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Numerical modelling of mixing in underexpanded jets from castellated nozzles

Published online by Cambridge University Press:  04 July 2016

A. J. Saddington ,
K. Knowles  and
R. Y. T. Wong
A. J. Saddington
Affiliation:
Aeromechanical Systems Group, Department of Aerospace, Power and Sensors, Cranfield University, RMCS, ShrivenhamSwindon, UK
K. Knowles
Affiliation:
Aeromechanical Systems Group, Department of Aerospace, Power and Sensors, Cranfield University, RMCS, ShrivenhamSwindon, UK
R. Y. T. Wong
Affiliation:
Aeromechanical Systems Group, Department of Aerospace, Power and Sensors, Cranfield University, RMCS, ShrivenhamSwindon, UK
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Abstract

A computational fluid dynamics (CFD) model of three different axisymmetric convergent nozzle designs was created. Two of the nozzles had trailing-edge modifications (TEMs), which consisted of four regularly-spaced tooth-gap castellation pairs. One of the castellated nozzles had a divergent gap profile (regular castellated) and the other had a convergent gap profile (convergent-chamfered castellated). The remaining plain nozzle had no castellations.

Calculated CFD results were examined at a nozzle pressure ratio (NPR) of 6·5 in the near field of the regular castellated and convergent-chamfered castellated nozzles. Jet fluid was seen to be ejected radially from the jet core through the gaps between the castellations at the nozzle exit plane. Contra-rotating streamwise vortices were seen to develop at the interface between the gap and castellation. These were of the sense that acts to pump jet fluid out in the plane of the gap and entrain ambient fluid in the plane of the tooth. Peak increases of approximately 45% in non-dimensional mass flow rate over the plain nozzle were seen.

The ejections from the four-tooth regular-castellated nozzles extended far enough radially that they became detached from the main jet core. The contra-rotating vortices moved closer together with downstream distance reinforcing the pumping action between them. The jet ejections from the convergent-chamfered castellated nozzles did not extend far enough radially to detach from the main core (at the NPR tested) nor did the contra-rotating vortices move significantly closer together. Peak increases of approximately 45% and 25% (for the regular-castellated and convergent-chamfered castellated nozzles respectively) in non-dimensional mass flow rate over the plain nozzle were seen.

The strength of the contra-rotating vortices had dropped by two orders of magnitude at five nozzle diameters downstream from the exit plane. This suggested that the extent of fluid ejection may be more important than the initial strength of the vortices (although stronger vortices may help to eject fluid further out radially). Thus, configurations of nozzle geometry which cause the resulting streamwise vortices to interact favourably in ejecting fluid, would be desirable.

Type
Research Article
Information
The Aeronautical Journal , Volume 106 , Issue 1066 , December 2002 , pp. 643 - 652
Copyright
Copyright © Royal Aeronautical Society 2002 

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