Precision Airdrop Systems (PADS) face difficulties in controlling their landing accuracy when flow-induced vibrations of the suspension lines occur. Recent research has identified a previously unknown cause of these vibrations: galloping of the suspension cables. Galloping is a type of vibration that can occur in cylinders with non-circular cross-sections. The suspension cables in PADS have a cross-section that is approximately rectangular in shape with rounded corners, but with the added complexity of surface topology (due to braiding of the lines). Using load measurements, recent experiments have shown that the presence of surface topology can alter the stability of rectangular cylinders to galloping; an effect that is dependent on Reynolds numbers. Knowledge of the corresponding topology effect on the flow around the cylinders is presently lacking. Therefore, this study aims to investigate the impact of surface topology on the boundary layer and near-wake flow around a rectangular cylinder with a side-ratio of 2.5 and fully-rounded corners (half-circular leading and trailing edges). The Reynolds number based on the cylinder thickness (d) is in the range Re_d=800-2500. The surface topology is defined using spatial Fourier modes with an amplitude of 5% of d, applied along the perimeter only (2D geometry) and along both the perimeter and the span (3D geometry) of the cylinder. While not an exact replica, this surface topology represents the characteristics of the actual suspension cable reasonably well. The study also investigates the effects of different topology amplitudes by using cylinders with 2.5% and 10% of d. Single-component molecular tagging velocimetry is employed to measure the streamwise velocity and visualize the flow field at various locations above the surface and in the wake of the cylinder. Mean and root-mean-square velocity profiles are analyzed to examine the development of the boundary layer and separated flow on the top and bottom surfaces of the cylinder. The mean separation bubble and the development of the shear layer unsteadiness over the surface of the cylinders are discussed at α=0° and at different Reynolds numbers. The results demonstrate the Reynolds number-dependent effect of the surface topology cross-sectional geometry and its variation along the span. An interpretation is provided of how these results could influence the galloping instability of the cylinder. The wake flow is investigated to help better understand the relationship between wake structures, surface topology, and the characteristics of the boundary layer. To achieve this, wake mean and rms velocity profiles are interrogated and the effect of the geometry on the Strouhal number of the wake vortex shedding is analyzed. An examination is also conducted to investigate the unsteady flow physics of the boundary layer and its relationship to the wake flow. This examination uses quantitative measures and flow visualization, and focuses on the smooth-surface cylinder. The analysis identifies and compares different Reynolds number dependent boundary-layer flow regimes. The correlation between the wake vortex shedding structure and various boundary-layer regimes is examined and compared to established understanding in literature for a sharp-corner rectangular cylinder. The results reveal that the details of the topology near the leading edge of the cylinder are most significant in affecting the behavior of the boundary layer flow. For the particular topology wavelength used in the present study, the biggest effect is found when a topology peak is present at the leading edge for the 2D (2Dp) geometry. In comparison to the smooth cylinder, the 2Dp topology substantially increases the separation zone thickness and the separated shear layer unsteadiness. The ensuing wake flow, exhibits an increased wake closure length, slower recovery of the mean centerline velocity, lower vortex shedding Strouhal number, and disrupted wake vortex organization.
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