3:30 PM - 3:45 PM
[SVC27-06] Influences of buildings and ground objects on ashfall distribution in built-up areas in cities
Keywords:Ashfall, Road, Buildings, Cities, Simulation
Effects of buildings and ground objects on the ash distribution in cities are investigated through performing numerical ashfall simulations. Two narrow, densely built-up rectangular districts having different layout of intra-city major roads are considered as the simulation areas. The one of which is 400m x 400m in area and has a four-lane wide road passing through the district curving rather sharply on its midway by about 20 deg. The other of which is 200m x 550m in size and has north-south and east-west directed two major roads crossing almost perpetually to each other about the center of the district, where a four-lane wide viaduct, which runs north-south direction and bridges over the east-west road, of 450m in length and 10m in height, equipped with 3.5m tall noise-proof walls on its both sides, is crudely but explicitly expressed in the numerical simulations. In the present study, ash accumulation characteristics on road and adjoining facilities are among our present concerns, because they play important roles in rescuing as well as recovering activities in the times of ashfall events. Buildings and inter-building ground spaces including roads and streets are discretized on the rectangular computational grid cells with the minimum horizontal and vertical grid spacing respectively of as small as 2m and 0.5m.
The numerical code used here consists of the Eulerian form of the equations of motion of ash particles and the transport equations of the spatial concentration of airborne ash with gravitational sedimentation and aerodynamic drag force between ash particles and air1) being included. The equations are all particle size dependent. With the code, distributions of the airborne ash concentration and ash fall on the ground on windy days are calculated. In the calculation, the ash of unit concentration is assumed to flow into the domain through the upper and the upwind lateral boundaries with the velocities equal to the vector sum of the approaching horizontal wind and the particle’s terminal fall velocities. The six particle diameters of 62.5, 125, 250, 500 and 2000μm are considered.
The results indicate the existence of surface roughness elements such as buildings significantly affects the ashfall on the ground in cities, displaying rather strong dependences on particle sizes and wind directions. Changes of the directions of road by the degree of magnitude considered here can lead to producing quite different ash accumulation conditions on the road between before and after passing the curve. There also found features implying the occurrence of significant differences in ash accumulation on the sloped approaches of the viaduct depending on whether the slope is directed windward or leeward, whose tendency is more to be pronounced as particle size decreases. These are the affairs we face in ashfall events. The implication of airborne ash concentrations in the context of mitigating ash induced functional damages of buildings will also be referred to in the presentation.
The numerical code used here consists of the Eulerian form of the equations of motion of ash particles and the transport equations of the spatial concentration of airborne ash with gravitational sedimentation and aerodynamic drag force between ash particles and air1) being included. The equations are all particle size dependent. With the code, distributions of the airborne ash concentration and ash fall on the ground on windy days are calculated. In the calculation, the ash of unit concentration is assumed to flow into the domain through the upper and the upwind lateral boundaries with the velocities equal to the vector sum of the approaching horizontal wind and the particle’s terminal fall velocities. The six particle diameters of 62.5, 125, 250, 500 and 2000μm are considered.
The results indicate the existence of surface roughness elements such as buildings significantly affects the ashfall on the ground in cities, displaying rather strong dependences on particle sizes and wind directions. Changes of the directions of road by the degree of magnitude considered here can lead to producing quite different ash accumulation conditions on the road between before and after passing the curve. There also found features implying the occurrence of significant differences in ash accumulation on the sloped approaches of the viaduct depending on whether the slope is directed windward or leeward, whose tendency is more to be pronounced as particle size decreases. These are the affairs we face in ashfall events. The implication of airborne ash concentrations in the context of mitigating ash induced functional damages of buildings will also be referred to in the presentation.