JZUS - Journal of Zhejiang University SCIENCE

Journal of Zhejiang University SCIENCE A 2017 Vol.18 No.5 P.329-345

Characteristics of orifices for modeling nonlinear power take-off in wave-flume tests of oscillating water column devices

Author(s): Fang He, Zhenhua Huang
Affiliation(s): Ocean College, Zhejiang University, Hangzhou 310058, China; more
Corresponding email(s): zhenhua@hawaii.edu
Key Words: Wave power extraction, Oscillating water column (OWC), Orifice characteristics, Quadratic loss coefficient, Contraction coefficient, Hydrodynamic efficiency

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Abstract: oscillating water column (OWC) devices for wave power extraction are appealing, but are still in need of research. In this study, a series of wave-flume experiments was conducted to examine the hydrodynamic performance of a rectangular OWC device fixed in regular waves. Two types of orifices, slot orifices and circular orifices, were used to simulate the nonlinear power take-off (PTO) mechanism, and the effects of orifice geometry were examined. A two-point measurement method was proposed to reconstruct the instantaneous spatial profile of the water surface inside the OWC chamber for reducing bias in the measured wave power extraction efficiency. The flow characteristics of PTO were described by a quadratic loss coefficient, and our experimental results showed that the quadratic loss coefficient of the slot orifices varied with wave period and slot geometry. Empirical formulas were proposed for the quadratic loss coefficients of the two types of orifices. The ability to determine the quadratic loss coefficient of an orifice will allow us to design orifices for small-scale tests and calculate the power extraction using only pressure measurement. Our results also suggested that the pressure coefficient should be more reliable than the amplification coefficient as an indicator of the power extraction performance of an OWC device.

振荡水柱装置波浪水槽试验中用于模拟非线性能量俘获系统的孔口特性

目的：在振荡水柱装置研究中，通常通过不同的孔口几何特征来改变能量俘获系统的特性，但其具体流动特性却鲜有报道。本文探讨孔口几何特征（形状、尺寸和开孔率等）对流动特性的影响机制，理解影响能量俘获系统特性的关键因素，研究其对振荡水柱装置水动力特性和波能提取的影响规律，并评估波能提取性能指标的有效性。
创新点：1. 提出了两点测量法来重构振荡水柱腔室内液面；2. 建立了孔口流动特性与孔口几何特征的关系式；3. 提出了仅测量腔室内气压即可获得波能提取功率的方法；4. 该方法可扩展至非二维矩形腔室及斜向波。
方法：1. 采用不同尺寸狭缝孔和圆形孔来模拟非线性能量俘获系统；2. 通过一系列波浪水槽试验，对振荡水柱装置的水动力特性及波能的提取展开研究；3. 采用二次损耗系数和收缩系数来描述孔口往复流动特性，并构建其与孔口几何特征的关系；4. 通过两点测量法获取振荡水柱腔室内的准确信息；5. 评估压力波动系数和液面放大系数作为振荡水柱装置波能提取性能指标的有效性。
结论：1. 两点测量法能够重建二维矩形振荡水柱腔室内液面的瞬时空间分布，消除了单点法的测量偏差；2. 孔口相对厚度及振荡气流对可被视为薄壁的圆形孔的影响可以忽略不计，但对不能视为薄壁的狭缝孔的影响显著；3. 本文提出的二次损耗系数经验公式可用于（1）通过孔口几何尺寸设计其流动特性和（2）通过仅测量腔室内气压来计算波能提取功率；4. 用作振荡水柱装置的波能提取性能指标时，压力波动系数比液面放大系数更为可靠。

关键词：波能提取；振荡水柱；孔口特性；二次损耗系数；收缩系数；水动力效率

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