Numerical investigation of scalloping effect on lobed nozzle flow features and thermal mixing performance

The lobed nozzle is usually used in the exhaust system of military helicopters to improve survivability. The thermal and flow characteristics of the lobed mixer with and without the scalloping structure are investigated to provide a theoretical basis. The delayed detached eddy simulation is used after validated against Hu's experiment of stereoscopic particle image velocimetry. The specific position, where the scalloping structure improves the mixing performance, is summarized. The dominant frequency of downstream vortex is quantitatively characterized by the power spectral density analysis. The results show that the scalloping structure brings a rapid improvement in the mixing performance near Z/D=0.75 and a faster mixing completion rate. At Z/D=1, the mixing efficiency of the scalloping structure is about 0.46 and the efficiency is enhanced by about 13.5%. The mixing characteristics correspond to the streamwise vortices with a larger spatial scale and stronger intensity in the downstream time-averaged flow field. In terms of unsteady flow, the downstream coherent structures include K-H vortex, hairpin vortex, streamwise vortex, normal vortex, and shedding vortex for the lobed mixer with the scalloping structure. Shedding vortices destroy the complete structure of the normal vortex. Meanwhile, shedding vortex enhances the vorticity intensity and spatial scale of streamwise vortices.(c) 2023 Elsevier Masson SAS. All rights reserved.