Numerical investigation of drag and wake characteristics of fairings for low aspect ratio surface-piercing cylinders

The flow past a vertical surface-piercing finite circular cylinder with a low aspect ratio (AR = h/D) exhibits strong three dimensional effects, which due to the influence of its free end. Therefore, the design of drag reducing fairings for low aspect rational cylinder, must account for these three dimensional complexities. This study designs three fairing profiles, including triangular, bell-shaped, and S-shaped configurations, and systematically investigates their hydrodynamic performance under two flow conditions characterized by Reynolds numbers of 8.16 × 104and 1.02 × 105, and Froude numbers of 0.186 and 0.232, for four length-to-diameter ratios (L/D = 0.82, 1.00, 1.31, and 1.93). Computational fluid dynamics (CFD) is employed to investigate the drag reduction performance, free-surface elevation, surface pressure coefficient distribution, flow separation point location, and wake characteristics of the fairings. Results indicate that all fairings effectively reduce the drag coefficient, with the bell-shaped fairing at L/D = 1.93 achieving the optimal drag reduction efficiency. The bell-shaped and S-shaped fairings exhibit a more uniform surface pressure coefficient distribution and outperform the triangular fairing in delaying the flow separation point. Special emphasis is placed on analyzing the wake characteristics of the three fairing configurations at L/D = 1.93, including the time-averaged streamwise velocity distributions at characteristic depths and the development length of the recirculation zone, with the spanwise wake also being a key focus. The wake characteristics reveal that the free end exerts a significant influence on the flow around the cylinder at low aspect ratios.