Investigation of Fluid–Structure Interactions of an Uncontrolled Flexible Hemisphere Utilising Non-Simultaneous Stereo Particle Image Velocimetry and Digital Image Correlation

This research investigates the interaction between a flexible thin-walled hemisphere and the surrounding wake at ReD=2×105 acting as a simplified model of a flexible surface protuberance immersed within a turbulent boundary layer (BL). A flexible model and a rigid model, both 100 mm in diameter, are experimentally tested to observe and contrast the flow variation between a rigid structure and a freely deforming structure. Two experiments were conducted. To capture fluid flow behaviour, stereo particle image velocimetry (SPIV) was used. To capture structural deformation of the model, digital image correlation (DIC) was utilised. Experimental testing was conducted non-simultaneously. From the experimental testing, it was observed that the flexible model experienced a leading edge (LE) deformation at 29° of the altitude angle (θ), showing an average deformation of 2.11 mm. All regions of the structure experienced non-zero distortion due to the incoming wind load. This was similar to behaviour observed in previous literature. This caused a modulation in the wake region, giving a parabolic wake velocity contour to form about θ≈20°. A velocity inflection point is observed for the flexible model at an average of θ=23.39° within the wake. This inflection region extends surrounding the area of maximum structural deflection up to θ≈40°. This indicates that the deflection across the LE centreline has a direct interaction with location and size of the near wake. Turbulent kinetic energy (TKE) in the wake was observed to drop with the introduction of the flexible model, with a lower dissipation rate observable. This is indicative of energy transfer from the flow to the structure, allowing deformation. The maximum region of TKE coincides with the recirculation vortex core region, which was shown to move from z/D= 0.19 to z/D= 0.35 for the rigid and flexible models, respectively. The results indicate that, with the Reynolds number tested, the rigid behaviour is in line with previous literature trends. The flexibility of the model, therefore, highly influences the wake region, with general shape deformation causing a decrease in near wake TKE and change in wake shape and recirculation core location.