APPLICATION AND VALIDATION OF AEROACOUSTIC TOOL FOR RANS-BASED JET NOISE PREDICTION

In the last decades noise produced by aircraft has become one of the major concerning issues for civil aviation. Everyday, residents living around busy airports experience high noise levels during landing and take-off phases, making the authorities act in terms of noise standard regulations. Current interest in next-generation supersonic aircraft represents a hard challenge for aeroacoustics research community, requiring fast and accurate methods for noise prediction and new technologies for noise suppression. Most common computational aeroacoustics methods rely on noise source reconstruction based on unsteady CFD analysis such as Large Eddy Simulation (LES), due to the necessity to properly capture turbulent fluctuations evolution in near-field domain. In addition, acoustic propagation through a non-uniform medium is generally covered by the Linearized Euler Equations (LEE) governing on high-order finite difference spatial schemes, followed by temporal integration schemes. The ACO_JNS tool, developed at CIRA (Centro Italiano Ricerche Aerospaziali), deals with a numerical implementation of an acoustic analogy developed for turbulent jet noise prediction, accounting for both turbulent mixing and shock-associated components, using only the mean flow information from steady RANS simulations. Great reduction in computational cost is then achieved with respect to unsteady-based schemes. Moreover, the method is based on Green's functions formalism, meaning that the acoustic propagation is not required to be explicitly solved. Real propagation effects should be considered by calculating the Green's functions of the LEE for a given mean flow, but a numerical procedure is not currently available. Hence, Helmholtz equation Green's functions are implemented, corrected by a weighting amplification factor based on empirical observations to account for convective effects. The entire calculation routine is reviewed in detail from the CFD simulations setup and post-processing to the acoustic prediction in terms of Sound Pressure Level (SPL) radiation in the far-field. Several test cases are analyzed, involving different nozzle geometries and operating conditions, starting from the widely studied SMC000 single-stream round nozzle by NASA. Next, a completely different nozzle typology is considered, i.e. the dual-stream configuration simulating a turbofan engine, which constitutes today almost the totality of engines installed on civil aviation aircraft. Ultimately, the tool is tested on innovative nozzle configurations in the context of SENECA consortium projects, aiming to fulfill the objectives of local and global environmental impact assessment and to develop technologies to minimize the impact of a supersonic fleet. All the numerical acoustic results are compared with experimental data available in literature, displaying a great capability of the tool to capture general trends for both mixing and broadband shock-associated noise components. Some difficulties still remain for observer points located at high downstream angles, where prediction is harder due to the convective effects, while for low to medium angles the predictions show overall good agreements for a preliminary design phase. Moreover, the analysis carried out on SENECA nozzle configurations show a trend of improvement with respect to the existing numerical results derived from the application of the empirical Stone’s model, due to the additional pressure related noise sources captured by the code.