On the Estimation and Relevance of Internally Generated Turbine Broadband Noise

Abstract

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.

Investigation on Turbine Tone Broadening in Static Engines

“…). Some of these findings are also observed by Matthews on their parametric studies for the JTD8 engine 7 Tone energy measured from cold flow rigs are converted into hot conditions 15,16 and the ground reflection and atmospheric absorption are applied so that the differences with the measured far field tone are expected to be the haystacking attenuation delta. If this phenomena can be modeled with one unique parameter, H, the delta obtained from different turbine tones, different rigs measured in different facilities and long and short cowl engines would correlate when plotted against H. Figure 9 shows these deltas for the three turbines powering engines labeled as A, B, C. The nomenclature for the turbine tones is XLPTY, where X is the temporal harmonic number and Y stands for the stage with the corresponding number of rotors.…”

“…The other explanation can be that the broadband source has not been included in the rig to engine method. Since that is a significant turbine source 15 , its addition to the haystacking around the tones might lead to an overall frequency shape similar to that obtained with the liner diagnostic method.…”

“…This second approach consists of turbine noise estimates from the comparison of engine measurements using acoustically treated and untreated hot ducts and has been applied only where reliable back to back data is available, i.e. Engine C. Discrepancies found between the two methods may possibly be due to mispredictions in the haystacking empirical model but are thought to be more likely due to the internally generated broadband noise that was not included in the rig to engine process and that can significantly affect the turbine source for certain operating conditions 15 .…”

“…As mentioned before, the present work will investigate if rig measurements are representative of the engine, and also if an existing semi-empirical model for haystacking is reasonably capable of modeling the phenomena. The first is something demonstrated 'in-duct' for the ANTLE Engine 15,16 , and now is investigated for three Rolls-Royce production engines representative of various design styles and thrust applications, and in the farfield for a static engine test. Since the haystacking can be highly dependent on the cold shear layer structure, and that significantly changes from static to flight conditions, some of the results and conclusions obtained here may differ when doing the analysis inflight, and that is something not addressed in the present work.…”

Turbine Broadband Noise Predictions Using Linearised Frequency Domain Navier-Stokes Solvers

“…Nesbitt claimed that it is necessary to develop tools that can predict turbine noise with sufficient accuracy to understand its sources and reduce them in the future. Regarding turbine noise sources break-down, Serrano et al [2] quantified experimentally the relative importance of turbine broadband noise, which can result in being dominant in many operating points and designs and hence, there is a need of industrial methods to predict it accurately. Turbulent wakes generated by a turbine row (see Figure 1) impinge on the downstream one, giving rise to acoustic waves with a broadband energy content.…”