Abstract
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.
Highlights
Spitting cobras belong to the Elapidae, a large family of venomous snakes that includes mambas, taipans, and death adders (e.g., [1])
These usually paired ridges span from the last third of the total length of the venom channel to its distal end, and end just before the sharp 90u bend that redirects the flow towards the discharge orifice [11]
The ridges are an adaptation to the requirements of venom spitting because they are not found in non-spitting cobras
Summary
Spitting cobras belong to the Elapidae, a large family of venomous snakes that includes mambas, taipans, and death adders (e.g., [1]). The venom channel of the fang has ridges that are unique to spitting cobras. These usually paired ridges span from the last third of the total length of the venom channel to its distal end, and end just before the sharp 90u bend that redirects the flow towards the discharge orifice [11]. The aim of this study was to investigate the function of the ridges in the venom channel of the spitting cobra Naja pallida, to determine how the venom jet is generated and what level of pressure loss must be compensated for by the venom gland contraction to achieve high jet velocities at the exit.
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