Ryuichi Nagaosa

Direct numerical simulation of vortex structures and turbulent scalar transfer across a free surface in a fully developed turbulence

Dynamics of well-organized tube-like coherent structures under a free surface and turbulent scalar transfer across the free surface in fully developed turbulent flow in an open channel is investigated. A direct numerical simulation of the three-dimensional Navier–Stokes equations is used to obtain the structure of the free-surface turbulence. First, the effect of the free surface on fully developed turbulence statistics is described. Anisotropy of velocity and vorticity under the free surface are given. Next, the dynamics of the intermittent vortex tubes beneath the free surface are stated. The genesis and development of these coherent structures and their interactions with the free surface are demonstrated. The role of the vortex/surface interactions on the dynamics of turbulence under the free surface, particularly intercomponent energy transfer due to the pressure–strain effect, is discussed. In addition, passive scalar transfer across the free surface is studied. Finally, the promotion of turbulent sc...

Surfactant effects on passive scalar transport in a fully developed turbulent flow

Direct numerical simulations of fully developed turbulence in an open channel were performed. Effects of surfactants on heat transfer and the underlying turbulent structures were investigated. As surface elasticity is increased turbulent fluctuations are damped and the mean surface temperature is decreased. A surface strain model is introduced to explain this behavior in a heuristic manner. A nondimensional parameter representing the ratio of surface elastic forces to local inertial forces is introduced. It is concluded that for values of the parameter of order one, surfactants have strong effects on surface turbulence, whereas an effectively clean surface can be obtained for parameter values less than O(10−3).

A new numerical formulation of gas leakage and spread into a residential space in terms of hazard analysis.

This study proposes a new numerical formulation of the spread of a flammable gas leakage. A new numerical approach has been applied to establish fundamental data for a hazard assessment of flammable gas spread in an enclosed residential space. The approach employs an extended version of a two-compartment concept, and determines the leakage concentration of gas using a mass-balance based formulation. The study also introduces a computational fluid dynamics (CFD) technique for calculating three-dimensional details of the gas spread by resolving all the essential scales of fluid motions without a turbulent model. The present numerical technique promises numerical solutions with fewer uncertainties produced by the model equations while maintaining high accuracy. The study examines the effect of gas density on the concentration profiles of flammable gas spread. It also discusses the effect of gas leakage rate on gas concentration profiles.

A gas-lift system for CO2 release into shallow seawater

The GLAD system is proposed for the injection and dissolution of CO 2 gas into the ocean. It is a pipeline of inverse U shape settled in the shallow sea. CO 2 bubbles injected into the pipe form a buoyant plume, but they dissolve while rising. The dense solution is released from the other side of the pipe. In the present paper, possibility of our concept is confirmed by the numerical simulation of gas-liquid two-phase flow with a primitive model of CO 2 dissolution

Reproductive and developmental toxicity of degradation products of refrigerants in experimental animals.

The present paper summarizes the results of animal studies on the reproductive and developmental toxicity of the degradation products of refrigerants, including trifluoroacetic acid (TFA), carbon dioxide (CO(2)), carbon monoxide (CO), carbonyl fluoride (CF), hydrogen fluoride (HF) and formic acid (FA). Excessive CO(2) in the atmosphere is testicular and reproductive toxic, embryolethal, developmentally neurotoxic and teratogenic in experimental animals. As for CO, maternal exposure causes prenatal and postnatal lethality and growth retardation, skeletal variations, cardiomegaly, blood biochemical, immunological and postnatal behavioral changes, and neurological impairment in offspring of several species. Very early studies of CO in rats and guinea pigs reported fetal malformations in exposed dams. The results of toxicological studies on sodium fluoride (NaF) were used to obtain insight into the toxicity of CF and HF, because CF is rapidly hydrolyzed in contact with water yielding CO(2) and HF, and NaF is similar in kinetics and dynamics to HF. Increased fetal skeletal variation, but not malformation, was noted after the maternal administration of NaF. Rat multiple-generation studies revealed that NaF caused retarded ossification and degenerative changes in the lung and kidney in offspring. There is a lack of information about the toxicity of TFA and FA.

Low shear turbulence structures beneath stress-driven interface with neutral and stable stratification

This study deals with turbulent flow below a stress-driven interface with stable density stratification, as a simplified configuration of a turbulent flow below a calm gas-liquid interface with wind forcing. A simple shear stress, which is constant in time and uniform at the interface, is enforced, hence only turbulence below the interface is studied. A flat interface is assumed to simplify transport phenomena beneath the interface by limiting the strength of the enforced wind stress and avoiding violent wave motions. A direct numerical simulation technique is employed to obtain three-dimensional turbulence structures below the interface. Results from the present study indicate two major changes of turbulence structures below the sheared interface in the presence of stable density stratification; one is a reduction of the Reynolds stress at the near-interface region, which leads to a coincidental increase of mean velocity gradient. The other effect of stable stratification is the quantitative variation of...

