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Any cephalophile engineers out there?

The frighteningly prolific PZMyers has helpfully alerted us to a recent paper in the Journal of Experimental Biology, which looks in detail at Jet flow in steadily swimming adult squid.

However, just like PZMyers, most of the paper was way over my head. I’m sure it is all quite fascinating for someone well-schooled in hydrodynamics. But for the majority of lay-cephalophiles it doesn’t much enlighten us as to how large squid propel themselves. The problem, I think, lies not only in the complicated mathematics and technical jargon. What this study really needs is some sexy animations of squid jet-flow in action, to show us just how these beasts move. Is there anyone up to this task?

Fortunately, there is a readable summary of the work here (along with a nice squid sketch and poem):

Watching ideal smoke rings in their laboratories, engineers with an interest in biology have often wondered whether squid use vortex ring propulsion to glide around their aquatic homes. But Mark Grosenbaugh and Erik Anderson of Woods Hole suspected that something quite different was going on, and set out to examine squid locomotion. It all began with an innocent equation for the ideal smoke ring: L/D=4, meaning that an ideal vortex ring results if the length of a fluid jet is four times the diameter of the pipe emitting the fluid. From his previous work, Anderson knew that this wasn’t true for squid pushing water out of their 1 cm-wide jet nozzle. To settle the matter, Anderson and Grosenbaugh applied advanced strobe photography to the extremely tricky problem of visualising squid jet flow (p. 1125).

To see the flow of water behind steadily swimming squid, they placed microscopic silver-coated spheres into the water and illuminated them with a sheet of laser light. Pointing a digital camera at the light-reflecting spheres, they compared successive frames to see exactly how the spheres, and therefore the water, moved as squid swam through a flume. Anderson and Grosenbaugh saw that squid don’t puff along on individual vortex rings, but instead eject a prolonged fluid jet that is normally 8 to 34 times the diameter of the squid’s jet nozzle.

“But the `Aha!’ moment really came when we realised that the background flow of water past the squid tends to discourage vortex ring propulsion in steady swimming,” Anderson says. He explains that most previous research examined jet flow issued into stationary water, but clearly, water flowing around the squid in its natural home influences jet structure. Ultimately, Anderson says, `all that’s important is what the data tell us is really happening in nature. What may be ideal from an engineering standpoint may not be ideal for a particular niche. Our work illustrates the need to let biology speak for itself, especially in biomechanics.’

Read on for the full summary of the Journal of Experimental Biology paper:

Jet flow in steadily swimming adult squid

Erik J. Anderson and Mark A. Grosenbaugh*

Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

* Author for correspondence (e-mail: mgrosenbaugh@whoi.edu)

Accepted 24 January 2005

Although various hydrodynamic models have been used in past analyses of squid jet propulsion, no previous investigations have definitively determined the fluid structure of the jets of steadily swimming squid. In addition, few accurate measurements of jet velocity and other jet parameters in squid have been reported. We used digital particle imaging velocimetry (DPIV) to visualize the jet flow of adult long-finned squid Loligo pealei (mantle length, Lm=27.1±3.0 cm, mean ± S.D.) swimming in a flume over a wide range of speeds (10.1-59.3 cm s-1, i.e. 0.33-2.06 Lm s-1). Qualitatively, squid jets were periodic, steady, and prolonged emissions of fluid that exhibited an elongated core of high speed flow. The development of a leading vortex ring common to jets emitted from pipes into still water often appeared to be diminished and delayed. We were able to mimic this effect in jets produced by a piston and pipe arrangement aligned with a uniform background flow. As in continuous jets, squid jets showed evidence of the growth of instability waves in the jet shear layer followed by the breakup of the jet into packets of vorticity of varying degrees of coherence. These ranged from apparent chains of short-lived vortex rings to turbulent plumes. There was some evidence of the complete roll-up of a handful of shorter jets into single vortex rings, but steady propulsion by individual vortex ring puffs was never observed. Quantitatively, the length of the jet structure in the visualized field of view, Lj, was observed to be 7.2-25.6 cm, and jet plug lengths, L, were estimated to be 4.4-49.4 cm using average jet velocity and jet period. These lengths and an average jet orifice diameter, D, of 0.8 cm were used to calculate the ratios Lj/D and L/D, which ranged from 9.0 to 32.0 and 5.5 to 61.8, respectively. Jets emitted from pipes in the presence of a background flow suggested that the ratio between the background flow velocity and the jet velocity was more important than L/D to predict jet structure. Average jet velocities in steadily swimming squid ranged from 19.9 to 85.8 cm s-1 (0.90-2.98 Lm s-1) and were always greater in magnitude than swimming speed. Maximum instantaneous fluid speeds within squid jets ranged from 25.6 to 136.4 cm s-1. Average jet thrust determined both from jet velocity and from three-dimensional approximations of momentum change in successive jet visualizations showed some differences and ranged from 0.009 to 0.045 N over the range of swimming speeds observed. The fraction by which the average jet velocity exceeded the swimming speed, or `slip’, decreased with increasing swimming speed, which reveals higher jet propulsive efficiency at higher swimming speeds. Jet angle, subtended from the horizontal, decreased from approximately 29° to 7° with increasing swimming speed. Jet frequency ranged from 0.6 to 1.3 Hz in the majority of swimming sequences, and the data suggest higher frequencies at the lowest and highest speeds. Jet velocity, angle, period and frequency exhibited increased variability at speeds between 0.6 and 1.4 Lm s-1. This suggests that at medium speeds squid enjoy an increased flexibility in the locomotive strategies they use to control their dynamic balance.

Key words: swimming, squid, Loligo pealei, jet propulsion, hydrodynamics, DPIV

First published online March 14, 2005
Journal of Experimental Biology 208, 1125-1146 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01507

This entry was posted on Tuesday, November 22nd, 2005 at 12:11 pm and is filed under Squid Facts. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.