Hydrodynamics of pulsed jetting in juvenile and adult brief squid Lolliguncula brevis: evidence of multiple jet `modes' and their implications for propulsive efficiency

• Published in: Journal of Experimental Biology
• Main Authors: Bartol, Ian K., Krueger, Paul S., Stewart, William J., Thompson, Joseph T.
• Format: Text
• Language: English
• Published: Company of Biologists 2009
• Subjects: Research Article; DML
• Online Access: http://jeb.biologists.org/cgi/content/short/212/12/1889; https://doi.org/10.1242/jeb.027771

The dynamics of pulsed jetting in squids throughout ontogeny is not well understood, especially with regard to the development of vortex rings, which are common features of mechanically generated jet pulses (also known as starting jets). Studies of mechanically generated starting jets have revealed a limiting principle for vortex ring formation characterized in terms of a `formation number' ( F ), which delineates the transition between the formation of isolated vortex rings and vortex rings that have `pinched off' from the generating jet. Near F , there exists an optimum in pulse-averaged thrust with (potentially) low energetic cost, raising the question: do squids produce vortex rings and if so, do they fall near F , where propulsive benefits presumably occur? To better understand vortex ring dynamics and propulsive jet efficiency throughout ontogeny, brief squid Lolliguncula brevis ranging from 3.3 to 9.1 cm dorsal mantle length ( DML ) and swimming at speeds of 2.43–22.2 cms–1 (0.54–3.50 DML s–1) were studied using digital particle image velocimetry (DPIV). A range of jet structures were observed but most structures could be classified as variations of two principal jet modes: (1) jet mode I, where the ejected fluid rolled up into an isolated vortex ring; and (2) jet mode II, where the ejected fluid developed into a leading vortex ring that separated or `pinched off' from a long trailing jet. The ratio of jet length [based on the vorticity extent ( L ω )] to jet diameter [based on peak vorticity locations ( D ω )] was <3.0 for jet mode I and >3.0 for jet mode II, placing the transition between modes in rough agreement with F determined in mechanical jet studies. Jet mode II produced greater time-averaged thrust and lift forces and was the jet mode most heavily used whereas jet mode I had higher propulsive efficiency, lower slip, shorter jet periods and a higher frequency of fin activity associated with it. No relationship between L ω / D ω and speed was detected and there was no apparent speed ...