长鳍波动推进流向涡结构PIV试验研究

鱼类的高效、低噪声、高机动游动为水下航行体推进技术研究提供了很好的启发与借鉴。尼罗河魔鬼鱼依靠长背鳍波动推进，可以在主体基本不变形下巡游，还可以通过改变波动方向敏捷地倒退或前进。对于这种推进模式国内外已经开展了一些仿生推进水动力实验研究，对流动涡结构也有一些数值计算研究，但对流动涡结构试验研究开展很少，因而数值计算方法也缺乏充分的验证。本文针对MPF（Median and/or Paired Fin，中央/对鳍）模式长鳍波动推进，采用相位同步PIV技术测量了系泊状态下首、中、尾部及尾流不同相位横截面流场，采用相位平均方法计算获得平均速度场，进一步提取分析了流向涡涡结构特征及其随相位的演变规律，为长鳍波动推进涡结构数值预报和水动力机理揭示提供了试验依据与支撑。

PIV measurement on streamwise vortex generated by undulating fins

The Gymnarchus Niloticus Fish (GNF) with long undulating fins generally cruises with high efficiency and extra-ordinal maneuverability while keeping its body for straight line. The flow field around andulating fins are measured by phase-locked PIV in the stationary water. Four cross sections including front part, middle part, after-body and wake flow are measured, respectively. Eight equal spaced phases are measured at each cross section, and one hundred instantaneous velocity fields are phase-averaged analyzed to extract the characteristics of vortex structure at each phase. The results indicate in cross sections the streamline vortexes are periodically generated near the fin tip as the fin swings to nearby maximum angle, and shed from the tip of the fin surface, There are two streamwise vortexes generated during each period at the two side maximum angle. On the other hand, as fin swings to the near mid-sagittal plane in the cross section, the fully developed streamwise vortexes begin to decay, and finally disappear. Meanwhile a high speed jet is generated in the direction from the root to the tip of the fin. Comparing the flow structure in the forepart, middle part and posterior cross sections, it is obvious that the streamwise vortex structure and its evolution with phases are similar in these three sections. From the forepart to the posterior along the shaft axis, the flow structure becomes more and more complicated as waves spread downstream, the jet ejected outward from the fin tip becomes stronger, and its sphene of influence seems enlarged. But the flow structure in the wake is different from the above three cross sections, which is less changed with the wave phases. The two large streamwise vortexes are formed under the fin surface, and stably and symmetrically locate on two sides of the mid-sagittal plane with opposite rotation directions.