These structures, which evolve and amplify in the braid region between primary vortical structures, are shown to drastically alter the entrainment process in the near field and to increase the rate at which fluid is entrained into the jet. As the flow evolves downstream, the efficiency of the streamwise vorticity in entraining fluid increases relative to that of the azimuthal vorticity. Beyond the end of the potential core regime, the entrainment process is mainly controlled by streamwise vorticity. These processes are identified via flow visualization and confirmed by detailed global entrainment measurements.