## 1 Introduction  
In recent years, we have been faced with issues such as global warming and depletion of fossil fuels. Particularly in Japan, since the Eastern Japan earthquake disaster and the stopping of many nuclear power plants, there has been a significant effort to move towards renewable clean energy generation. As Japan is surrounded by the sea which provides stable ocean currents throughout the year, we propose an oscillatory-type tidal current-powered generator based on robotic fish technology. A sea bream robotic fish was developed in 1995, as shown in Figure 1. A flexure like a fish fin was produced from variously changing elastic modules of the flexible oscillating fin [1; 2; 3]. The author found that the fin had significant potential as an oscillatory-type tidal current-powered generator. In tidal power generation, propeller-based mechanisms are predominantly used. However, maintenance costs are high, due to marine fouling from incrustations and entanglement by seaweed; the rotating propeller itself also negatively impacts the marine environment in various ways. Furthermore, the relatively high complexity of a propeller combined, with the intense corrosion that occurs in sea water, results in high manufacturing and maintenance costs. Therefore, there have been many studies on fundamentally different approaches to marine energy generation. Among them is the VIVACE system of the University of Michigan [4]. This system uses the physical phenomenon of vortex-induced oscillation, in which water currents flowing around cylinders induce transverse motion. The energy resulting from the movement of the cylinders is then converted to electricity. There is also a small reciprocating-type generator in Japan, made by [PERSON] at Fukuoka Industrial University [5], in which the fluttering behavior of a wing is used. Further, a water flow-powered generator using pendulum-oscillation has been developed at Okayama University [6; 7], which uses the same principle as the VIVACE system. The authors' aim is to provide some assistance toward developing a more efficient reciprocating-type power generating system. Specifically, with regard to determining the optimal elastic modulus of a wing foil-type oscillator, and the optimum shape of a reciprocating oscillator head (from a group of six common shapes) to provide high lift coefficients when these oscillating heads are placed in transverse flows. The focus of this paper is on the optimization of this lift force. The magnitude of the \"coefficient of lift\" (\(Cl\)) varies with the shape of the oscillating mechanism. Therefore, as the oscillator is the main part of a reciprocating type generator, it's design is critical. The purpose of this study is to find the shape of oscillator head that produces the largest \(Cl\). To begin with, the optimum shape of the oscillator is found through numerical analysis; then, a physical machine using the oscillator will be created for further testing. A problem with coupling fluid flow to an object/mechanism must be solved, because the flow pressure causes deformation and displacement of the object (in this case, the oscillator), and the movement of the object in turn affects the flow (its velocity, pressure, etc.). The interdependent fluid-mechanical coupled system is numerically simulated in ANSYS. The fundamental equations of the two-way coupled problem are as follows:  
The unsteady flow field (incompressible) around the oscillator is governed by the following Navier-Stokes equation, and the equation of continuity, where \(t\) is the time, \(\rho\) is the mass of the flow, \(V\) is the velocity, \(F\) is the body force and \(p\) is the pressure:  
\[\rho\frac{dV}{dt}\ =\ \mathbf{F}-\
abla p+\mu\
abla^{2}\mbox{\boldmath$V$ },\ \
abla\cdot V\ =\ 0 \tag{1}\]  
Figure 1: Sea bream robotic fish.