背鳍扭波推进运动的附加转动惯量

Virtual moment of inertia in torsional wave motion of dorsal fin

由于背鳍扭波推进运动方式不同于鳗鲡模式鱼体横波运动,细长体理论不能直接运用于尼罗河魔鬼鱼(GNF)的长背鳍扭波推进水动力性能的计算。为此,建立了计算鳍面与流体相互作用力的附加质量方法和附加转动惯量方法。对于背鳍扭波鳍条的摆动方式,首先提出计算附加质量的分离泡模型;其次,用偏心率e接近1的扁椭圆柱代替鳍条,运用势流理论计算了鳍条的附加转动惯量。两种方法给出的单位面积鳍面的附加质量函数形式相同。对于扭波推进运动的水动力性能,则用大摆幅理论结合数值积分方法来研究,结果表明:推力呈周期性变化,其变化频率为鳍条摆动频率的2倍;侧向力和升力周期平均值为0。

The motion of dorsal fin propulsion by torsional wave is different from the anguilliform mo- tion propelled by body transverse wave. The interaction between dorsal fin and water cannot be described by means of virtual mass presented by M. J. Lighthill. A separation bubble model and a virtual moment of inertia model were presented based on the features of fin ray movement, and an ellipse with partiality ratio e near to 1 was used to replace the fin ray. The functions of virtual mass of per unit dorsal fin educed from the two models above are the same in form. With the new virtual mass model established, the thrust force and the lift force produced by the torsional wave were studied numerically. The results show that the thrust takes on periodical change, whose frequency is twice that of the fin swing, the periodic mean value of the lateral force and the lift force being zero.