Mixing Process in a Lobed Jet Flow

A high-resolution stereoscopic particle image velocimetry (PIV) system was used to conduct three-dimensional measurement of an air jet exhausted from a lobed nozzle. The characteristics of the mixing process in the lobed jet  ow are revealed clearly and quantitatively from the stereoscopic PIV measurement results. The instantaneous velocity and streamwise vorticity distributions revealed that the large-scale streamwise vortices generated by the lobed nozzle break into smaller but not weaker streamwise vortices as they travel downstream. This is the proposed reason why a lobed nozzle would enhance both large-scale mixing and small-scale mixing reported by other researchers. The overall effect of the lobed nozzle on the mixing process was evaluated by analyzing the ensemble-averaged streamwise vorticity distributions. The strength of the ensemble-averaged streamwise vortices was found to decay very rapidlyover the Ž rst two diameters (Ž rst six lobeheights), then to decay at amoremoderate rate farther downstream. The averaged turbulent kinetic energy proŽ le also indicated that most of the intensive mixing between the core jet and ambient  ow occurred within the Ž rst two diameters. These results indicate that the maximum effectiveness region for the lobed nozzle to enhance mixing is about the Ž rst two diameters of the lobed nozzle (Ž rst six lobe heights).