方形网箱水平波浪力的迭加计算和实验验证

CALCULATION AND EXPERIMENT OF HORIZONTAL WAVE FORCE AND PROVED BY EXPERIMENT IN WAVE POOL FOR A SQUARE SEA-CAGE

采用小直径圆柱体绕流理论和网渔具理论为基础的经验水动力计算法 ,分别考虑网箱框架是刚性体、网衣和绳索是柔性体的特征 ,结合正弦波理论对方形网箱及其构件受到的水平波浪力特性进行了分析研究 ,理论给出了波浪力迭加计算法 ,并把计算结果与小尺度的网箱水槽实验进行对比验证。结果表明 ,计算数值与试验数据较接近 ,平均误差率在 1 5 %以内。网箱水平波浪力呈周期性、不对称变化 ,在波浪周期小于 0 72s时 ,主要以框架的波浪力为主 ,反之以网衣的为主。在波长为 0 8m、周期为 0 72s、水深为 0 7m、波高为 68 3mm情形下 ,计算结果显示网箱的框架尺寸和形状及其系泊、布局的选择应根据实际海况而定 ,网箱高度可适当增加 ,当设置水层下降深度相对于网箱高度比率为 2 0 %— 40 %时 ,波浪力峰值下降相对率达68 0 3%— 88 2 0 %。

Sea cages are mechanically influenced by both waves and currents. An effective operation requires hydrodynamic research in the perspective of durability, security and productiveness of the cage. Current research is hindered by insufficient knowledge of wave force. Intrinsic factors of the cage, such as framework pattern (shape and size), and materials (rigid or flexible net and rope), must be considered in any simulation of the sort. Using sinusoidal wave theory and iterative calculation method, the authors simulated the scenario of a rectangular cage under lateral wave force and analyzed the characters of horizontal wave force for the sea cage. The simulated result was then compared with that of lab flume test. Two results were found to be similar to each other, and the average error ratio was <15% and occasionally was 20% when ratio of wave height to wavelength was <0 05 and the Keulegan Carpenter number was <30. This is because the cage inertia force is ignored in this case and the calculated hydrodynamic coefficient was based on the result of different material experiments that was conducted by early researchers more than 50 years ago. The horizontal wave force on the cage changed periodically and asymmetrically with two peaks per period. When wave period was <0 72s, the frame wave force was in effect, otherwise net wave force took over. In flume test, a constant condition was applied which was wavelength 0 8m, period 0 72s, depth 0 7m and height 68 3mm. The simulated results showed that under the condition, the magnitude of wave force have positive relationship to the wave height, the frame size, cage shape, anchor attachment and other specifications should be designed according to the actual condition. The simulated results showed that: first, when wave height increased by 1, 2, 3 times, the wave force increased about 3 5, 13, 40 times respectively, which suggested that the sea cage should be placed in lower height wave area. However, increase of wavelength would result in instability of the sea cages, thus placement of combined cage in the longer length wave and individual cage in shorter length wave area is suggested. Secondly, when wave motion direction does not change, we could lengthen the cage size parallel to the wave direction; when wave whirls and swirls, squared or circled cage should be used. Thirdly, when sunken depth to cage height was between 20% to 40%, the relative ratio of peak force decreased from 68 03% to 88 20%, indicating that the impact of wave force can be reduced with placing depth. Therefore, the sea cage height can be adjusted accordingly to some extent with the wave dada. But this paper dealt with only the situation mentioned above, in the future, study on mutual interaction of wave and current must be considered using vector iterative calculation method. The cage's cubage must be calculated through net figuration change by the Finite Element Method (FEM).