Computational Aero-acoustics of the coaxial flow exhaust system of gas turbine engine

Jet engine noise is an environmental problem that needs to be addressed. Several methods to reduce the jet noise have been proposed in the last decades. The main issue is to find methods that reduce noise without causing considerable loss of thrust. Experimental and computational tools are mandatory in successfully reducing jet engine noise emissions. One of the challenging issues of computing the jet engine noise is the presence of very large scales (associated with the wave length of the acoustic wave) and at the same time also small scales that are responsible for the acoustical sources. In the field of Computational Aero-Acoustics (CAA) different hybrid approaches have been introduced to handle the different scales using problem specific models and methods. Here, a decomposition of flow variables is used that allows separation of flow and acoustic computations. Large Eddy Simulation approach is employed to compute the flow field and the acoustic sources. An inhomogeneous wave equation is used to perform acoustic computations. The paper investigates numerically the flow and the near-field acoustic data from a coaxial jet case with chevrons on the core nozzle that are compared with those obtained from a baseline coaxial jet, showing the spatial character of the acoustic benefit when chevrons are used on the core nozzle. Comparisons in terms of sound pressure levels with experimental data performed with the same geometry show a good agreement.