LES PREDICTIONS OF NOISE EMISSIONS FROM A LOW-BYPASS RATIO MILITARY GAS TURBINE ENGINE

A practical framework for predicting jet structure and noise from military aircraft is described, which is developmental and has been examined for some fundamental jet flow problems. The framework currently utilizes RANS methodology for geometrically complex internal propulsive flowpaths and LES methodology for the jet structure downstream of the nozzle exit. Temporal data from the LES solution is stored on a flared-cylindrical surface surrounding the jet, to be used for noise propagation to the farfield. Earlier applications of RANS methodology combined with the use of analogy-based jet noise codes proved inadequate due to the inability of the noise codes to treat complex 3D flows such as those associated with multiple nozzles and/or with varied jet noise reduction concepts. Restricting the use of LES (or RANS/LES) methodology to free shear flows remedies the severe grid resolution issues that would be encountered with utilization of LES for modeling internal propulsive flows. The issue of "adequately" initiating the LES solution from a RANS solution profile just downstream of the nozzle exit has been the focus of our exploratory studies, and is clearly more complex than standard procedures such as recycling and resealing techniques used for simple wall bounded flows. Approaches examined are discussed and unified RANS / LES solutions for several flows are described. The application of this framework to more complex flows requires no fundamental modifications as will also be discussed.