Hi all, I recently passed through the crucible of Comprehensive exams. I have been studying a whole suite of topics for the last eight months or so most of which revolve around my research interests in how fish control their maneuvers. One interesting topic that I looked into was how fish interact with their flow environments. I wanted to take todays post to do an expose on some really interesting work done by James Liao out of The University of Florida at Gainesville. What you’re about to read is an answer from one of my exam questions,
Dead trout can swim upstream. Now that I have used up my clickbait, here is a more technical description of what we will be discovering: A freshly killed trout can passively achieve forward thrust when towed in a Karman wake. To understand how this work first we must understand the hydrodynamics of a Karman wake. Water flowing past a bluff body (a log in a stream) alternately sheds vortices clockwise and counterclockwise. These vortices rotate inwards, toward the center of the wake and are offset as a function of the vortex shedding frequency and the velocity of the wake. Flow visualization shows an expanding wake with a zig-zag pattern of vortex cores with opposite signs.
Between two vortices, the rotation of each constructively interferes forming a jet between. This jet is oriented perpendicular to the angle between the path of the two vortices and the direction of flow. These jets are linked as each vortex shares two neighbors. The result is a jet with a component of upstream flow as well as oscillating lateral flow.
Live trout swimming in uniform flow have small lateral body displacement and body curvature is lowest at the head and increases toward the tail.
However, in a Karman wake the trout adopts a slaloming gait, exhibiting large lateral displacements and body curvature. The lateral flow described above generates the lateral body displacements and probably aids in generating the body curvature. The frequency of lateral body displacements matches the frequency of vortex shedding. The upstream component propels the fish forward. EMG tests from trout swimming in these Karman wakes show reduced muscle activation. The kinematics of freshly killed trout very closely resemble the kinematics of live fish. The body resonates with the frequency of the vortex shedding and allows the fish to hold station in the wake. These wakes can even provide enough thrust to move the fish upstream, all the way up into the suction zone directly behind the bluff body. Live fish have more control over this Karman gait. They can selectively apply drag to maintain station in the optimal region of the wake. They can also modulate body stiffness to match the resonant frequencies of their bodies to the frequency of the Karman wake.
I particularly like this example of fish locomotor behavior because it is shocking and counter intuitive. It recognizes that a locomotor control can be simplified by “programming” the material properties of the system, in this case the body stiffness. This example has implications for increasing the energy efficiency of aquatic vehicles. I could envision a system where the energy extracted by a fish robot swimming in a Karman wake could be used to charge its batteries, and redeploy without having to be retrieved from the water to charge.
Full citations and for further reading see:
Liao, J. C. (2004). “Neuromuscular control of trout swimming in a vortex street: implications for energy economy during the Kármán gait.” Journal of Experimental Biology 207(20): 3495-3506.
Liao, J. C., D. N. Beal, G. V. Lauder and M. S. Triantafyllou (2003). “The Karman gait: novel body kinematics of rainbow trout swimming in a vortex street.” J Exp Biol 206(Pt 6): 1059-1073.