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

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.中文概要目 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。