CFD simulations and experimental measurements of flow past free-surface piercing, finite length cylinders with varying aspect ratios

Abstract

Highlights• We simulate (with CFD) and experimentally measure loads on surface-piercing cylinders with freeends of varying aspect ratios • A wider range of aspect ratios than has previously been investigated is covered • Drag coefficients and the presence of vortex shedding diminish greatly with decreasing aspect ratio • Furthermore, loads along the length of a cylinder are investigated• The free-surface and free-end affect the load behavior, with ramifications to offshore structural engineering 1 AbstractIn this work, the flow past surface-piercing cylinders with free lower ends is studied numerically with computational fluid dynamics (CFD) at varying aspect ratios (AR = H/D) from around 1 to 19, and compared against experimental measurement across the same range. Numerical and experimental work is carried out at Re = 2900 and Fr = 0.65. The aspect ratio of the cylinder is varied by raising and lowering the cylinder into the water. The resulting flow field and loads on the cylinder as a result of changing the cylinder's depth of submergence are investigated in terms of drag, lift and frequency content. First, the effects of changing the aspect ratio are examined. CFD simulations demonstrate a reduced drag coefficient, consistent with the experimental measurements, as compared to a fully-submerged, infinite cylinder at the same Reynolds number. A significant reduction in the drag coefficient is observed at an aspect ratio of 2. For aspect ratios 3 and below, Karman vortex shedding is completely suppressed due to the dominance of free surface effects. With increasing aspect ratio the Strouhal number is found to approach 0.2. Second, the flow regime is investigated as a function of cylinder length for a cylinder with aspect ratio of 12. Different regions of vortex shedding are observed along the length of the cylinder, as demonstrated by lift forces, Strouhal numbers and CFD flow visualizations. Strouhal numbers below 0.2 were observed for regions near the free surface. Additionally, the drag coefficient was found to decrease near the free surface. This study over a wide range of aspect ratios exhibits the effects of a free surface, free end and also the length of the cylinder on both the flow regime and loading.

Large-eddy simulation of shallow turbulent wakes behind a conical island

“…12 With increasing aspect ratios, the Strouhal number has been observed to remain constant. 12 For larger ratios (h D ≥ 9), periodic shedding and vortex sheets have been observed in the wake over the majority of the cylinder height, with the vertical axis of the vortex sheet no longer parallel to the cylinder axis. 12 This is thought to be due to the interaction of the tip vortices with the vortex shedding at the cylinder mid-height.…”

“…12 For larger ratios (h D ≥ 9), periodic shedding and vortex sheets have been observed in the wake over the majority of the cylinder height, with the vertical axis of the vortex sheet no longer parallel to the cylinder axis. 12 This is thought to be due to the interaction of the tip vortices with the vortex shedding at the cylinder mid-height. [12][13][14] The vortex sheet axis becomes less inclined further away from the cylinder's free end.…”

“…5,8 The critical aspect ratio at which vortex shedding is suppressed for a cylinder is around three. 8,12 The drag coefficient and Strouhal number are significantly reduced at an aspect ratio of around two or three due to free end effects. 12 With increasing aspect ratios, the Strouhal number has been observed to remain constant.…”

“…8,12 The drag coefficient and Strouhal number are significantly reduced at an aspect ratio of around two or three due to free end effects. 12 With increasing aspect ratios, the Strouhal number has been observed to remain constant. 12 For larger ratios (h D ≥ 9), periodic shedding and vortex sheets have been observed in the wake over the majority of the cylinder height, with the vertical axis of the vortex sheet no longer parallel to the cylinder axis.…”

“…12 This is thought to be due to the interaction of the tip vortices with the vortex shedding at the cylinder mid-height. [12][13][14] The vortex sheet axis becomes less inclined further away from the cylinder's free end. 12 The cylinder aspect ratio has also been observed to govern the occurrence of "symmetrical" vortex shedding (h D > 2) rather than the alternate shedding of a vortex from each cylinder side 5 when the height is significantly greater than the diameter (h >> D).…”