CFD Analysis of Vortex-Induced Motions of Bare and Straked Cylinders in Currents

Abstract

It is widely acknowledged that the use of helical strakes for mitigation of vortex-induced motions (VIM) of surface piercing cylinders, such as spar platforms, is only partially effective. Using computational fluid dynamics tools, we compare the oscillation characteristics of a bare cylinder and a straked cylinder in uniform currents.Our model comprised of a straked cylinder with diameter of 0.741 m, aspect ratio of 1:1.9 and three helical strakes of height 13% of cylinder diameter. This geometry corresponds to the hard tank geometry of a scaled truss spar model known to exhibit VIM in tow tank testing. In the CFD simulations the cylinder is moored with linear springs to provide a range of reduced velocities.The fluid domain is made of an unstructured grid comprising of hexahedral elements. Fluid structure interaction utilizes grid stretching and a user defined function for solving the equations of motion. Turbulence modeling uses Detached Eddy Simulation (DES) and the boundary layer is modeled using a wall function with a surface roughness of 0.0003 m. Reynolds numbers are in the range of 50,000 to 100,000.Results for straked cylinder compares reasonably with published results, but under-predicts the peak response. In comparing with corresponding results for a bare cylinder without strakes, the spectral features of the transverse displacement show variations, which are found to be due to the spoiling effect of the strakes.

State-of-the-Art Review of Vortex-Induced Motions of Floating Offshore Wind Turbine Structures

“…CFD has been used as an important tool for studying the vortex-induced motion of offshore floating structures since the early 2000s, with a major focus on the spar concept. Early studies also investigated the VIM of straked cylinders given the similarities between straked cylinder and spar platforms [112]. Later on, there have been a good number of publications that used CFD to simulate the VIM of spar, to name a few [113][114][115][116].…”

Investigation of the VIMs of a spar-type FOWT using a model test method

“…Moreover, the strength and fatigue were considered, and corresponding modifications were presented. Thiagarajan et al 27 compared the response characteristics of VIMs of bare and straked cylinders in uniform currents through computational fluid dynamics (CFD) analysis. The cylinder used in this research was identical to the hard tank part of the truss spar used in the model test conducted by Irani et al in 2005.…”

Experimental and numerical study on vortex-induced motions of a deep-draft semi-submersible

“…Halkyard et al [9], Oakley and Constantinides [10] combined the results from experimental and numerical studies in order to compare the VIM effects on Spar from experimental measurements and CFD predictions. Thiagarajan et al [11] further investigated a bare cylinder and a cylinder with strakes to study the VIM phenomenon. A guideline of numerical simulation of the VIM on the Spar platform was proposed by Lefevre et al [12].…”