In order to change the blade count (BC) of axial rotor designs, a scaling technique can be applied where the blades are scaled in axial and circumferential directions while maintaining the solidity. This technique allows to adjust the blade count without changing the steady-state aerodynamics, but the influences on the aerodynamic damping are unknown. The present study is focused on the investigation of the change in aerodynamic damping of a subsonic axial compressor rotor, if the blade count is changed between 13 and 25 blades. The investigation is focused on the first bending mode family and the influences of the hub geometry on the modes are neglected. First, a comparison between the influence coefficient (IC) method and the traveling wave mode (TWM) method is conducted, which shows that the application of the IC method in combination with harmonic balance simulations offers a fast way to compute the aerodynamic damping without introducing significant errors compared with the TWM method simulations. Regarding the aerodynamic damping of the different rotor geometries, it can be noted that the amplitude of the work per cycle influence coefficient of the center blade scales linearly with the blade scaling factor and an increase in aerodynamic damping for an increase in blade count is observed. Furthermore, a simplified analytic theory is established, which explains the phase angle change of the blade influence coefficients due to the blade scaling. In the last part of the paper, an extrapolation method is proposed for the investigated geometry and mode family, which allows for the estimation of damping S-curves for scaled rotor geometries based on only one IC method simulation.

The mistuning problem of quasi-periodic structures has been the subject of numerous scientific investigations for more than 50 years. Researchers developed reduced-order models to reduce the computational costs of mistuning investigations including finite element models. One question which has also high practical relevance is the identification of mistuning based on modal properties. In this work, a new identification method based on the subset of nominal system modes method (SNM) is presented. Different to existing identification methods where usually the blade stiffness of each sector is scaled by a scalar value, N identification parameters are used to adapt the modal blade stiffness of each sector. The input data for the identification procedure consist solely of the mistuned natural frequencies of the investigated mode family as well as of the corresponding mistuned mode shapes in the form of one degree-of-freedom per sector. The reduction basis consists of the tuned mode shapes of the investigated mode family. Furthermore, the proposed identification method allows for the inclusion of centrifugal effects like stress stiffening and spin softening without additional computational effort. From this point of view, the presented method is also appropriate to handle centrifugal effects in reduced-order models using a minimum set of input data compared to existing methods. The power of the new identification method is demonstrated on the example of an axial compressor blisk. Finite element calculations including geometrical mistuning provide the database for the identification procedure. The correct functioning of the identification method including measurement noise is also validated to show the applicability to a case of application where real measurement data are available.

One of the main design decisions in the development of low-speed axial fans is the right choice of the blade loading versus rotational speed, since a target pressure rise could either be achieved with a slow spinning fan and high blade loading or a fast spinning fan with less flow turning in the blade passages. Both the blade loading and the fan speed have an influence on the fan performance and the fan acoustics, and there is a need to find the optimum choice in order to maximize efficiency while minimizing noise emissions. This paper addresses this problem by investigating five different fans with the same pressure rise but different rotational speeds in the design point (DP). In the first part of the numerical study, the fan design is described and steady-state Reynolds-averaged Navier–Stokes (RANS) simulations are conducted in order to identify the performance of the fans in the DP and in off-design conditions. The investigations show the existence of an optimum in rotational speed regarding fan efficiency and identify a flow separation on the hub causing a deflection of the outflow in radial direction as the main loss source for slow spinning fans with high blade loadings. Subsequently, large eddy simulations (LES) along with the acoustic analogy of Ffowcs Williams and Hawkings (FW–H) are performed in the DP to identify the main noise sources and to determine the far-field acoustics. The identification of the noise sources within the fans in the near-field is performed with the help of the power spectral density (PSD) of the pressure. In the far-field, the sound power level (SWL) is computed using different parts of the fan surface as FW–H sources. Both methods show the same trends regarding noise emissions and allow for a localization of the noise sources. The flow separation on the hub is one of the main noise sources along with the tip vortex with an increase in its strength toward lower rotational speeds and higher loading. Furthermore, a horseshoe vortex detaching from the rotor leading edge and impinging on the pressure side as well as the turbulent boundary layer on the suction side represent significant noise sources. In the present investigation, the maximum in efficiency coincides with the minimum in noise emissions.

The mistuning problem of quasi periodic structures is subject of numerous scientific investigations for more than 50 years. Researchers developed reduced order models to reduce the computational costs of mistuning investigations including finite element models. One question which has also high practical relevance is the identification of mistuning based on modal properties. In the present work, a new identification method based on the Subset of Nominal System Modes method (SNM) is presented. The input data for the identification procedure consists solely of the mistuned natural frequencies of the investigated mode family as well as of the corresponding mistuned mode shapes in the form of one degree of freedom per sector. The reduction basis consists of the tuned mode shapes of the investigated mode family. Furthermore, the proposed identification method allows for the inclusion of centrifugal effects like stress stiffening and spin softening without additional computational effort. From this point of view the presented method is also appropriate to handle centrifugal effects in reduced order models using a minimum set of input data compared to existing methods. The powerfulness of the new identification method is demonstrated on the example of an axial compressor blisk. Finite element calculations including geometrical mistuning provide the data base for the identification procedure. The correct functioning of the identification method including measurement noise is also validated to show the applicability to a case of application where real measurement data is available.

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