Eunmo Koo

Firebrands and spotting ignition in large-scale fires

Spotting ignition by lofted firebrands is a significant mechanism of fire spread, as observed in many large-scale fires. The role of firebrands in fire propagation and the important parameters involved in spot fire development are studied. Historical large-scale fires, including wind-driven urban and wildland conflagrations and post-earthquake fires are given as examples. In addition, research on firebrand behaviour is reviewed. The phenomenon of spotting fires comprises three sequential mechanisms: generation, transport and ignition of recipient fuel. In order to understand these mechanisms, many experiments have been performed, such as measuring drag on firebrands, analysing the flow fields of flame and plume structures, collecting firebrands from burning materials, houses and wildfires, and observing firebrand burning characteristics in wind tunnels under the terminal velocity condition and ignition characteristics of fuel beds. The knowledge obtained from the experiments was used to develop firebrand models. Since Tarifa developed a firebrand model based on the terminal velocity approximation, many firebrand transport models have been developed to predict maximum spot fire distance. Combustion models of a firebrand were developed empirically and the maximum spot fire distance was found at the burnout limit. Recommendations for future research and development are provided.

Fire spread in chaparral – a comparison of laboratory data and model predictions in burning live fuels

Fire behaviour data from 240 laboratory fires in high-density live chaparral fuel beds were compared with model predictions. Logistic regression was used to develop a model to predict fire spread success in the fuel beds and linear regression was used to predict rate of spread. Predictions from the Rothermel equation and three proposed changes as well as two physically based models were compared with observed spread rates of spread. Flame length–fireline intensity relationships were compared with flame length data. Wind was the most important variable related to spread success. Air temperature, live fuel moisture content, slope angle and fuel bed bulk density were significantly related to spread rate. A flame length–fireline intensity model for Galician shrub fuels was similar to the chaparral data. The Rothermel model failed to predict fire spread in nearly all of the fires that spread using default values. Increasing the moisture of extinction marginally improved its performance. Modifications proposed by Cohen, Wilson and Catchpole also improved predictions. The models successfully predicted fire spread 49 to 69% of the time. Only the physical model predictions fell within a factor of two of actual rates. Mean bias of most models was close to zero. Physically based models generally performed better than empirical models and are recommended for further study.

Visualization and Analysis of Vortex-Turbine Intersections in Wind Farms

Characterizing the interplay between the vortices and forces acting on a wind turbines blades in a qualitative and quantitative way holds the potential for significantly improving large wind turbine design. This paper introduces an integrated pipeline for highly effective wind and force field analysis and visualization. We extract vortices induced by a turbines rotation in a wind field, and characterize vortices in conjunction with numerically simulated forces on the blade surfaces as these vortices strike another turbines blades downstream. The scientifically relevant issue to be studied is the relationship between the extracted, approximate locations on the blades where vortices strike the blades and the forces that exist in those locations. This integrated approach is used to detect and analyze turbulent flow that causes local impact on the wind turbine blade structure. The results that we present are based on analyzing the wind and force field data sets generated by numerical simulations, and allow domain scientists to relate vortex-blade interactions with power output loss in turbines and turbine life expectancy. Our methods have the potential to improve turbine design to save costs related to turbine operation and maintenance.

Large-Eddy Simulations of Air Flow and Turbulence within and around Low-Aspect-Ratio Cylindrical Open-Top Chambers

AbstractThe flow around cylindrical open-top chambers (OTCs) with aspect ratios (i.e., height-to-diameter ratios) much less than unity is investigated using a large-eddy simulation (LES) model. The solid structures are represented using the immersed boundary method, and the ambient flow in which the OTCs are embedded is representative of a turbulent atmospheric boundary layer. Results from the LES model show that the flow inside OTCs depends strongly on the height of the chamber wall. In particular, as chamber height increases the flow impinging on the upstream wall is deflected more in the vertical direction, a stronger recirculation flow develops inside the chamber, turbulence intensities are greater, and there is stronger vertical transport and mixing within the OTC, even at or near the ground. For low wall heights (i.e., very low aspect ratios), however, the flow impinging on the OTC is only diverted weakly in the vertical direction; aside from a small recirculation zone inside the OTC near the upstre...