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Nicholas Gladman - The hydrodynamics of cuttlefish jet propulsion
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The hydrodynamics of cuttlefish (Sepia officinalis) jet propulsion
Nicholas Gladman and Graham Askew
Rapid and escape locomotion in cephalopods is driven by jet propulsion. Water is taken into the mantle cavity which is compressed by contraction of surrounding circular muscles, expelling water via the siphon propelling the animal. In creating jets, animals transfer energy from their locomotory muscles to the surrounding water resulting in movement. Cuttlefish (Sepia officinalis) use jet propulsion swimming for activities ranging from migratory journeys and foraging to interactions with conspecifics and predators. Jet propulsion is generally considered a relatively inefficient locomotive system, as small volumes of water are propelled at high velocity to produce thrust, in comparison with undulatory swimming. During jet propulsion swimming, jets that maximise thrust while minimising energy costs are considered optimal. In mechanically generated jets, optimal structure occurs when the ratio of jet length to diameter is approximately 4; this ratio is known as the formation number, F.
Here we investigated the structure of jets produced by cuttlefish in order to determine if jet formation in cuttlefish is optimal for thrust generation, and to quantify the hydrodynamic efficiency (the amount of jet energy transferred into useful momentum). We analysed jets using particle image velocimetry (PIV). In this technique, suspended particles in the water are illuminated in a laser light sheet and their movements tracked using high-speed recordings.
We found cuttlefish jets could be categorised qualitatively into two jet types or jet modes (termed mode I and II, following previous nomenclature). Mode I jets consisted of an isolated vortex ring, while mode II jets consist of a vortex ring followed by a trailing jet. Analysis of these two jet modes revealed jet structure was close to optimal for thrust generation during mode I, but F was suboptimal and significantly higher in mode II jets (in jet mode II, F = 5.59 ± 0.74 compared with jet mode I F = 3.64 ± 0.41). There was no relationship between jet mode and the animal’s swimming velocity and no significant difference in hydrodynamic efficiency between jet modes. However, hydrodynamic efficiency did increase with increasing swim speed for both jet modes (ranging from approximately 44 to 84%). Contrary to the literature on mechanically generated jets, it was found while jet structure and formation number did vary, there was no difference in hydrodynamic efficiency or the swimming speed at which the jets were utilised.
Undulatory swimmers, such as eels and knifefish achieve hydrodynamic efficiencies between 70 and 90%, our results suggest cuttlefish are able to achieve comparable hydrodynamic efficiencies through jet propulsion.
Nicholas Gladman and Graham Askew
Rapid and escape locomotion in cephalopods is driven by jet propulsion. Water is taken into the mantle cavity which is compressed by contraction of surrounding circular muscles, expelling water via the siphon propelling the animal. In creating jets, animals transfer energy from their locomotory muscles to the surrounding water resulting in movement. Cuttlefish (Sepia officinalis) use jet propulsion swimming for activities ranging from migratory journeys and foraging to interactions with conspecifics and predators. Jet propulsion is generally considered a relatively inefficient locomotive system, as small volumes of water are propelled at high velocity to produce thrust, in comparison with undulatory swimming. During jet propulsion swimming, jets that maximise thrust while minimising energy costs are considered optimal. In mechanically generated jets, optimal structure occurs when the ratio of jet length to diameter is approximately 4; this ratio is known as the formation number, F.
Here we investigated the structure of jets produced by cuttlefish in order to determine if jet formation in cuttlefish is optimal for thrust generation, and to quantify the hydrodynamic efficiency (the amount of jet energy transferred into useful momentum). We analysed jets using particle image velocimetry (PIV). In this technique, suspended particles in the water are illuminated in a laser light sheet and their movements tracked using high-speed recordings.
We found cuttlefish jets could be categorised qualitatively into two jet types or jet modes (termed mode I and II, following previous nomenclature). Mode I jets consisted of an isolated vortex ring, while mode II jets consist of a vortex ring followed by a trailing jet. Analysis of these two jet modes revealed jet structure was close to optimal for thrust generation during mode I, but F was suboptimal and significantly higher in mode II jets (in jet mode II, F = 5.59 ± 0.74 compared with jet mode I F = 3.64 ± 0.41). There was no relationship between jet mode and the animal’s swimming velocity and no significant difference in hydrodynamic efficiency between jet modes. However, hydrodynamic efficiency did increase with increasing swim speed for both jet modes (ranging from approximately 44 to 84%). Contrary to the literature on mechanically generated jets, it was found while jet structure and formation number did vary, there was no difference in hydrodynamic efficiency or the swimming speed at which the jets were utilised.
Undulatory swimmers, such as eels and knifefish achieve hydrodynamic efficiencies between 70 and 90%, our results suggest cuttlefish are able to achieve comparable hydrodynamic efficiencies through jet propulsion.
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