Quantification of ember accumulation on the rooftops of isolated buildings in an ember storm

Abstract

Results are presented from a series of small-scale wind tunnel experiments to improve understanding how embers accumulate on isolated building rooftops. Non-combusting model embers were introduced into a wind tunnel and the embers that stayed on the roof were collected and weighed. The results indicate that only a small fraction of embers (typically less than 10%) remain in contact with a building rooftop compared to the mass that would have fallen on the building footprint were the building not there. Embers landed at a broad range of locations on the model rooftops. Some of those that landed did not remain in contact with the rooftop but were blown off. The vast majority that did stay on the roof moved from their landing location to the rooftop internal corners where they came to rest. The accumulation rates measured were very sensitive to the building shape and wind angle as both these parameters control the flow separation over and around the building that in turn controls the ember's path and stability. Tests run for the same geometry at different wind speeds indicate that there is a critical wind speed above which embers will no longer accumulate on the roof top.