S. Kida 교수

The interaction of two identical circular viscous vortex rings starting in a side-by-side configuration is investigated by solving the Navier–Stokes equation using a spectral method with 64 3 grid points. This study covers initial Reynolds numbers (ratio of circulation to viscosity) up to 1153. The vortices undergo two successive reconnections, fusion and fission , as has been visualized experimentally, but the simulation shows topological details not observed in experiments. The shapes of the e

A direct numerical simulation of the three-dimensional Navier–Stokes equation at high Reynolds numbers is performed by the spectral method with 3×3403 effective modes (853 independent degrees of freedom) starting from a high-symmetric flow. Kolmogorov’s [C. R. (Dokl.) Acad. Sci. URSS 30, 301 (1941)] similarity forms of the energy spectra (the one-dimensional longitudinal and lateral energy spectra as well as the three-dimensional one) in the universal range are observed in a decaying period afte

The reconnection of two identical viscous vortex rings is investigated numerically. The initial state consists of two rings (of Re≡Γ/ν=578) of a circular cross section placed side by side in a periodic box with their coplanar axes inclined by 30° to the symmetric plane. The subsequent motion is examined by solving the Navier–Stokes equation by a dealiased pseudospectral method in a 643 box. We found two prominent reconnection processes: first, the two vortex rings merge into one by viscous annih

The response of the Indonesian Seas to the monsoonal winds is investigated using a regional ocean model with an emphasis on understanding the mechanisms behind the observed seasonal sea surface temperature (SST) variability. Understanding the seasonal SST variability in the Indonesian Seas is crucial for understanding tropical climate variability since atmospheric deep convection is located directly above these seas. Our model results suggest that the monsoonal winds play a dominant role in crea

The mechanism of vortex reconnection is investigated by solving the Navier–Stokes equation numerically starting with a closed knotted vortex tube, which gives a nonzero helicity. A new type of vortex reconnection mechanism—bridging—is observed. Small regions of high vorticity bursting out of the vortex tube become larger and bridge different portions of the tube. A relation between the change of the helicity and the mechanism of the vortex reconnection is discussed.

Abstract A long-term increasing trend in the transport of the Japan Sea Throughflow is observed from sea-level differences across the Tsushima Strait. Tidal gauge observations show sea level at Hakata, Japan, increasing at a higher rate than that at Busan, Korea. Numerical modeling results suggest that this increasing trend is forced by a northward shift in the Kuroshio axis. As the Kuroshio axis moves northward, sea level along the southern coast of Japan increases. The signal then propagates a

A numerical model is used to investigate how the Indonesian Throughflow and tidal mixing are affecting the seasonal cycle of the sea surface temperature (SST) in the Indonesian Seas. The SST in these seas is considered to play a major role on the development of the Australian Summer Monsoon. Based on a quantitative assessment of the heat budget, the Indonesian Throughflow is found to affect the SST in the western Indonesian Seas primarily during Austral summer. The Throughflow advects the warm w

Abstract The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows enter the open ocean as dense gravity currents that flow along and down the continental slope. While descending the slope, overflows typically double their volume transport by entraining upper oceanic water. The upper oceanic layer must balance this loss of mass, and the resulting convergent flow produces significant vortex stretching. Overflows thus represent an intense and localized mass and vor

The mechanism of vortex reconnection is investigated by solving the Navier-Stokes equation numerically starting with a trefoiled closed knotted vortex tube. A new type of vortex reconnection mechanism–bridging–is observed. Small regions of high-vorticity burst out of the vortex tube. grow up and bridge different portions of the tube. A relation between the change of the helicity and the mechanism of the vortex reconnection is discussed.

Abstract The mechanism responsible for the annual cycle of the flow through the straits of the Japan Sea is investigated using a two-layer model. Observations show maximum throughflow from summer to fall and minimum in winter, occurring synchronously at the three major straits: Tsushima, Tsugaru, and Soya Straits. This study finds the subpolar winds located to the north of Japan as the leading forcing agent, which first affects the Soya Strait rather than the Tsushima or Tsugaru Straits. The sub

The distortion by large-scale random motions of small-scale turbulence is investigated by examining a model problem. The changes in energy spectra, velocity and vorticity moments, and anisotropy of small-scale turbulence are calculated over timescales short compared with the timescale of small-scale turbulence by applying rapid distortion theory with a random distortion matrix for different initial conditions: irrotational or rotational, and isotropic or anisotropic large-scale turbulence with o