## 4 Conclusions  
Using a co-rotating LSV setup, it was possible to measure the blade vibration of an axial fan with high spectral and spatial resolution. Additionally, the experiment was set up inside a wind tunnel test section to investigate the effect of a uniform inlet cross-flow on the fan blade vibration, which is known to introduce a significant axial asymmetry of the flow field in the rotor section [12; 13; 14].  
Up to this point, similar investigations have only been made numerically [15; 16; 18] or by using strain gauges on larger-diameter fans [17; 24]. Using LSV in this setup, it was possible to gain more insight into the spatial resolution of the fan blade vibration under cross-flow influence, and to compare the resulting motion distributions to numerical mode analysis.  
Two distinct fan operating points were examined, with very similar spectral distributions of the surface-averaged blade vibration amplitude in the reference configuration, that is, without ambient flow field. The first and, to a lesser extent, the second natural frequency showed peaks in the vibration amplitude spectrum that fit the predicted natural mode very well in shape, but a stronger blade vibration was measured for the rotation-rate harmonics. Under the influence of inlet cross-flow, the blade-pass frequency and the sixth rotation-rate harmonic were excited strongly, while no significant amplification of the natural frequencies was measured. These findings agree with previously observed cross-flow excitation effects [17; 18]. Integral fan blade vibration almost doubled at the low fan flow rate, and increased by 20% at the higher fan flow rate.  
While a synchronous flapping of all five blades' tips was observed to be instigated at the blade-pass frequency, the nature and cause of the sixth rotation rate harmonic's excitation remains uncertain. It may have been attributed to an additional resonance effect of the combined elastic system, including the fan and its fixation to the duct section, which was not represented in the modal computation setup.  
The experimental results help to identify the affected frequencies and respective mode shapes under cross-flow excitation. Measuring capacity restrictions allowed for a few configurations only; clearly it would be interesting to modify, for example, the fan blade-shape, the hub and shroud shape, and cross-flow coefficients. A generic fan is suggested with improved axial alignment to better eliminate perspective distortion. Attention has to be paid to realize a maximum rigid fan-axis fixation in the test rig. For future investigation, it is also advised to not only record the relative phase relation of the measured scanning points, but also to identify the azimuthal position in relation to the cross-flow. Such results could yield a better comparison to the findings of e.g., [PERSON] et al. [17; 18; 24].  
The research leading to these results received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement number 256797, within the \"MACCSol\" project. The authors thank Polytec Inc. for their support.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments, analyzed the data and wrote the paper.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} ACC & Air-cooled condenser \\ LSV & Laser scanning vibrometry \\ FEM & Finite element method \\ OP\(xy\) & Operating point \(x\) with (\(y=1\)) or without (\(y=0\)) cross-flow \\ \end{tabular}