Book Description

The thrust of this thesis is to initiate an investigation into the acoustic effects related to the presence of a pylon installed on a high bypass ratio turbofan engine. It is well known that the presence of a pylon bifurcation generates an asymmetric sound field and modifies the characteristics of the exhaust flow. This study was designed to gain an understanding between these two results of the pylon's presence. To accomplish this, a pylon was designed and built to modify the existing bypass ratio 5 nozzle in the Aeroacoustic Test Facility at the University of Cincinnati's Gas Dynamics and Propulsions Laboratory. This pylon and bottom bifurcation modifies the baseline nozzle in a manner geometrically similar to that of a real engine configuration. Experiments were carried out to measure the acoustic properties of the pylon configuration and understand their connection to the observed flow field. Both near and far field recordings were made of the baseline nozzle and the pylon nozzle at several azimuthal positions. Velocimetry measurements were also taken for these configurations. It was seen that the classic pylon effects were present on the tested configuration; the core flow was turned towards the pylon, the fan stream was directed away from the pylon. The resulting far field and near field signatures were asymmetric. In the far-field, the presence of the pylon at the highest bypass cycle condition exhibited a maximum increase in noise production of 2.2 EPNL dB, at the sideline angle, and a minimum increase of 1.1 EPNL dB directly under the pylon. Increasing the shear velocity lowered the increase in sound production due to the pylon, but the azimuthal variation was largely unaffected. A chevron nozzle, an existing noise reduction technology, was tested on the pylon nozzle configuration to study how the pylon affects the acoustic benefits of this technology across a range of cycle conditions. Also, a new technology known as an internal chevron nozzle was designed and tested with the baseline and pylon configurations. This internal chevron nozzle was designed as an alternative to the existing chevron technology; intended to reduce the sensitivity to shear velocities exhibited by traditional chevron nozzles. The 8LP core chevron reduced the EPNL of the baseline nozzle by up to 1.6 dB, and the internal chevron nozzle provided up to a 0.8 EPNL dB reduction. However, the presence of the pylon modified the effectiveness of these nozzles. The chevron nozzle increased sound production at high shear velocity, but reduced noise up to 2.0dB for lower shear cases. The effectiveness of the internal chevron nozzle grew at both the medium and low shear conditions for all azimuthal positions, up to a 1.3 EPNL dB reduction. However, reductions seen at high shear velocity were reduced by the presence of the pylon. The noise reduction of the internal chevron nozzle was less than the chevron nozzle, but its design was successful in being less dependent on the cycle condition.