Feasibility of an Innovative Methodology for Testing Marine Current Turbines in Unsteady Flow

在海上石油行业的早期，由于对设计载荷的适当值缺乏基本的理解，许多设计挑战都是通过纳入大的安全因素来解决的。石油业务的高盈利能力允许在短期内采用高成本的解决方案；虽然偶尔的失败不会威胁到这样一个资金充足的行业。相比之下，海洋可再生能源的经济不允许这种高成本的方法，而高调项目的早期失败可能会对这样一个新兴行业的信心造成巨大损害。为了在技术和经济上取得成功，MCT设计必须符合严格的环境法规，必须高效、经济、环保，最重要的是可靠。因此，重要的是不仅要对转子本身的性能有一个基本的了解，而且要对系统的整体水动力载荷有一个基本的了解，以便能够安全、经济地设计系泊/锚定系统。多年来，人们进行了广泛的研究，旨在寻找确定非定常流动（即随时间变化的流动，如波浪或波动电流）中固定和浮动海上结构的阻力、附加质量和阻尼效应的方法，然而，这些方法对MCT转子旋转叶片的适用性非常值得怀疑，这增加了感知知识的不确定性。考虑转子尺寸时，转子非定常载荷的重要性更为明显；目前的建议涉及直径超过10米的转子对，其扫面面积可能是支撑结构投影面积的50倍。因此，全局载荷显然是由运动转子控制的。非定常水动力载荷的重要性通常以附加质量和阻尼系数为特征，与传统的海洋和/或海上结构相比，这些装置的实际质量相对较小，这进一步强调了非定常水动力载荷的重要性。无论是固定结构还是柔性结构，最大载荷和疲劳性能都是重要的问题；对于柔性结构，非定常旋翼力对整体动力响应的影响也可能成为系泊设计中的一个关键参数。mct可能会在大电流和陡波明显的环境中运行，并且不可避免地会出现明显的不稳定流。在这种情况下，转子的整体附加质量和阻尼能够在大范围的非定常流动中可靠地评估是至关重要的。虽然使用大型模型的海上试验可以在理解非定常响应方面取得一些进展，但现实环境的可控性和可重复性的缺乏对这些影响的基本理解提出了重大挑战，特别是在极端条件下。因此，需要一种以实验室为基础的实验技术，可以通过直接的实验测量和间接的数值模拟方法的验证来发展这种基本的理解。因此，本文提出的研究旨在建立一种模型试验方法的可行性，以预测这些非定常全局载荷以及水平轴海流涡轮机的运行效率参数，解决一个有趣且具有挑战性的具有重要实际意义的水动力问题。

In the early years of the offshore oil industry, many design challenges were met by the incorporation of large factors of safety to allow for a lack of fundamental understanding of appropriate values of design loads. The high profitability of the oil business allowed the resulting high-cost solutions in the short-term; whilst occasional failures could not threaten such a well financed industry. In contrast, the economics of marine renewable energy do not allow such a high-cost approach, whilst early failures in high-profile projects could cause immense damage to the confidence placed in such a fledgling industry. In order to succeed technically and economically, MCT designs must meet stringent environmental legislation, must be efficient, cost effective, environmentally friendly and above all reliable. For this reason it is important to have a fundamental understanding not only of the performance of the rotor itself, but also of the global hydrodynamic loading on the system, in order to allow safe and cost-effective design of the mooring/anchoring system. There has been extensive research over many years aimed at finding approaches for determining drag, added mass and damping effects of fixed and floating offshore structures in unsteady flows - that is flows which vary with time, such as waves, or fluctuating currents - however the applicability of these approaches to the rotating blades of an MCT rotor is highly questionable raising uncertainties in perceived knowledge. The importance of the unsteady loading on the rotors is all the more apparent when their dimensions are considered; current proposals involve pairs of rotors of over 10m diameter, with a swept area which may be 50 times the projected area of the support structure. The global loading is therefore clearly dominated by the moving rotor. The importance of the unsteady hydrodynamic loads, often characterized by added mass and damping coefficients, is further emphasized by the relatively small real mass of these devices in comparison to conventional marine and/or offshore structures. For both fixed and compliant structures, maximum loading as well as fatigue performance are important issues; for compliant structures the influence of the unsteady rotor forces on the global dynamic response is also likely to be a critical parameter in the mooring design. MCTs are likely to operate in environments in which large currents and steep waves will be evident, and appreciable unsteady flows are inevitable. In such cases it is essential that the global added mass and damping of the rotors can be reliably evaluated for a wide range of unsteady flows.Whilst some progress towards the understanding of the unsteady response can be made with sea trials using large scale models, the lack of controllability and repeatability of the real-world environment presents substantial challenges to the development of fundamental understanding of these effects, particularly with regard to extreme conditions. A laboratory-based experimental technique is thus required which can be used to develop this fundamental understanding both directly, through experiment measurement, and indirectly, via the verification and validation of numerical simulation approaches. The study proposed here therefore aims to establish the feasibility of a model-testing methodology to predict these unsteady global loads as well as operational efficiency parameters on horizontal axis marine current turbines, addressing an interesting and challenging hydrodynamic problem of significant practical relevance.