CFD Predictions of Jet Engine Exhaust Plumes

Abstract

Laser beam propagation in severe environment such as a jet engine exhaust may influence performance of e.g. laser countermeasures and active imaging systems located on airborne platforms. Beam propagation in close vicinity to the engine plume causes performance degradation due to beam wander and beam broadening effects. In this study, aero-optical effects, as well as conventional fluid dynamical issues, are computationally examined using Large Eddy Simulations (LES) in round and square shaped co-annular jets having the same cross-sectional nozzle area. The target configuration for these LES computations is a downscaled jet engine, operated by Volvo Aero, providing a characteristic exhaust in comparison to a full-scale engine. The LES model is separately validated against velocity and temperature data from the Seiner experiments before being employed to compute the target engine configuration. The computed results are used to elucidate the fluid dynamics in a supersonic jet plume, and to help in further understand the complicated aero-optical processes in the engine plume. In particular, we estimate the refractive index and structure parameter that in turn can be used to estimate the beam wander, distortion and broadening. Laser beam propagation through adverse turbulent regions, such as the environment close to a jet engine exhaust, needs to be studied in order to understand the mechanisms affecting the performance of airborne laser systems. During recent years, development of several laser systems aimed primarily at airborne applications has attracted interest. In security applications, Directed Infra-Red Counter-Measures (DIRCM) are now being developed to protect civilian airliners from heat seeking missile threats that potentially could be used by terrorists. DIRCM systems operate by directing modulated laser irradiation towards the missile seeker causing the optical sensor within the seeker to be jammed or decoyed, hence prohibiting target tracking. As a result of a successful jamming, the missile will break-lock and miss the intended target. Other airborne laser systems include e.g. active imaging and free-space optical communication. There are several sources that may perturb the emitted laser radiation such as, atmospheric turbulence, structural vibrations, aero-optical effects and jet engine exhaust plumes. In this study, the effects of the jet engine exhaust are emphasized using Computational Fluid Dynamics (CFD) together with supporting experimental measurement data. The jet engine exhaust plume is turbulent, and may therefore introduce severe aero-optic perturbations that accumulate and cause laser beam degradation in terms of beam wander, intensity scintillations and beam broadening at long range. By characterizing and evaluating perturbations, schemes for compensation or evasion of performance degradation can be devised. Jet engine exhaust plumes, figure 1, are complicated in nature as being created by the combustion process in the combustion chamber and being affected by the turbine, afterburner and nozzle. The engine processes are unsteady and sometimes affected by combustion instabilities, whereas the flow in the nozzle sometimes experience intermittent separation close to the walls. These jet engine plumes are supersonic, hot and with nozzle orifice pressures either just above or below the ambient pressure. The pressure mismatch causes the jet boundary to oscillate as it attempts to match the ambient pressure, and since the sound waves, by which information is transferred across the jet, travel slower than the supersonic flow, the jet repeatedly overshoots its equilibrium position. The sound waves converge to develop a network of shock diamonds that alternate with rarefaction fans. The gas in the jet interior expands and cools off as it flows