In a continuing effort to study the mixing enhancement by large-scale streamwise vortices generated by a lobed nozzle, a high-resolution stereoscopic particle image velocimetry (PIV) system was used in the present study to conduct three-dimensional measurements of air jet flows exhausted from a lobed nozzle and a conventional circular nozzle. The three-dimensional instantaneous and ensemble-averaged velocity fields, instantaneous and ensemble-averaged streamwise vorticity distributions and turbulent kinetic energy distributions were used to analyze the characteristics of the mixing process in the lobed jet flow compared with conventional circular jet flow. The existence of large-scale streamwise vortices in the lobed jet mixing flow is revealed clearly from the stereoscopic PIV measurement results. The instantaneous streamwise vorticity distributions revealed that the large-scale streamwise vortices generated by the corrugated trailing edge of the lobed nozzle break into smaller, but not weaker streamwise vortices gradually 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 in the lobed jet mixing flow was evaluated by analyzing the ensemble-averaged streamwise vorticity distributions. It was found that ensemble-averaged streamwise vortices in the lobed jet mixing flow grew up and expanded radially over the first diameter of the test nozzle, then began to break down into smaller and weaker vortices further downstream. The averaged turbulent kinetic energy profile indicated that most of the intensive mixing between the core jet flow and ambient flow, due to the special geometry of the lobed nozzle, occurred within the first two diameters of the lobed nozzle, which corresponds to the upstream region where the ensemble-averaged streamwise vortices break down.

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