## 2 Experimental Investigation of Ice Accumulation (Materials and Methods)
### Experimental Setup and Design  
The experimental setup consists of a rotor test rig positioned inside a climate chamber. Figure 3 contains a schematic view of the test stand assembly. It is equipped with four different measurement  
Figure 2: Fibre alignment and dimensions of the radial and tangential sub-preform types with the total layup of the GFRP rotor (**a**). Placement of the actuators and sensors (**b**). systems in order to provide the ice mass, the ice distribution and thickness, as well as the structural dynamic behaviour of the rotor. The climate chamber (KBV 2000-C, Feutron) varies the temperature between 25 \({}^{\circ}\)C down to 20 \({}^{\circ}\)C, and it is equipped with a video camera (HERO7 Black, GoPro), which monitors the tests and allows for live viewing. A containment surrounds the rotor inside the chamber in order to prevent the water or detached ice from damaging any surrounding electrical components. As mentioned above, the experimental setup consists of four subsystems, an ice-generating subsystem, a vibrational measurement sub-system, an ice-detecting system, and an acquisition and processing system, as shown in Figure 3 and described in more detail below. An ice-generating subsystem containing a water spray unit is positioned in front of a rotor that is attached to the motor. The three nozzles shown in Figure 3 spray cold water in the direction of the rotor while an electric motor (M21R 100L, VEB) rotates at different speeds to achieve an adjustable ice accumulation. Nozzle no. 1 (N1) is positioned at the root of the rotor (\(r_{1}\approx\) 125 mm), nozzle no. 2 (N2) at the middle of the rotor (\(r_{2}\approx\) 175 mm), and nozzle no. 3 (N3) at the outer side of the rotor (\(r_{3}\approx\) 225 mm), forming a (N1, N2, N3) setup. The nozzles are mounted onto an aluminium bar, which is attached to the containment of the test bench, pointing towards the centre of the rotor at an angle of approximately \(\phi_{N}\approx-35^{\circ}\). Each nozzle can be activated/opened (1) or deactivated/closed (0) separately. The structural dynamic behaviour of the rotor is determined by the vibration measurement subsystem consisting of three accelerometers, placed on the back of the rotor, providing the dynamic response in the time domain. Two macro fibre composite piezo actuators (MFC) enable a frequency band-limited excitation with a frequency range of up to 1000 Hz. The MFCs are placed radially and perform an expansion along the same direction resulting in bending forces on the rotor. Various excitations have been tested; due to the relatively high stiffness of the rotor, eventually only chirp signals were found to produce proper excitation. More specifically, a chirp signal with a duration of 40 s is sent to the actuators. In order to measure the ice thickness distribution and/or mass, an ice-detecting subsystem is equipped with a triangulation laser profiler (IJ-V7300, KEYENCE) that constantly scans the profile along a radial line on the rotor's surface. The rotor is mounted to the motor, which is placed on a pendulum, thus being free to rotate on an axle via pivots (point C in Figure 4). Ice accumulation will cause a small rotation of the lever, which will be measured by a force sensor on the back. Additional mass is added to the centre of gravity to increase inertia, reducing oscillations caused by the spinning rotor. Figure 3: Picture of the nozzle setup on the test bench (**a**) and the corresponding schematic illustration (**b**). The vibration analysis is performed in several basic steps and using the vibration excitation and acquisition described above via MFCs or accelerometers. First, the chamber is cooled down to the desired temperature and kept constant till the end of one measurement series. Subsequently, the initial vibration response of the rotor is determined under static conditions. The rotor is then rotated at a certain speed and a further analysis is carried out to eliminate harmonics and other rotation-induced side effects.