帆张网帆布面积对网具作业性能的影响

帆布面积增加在促进网口扩张的同时,也使网具整体阻力增加。为了明晰帆布面积对网具整体作业性能的影响,以网口面积与网具阻力的比值(K)作为帆张网网具性能比较参数,通过模型实验分析帆布面积对网具作业性能的影响,结果显示,流速为0.24 m/s时,K值随帆布面积增加而递增,但增速逐渐趋缓,帆布面积趋于0.288 m~2时,K呈现接近峰值的特征;流速≥0.4 m/s时,K值保持平稳,与帆布面积相关性不明显;因此,帆布面积趋于0.288 m~2时,此时帆布面积与网具匹配系数(P)趋于0.072,网具可在各种流速条件下均保持较好的作业性能,根据实验观察,此时背腹网网衣受力均匀,网型较好;网具整体阻力(F)与帆布面积(A)和流速(V)呈幂函数关系,其表达方式为F=334.32×A~(0.5)×V~(1.61);帆布展弦比对K值变化趋势影响较小;以上结论可为帆张网帆布、锚与网具的合理配置提供科学依据。

Relationship between canvas area and performance of the canvas spreader stow nets

The increase of canvas area not only promotes the net mouth opening, but also increases the overall resistance of the net. In order to understand the relationship between canvas area and net performance, 7 groups for model test were designed, the ratio of canvas area/net resistance (K) was used as an indicator for net performance. The 7 groups were divided into two parts according to research objective, (1) 5 groups within 7 groups were chosen to study the effect of the canvas areas on net performance, which the canvas areas increased gradually, but the canvas width/length ratio was similar; (2) 4 groups within 7 groups were chosen to estimate the effect of the canvas width/length ratio on the research result of part (1). The results show that: when the current speed was 0.24 m/s, the K rose with the canvas area increasing, but the rise rate slowed down gradually, as the canvas area was close to 0.288 m², the K was close to the peak; when the current speed was more than 0.4 m/s, the K was a constant, and the correlation between k and canvas area was not obvious. So when the canvas area was close to 0.288 m², the matching coefficient (P) of canvas area and net size was close to 0.072, the stow net can keep good performance under different current conditions. The effect of aspect ratio of canvas on K trend was not obvious. The power function relation was found between net resistance with canvas area and current speed, and the empirical formula was expressed as F=334.32×A^0.5×V ^1.61. As a common sense, the matching coefficient (P) and net resistance were affected by multiple factors, not only by canvas area and current speed. So the results were suitable for using in similar condition. If the conditions were different obviously, the coefficient and formula should be modified. The above findings can be used for the performance improvement of canvas spreader stow nets.