Maximilian Behn

For the development of low-noise fan designs the sound generation must be reduced at the source, but also sound transmission effects through the rotor can be exploited to reduce the noise emitted to the far-field. Rotor shielding has been identified as a means of noise abatement, where rotor counter-rotating modes are blocked by the rotor and transmit poorly to the inlet. An effect that is rarely considered in this context is mode-frequency scattering at the rotor, which can circumvent the blockage effect by shifting the sound energy in frequency and mode order, typically to a rotor co-rotating mode order that transmits efficiently to the inlet. An experimental investigation is presented, where the transmission, reflection, and scattering of individually generated modes through a fan stage is measured by means of a loudspeaker array and microphone arrays, upstream and downstream. The highly resolved measured data comprise the variation of mode order, frequency, rotor speed, and aerodynamic operating conditions. In this study, the focus is on mode-frequency scattering, for which results are presented for high and low rotor speeds.

Turbulence parameters, in particular integral length scale (ILS) and turbulence intensity (Tu), are key input parameters for various applications in aerodynamics and aeroacoustics. The estimation of these parameters is typically performed using data obtained via hot-wire measurements. On the one hand, hot-wire measurements are affected by external disturbances resulting in increased measurement noise. On the other hand, commonly applied turbulence parameter estimators lack in robustness. If not addressed correctly, both issues may impede the accuracy of the turbulence parameter estimation. In this article, a procedure consisting of several signal processing steps is presented to filter non-turbulence related disturbances from the unsteady velocity data. The signal processing techniques comprise time- and frequency-domain approaches. For the turbulence parameter estimation, two different models of the turbulence spectra—the von Kármán model and the Bullen model—are fitted to match the spectrum of the measured data. The results of several parameter estimation techniques are compared. Computational Fluid Dynamics (CFD) data are used to validate the estimation techniques and also to assess the influence of the variation in window size on the estimated parameters. Additionally, hot-wire data from a high-speed fan rig are analyzed. ILS and Tu are assessed at several radial positions for two fan speeds. It is found that most techniques yield similar values for ILS and Tu. The comparison of the fitted spectra with the spectra of the measured data shows a good agreement in most cases provided that a sufficiently fine frequency resolution is applied. The ratio of ILS and Tu of the velocity components in longitudinal and transverse direction allows the assessment of flow-isotropy. Results indicate that the turbulence is anisotropic for the investigated flow fields.

The present study is dealing with the analytical modelling of sound transmission through turbomachinery stators. Two-dimensional cascade models are applied in combination with a newly proposed impedance model to account for the effect of flow deflection on the propagation of acoustic modes in annular flow ducts. Further, two different approaches for the calculation of the 3D mode transmission using 2D cascade models are used. The proposed methods allow the consideration of vane twist and mode scattering w.r.t. the radial mode order. The results obtained with the different approaches are compared with the outcome of numerical calculations for several flat plate cascades and a generic LPT vane geometry. The similarities and differences between the different analytical approaches are examined. It is shown, that the flow deflection through a stator row has a significant influence on the transmitted and reflected sound power. Furthermore, the occurrence of total transmission resp. zero reflection along with non-zero transmission is identified for particular mode orders and related to special conditions occurring in 2D cascade models