TY - BOOK

T1 - Similarity Conditions for Modal Sound Propagation in Turbomachinery Test Rigs

AU - Hurfar, Carolin Mandanna

N1 - Doctoral thesis

PY - 2021/7/30

Y1 - 2021/7/30

N2 - Psychoacoustic studies show that the level of perceived annoyance with respect to noise emitted by aircrafts engine positively depends on the sound pressure level of the tonal components in the frequency spectrum. Acoustic modes generated by the rotation of the rotor and the periodic interaction of the stationary and rotating blades in the turbomachinery parts of the aircraft engine are the main contributors to the emitted tonal noise. In order to develop mitigation measures for these modal sound fields, experimental investigations are carried out on turbomachinery test rigs. For high quality research data, similarity between measurements has to be accomplished, as even minor changes to reference conditions might impair the significance of measurements. For time-harmonic sound propagation in an inviscid and ideal flow, the Helmholtz number and the Mach number establish similarity conditions. This work addresses the establishment of similarity of modal sound propagation on test rigs where equality of the Mach number and / or the Helmholtz number cannot be met reliably between measurements. The investigation focuses on individual acoustic modes propagating in a circular duct carrying an axial uniform flow. Experimental investigations are carried out on a low-pressure air turbine test rig. The test rig is equipped with a so-called sound generator for the controlled excitation of acoustic modes of arbitrary order and at specific frequencies. Similarity of the propagation of excited modes is evaluated by analyzing the modal response function. Based on an analytical analysis of the collected measurement data and a post-hoc scaling approach, it is shown that the axial angle of the group velocity vector is a suitable parameter to establish partial similarity with respect to modal sound propagation in cases where similarity of the Helmholtz number and / or the Mach number cannot be achieved. A similarity relation between the three parameters is derived. A sensitivity analysis shows that the impact of variations in the Helmholtz number and the Mach number can differ considerably depending on the operating point. For the two operating points investigated here, it is found accordingly that the variations in the modal response strongly correlate with the axial angle of the group velocity vector and the Helmholtz number, but only moderately or even negligible with the Mach number. A reduced frequency is introduced which, similar to the reduced rotational speed and reduced mass flow rate, can be used as an acoustic operating parameter to establish similarity of modal sound propagation. For test rigs equipped with a sound generator, partial similarity can thus be established between measurements even though equality of the parameters, Helmholtz number and Mach number, is not met.

AB - Psychoacoustic studies show that the level of perceived annoyance with respect to noise emitted by aircrafts engine positively depends on the sound pressure level of the tonal components in the frequency spectrum. Acoustic modes generated by the rotation of the rotor and the periodic interaction of the stationary and rotating blades in the turbomachinery parts of the aircraft engine are the main contributors to the emitted tonal noise. In order to develop mitigation measures for these modal sound fields, experimental investigations are carried out on turbomachinery test rigs. For high quality research data, similarity between measurements has to be accomplished, as even minor changes to reference conditions might impair the significance of measurements. For time-harmonic sound propagation in an inviscid and ideal flow, the Helmholtz number and the Mach number establish similarity conditions. This work addresses the establishment of similarity of modal sound propagation on test rigs where equality of the Mach number and / or the Helmholtz number cannot be met reliably between measurements. The investigation focuses on individual acoustic modes propagating in a circular duct carrying an axial uniform flow. Experimental investigations are carried out on a low-pressure air turbine test rig. The test rig is equipped with a so-called sound generator for the controlled excitation of acoustic modes of arbitrary order and at specific frequencies. Similarity of the propagation of excited modes is evaluated by analyzing the modal response function. Based on an analytical analysis of the collected measurement data and a post-hoc scaling approach, it is shown that the axial angle of the group velocity vector is a suitable parameter to establish partial similarity with respect to modal sound propagation in cases where similarity of the Helmholtz number and / or the Mach number cannot be achieved. A similarity relation between the three parameters is derived. A sensitivity analysis shows that the impact of variations in the Helmholtz number and the Mach number can differ considerably depending on the operating point. For the two operating points investigated here, it is found accordingly that the variations in the modal response strongly correlate with the axial angle of the group velocity vector and the Helmholtz number, but only moderately or even negligible with the Mach number. A reduced frequency is introduced which, similar to the reduced rotational speed and reduced mass flow rate, can be used as an acoustic operating parameter to establish similarity of modal sound propagation. For test rigs equipped with a sound generator, partial similarity can thus be established between measurements even though equality of the parameters, Helmholtz number and Mach number, is not met.

U2 - 10.15488/11156

DO - 10.15488/11156

M3 - Doctoral thesis

T3 - Berichte aus dem Institut für Turbomaschinen und Fluid-Dynamik, Leibniz Universität Hannover

CY - Hannover

ER -