Appendix 2: Technical Report. Study of turbulent coaxial jet efflux mixing using LES [in, Final Report: Dispersion of Aircraft Efflux in Proximity to Airports]

Abstract

The scalar mixing of gas turbine exhaust jets is important to many engineering issues in turbomachinery propulsion and power systems, but especially so with regard to current aircraft emissions concerns. There have been many laboratory scale studies of self-similarity, scalar dissipation, temporal intermittency of passive scalar mixing in isothermal jets, and of fuel/air mixture in reacting jets, but no previous detailed investigations of co-flowing jet efflux development in the context of engine exhaust interactions with aircraft trailing vortices for emissions impact.

The objective of the present project has been to use latest advanced computational simulation techniques to study passive scalar mixing of turbulent co-flowing jets in their initial exhaust development region, as a complement to separate experimental studies, to provide better defined initial conditions for subsequent simpler model predictions of the effect of engine plume/vortex interactions on air quality. Current assessment methods start with either assumed engine exit conditions (which are not fully characterised and whose initial development cannot be accurately reproduced) or take as a starting point the assumed establishment of a fully developed jet plume (for which the degree of development and downstream origin are equally uncertain). However progress towards the application of large eddy simulation (LES) methods to such flows has been growing since they directly solve for the largest scales of the turbulent flow responsible for mixing.

Accordingly a well-established Large Eddy Simulation technique has first been validated against new experimental data for a low-speed, isolated round jet, seeded with a gas tracer, and then used to perform a parametric series of simulations, numerical experiments, for a coaxial jet representative of a modern, large by-pass ratio jet engine exhaust under a variety of conditions with passive scalar introduced into either the core or bypass flow. Effects of mesh resolution, free-stream velocity, swirl, and boundary proximity have all been considered and some conclusions drawn.

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ORCID for K.K.J. Ranga Dinesh: orcid.org/0000-0001-9176-6834

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