14th International Symposium on Particle Image Velocimetry – ISPIV 2021 August 1–5, 2021 An experimental investigation on stall flutter over a vertically mounted rigid finite wing R. F. Soares1*, I.Karasu2, B. Ganapathisubramani1 1 University of Southampton, Southampton, SO17 1BJ, United Kingdom 2 Adana Alparslan Turkes Science and Technology University, 01250, Saricam, Turkey ∗ R.F.Soares@soton.ac.uk Abstract As stall flutter has relevant engineering implications, such as in blades of wind turbine and HALE (high- altitude long-endurance aircraft). This work presents the experimental investigation of rigid wing setup in a closed-circuit wind tunnel having 2.1 m × 1.5 m test section. The experimental campaign reached stable and symmetrical LCO within the freestream range from 9 m/s up to 14 m/s (1.69 × 105 < Re < 2.63 × 105). Two techniques were used for position tracking: one mechatronic and one image-based. The latter used ‘shake- the-box’ method applied to a body, which has proven a successful approach as a non-intrusive tool. 1 Introduction The experiments were carried out at the 7x5 Wind Tunnel; a closed-circuit facility with two test sections in serial. For this experiment, the setup was installed at the high-speed test section (2.1 m × 1.5 m). The wing is a NACA0012 cross-section model of aluminium (.e. structure) and carbon-fibre composite (e.g. skin) of 0.3m chordwise and a span length of 0.77 m. A servo motor with cylindrical shield is the base for the wing, which was connected to a 6-axis Delta IP65 load cell. In the heaving axis, the system was attached to a pair of linear-guided carriages, and the nominal rest position is found after two identical, counterbalance springs were linked, providing a spring constant (k) of 400 N/m. In the pitching axis, one pair of identical rubber bands were used on each side, connecting the wing leading edge to the cylindrical base. The image-based tracking solution involved Particle Tracking Velocimetry (PTV) with ‘shake-the- box’ methodology, where 5 Phantom V641 high-speed cameras recorded a scattered pattern of 8mm dots on the wing surface. Experimental setup is illustrated in Figure 1. Figure 1: Wing system and ‘shake-the-box’ hardware setup for structural analysis. 2 Summary of Results Both position tracking tools were performed simultaneously and lately compared in order to assess the quality of the image-based technique as a surface tracking tool. The later were processed in two steps: (i) obtaining the time-resolved position tracking with ‘shake-the-box’ methodology, then (ii) reconstruction of the wing surface and estimation of the angular and heaving history. Assuming the mechatronic position tracking results as baseline, the non-intrusive results are compared and presented in Figure 2. A nominally harmonic dataset were found for each self-sustained LCO, which allowed analysis in phase- averaged format (Figure 3), where the properties are averaged as a function of the angle of attack cycle (i.e. pitching motion) discretised within 1° resolution. Figure 2: Position tracking validation: mechatronic- vs image-based systems. Figure 3: Phase-averaged LCO’s. 3 Conclusions The occurrence of a self-sustained LCO were found of the system with a NACA0012 rigid wing. An initial perturbation was required to trigger the phenomenon; however, the motion amplitudes were noticeably stable and reproducible for a same trigger input.The absolute minimum freestream able to sustain the motion was 8.5m/s with minor decay instabilities, and any freestream higher than 14 m/s would reach the physical end-stops. The ‘Shake-the-Box’ method applied as a body tracking solution has been successfully validated, allowing the use of this approach as a non-intrusive tool for further applications. Acknowledgements We gratefully acknowledge the financial support from EU H2020 project HOMER (Grant Ref No: 769237). References [1] G. Dimitriadis and J. Li, “Bifurcation Behavior of Airfoil Undergoing Stall Flutter Oscillations in Low- Speed Wind Tunnel,” AIAA Journal, vol. 47, no. 11, pp. 2577–2596, Nov. 2009. [2] D. Poirel and F. Mendes, “Experimental Small-Amplitude Self-Sustained Pitch–Heave Oscillations at Transitional Reynolds Numbers,” AIAA Journal, vol. 52, no. 8, pp. 1581–1590, Aug. 2014.