Author(s): Z. Zhang; C. Ji
Linked Author(s): Zhixiong Zhang, Ji Chen
Keywords: Local scour; Tandem piles; Shallow flow; Offshore wind turbines; Numerical simulation
Abstract: With the fast development and construction of Offshore Wind Turbines (OWT), the local scour around OWT piles in coastal areas provokes wide interests in academic and industrial communities. Different from the traditional pile foundation sufficiently submerged in water, the near-shore OWT steel pipe monopile often has a diameter ranging from 5 to 8 m, being larger than the water depth (Jensen et al., 2006). Therefore, the prediction of the local scour at the OWT steel pipe monopile is difficult because of the relatively low aspect ratio (flow depth-to-pile diameter ratio) and the low Froude number when an OWT situates in a shallow-water coastal area – a condition for which current scour equations do not account. In addition, the array arrangement of the large-diameter monopiles further increases the complexity of the problem, and a comprehensive understanding of the interaction mechanism among the tandem piles (the simplest array arrangement) in shallow water is still lacking. In this paper, a series of numerical simulations of the local scour around two tandem circular piles with a wide range of aspect ratios are carried out, and the effect of the aspect ratio (h/D) and pile spacing (G/D) on the scour process and scour morphology is investigated. The characteristics of the scour process, depth, and flow fields are shown to be significantly influenced by the pile gap ratio (G/D = 1.0–5.0) and water depth ratio (h/D = 0.5–3.0). Under the same G/D, the dimension of the scour hole and the maximum scour depth around two piles become larger with the increase of h/D. Under the shallowest water depth condition (h/D = 0.5), the scour dimension and maximum scour depth of the rear pile are larger than that of the front pile, due to the weak upflow and strong downflow upstream of the rear pile. However, under the deepest water depth condition (h/D = 3.0), the scour hole becomes smaller than that of the front pile, due to the significant upflow downstream the front pile by less confinement from the high water surface. The vortex shedding patterns and their interaction with the sediment transport in different G/D and h/D cases are also discussed.
Year: 2023