In the present paper, a comparison of the numeric and experimental results obtained from the acoustic mode analysis for a turning mid turbine frame (TMTF) is presented. The investigated turning mid turbine frame is part of the two-stage two-spool test turbine located at the Institute for Thermal Turbomachinery and Machine Dynamics of Graz University of Technology. In this specific test turbine a transonic turbine stage (HP) is followed by a low pressure turbine stage (LP) consisting of a TMTF with 16 highly 3D-shaped turning struts and a counter-rotating low pressure rotor.

The experimental dataset is obtained by a measurement section downstream of the low pressure rotor which is instrumented with acoustic sensors. This microphone array is wall flush mounted in the outer casing which is traversable over 360 degrees in circumferential direction. The numerical setup consists of the whole test turbine including the experimental measurement section downstream of the low pressure rotor. Since the periodicity of the test setup equals 90 degrees the CFD calculations were performed accordingly using the unsteady inhouse Navier-Stokes code LINARS.

For both, numerical and experimental datasets the same post-processing tools are used in order to perform the acoustic mode analysis of the unsteady data. At first a comparison in terms of noise generation and propagation of the results is done by the frequency spectra, the emitted sound pressure and sound power level of both rotors independently. Since the emitted sound pressure level rises to a maximum at the first blade passing frequency of the HP rotor as well as the LP rotor the further analysis focuses on these two specific frequencies only. Therefore the acoustic field at those frequencies is characterized by azimuthal and radial modes. For a correct comparison between the numerical and experimental results numeric data taken from the same geometric locations as the microphones’ positions (thus the measurement locations) is processed.

Hence, this paper provides a deep insight into the capability of using unsteady CFD calculation in combination with the acoustic mode analysis in order to obtain the noise generation and propagation in turbines.

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