Lipinski Doug, Mohseni Kamran
College of Engineering and Applied Science, University of Colorado at Boulder, Boulder, CO 80309, USA.
J Exp Biol. 2009 Aug;212(Pt 15):2436-47. doi: 10.1242/jeb.026740.
The flow structures produced by the hydromedusae Sarsia tubulosa and Aequorea victoria are examined using direct numerical simulation and Lagrangian coherent structures (LCS). Body motion of each hydromedusa is digitized and input to a CFD program. Sarsia tubulosa uses a jetting type of propulsion, emitting a single, strong, fast-moving vortex ring during each swimming cycle while a secondary vortex of opposite rotation remains trapped within the subumbrellar region. The ejected vortex is highly energetic and moves away from the hydromedusa very rapidly. Conversely, A. victoria, a paddling type hydromedusa, is found to draw fluid from the upper bell surface and eject this fluid in pairs of counter-rotating, slow-moving vortices near the bell margins. Unlike S. tubulosa, both vortices are ejected during the swimming cycle of A. victoria and linger in the tentacle region. In fact, we find that A. victoria and S. tubulosa swim with Strouhal numbers of 1.1 and 0.1, respectively. This means that vortices produced by A. victoria remain in the tentacle region roughly 10 times as long as those produced by S. tubulosa, which presents an excellent feeding opportunity during swimming for A. victoria. Finally, we examine the pressure on the interior bell surface of both hydromedusae and the velocity profile in the wake. We find that S. tubulosa produces very uniform pressure on the interior of the bell as well as a very uniform jet velocity across the velar opening. This type of swimming can be well approximated by a slug model, but A. victoria creates more complicated pressure and velocity profiles. We are also able to estimate the power output of S. tubulosa and find good agreement with other hydromedusan power outputs. All results are based on numerical simulations of the swimming jellyfish.
利用直接数值模拟和拉格朗日相干结构（LCS）研究了水螅水母萨氏真囊水母（Sarsia tubulosa）和维多利亚多管水母（Aequorea victoria）产生的流动结构。对每个水螅水母的身体运动进行数字化处理，并输入到计算流体动力学（CFD）程序中。萨氏真囊水母采用喷射式推进方式，在每个游泳周期发射一个单一、强烈、快速移动的涡环，而一个反向旋转的次生涡则被困在伞下区域内。喷出的涡能量很高，并且非常迅速地远离水螅水母。相反，桨式水螅水母维多利亚多管水母则是从钟形顶部表面吸入流体，并在钟形边缘附近以成对的反向旋转、缓慢移动的涡的形式喷出这种流体。与萨氏真囊水母不同，维多利亚多管水母的两个涡都是在游泳周期中喷出的，并在触手区域停留。事实上，我们发现维多利亚多管水母和萨氏真囊水母游动时的斯特劳哈尔数分别为1.1和0.1。这意味着维多利亚多管水母产生的涡在触手区域停留的时间大约是萨氏真囊水母产生的涡的10倍，这为维多利亚多管水母在游泳时提供了绝佳的捕食机会。最后，我们研究了两种水螅水母钟形内部表面的压力以及尾流中的速度剖面。我们发现萨氏真囊水母在钟形内部产生非常均匀的压力，并且在缘膜开口处的喷射速度也非常均匀。这种游泳方式可以很好地用蛞蝓模型来近似，但维多利亚多管水母产生的压力和速度剖面更为复杂。我们还能够估算出萨氏真囊水母的功率输出，并发现与其他水螅水母的功率输出吻合良好。所有结果均基于对游动水母的数值模拟。
