Vortex structures and entertainment in circular and daisy-shaped orifice jets

Abstract

Paper presented at the 7th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Turkey, 19-21 July, 2010.

The role of streamwise and spanwise vortex structures in the near-field entrainment of a circular and a daisy-shaped orifice jet is investigated experimentally in this paper. Timere-solved stereoscopic PN measurements are performed in several cross sections perpendicular to the jet axis. Three­-dimensional flow field reconstruction based on the stereoscopic PIV results gives better understanding of the three-dimensional features and the different development stages of the jet. The variation of the entrainment rate along the jet axis has been evaluated from the mean velocity field. A higher entrainment is observed in the daisy shaped jet as compared to that of the circular jet. This is due to large-scale streamwise vortical structures generated by the lobed geometry. Strong correlation exists between the entrainment rate and the Kelvin-Helmholtz (K-H) vortices passing for the circular jel The instantaneous entrainment rate in the daisy jet presents small amplitude vatiations and is affected by the dynamics of both the azimuthal and the streamwise vortices. The longitudinal development of the mean streamwise vorticity is quite different between the circular and the lobed jets. In the circular jet, the streamwise vorticity becomes more important when traveling downstream along the jet axis and their trajectory is not necessarily linear. The expaosion rate of the jet is higher in the minor plane as compated to the major plane in the region of the axis-switching for the daisy jet.