Author(s): Ichiro Kimura; Takashi Hosoda
Linked Author(s): Ichiro Kimura
Keywords: Tidal current; Tidal vortex; Coherent structure; Shear instability; Numericalsimulation
Abstract: The Naruto Straits is famous for its violent tidal currents, in which hundreds of tidal vortices (Naruto Whirlpools) are created due to the shear instability. The diameter of a well-grown vortex is up to 20m and the maximum depth of the hollow of vortex is more than 1. 5m. The tidal vortices play an important role in the tidal exchange through the straits, which closely concerns with the water pollution in the Inland Sea. It has been pointed out that the tidal vortices are generated by the shear instability in the horizontal flow field and have the quasi-2D columnlike structure reaching from the water surface to the sea bottom. Therefore, it seems to be possible to reproduce the tidal vortices numerically by the depth-averaged plane 2-D open channel flow equations to some extent. The applicability of the plane 2-D equations to the unsteady vortex formation induced by the shear instability in open channels has already been studied theoretically using the linear instability analysis. In this paper, we apply the depth averaged plane 2-D flow equations to the tidal currents in the Naruto Straits and clarify the applicability of the numerical method to the tidal currents with vortices. The basic equations are composed of the depth-averaged continuity and momentum equations with the eddy viscosity terms. The equations were discretized using the finite volume method with the QUICK scheme for the convective inertia term and the Adams-Bashforth method for the time integration. The water level variations at both sides of the straits were given at two boundary sections according to the results of the field observations. The calculated velocity field of the tidal currents was in good agreement with the observed one. The formation of tidal vortices was simulated numerically and the characteristics of vortices were compatible with the observed ones. It has been pointed out that the tidal vortices are developed through the several stages of the pairing, which are called the “Cascade-Up”. It was indicated that “Cascade-Up” of the tidal vortices can be simulated qualitatively by the present numerical model.