Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform
Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform
复制标题
半潜式 FOWT 平台非线性差频波浪载荷 CFD 研究的不确定度评估
作者：
Lu Wang;A. Robertson;J. Jonkman;Yi
人们发现，目前用于浮动海上风力涡轮机（FOWT）的中保真建模方法低估了非线性、低频波浪激励和半潜式FOWT的响应。为了检查这种预测不足的原因，OC 6项目正在使用计算流体动力学（CFD）工具来研究OC 5-DeepCwind半潜式平台上的波浪载荷，重点是非线性差频激励。本文重点评估双色波载荷下固定条件下半潜式平台模拟的CFD预测的不确定性，并建立用于改进中保真度模型的结果置信度。非线性波激励的不确定性被认为是可以接受的，但大于波频激励，空间离散化误差是主要的贡献者。此外，在计算流体力学解决方案中已经识别出差频处的不需要的自由波。为了消除自由波对计算结果的干扰，提出了一种分波和波浪载荷修正的方法。与二阶势流理论的初步比较表明，计算流体力学模型预测的差频波激励明显更高，尤其是在涌浪中，这表明计算流体力学结果可用于更好地校准中保真度工具。
Current mid-fidelity modeling approaches for floating offshore wind turbines (FOWTs) have been found to underpredict the nonlinear, low-frequency wave excitation and the response of semisubmersible FOWTs. To examine the cause of this underprediction, the OC6 project is using computational fluid dynamics (CFD) tools to investigate the wave loads on the OC5-DeepCwind semisubmersible, with a focus on the nonlinear difference-frequency excitation. This paper focuses on assessing the uncertainty of the CFD predictions from simulations of the semisubmersible in a fixed condition under bichromatic wave loading and on establishing confidence in the results for use in improving mid-fidelity models. The uncertainty for the nonlinear wave excitation is found to be acceptable but larger than that for the wave-frequency excitation, with the spatial discretization error being the dominant contributor. Further, unwanted free waves at the difference frequency have been identified in the CFD solution. A wave-splitting and wave load-correction procedure are presented to remove the contamination from the free waves in the results. A preliminary comparison to second-order potential-flow theory shows that the CFD model predicted significantly higher difference-frequency wave excitations, especially in surge, suggesting that the CFD results can be used to better calibrate the mid-fidelity tools.