The ratio of orifice and cavity diameters is adjusted by selecting the appropriate orifice plate ( Fig. 1B). Reservoir pressure and the time the valve is open are controlled using a regulator (Norgren B07-202-A1KA) and microcontroller (Arduino Duemilanove, ATmega328), respectively. Vortex characteristics are tuned by adjusting the pressure in the reservoir, the duration for which the solenoid valve is open, the ratio of the orifice to the cavity diameter, and the ball's initial position. The vortex causes a large deflection in the caudal fin, which is used to estimate the compliance of the fin. The images show the response of the caudal fin, in the form of deflection from its original position, to a vortex perturbation of known strength. (E) Snapshots from an experimental trial with a live bluegill sunfish swimming at 0.65 BL s −1 (∼107 mm s −1). L stroke, stroke length D, cavity diameter d, orifice diameter.
At a certain distance from the ejection point, the vortex separates from the starting jet, and the residual jet is called the trailing jet. (D) Illustration of the vortex formation process – when the ball accelerates water out of the cavity, an axial jet is attached to the vortex, called the starting jet. The maximum velocity is observed in the front and center of the vortex. The arrows represent the resultant velocity vectors for the particles constituting the vortex. (C) Digital particle image velocimetry image of a vortex ring close to impacting the caudal fin of a bluegill sunfish swimming at 0.65 BL s −1. Food coloring (dye) was used to visualize the vortex ring. (B) Image of a vortex ring produced using the prototype 1 (of 2) vortex generator during experiments. (A) Vortex generator set up in the experiment tank with a bluegill sunfish swimming naturally with minimal obstruction.
The vortex generator, showing vortex ring formation and its use as a short-duration stimulus to fish during natural swimming. To meet these needs, we have developed a device that generates vortices suitable for conducting perturbation studies with sunfish swimming at speeds between 0 and 350 mm s −1. The strengths of the vortex rings can be estimated using the vortex size, speed and circulation ( McErlean, 2011). The relative stiffness changes in the fin can be quantified by comparing the deflections in the fin caused by vortex rings of known strengths. The perturbation would have to be aimed and modulated in strength for different fins, develop and travel in free-stream flows of different speeds, be applied as the fish swims freely, interact with the fin over a short period relative to the fin beat duration, and cause measurable deflection in the fin without stopping the fish's natural swimming. Based on perturbation studies conducted to evaluate human physiological responses ( Marmarelis and Marmarelis, 1977 Kearney and Hunter, 1982 Bennett et al., 1992 Tangorra, et al., 2003), it was hypothesized that a fin's stiffness could be evaluated by measuring its response to an external fluidic perturbation.
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