A RANS-based jet noise prediction method using ray tracing method

Abstract

Hybrid tools based on Reynolds Averaged Navier-Stokes (RANS) and Computational Aero-Acoustics (CAA) methods have been the current focus of industries to aid the design and development of quieter nozzles with novel configurations for future aircraft. In the current work, improvements to a RANS-based jet noise prediction method are proposed. The source model is based on Lighthill’s acoustic analogy and ray theory (geometric acoustics) is used to compute the propagation effects. The method is named, Lighthill Ray Tracing, ‘LRT’. In this thesis, improvements within the RANS informed source and propagation model are proposed. These improvements are applied to a wide range of nozzle geometries, both single stream and coaxial stream, with varying flow conditions. The LRT predictions are validated against experimental data. All RANS-based jet noise methods include model coefficients associated with eddy length-scales and time-scales. Computation of these model coefficients are dependent on measured acoustic spectrum at the 90o polar angle, thus making it difficult to predict noise from new conceptual nozzles. The current work focuses on arriving at a formalism to determine the RANS-associated length-scale and time-scale coefficients with reduced dependency on experimental data. This procedure is used to perform a parametric study on isothermal single stream and coaxial stream nozzles with varying geometry and flow parameters. Effect of velocity ratio, area ratio, bypass ratio and the effect of the primary nozzle in different noise producing regions of the nozzle are investigated. Along with the LRT method, the 4-source semi empirical ESDU noise prediction program has been extensively used to perform a characterisation study of far-field noise from ultra high bypass ratio (UHBR) nozzles. The study showed that noise from nozzles with higher area ratios can be modelled using a single equivalent jet. RANS-based prediction methods coupled with ray theory have been proven to be computationally fast and capable of providing good jet noise predictions. While the use of real rays to calculate propagation effects is not new, the calculation of so-called complex rays - that allow for the modelling of mevanescent waves - is typically not accounted fornin generic ray programmes. This presents serious shortcomings when dealing with highspeed jets, where high-frequency waves refract away from the downstream axis giving rise to a region exclusive to complex rays commonly referred to as the cone of silence (CoS). In the current work, two methods are proposed to model the field in the CoS. The first method is based on modelling the exponential decay using an approximate WKB solution for a parallel shear flow; the second method exploits the canonical nature of the cone of silence boundary to continue solutions inside the CoS via a complex ray embedded inside the Airy function and its derivative. To demonstrate both methods, the results are compared against Lilley’s Green’s function solution for a parallel shear flow using a variety of parametric studies. The first method is applied to an isothermal single stream jet and the noise predictions at the CoS angles are compared with experimental data.

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