International Journal of Aeroacoustics

This article examines Low Pressure Turbine (LPT) noise in modern aircraft and shows how difficult it is to design a quiet LPT. LPT noise has long been neglected in terms of research and prediction capability due to it being considered of lesser importance. Recent developments in LPT design have led to what is termed a “turbine noise storm,” in which the LPT is designed to be cut-off but in the end there is significant LPT noise and little or no ability to reduce it. These developments include the trend toward lower blade counts and solidities. The situation is further hurt due to the inability to predict LPT noise, particularly for the new type of designs. The inability to reduce the LPT noise once the engine and airplane is designed is due to there being no available light-weight high-temperature acoustic-lining technology that can be added to fix a LPT noise problem without a significant penalty. In an attempt to understand what makes some engines' LPT quiet while others are very loud, modern turbine designs are characterized into several LPT noise “classes” and important aspects of each class are shown. Finally, the need for a semi-empirical prediction is discussed including what essential elements this prediction needs to have to be relevant to modern aircraft preliminary design.

We consider the scattering of sound by turbulence in a jet shear layer. The turbulent, time-varying inhomogeneities in the flow scatter tonal sound fields in such a way as to give spectral broadening, which decreases the level of the incident tone, but increases the broadband level around the frequency of the tone. The scattering process is modelled for observers outside the cone of silence of the jet, using high-frequency asymptotic methods and a weak-scattering assumption. An analytical model for the far-field power spectral density of the scattered field is derived, and the result is compared to experimental data. The model correctly predicts the behaviour of the scattered field as a function of jet velocity and tone frequency.

The intention of this paper is quantifying the relevance of the internally generated broadband noise in the overall turbine signature. Results from five cold flow rigs of several low pressure turbines have been used to split the overall turbine noise in the two main sources: tonal and broadband. A comparison between both sources has allowed to determinate the relevance of the broadband noise and its dependence on the design style, operating conditions and turbine application. The methodology to get the broadband noise from a cold flow rig is also discussed and for one specific turbine, the broadband obtained from the rig is compared to engine results to demonstrate that rig noise measurements are enough representative of an engine. Finally, the broadband noise is evaluated at engine level and a turbine source breakdown EPNL is estimated for the three noise critical conditions of Approach, Cutback and Sideline. The main conclusion is that turbine broadband is a relevant source of noise at total aircraft level.

As turbofan engine bypass ratios continue to increase, the contribution of the turbine to the engine and aircraft noise signature is receiving more attention. Understanding the relative importance of the various turbine noise generation mechanisms and the characteristics of the turbine acoustic transmission loss are essential ingredients in developing robust reduced-order models for predicting the turbine noise signature. A computationally based investigation has been undertaken to help guide the development of a turbine noise prediction capability that does not rely on empiricism. Under the auspices of the Fundamental Aeronautics Program, NASA is currently funding core noise research focused on the generation and propagation mechanisms of combustor and turbine noise in the combustor-turbine-nozzle system. Highly detailed numerical simulations of the unsteady flow field inside the first stage of a modern high-pressure turbine were carried out using TURBO. Spectral and modal analysis of the unsteady pressure data from the numerical simulation of the turbine stage show a circumferential modal distribution that is consistent with the Tyler-Sofrin rule, which gives confidence in the approach. Additionally, initial steps have been taken in determining the source strength hierarchy.