Vortex dynamics and mass entrainment in turbulent lobed jets with and without lobe deflection angles

Passive control of jet flows in order to enhance mixing and entrainment is of wide applicative interest. Our purpose is to develop new air diffusers for HVAC systems, by using lobed geometry nozzles, in order to ameliorate users the thermal comfort. Two turbulent 6-lobed air jets with and without lobe deflection angles were studied experimentally and compared with a reference circular jet having the same initial Reynolds number. The main objective was to analyze the modifications occurring in the vortex dynamics of the flow, firstly by replacing a circular tube with a straight lobed tube, and secondly by a lobed tube having a double inclination of the lobes. Rapid visualizations of the flows and hot-wire measurements of the streamwise velocity spectra allow understanding the vortex roll-up mechanisms. Unlike the circular jet, where the primary rings are continuous, the Kelvin-Helmholtz vortices in the lobed jet flows were found to be discontinuous. The resulting "ring segments" detach at different frequencies whether they are shed in the lobe troughs or at the lobe sides. One explanation relies on the strong variation of the exit plane curvature. Additionally, a speculative scenario of the vortical dynamics is advanced by the authors. The discontinuous nature of the K-H vortices enables the development of secondary streamwise structures, non-influenced by the passage of the primary structures as in the case of the circular jet. Thus, the momentum flux transport role played by the streamwise structures is rendered more efficient and leads to a spectacular increase in the entrainment rate in the initial region. The amount of fluid being entrained in the lobed jet by the streamwise structures is drastically amplified by the double inclination of the nozzle exit boundary.