The Kármán gait: novel body kinematics of rainbow trout swimming in a vortex street

Abstract

SUMMARYMost fishes commonly experience unsteady flows and hydrodynamic perturbations during their lifetime. In this study, we provide evidence that rainbow trout Oncorhynchus mykiss voluntarily alter their body kinematics when interacting with vortices present in the environment that are not self-generated. To demonstrate this, we measured axial swimming kinematics in response to changes in known hydrodynamic wake characteristics. We compared trout swimming in the Kármán street behind different diameter cylinders (2.5 and 5 cm) at two flow speeds (2.5 and 4.5 Ls-1, where L is total body length) to trout swimming in the free stream and in the cylinder bow wake. Trout swimming behind cylinders adopt a distinctive, previously undescribed pattern of movement in order to hold station, which we term the Kármán gait. During this gait,body amplitudes and curvatures are much larger than those of trout swimming at an equivalent flow velocity in the absence of a cylinder. Tail-beat frequency is not only lower than might be expected for a trout swimming in the reduced flow behind a cylinder, but also matches the vortex shedding frequency of the cylinder. Therefore, in addition to choosing to be in the slower flow velocity offered behind a cylinder (drafting), trout are also altering their body kinematics to synchronize with the shed vortices (tuning), using a mechanism that may not involve propulsive locomotion. This behavior is most distinctive when cylinder diameter is large relative to fish length. While tuning, trout have a longer body wavelength than the prescribed wake wavelength, indicating that only certain regions of the body may need to be oriented in a consistent manner to the oncoming vortices. Our results suggest that fish can capture energy from vortices generated by the environment to maintain station in downstream flow. Interestingly, trout swimming in front of a cylinder display lower tail-beat amplitudes and body wave speeds than trout subjected to any of the other treatments, implying that the bow wake may be the most energetically favorable region for a fish to hold station near a cylinder.

Nature in engineering for monitoring the oceans: comparison of the energetic costs of marine animals and AUVs

“…Away from free stream operating conditions the preference of fish to use unsteady flow features has been observed in both the laboratory Webb (1998); Liao et al (2003) and the field Hinch and Rand (2000); Fausch (1993);McLaughlin and Noakes (1998). The potential of replicating such behaviours with AUVs is discussed in Philips et al (2010).…”

Learning from fish: Kinematics and experimental hydrodynamics for roboticists

“…But many fish swim naturally in flowing waters of high turbulence. Liao et al [86,99,100] studied fishes swimming in the Karman vortex wake behind cylinders placed in flowing water and demonstrated that trout can greatly alter their locomotor kinematic pattern to tune the pattern of body bending to the wavelength between oncoming vortices. Remarkably, fish swimming in a Karman street can completely shut off body muscle activity and generate thrust passively by adjusting the angle of their body airfoil in the vortex street to generate thrust as vortices pass by.…”

Study the Flow behind a Semi-Circular Step Cylinder (Laser Doppler Velocimetry (LDV) and Computational Fluid Dynamics (CFD))

“…These vortices are influenced by wake and shedding frequency, but these wake and vortex shedding frequencies are greatly dependent on the cylinder diameter [8]. A cylinder with different diameters, such as a step cylinder, could generate an interesting flow behind it and variable wake and shedding frequencies.…”

“…The concept is to create periodic vortices that the fish uses in an energy efficient way (less energy than in a free stream) to maintain a position in the flow. Liao [8] describes the structure of the flow around a half cylinder and the positions of fish swimming within the wake behind and around the cylinder. It was found that the fishes exploit the flow conditions created by the semicircular cylinder when swimming upstream around it.…”