Institut für Mechanik und Fluiddynamik, TU Bergakademie Freiberg, Freiberg, Germany.
PLoS One. 2013 May 6;8(5):e61548. doi: 10.1371/journal.pone.0061548. Print 2013.
The spitting cobra Naja pallida can eject its venom towards an offender from a distance of up to two meters. The aim of this study was to understand the mechanisms responsible for the relatively large distance covered by the venom jet although the venom channel is only of micro-scale. Therefore, we analysed factors that influence secondary flow and pressure drop in the venom channel, which include the physical-chemical properties of venom liquid and the morphology of the venom channel. The cobra venom showed shear-reducing properties and the venom channel had paired ridges that span from the last third of the channel to its distal end, terminating laterally and in close proximity to the discharge orifice. To analyze the functional significance of these ridges we generated a numerical and an experimental model of the venom channel. Computational fluid dynamics (CFD) and Particle-Image Velocimetry (PIV) revealed that the paired interior ridges shape the flow structure upstream of the sharp 90° bend at the distal end. The occurrence of secondary flow structures resembling Dean-type vortical structures in the venom channel can be observed, which induce additional pressure loss. Comparing a venom channel featuring ridges with an identical channel featuring no ridges, one can observe a reduction of pressure loss of about 30%. Therefore it is concluded that the function of the ridges is similar to guide vanes used by engineers to reduce pressure loss in curved flow channels.
唾蛇（Naja pallida）可以将毒液从两米远的距离喷射到冒犯者身上。本研究的目的是了解尽管毒液通道只有微观尺度，但毒液射流却能覆盖相对较大距离的机制。因此，我们分析了影响毒液通道中二次流和压降的因素，包括毒液液体的物理化学性质和毒液通道的形态。蛇毒显示出剪切稀化特性，并且毒液通道具有从通道的最后三分之一延伸到其远端的成对脊，侧向终止并靠近排放口。为了分析这些脊的功能意义，我们生成了毒液通道的数值和实验模型。计算流体动力学（CFD）和粒子图像测速（PIV）表明，成对的内部脊在远端的 90°急弯的上游形成了流结构。可以观察到在毒液通道中出现类似于 Dean 型旋涡结构的二次流结构，这会导致额外的压力损失。比较具有脊的毒液通道和具有相同脊的无脊毒液通道，可以观察到压力损失减少了约 30%。因此，可以得出结论，脊的功能类似于工程师用于减少弯曲流道中压力损失的导向叶片。
