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Characterization of the Aerodynamics of Rectangular Cylinders with Surface Topology

The main motivation behind this study is to understand the susceptibility of the suspension lines of Precision Airdrop Systems (PADS) to flow-induced vibrations by galloping instability. Such vibrations are undesirable as they negatively affect the controllability of PADS. Suspension cables of PADS have a cross-section of a roundcorner rectangle with surface geometrical features (topology) produced by the braided structure of the cables. Therefore, an experimental study is conducted to provide fundamental understanding of the effects of surface topology on the instability of round-corner rectangular cylinders to galloping. The study utilizes a baseline cylinder with smooth (topology-free) surface, a side (chord-to-width) ratio of 2.5 and fully-round corners. Results from the baseline model are compared against those from cylinders with the same geometry but with added surface topology. Fourier modes are employed to define different surface topologies with an amplitude of 5% of the cylinder width d along the perimeter only (2D geometry), as well as along the span (3D geometry). Boundary-layer-resolved measurements are conducted using single-component molecular tagging velocimetry at Reynolds numbers based on d of Red = 1, 100 and 2, 500, and cylinder angle of attack of 0°, 2° and 5°. A complementary investigation using direct force measurements is also carried out to determine the galloping stability characteristics of the cylinders examined. Results suggest that the topology could lead to destabilization to galloping when the associated geometrical features cause the flow to separate along the corner region of the cylinder, promoting earlier separation relative to the baseline smooth geometry. On the other hand, reattachment of the separated shear layer, which has a stabilizing effect on galloping, is promoted by the presence of spanwise topology variation (3D geometry) and increasing Red.