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[SVC35-P16] On inhomogeneity of ashfall distribution in an urbanized area
Keywords:Ashfall Simulation, Urban Area, Effects of Building
In densely built-up areas in cities, ashfall distribution can be inhomogeneous to a certain extent even though the ash falls from aloft rather uniformly because buildings disturb the flows of air and ash particles. Estimations of ashfall-induced damages are usually made based on some predicted ashfall amounts provided by the large-scale atmospheric ash dispersion-sedimentation simulations1) with the horizontal resolution of the order of kilometers. However, variation of the ashfall over the spatial scales smaller than those of the horizontal grid of the large-scale models will not be negligible in assessing the ashfall impacts on the areas covered with narrowly standing buildings and houses.
In the present study, we will investigate the influences of buildings on small-scale variations of ashfall in the urban area through numerical ashfall calculation with spatial resolution high enough to roughly resolve individual buildings. A small, really existing densely built-up area with low-rise buildings is chosen as the test area of the present simulation. The three-dimensional rectangular computational domain is 200m and 280m in lengths in the E-W and N-S directions respectively with horizontal resolution of 2m, while in the vertical direction, grid spacing is 1m from the ground to the altitude of 40m and from there smoothly increase toward the top boundary of 450m above ground. The present model consists of the Eulerian form of the equations of motion of ash particles and the transport equations of the spatial concentration of the airborne ash including gravitational sedimentation and aerodynamic drag between the ash particles and air. The ash of unit spatial concentration flow into the domain through the upper and the upwind lateral boundaries at the velocities equal to the vector sum of the wind and the terminal fall velocities.
The ashfall calculation are performed for sixteen wind directions with respect to the particle diameters of 62.5, 125, 250, 500, 1000 and 2000μm. It is found from the calculation that the non-uniformity of the distributions of the ashfall rate at the ground as well as the airborne ash concentration are more to be pronounced as the diameter decreases. This will be due to the tendencies that small particles respond more quickly to the changes in the airflow near the buildings than large particles. The results also show that more than 10 to 20% of the area of the ground, the ashfall rates exceed 1.2, while they exceed 1.4 for several percent of the ground area. As to ash concentration, similar values are obtained for the grid points in the air adjacent to the building surface. Note that it is the airborne ash concentration close to the building surfaces that primarily affects the performances of the air conditioning equipment because air intakes are usually placed on the building walls or on the roofs. From the present results, it is suggested possible for the ashfall related quantities of larger than the large-scale values by a factor of 1.4-1.5 to occur locally in cities.
1)Dare, R.A., 2015, CSIRO, CAWCR Tech. Rep. No.79 34pp.
In the present study, we will investigate the influences of buildings on small-scale variations of ashfall in the urban area through numerical ashfall calculation with spatial resolution high enough to roughly resolve individual buildings. A small, really existing densely built-up area with low-rise buildings is chosen as the test area of the present simulation. The three-dimensional rectangular computational domain is 200m and 280m in lengths in the E-W and N-S directions respectively with horizontal resolution of 2m, while in the vertical direction, grid spacing is 1m from the ground to the altitude of 40m and from there smoothly increase toward the top boundary of 450m above ground. The present model consists of the Eulerian form of the equations of motion of ash particles and the transport equations of the spatial concentration of the airborne ash including gravitational sedimentation and aerodynamic drag between the ash particles and air. The ash of unit spatial concentration flow into the domain through the upper and the upwind lateral boundaries at the velocities equal to the vector sum of the wind and the terminal fall velocities.
The ashfall calculation are performed for sixteen wind directions with respect to the particle diameters of 62.5, 125, 250, 500, 1000 and 2000μm. It is found from the calculation that the non-uniformity of the distributions of the ashfall rate at the ground as well as the airborne ash concentration are more to be pronounced as the diameter decreases. This will be due to the tendencies that small particles respond more quickly to the changes in the airflow near the buildings than large particles. The results also show that more than 10 to 20% of the area of the ground, the ashfall rates exceed 1.2, while they exceed 1.4 for several percent of the ground area. As to ash concentration, similar values are obtained for the grid points in the air adjacent to the building surface. Note that it is the airborne ash concentration close to the building surfaces that primarily affects the performances of the air conditioning equipment because air intakes are usually placed on the building walls or on the roofs. From the present results, it is suggested possible for the ashfall related quantities of larger than the large-scale values by a factor of 1.4-1.5 to occur locally in cities.
1)Dare, R.A., 2015, CSIRO, CAWCR Tech. Rep. No.79 34pp.