[S-VC30] Mitigation of volcanic disasters - basic and applied researches

[SVC30-10] Preliminary study on the effects of buildings on ashfall distribution in small areas

*Kiyotoshi Otsuka¹, Arihide Nobata¹, Hitoshi Suwa¹, Tomohiro Kubo², Yousuke Miyagi³ (1.Obayashi Corporation, 2.Mount Fuji Research Institute Yamanashi Prefectural Goverment, 3.National Research Institute for Earth Science and Disaster Resilience)

Keywords:ashfall , building functional damage, numerical calculation, ash particle concentration, ashfall depth, air conditioning

Widespread ashfalls brought about by explosive eruptions of volcanos can seriously impacts on cities which are situated far from the volcanos (distal area) and have ashfall much smaller than that collapsing houses due to ash load. It is said that significant nonstructural or functional damages can be induced even with light to moderate ashfalls. Severity of ashfall-induced functional damages are primarily dependent on the ash amount and the damage estimations are usually made based on the ash depths predicted by some appropriate atmospheric ash dispersion models which, in many cases, employ computational grid having horizontal resolutions of the order of kilometers¹⁾. However, since the surfaces of the urbanized areas are highly nonuniform due to the existence of ground objects such as buildings, the ashfall within narrow areas can be deviated from the prediction of large-scale ash dispersion models where the ground surface within individual computational grid cells are assumed to be uniform. In the present study, the influence of buildings on the spatial variation of the ashfall distribution are investigated through numerical calculation.
The numerical code used here consists of an incompressible Reynolds-averaged computational fluid dynamics code with the standard k-ε turbulence closure combined with the conservation equations of momentum and concentration of ash particles. A rectangular building of 32m(E-W)×60m(N-S) ×24m(Height) in size having a pent house (12m(E-W)×16m(N-S) ×6m(Height)) at the north-east corner of the roof of it is place at the center of the bottom of the innermost computational grid of the three-fold nested grids. The horizontal and the vertical grid resolutions of the innermost grid is 2m and 1m, respectively, near the building. The ash with unit concentration is fallen from the top boundary of the computational domain at the size-dependent terminal fall velocity and the relative variations of the airborne ash concentration is calculated. The ash also inflows from the lateral boundaries when wind exist. The simulation with the wind of moderate strength (5m/s at 10m above ground) shows that the influence of the building on the distribution of airborne ash concentration is apparent. The area with low ash concentration extends leeward of the building while a high concentration area is formed in front of the upwind side of the building. The extent of the low ash concentration area in the lee side is longer/shorter for smaller/larger particles, while the high concentration area in the upwind side is more pronounced for larger particles than smaller ones. The results suggest that the non-negligible influences of the spatial inhomogeneity of the ashfall rate on the ash ingress into the air conditioning, ash accumulation on the ground, and so on, are expected when the deformation of wind field by buildings are considered. In the presentation in the annual meeting, the results with multiple buildings will also be presented.