Keisuke Himoto

Transport Of Disk-shaped Firebrands In A Turbulent Boundary Layer

A transport model for a disk-shaped firebrand in 3D space has been formulated for the purpose of spotting simulation. In the model, firebrand transport is described by solving the conservation equations of momentum and angular-momentum, simultaneously. The airborne firebrand receives the aerodynamic forces and moment from the surrounding fluid, which varies in time and space due to the change in its location and orientation. An approximate form of the Navier-Stokes equations appropriate for low-Mach number flow was applied for the fluid motion. LES (Large Eddy Simulation) on buoyant flow downstream of a square heat source was carried out, and transport behaviors of various firebrands were investigated. Then, a scaling dimensionless parameter * B for the transport was derived. Numerically obtained mean travel distance to the windward direction, as well as its s.d. (standard deviation), were correlated with * B with reasonable accuracy.

A Physically-Based Model for Urban Fire Spread

An attempt is made to develop a physically-based model for simulating urban fire spread. In the model, urban fire is regarded as an ensemble of multiple building fires. The model consists of two sub-models, i.e. the model to predict the building fire behavior under the exposure of heating from other building fires and the model to predict the thermal environment caused by building fires. The building fire model is based on single zone method, applying control volumes to compartments in a building. When the external heating, whether it is from the same building or from other buildings, exceeds the critical heat flux, the fire load in the compartment ignites and burns. For the thermal environment model, thermal radiation and fire-induced plume are considered as the factors of building-to-building fire spread. The model is applied to a fictitious urban district where 49 multi-room buildings are arrayed in a simple configuration.

A Post-Earthquake Fire Spread Model considering Damage of Building Components due to Seismic Motion and Heating of Fire

KEISUKE HIMOTO, KYOSUKE MUKAIBO, RYO KURODA, YASUO AKIMOTO, AKIHIKO HOKUGO, and TAKEYOSHI TANAKA Disaster Prevention Research Institute Kyoto University Gokasho, Uji, Kyoto 611-0011, Japan Graduate School of Science and Technology Ritsumeikan University Noji-Higashi, Kusatsu, Shiga 525-8577, Japan Ichijo Corporation Kiba, Kohtoh, Tokyo 135-0042, Japan Mitsubishi UFJ Research and Consulting Imabashi, Chuo, Osaka 541-0042, Japan Research Center for Urban Safety and Security, Kobe University Rokkodai 1-1, Nada, Kobe 657-8501, Japan

Risk and Behavior of Fire Spread in A Densely-built Urban Area

A fire staring in a densely-built urban area easily spreads to adjacent buildings. In the case of large earthquakes in which multiple fires may break out simultaneously, the spread of such fires may overwhelm the ability of firefighters and damage large areas. In this study, fire spread simulations were carried out in order to investigate behavior of fire spread in Kyoto Higashiyama area, one of the representatives of densely-built urban areas in Japan. A physics-based model was used in which the following mechanisms of fire spread are considered: thermal radiation heat transfer from fire-involved buildings; elevation of ambient temperature by wind-blown fire plumes; and spotting ignition by firebrands. The risk of fire spread was also analyzed by the Monte Carlo method to evaluate the expected magnitude of loss. The effect of uncertainty of the location of fire origin and the weather conditions was investigated.

A Physics-Based Model for Post-Earthquake Fire Spread Considering Damage to Building Components Caused by Seismic Motion and Heating by Fire

The prototype model previously developed by the authors was improved in order to simulate the behavior of fire spread in an earthquake-affected urban area. In the new model, seismic motion and heating by fire are both considered as the causes of damage to building components. The damage affects the burning behavior of a fire-involved building, as well as the behavior of building-to-building fire spread. For validation of the new model, a simulation of the fire spread that followed 1995 Kobe earthquake was conducted. The behavior of the fire spread obtained by the numerical simulation was compared with the observed data. Reasonable agreement was obtained with regard to the number of burned buildings.

Validation of a Potential-based Evacuation Model of City Residents in Post-earthquake Fire

Evacuation of a large number of residents is conceivable in case of urban fires following a large earthquake in a city. It is essential to implement effective evacuation measures for ensuring residents’ safety in the regional disaster prevention plan. We have been developing a simulation model based on a potential method for the evacuation behaviors of city residents in a post-earthquake fire. The model has been validated by comparing the evacuation behaviors of Tokyo City residents in the Kanto Earthquake Fire, where the distribution of fatalities calculated by this model was qualitatively similar to that reported by the survey of that time. In this paper, the evacuation behaviors of Tokyo City residents in the Kanto Earthquake Fire were simulated for validating the prediction function of this model in terms of the traveling trajectory of an evacuee.

Evaluation of Temperature Rise Under an Eave Due to Flame Impingement: Toward the Mitigations of Fire Spread Risk in Japanese Historic Urban Areas

Flame spread along eaves has been one of the major causes of fire spread for Japanese historic urban areas. For the fire safety assessment of such an environment, this paper investigates basic characteristics of heat transfer under eaves due to flame impingement. A series of model experiment was conducted measuring temperature rise and incident heat flux under nonflammable eaves with varying parameters on burner geometry, burner installation position, and fuel supply rate. Flow behavior under an eave was analyzed following the approach of the classical plume theory. The analysis result showed that the temperature rise is constant in the flaming regime and decreases proportionally to the distance to the power of −1 in the nonflaming regime. Reasonable correlation was obtained with the experimental results; the correlation was better when using the distance along trajectory from the burner as the reference length scale, rather than the distance from the impingement point.

A Model for the Spatio-temporal Distribution of Population using Country-Wide Statistical Data and Its Application to the Estimation of Human Exposure to Disasters

Extent of fatality due to natural disaster depends largely on spatial distribution of population at the moment disaster occurs. In this study, a computational model for estimating day-long spatio-temporal distribution of commuters (workers and students) was developed using a number of country-wide statistical data of Japan such as “population census”, “survey on time use and leisure activities”, and “economic census”. The model estimates behavior of individual commuters by considering their attributes including gender, occupation, place of work (or school), and place of residence. As a case study, the proposed model was applied to the Keihanshin (Kyoto-Osaka-Kobe) metropolitan area, one of the largest metropolitan areas in Japan. Estimated maximum number of commuters unable to return home in an expecting earthquake scenario was between 1.1 and 1.9 million depending on the assumptions on traffic disruption following disaster and the maximum walkable distance of each commuter.

Modeling of Recognition Degree of Refuge Areas by Kyoto City Residents in Post-earthquake Fire Event

In this study, a model for recognition degree of refuge areas by Kyoto City residents in post-earthquake fire event was developed based on the data obtained by an interview survey. When a large earthquake hits a city in Japan, a tremendous number of evacuees may wander around in urban area escaping from hazards due to conflagrations following the earthquake. We have been developing a simulation model for city evacuation of residents in post-earthquake fire as a tool to evaluate effectiveness of evacuation safety measures. For the further refinement, it is important to accurately estimate evacuation destination of residents considering bias of recognition degree of refuge areas. The interview survey was carried out at 104 spots on busy streets in Kyoto City and replies from 2,267 residents were collected from 1 September 2009 to 30 November 2009. In the interview, the names of open-spaces recognized as evacuation destination were asked having the subjects imagine that conflagrations had taken place in urban area. The governing factors of recognition degree of refuge areas were identified and evacuation destination of residents was formulated by adopting a probabilistic model.