Vortex Dynamics and Instability Mechanisms in a Radially Lobed Nozzle

The application of radially lobed nozzles has seen renewed challenges in the recent past with their roles in combustion chambers and passive flow control. The free jet flow from such nozzles has been studied for different flow conditions and compared to jets from round nozzles, verifying their improved mixing abilities. The precise mixing mechanisms of these nozzles are, however, not entirely understood and yet to be analyzed for typical jet parameters and excitation modes. The present study carries out three-dimensional Large Eddy Simulations (LES) of the flow from a tubular radially lobed nozzle to identify instability mechanisms and vortex dynamics that lead to enhanced mixing. The flow is studied at two Reynolds numbers of around 6000 and 75,000, based on the effective jet diameter. The low Reynolds number jet is compared to that from a round nozzle and experimental data to demonstrate changes in mixing mechanisms. The present simulations confirmed the presence of K-H-like modes and their evolution. The analysis also confirms the evolution of three distinct types of structures - the large-scale streamwise modes at the lobe crests, corresponding K-H structures at the troughs and an additional set of structures generated from the lobe walls. The higher Reynolds number simulations indicate changes in the mechanics with a subdued role of the lobe walls.

Analysis of the Vortex Dynamics and Instability Mechanisms for a Lobed Nozzle Jet

The application of radially lobed nozzles has seen renewed challenges in the recent past with their roles in combustion chambers and passive flow control. The free jet flow from such nozzles has been studied for different flow conditions and compared to jets from round nozzles, verifying their improved mixing abilities. The precise mixing mechanisms of these nozzles are, however, not entirely understood and yet to be analyzed for typical jet parameters and excitation modes. While past studies have proposed the presence of spanwise Kelvin-Helmholtz instability modes, the roll-up frequencies of the structures indicate more than one primary structure, which is challenging to resolve experimentally. The present study carries out three dimensional CFD simulations of the flow from a tubular lobed nozzle to identify instability mechanisms and vortex dynamics that lead to enhanced mixing. We initially validate the model against existing hotwire and LDV data following which a range of Large Eddy Simulations (LES) are carried out. The free jet flow was at a Reynolds number of around 5 × 104, based on the effective jet diameter. Initial results are compared to that of a round nozzle to demonstrate changes in mixing mechanisms. The lobed nozzle simulations confirmed the presence of K-H-like modes and their evolution. We also track the formation and the transport of coherent structures from the tubular part of the nozzle to the core flow, to reveal the evolution of the large-scale streamwise modes at the crests and corresponding horseshoe-like structures at the troughs.

Numerical predictions of the flow characteristics of subsonic jet emanating from corrugated lobed nozzle

Purpose The purpose of this study is to investigate the aerodynamic characteristics of subsonic jet emanating from corrugated lobed nozzle. Design/methodology/approach Numerical simulations of subsonic turbulent jets from corrugated lobed nozzles using shear stress transport k-ω turbulence model have been carried out. The analysis was carried out by varying parameters such as lobe length, lobe penetration and lobe count at a Mach number of 0.75. The numerical predictions of axial and radial variation of the mean axial velocity, u′u′ ¯ and v′v′ ¯ have been compared with experimental results of conventional round and chevron nozzles reported in the literature. Findings The centreline velocity at the exit of the corrugated lobed nozzle was found to be lower than the velocity at the outer edges of the nozzle. The predicted potential core length is lesser than the experimental results of the conventional round nozzle and hence the decay in centreline velocity is faster. The centreline velocity increases with the increase in lobe length and becomes more uniform at the exit. The potential core length increases with the increase in lobe count and decreases with the increase in lobe penetration. The turbulent kinetic energy region is narrower with early appearance of a stronger peak for higher lobe penetration. The centreline velocity degrades much faster in the corrugated nozzle than the chevron nozzle and the peak value of Reynolds stress appears in the vicinity of the nozzle exit. Practical implications The corrugated lobed nozzles are used for enhancing mixing without the thrust penalty inducing better acoustic benefits. Originality/value The prominent features of the corrugated lobed nozzle were obtained from the extensive study of variation of flow characteristics for different lobe parameters after making comparison with round and chevron nozzle, which paved the way to the utilization of these nozzles for various applications.