*3.1. Measurement*

The turbine rotor was mounted on a special highly flexible jig, so that the effect of boundary conditions was negligible. In the sense of a classic modal analysis, a small modal hammer excites the individual rotor blades at a suitable position. A laser Doppler vibrometer measures the blade vibration speeds contactless. As part of the experimental investigation regarding the blade vibrations, only the vibration response of one suitable selected point per blade was measured. This is acceptable if the measurements match the expected mode shapes of the computational model prediction.

The measuring chain was assembled for an experiment from POLYTEC Sensor Head OFV-505, Vibrometer Controller OFV-5000 and cDAQ-9179 with modules NI-9229 and NI-9234. The laser head was positioned 438 mm from the top of the turbocharger blade according to the datasheet (Figure 3). The beam was directed perpendicularly to the scanning area.

With the individual blade mistuning measurement, the deviation from the mean value of all blades due to the material property or geometry deviations is converted into a calculation value in an experimental way. To decouple the oscillation movement of an individual blade when measuring, all other blades are additionally detuned by attaching additional masses of appropriate size to the blade tips (see Figure 4 right). The small modal hammer Brüel & Kjær Type 8204 (Figure 4 left) excites the blade to vibrate, while the laser vibrometer measures the vibration speed at the tip of the blade. Through a suitable choice of size and position of the additional masses as well as through variation of the measuring and excitation point, the mistuning behaviour in the frequency range of the blade dominated mode shapes can be identified. The measurement was carried out without and with additional masses on the blades to make clear their positive effect.

**Figure 3.** Laser equipment for blade vibration measurement.

**Figure 4.** Turbocharger turbine wheel with additional masses on blades and the used modal hammer.

Data monitoring was performed using a programmed script in NI LabVIEW software and 20 second sections were saved. Their data includes at least five responses from the hammer blow to the blade. At the same time, the experiment was checked for a double hammer blow to prevent any adverse effects of the subsequently processed data. Thereafter, five responses lasting 0.2 s and the corresponding time records of the modal hammer force pulses were selected for frequency analysis.
