Nov 4 2020
Cephalopods such as squids use a kind of jet propulsion that is not clearly understood, specifically with respect to their hydrodynamics under turbulent flow conditions.
Development of the vorticity pattern into symmetry-breaking instability. Image Credit: Yang Luo.
Finding out their secrets could be useful in making new designs for bioinspired underwater robots and vehicles that must function in such settings.
Scientists in the United States, China, and Scotland have been analyzing the basic mechanism behind the pulsed-jet propulsion of squids.
In the Physics of Fluids journal, from AIP Publishing, the researchers explain their numerical study of the jet propulsion of cephalopods, where the turbulent flow was considered for the first time. In their studies, they identified that thrust production and efficiency are underrated in laminar, or nonturbulent, flows.
For this study, the researchers used a 2D squid-like swimmer as a model, with a flexible mantle body that features a pressure chamber and a nozzle that acts as the inlet and outlet of water. An external force, which imitates the constriction of the squid muscle, was applied on the model’s flexible mantle surface.
As a result, the internal volume of the body decreases and water inside the chamber is ejected to form a jet flow. The squid is propelled forward by the strong jet in the opposite direction, then the mantle inflates automatically as a result of stored elastic energy. During inflation of the mantle, water is sucked into the chamber and gets ejected during the next mantle deflation.
Yang Luo, Study Author and Research Assistant, University of Strathclyde
Luo added that jet propulsion could be highly effective when the turbulent flow is taken into account. Moreover, the researchers identified the symmetry-breaking uncertainty of vortices near the jetter, which discharges jets of water, following several continuous jet cycles.
This may help provide a better understanding of why burst-and-coast swimming is used by juvenile and adult squids that operate within turbulent flows more frequently compared with squid hatchlings that operate within laminar flows.
Yang Luo, Study Author and Research Assistant, University of Strathclyde
Apart from jet propulsion, adult and juvenile squids also depend on fin oscillation on their heads to swim very often. The researchers discovered that this burst-and-coast style might help squids prevent the symmetry-breaking instability of the surrounding flow vortex that could result in thrust and a drop in efficiency.
The findings of our work about the mechanism of symmetry-breaking instability provides guidance for the design of squid-inspired underwater robots and vehicles.
Yang Luo, Study Author and Research Assistant, University of Strathclyde
Luo continued, “Continuous jet propulsion may not be favorable, and specific measures are needed to mitigate the effect of this instability during the design of jet propulsion-inspired underwater vehicles or propulsors via active control of body deformation to change the evolution of the internal vortices pattern.”
Will new jet propulsion-based submarines be developed any sooner?
“It’s difficult to determine at this point. But as a relatively less extensively studied form of underwater propulsion, it is advantageous in terms of a straightforward mechanism for effective instantaneous escape and high maneuverability. This makes it promising for integrating with typical thruster propulsion to achieve on-demand maneuverability,” answered Luo.
Journal Reference:
Luo, Y., et al. (2020) Pulsed-jet propulsion of a squid-inspired swimmer at high Reynolds number. Physics of Fluids. doi.org/10.1063/5.0027992.