Turbulent swirling flow in a straight pipe is one of turbulent flows that are difficult to predict accurately by the eddy viscosity model. In this work, the Reynolds stress equation model was integrated along the fluid particle path to obtain a temporally nonlocal model. It was shown that the history effect associated with the swirling motion leads to the decrease of the eddy viscosity. The velocity profiles in a swirling pipe flow obtained from the present model agree well with experiment; the reduced eddy viscosity can account for the slow decay of the swirling motion in a pipe flow.

Inhomogeneous helicity enters expression for the Reynolds stress as the coupling coefficient of absolute vorticity. In contrast to the eddy viscosity, this helicity effect may play for the transport suppression and structure generation in turbulence. Large-scale flow generation due to inhomogeneous helicity is confirmed by direct numerical simulations '(DNSs) of rotating turbulence. As interesting applications of this effect, the roles of helicity in the generation and sustainment of cyclone and angular-momentum transport in the solar convection zone are suggested.

A turbulence model is proposed for the numerical simulation of turbulent flows. The Reynolds-averaged numerical simulation (RANS) solves mean variables with the Reynolds stress to be modeled for closing the system of equations. The k－ε model is a typical turbulence model and the most widely used in RANS. However, the Reynolds stress model (RSM), which gives the stress by its transport equation, is used when we encounter complex flow fields which cannot be accurately predicted by the k－ε model. Although the RSM also has a weak point that it costs much time to solve its six components. Recently, Yoshizawa introduced an eddy-viscosity transport equation which was extracted from the RSM and utilized the equation as his model. In this study, taking advantage of its better simplicity and saving more computational time, we aim to extend his model to being applicable to buoyancy-driven flows under the Boussinesq approximation. As a result, a transport equation for eddy thermal diffusivity is introduced as well as that for eddy viscosity.

The effects of helicity are considered to be significant in the magnetohydrodynamics. However, they did not attract much attention in the turbulence model of the neutral fluid. Recent numerical simulations revealed that helicity has notable role in the neutral fluid. We discuss the effects of helicity on the turbulent kinetic energy equation. We focus on the pressure diffusion term to derive the term which indicates the spacial energy transfer due to the rotating motion of the fluid and the helicity. It is suggested that this effect corresponds to the energy transfer due to the inertial wave.

Modeling the destruction term in the energy dissipation transport equation, whose physical origin is not clear, was examined using the two-scale direct-interaction approximation. When two terms corresponding to the destruction term are expanded in powers of the viscosity, the first-order parts of the two terms need to cancel each other. In this work, the cancelation was used as a constraint condition to obtain the form of the energy spectrum. It was expected that a model expression can be derived from the second-order parts by modifying the response function.

Water in the bottom layer has been in hypoxic condition due to water utilization initiated from 1982 and recent climate change in Lake Ikeda, which locates in the south of Kyusyu District. As a mitigation of the hypoxia, sucking water from the bottom layer is proposed and its effects were predicted by numerical simulation using a hydrodynamic and ecosystem coupled model. The results of numerical simulation represent that the dissolved oxygen concentration increases below 50m from water surface mainly due to acceleration of advection in the vertical direction.

Wind integration studies are important to estimate the effect which deployment of large amounts of variable and uncertain power generation will have on the grid. One of the most important aspects of reliable power system operations is the balance between power demand and generation. Different time frames require different techniques of balancing the generation and demand. Ramp events, which are sudden and large changes of power generation （it can be increasing or decreasing), are critical events. In this paper, we report the results of an analysis of wind power fluctuation and ramps from the measured power generation at existing wind farms located in Tohoku region of Japan. We also evaluate ramps of residual load (demand minus wind power generation).