The dramatic growth that air traffic has experienced in the last years is not likely to slow down in the future. The situation for the airlines has however been critical due to the large share of the operating costs corresponding to fuel. On the other hand, the society demands quieter aircraft which is then translated into stricter regulations. The Advisory Council for Aeronautics Research in Europe (ACARE) has set an ambitious array of objectives to be accomplished by 2020. It is often claimed that complying with those targets will not require evolution but, rather, revolution. One of the potential future engine configurations being considered is the counter-rotating turbofan (CRTF) concept. This paper addresses the possibilities of improvement that the CRTF can offer with respect to the specific fuel consumption, emissions and noise as compared to the baseline engine, the GE90. Semi-empirical correlations and methodologies have been used for the study. First a Blade Element Method (BEM) is developed to estimate the performance of the fan and to build confidence upon the applied loss and deviation angle models. Next, the design methodology is applied to three cases: a single-stage fan featuring the reference properties of the GE90 engine; a counter-rotating fan (CRF) fan with similar properties as a GE90 fan, but with a lower rotational speed; and a CRF with higher fan pressure ratio (FPR) for lower specific fuel consumption. Finally, noise emission by all the three configurations are estimated by noise models available in the literature. Reductions of equivalent perceived noise level (EPNL) were found to be possible if a CRF is used instead of the baseline single-stage arrangement. Other noise descriptors are also reduced by a similar amount. Approximately equal noise levels are expected if the CRF is of higher pressure than the baseline.
Whistling due to Flow-Induced Pulsations can occur in gas transport and gas export pipe systems with low Mach number flow. These flow-induced pulsations can lead to piping vibration or acoustic fatigue of piping elements. The study presents a simple numerical pulsation source identification method applied to restriction orifices. In case of whistling, small acoustic perturbations incident on some pipe elements such as restriction orifices and T-junctions are amplified by the shear layer flow at certain frequencies, propagate downstream and upstream of the elements, and are reflected by the ends of the piping or by large section changes. The element acts as an acoustic amplifier for the incident acoustic energy at certain frequencies. The source identification method, earlier presented by Martinez-Lera et al [1] for the aeroacoustic source identification of T-junctions and improved by Nakiboglu et al [2] for circumferential cavities in corrugated pipes, combines incompressible numerical simulations with vortex sound theory. It is applied here to restriction orifices, commonly used in industrial pipe systems as measuring devices, to induce a pressure drop, or to reduce low frequency pulsations. Incompressible, laminar CFD simulations are used for the source identification instead of fully compressible LES. These simplifications enable much less CPU intensive computations, but require extra post processing. A particularly sensitive point is the estimation of the pressure drop due to the potential flow. This is estimated with a simulation made on a straight, empty pipe of same characteristics. This method is applied to restriction orifices in this paper, and the specific points for this particular geometry are reviewed. The results are compared to the experimental and numerical results of Testud et al [3] and Lacombe et al [4]. The application of the method to the source identification of whistling restriction orifices predicts whistling at Strouhal numbers similar to earlier experimental and numerical studies, with limited computational effort.
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