Abstract
The flow in the initial region of two jets, namely, a circular orifice jet and a lobed orifice jet, is considered in the present paper. The role played by the Kelvin-Helmholtz (K-H) azimuthal rings and the streamwise vortices in the entrainment process of both jets have been qualitatively and quantitatively analyzed, for a Reynolds number of 3600. This has been achieved using the high speed stereoscopic particle image velocimetry measurements and a proper orthogonal decomposition (POD) analysis. The mean entrainment rate observed in the daisy-shaped jet is higher than that of the circular jet. It is found that a strong correlation exists between the entrainment rate and the K-H vortex dynamics for the circular jet. The entrainment is mainly produced in the upstream part of the K-H ring as well as in the braid region where the streamwise vortices appear. In the downstream part of the K-H ring, the flow expands from the jet core to the surrounding. In the lobed jet, the amplitude of the entrainment variation is smaller than in the circular jet at the same axial position. The flow dynamics observed in the region of axis switching is highly complex. A snapshot POD analysis is performed to bring out the individual role played by the azimuthal and the streamwise vortices on the entrainment for both the jets. In the circular jet, the POD analysis confirms quantitatively the role of the K-H vortices as well as the streamwise vortices on jet entrainment. In the lobed jet, in addition to the main role of the streamwise vortex pairs in the entrainment, the K-H vortex also plays an undeniable role on the entrainment. In the experiments conducted, a steep rise in the momentum flux is observed for the axis-switching region of the lobed jet. The longitudinal distribution of the mean entrainment ratio of the daisy jet corroborates with that of the averaged streamwise vorticity and with the maximum streamwise vorticity. In the region of the axis switching, an important decrease in amplitude of the turbulent kinetic energy is evidenced. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3358465]