CSD Predictions of Jet Exhaust Plumes

Directed Infra-Red Counter Measures (DIRCM) are used to protect airborne platforms from heat seeking missiles. The basic operating principle for DIRCM systems is to direct energy (laser light) into the sensor of the missile and thereby destroy or confuse the missile seeker electronics. There are several sources for disturbance of the emitted light, e.g. vibrations, atmospheric turbulence, aero-optical effects and engine exhaust. Fluctuations in the refractive index of the air through which the laser propagates may lead to such as beam broadening, beam wander, intensity fluctuations and phase variations. A fast moving body will generate more turbulence than is usually found in the ambient atmosphere. Similarly, jet engine exhaust plumes create even higher levels of turbulence. This turbulence is not just stronger than the ambient atmospheric turbulence but also of a different kind, involving large gradients in temperature, velocity, pressure, and composition. In the present report various aspects of jet engine exhaust plumes are studied using computational fluid dynamics (CFD). Due to the requirements of the problem, detailed simulation of small scale variability in the refractive index, large eddy simulation (LES) was chosen for the simulations. Simulations are carried out for both an experiment performed by the Volvo Aero Corporation using a scaled down version of a jet engine, and idealized jets with high resolution to determine flow characteristics. For the idealized jets the nozzle shapes and bypass flow have also been varied.