A numerical modelling of gas exchange mechanisms between air and turbulent water with an aquarium chemical reaction

This paper proposes a new numerical modelling to examine environmental chemodynamics of a gaseous material exchanged between the air and turbulent water phases across a gas-liquid interface, followed by an aquarium chemical reaction. This study uses an extended concept of a two-compartment model, and assumes two physicochemical substeps to approximate the gas exchange processes. The first substep is the gas-liquid equilibrium between the air and water phases, A ( g ) ? A ( aq ) , with Henry?s law constant H. The second is a first-order irreversible chemical reaction in turbulent water, A ( aq ) + H 2 O ? B ( aq ) + H + with a chemical reaction rate ? A . A direct numerical simulation (DNS) technique has been employed to obtain details of the gas exchange mechanisms and the chemical reaction in the water compartment, while zero velocity and uniform concentration of A is considered in the air compartment. The study uses the different Schmidt numbers between 1 and 8, and six nondimensional chemical reaction rates between 10 - ∞ ( ? 0 ) to 101 at a fixed Reynolds number. It focuses on the effects of the Schmidt number and the chemical reaction rate on fundamental mechanisms of the gas exchange processes across the interface.

Reproductive and developmental toxicity of hydrofluorocarbons used as refrigerants.

The present paper summarizes data on the reproductive and developmental toxicity of hydrofluorocarbons (HFCs), including pentafluoroethane (HFC-125), 1,1,1,2-tetrafluoroethane (HFC-134a), 1,1,1-trifluoroethane (HFC-143a), 1,1-difluoroethane (HFC-152a), difluoromethane (HFC-32) and 1,1,1,3,3-pentafluoropropane (HFC-245fa), used as refrigerants, published in openly available scientific literature. No developmental toxicity of HFC-125 was found even at 50,000 ppm in rats or rabbits. Although HFC-134a exhibited no dominant lethal effect or reproductive toxicity in rats, it caused low body weight in pre- and postnatal offspring and slightly retarded skeletal ossification in fetuses at 50,000 ppm in rats. No maternal or developmental toxicity was noted after exposure to HFC-143a even at 40,000 ppm in rats or rabbits or HFC-152a even at 50,000 ppm in rats. HFC-32 is slightly maternally and developmentally toxic at 50,000 ppm in rats, but not in rabbits. HFC-245fa caused decreases in maternal body weight and food consumption at 10,000 and 50,000 ppm and fetal weight at 50 000ppm. No evidence of teratogenicity for these HFCs was noted in rats or rabbits. There is limited information about the reproductive toxicity of these HFCs. Animal studies remain necessary for risk assessments of chemicals because it is difficult to find alternative methods to determine the toxic effects of chemicals. It is required to reduce emissions of organic vapors containing HFCs to reduce the risk of exposure.

Reprint of

This paper proposes a new numerical modelling to examine environmental chemodynamics of a gaseous material exchanged between the air and turbulent water phases across a gas-liquid interface, followed by an aquarium chemical reaction. This study uses an extended concept of a two-compartment model, and assumes two physicochemical substeps to approximate the gas exchange processes. The first substep is the gas-liquid equilibrium between the air and water phases, A ( g ) Â? A ( aq ) , with Henrys law constant H. The second is a first-order irreversible chemical reaction in turbulent water, A ( aq ) + H 2 O Â? B ( aq ) + H + with a chemical reaction rate Â? A . A direct numerical simulation (DNS) technique has been employed to obtain details of the gas exchange mechanisms and the chemical reaction in the water compartment, while zero velocity and uniform concentration of A is considered in the air compartment. The study uses the different Schmidt numbers between 1 and 8, and six nondimensional chemical reaction rates between 10 - ∞ ( Â? 0 ) to 101 at a fixed Reynolds number. It focuses on the effects of the Schmidt number and the chemical reaction rate on fundamental mechanisms of the gas exchange processes across the interface.

On Suitable Grid Resolution in the Wall-Parallel Directions for a Direct Numerical Simulation of Wall Turbulence using a Second-Order Finite Difference Method.

This paper aims to discuss suitable grid resolution for a direct numerical simulation (DNS) of wall-bounded turbulence using a finite difference method (FDM). All spatial derivatives are approximated by a second-order central difference. The fully-developed turbulence statistics in a turbulent flow near a flat wall is evaluated using the FDM. The accuracy of the DNS based on the finite difference approximation is confirmed by the DNS database evaluated by a spectral method (Kasagi, N. et al. ASME J. Heat Transf. 114, (1992), 598). In order to discuss the effect of the grid resolution in both the streamwise and spanwise directions, the numbers of grid points in these directions are reduced. The effect of the grid resolution on the fully-developed turbulence statistics near the wall is finally discussed.