Hitoshi Suto

Computational fluid dynamics simulation and statistical procedure for estimating wide-area distributions of airborne sea salt considering local ground conditions

Abstract Steel corrosion under atmospheric conditions is a critical issue in the maintenance of structures such as electric transmission towers and bridges during their long-term operation, which are generally located at many places over a wide area. Since a major factor causing corrosion is airborne salt particles coming from the sea, wide-area distributions of the long-term cumulative amount of sea salt deposited on surfaces are needed. Moreover, since the amount of airborne sea salt varies locally with the topography, it is also important to consider the effects of topography. In this paper, a method combining a computational fluid dynamics model and a statistical procedure is proposed to efficiently estimate wide-area distributions of the cumulative amount of airborne sea salt by considering the local topography. The predicted amount of airborne sea salt decreases with increasing distance from the coast and varies with the topography and the offshore wind. A comparison between predicted and observed amounts revealed that: (1) this method appropriately estimates topographical effects on sea-salt transport and enables the estimation of deposited sea salt on structure surfaces, and (2) consideration of the trapping efficiency of sea-salt particles on structure surfaces improves the prediction accuracy.

Experimental Study on Fire Behavior in a Compartment Under Mechanical Ventilated Conditions: The Effects of Air Inlet Position

The purpose of this research is to understand the fire behavior expected in a mechanically ventilated compartment. To date, some experimental studies have been conducted for investigation of fire behavior under mechanical forced ventilation; however, it might be not enough to understand everything. We therefore carried out a series of experiments on fire behavior focused on the effect of air inlet position in a compartment with same size as an ISO 9705 room (width 2.4 m × length 3.6 m × height 2.4 m) under conditions of mechanical ventilation using a pool fire. In this paper, the effects of ventilation conditions such as air inlet position and flow rate were studied. We found that differences in the air inlet position and flow rate were one of the principal factors for determining the burning behavior.

Interaction of a Pool Fire in a Compartment with Negative Pressure Generated by Mechanical Ventilation

We have experimentally investigated a medium-size pool fire in a compartment, the dimensions of which corresponded to the ISO 9705 room. Airflow rates in ducts, pressure, mole fraction, and temperature of air in the compartment and a mass loss rate of the fuel were measured. The liquid fuel and pool diameter were ethanol and 600 mm, respectively, which rapidly increased the compartment pressure just after ignition. The compartment was ventilated at inlet and outlet ducts with natural and mechanical ventilation systems, which initially gave negative compartment pressures in the range of −2 to −85 Pa. The negative pressure was much weaker than the pressure increase, which restrained the air supply with natural ventilation and resulted in extinction due to lack of oxygen. On the other hand, the negative pressure with the stronger mechanical ventilation sustained the air supply and yielded the transition to a ventilation-controlled fire without extinction. The ventilation-controlled fire led to two kinds of oscillating flame: one was caused by poor oxygen supply, which is similar to that reported by previous studies, and the other was caused by repetition of ignition and extinction, which was attributed to the change in the flow rate and direction of fresh air at the inlet duct. This oscillation generated large pressure fluctuations but did not yield thermal energy with combustion.

Large-Eddy Simulation of a Buoyant Plume Past a Bluff Body

A large-eddy simulation (LES) of a buoyant plume past a bluff body is performed. A round heat source is placed at the center of a horizontal flat wall, and a bluff body in the shape of a thick round plate is floating right above the heat source. The modified Rayleigh number based on the total heat input is set at 1.2×1010 . On the basis of past studies, the Smagorinsky model is adopted as a subgrid-scale (SGS) model, and a partial slip boundary condition based on the wall law is applied to a horizontal flat wall and a disk surface. The validity of numerical results is ascertained by comparison with theoretical solution and experimental data. The blocking of upward flow and imparting turbulence through a bluff body vary the process of developing a buoyant plume, while properties of a fully developed plume rarely vary. With heat from a bluff body, another buoyant plume is formed near the center, piled with upward flow passing around the bluff body. Moreover, main positions of buoyant production of turbulent kinetic energy move a point from near the side of the bluff body to a point near the central axis. This affects the transition to a fully developed plume in turbulence statistics.Copyright

Turbulent Structure in Stably Stratified Flow over a Wavy Surface

Large eddy simulation was performed for flow and heat transfer under stable density stratification in a wavy wall channel to reveal the effect of stratification on turbulent structures. Low-Mach-number approximation was applied to treat density variation. The iso-surface of instantaneous temperature in the middle of the channel shows that the effect of stable stratification is strong and greatly reduces turbulence there. On the other hand, instantaneous structures and two-point correlations near the wavy wall suggest that characteristic near-wall structures and their scales are not strongly affected by the extent of stratification in spite of considerable reduction of turbulent intensity.

A Study on Coherent Structures of Turbulence in Reacting Jet

Large eddy simulation was performed for a turbulent jet of diluted hydrogen. Treated were the case where hydrogen reacts with surrounded oxygen and the case where only mixing occurs without reaction. Coherent structures of turbulence in these cases were investigated. In reacting case, temperature rise causes significant drop of flow density, and, thus, velocity is two times as higher as the counterpart at the same position of non-reacting case. In spite of dramatic change in order of velocity magnitude, there is close similarity between two-point correlation of reacting case and that of non-reacting case. In two cases, coherent structure inclined against jet central axis was suggested through distribution of two point correlation and PDF of vorticity fluctuation vector.