Forest fires are of common occurrence all over the world, which cause severe damages to valuable natural resources and human lives. In the recent California Fire, which burned 300,000 hectors of land, the disaster danger could reasonably be predicted, but early control of fires by means of aerial fire fighting might have been failed in that situation. Also in Japan, there are similar problems in the aerial fire fighting. Most forest fires occur in the daytime and the fires are freely in progress without any control during the nighttime. Therefore, it is important to attack the fires when there is daylight. The water dropped by helicopters is not always sufficient to control fires, since the quantity of water that can be carried aloft is a critical issue. Large amount of water can be dropped from aircrafts, but the high-speed flight of aircrafts may be dangerous in the mountain, where tall trees and steel towers with electric wires may exist. Therefore, those aircrafts have to fly at much higher altitudes than helicopters, while the water drop at high altitudes changes water into mist in the air. The objective of this study is to examine the methods to prevent the ignition by firebrands in the downwind area by applying water through the aerial fire fighting. However, tests by real aircrafts to obtain such information would be too costly. Therefore, the patterns of water drop from aircrafts were examined in CFD simulations, together with the investigation of needed water drop rate based on the forest fire statistics, the previous real aircraft tests and laboratory experiments. It has been found in the simulations that the water supply with the water density of 2 L/m2 is effective to control fires and the patterns of dropping water are reasonable.
Investigation of mechanisms leading to water drop breakup at Mach 4.4 and Weber numbers above 105
