INVESTIGATION OF REPRODUCTION OF SNOW DISTRIBUTION ON A TWO-LEVEL FLAT-ROOF BUILDING

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  • ２段屋根建物上の積雪分布の再現性の検討
  • 2段屋根建物上の積雪分布の再現性の検討 : CFDを用いた風による屋根雪偏分布の予測
  • 2ダン ヤネ タテモノ ジョウ ノ セキセツ ブンプ ノ サイゲンセイ ノ ケントウ : CFD オ モチイタ カゼ ニ ヨル ヤネユキヘンブンプ ノ ヨソク
  • Prediction of unbalanced snow distribution due to wind on roofs using CFD
  • CFDを用いた風による屋根雪偏分布の予測

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Abstract

<p> Unbalanced snow accumulation resulting from wind flow represents a difficult problem when predicting snow loads on building roofs. Accumulation occurs because of the complex interaction between snow particle motion and fluid flow, which is affected by building geometry. Recently, computational fluid dynamics (CFD) was applied to the prediction of snowdrift around buildings in several studies. However, few studies have applied CFD to the prediction of roof snow. In this study, the CFD simulation method of the snowdrift around buildings as previously proposed by the authors was applied to the prediction of roof snow accumulation on a two-level flat-roof building. The performance of the prediction model was validated by comparing the results of field observations and wind tunnel experiments. In particular, the importance of the effects of snow distribution change on the flow field was investigated.</p><p> The snowdrift model was implemented in ANSYS FLUENT 14.5 as a user-defined function. To model the suspension of snow, the transport equation for drifting snow density was solved. The deposition and erosion fluxes on the snow surface were determined as functions of the friction velocity at the snow surface. The proposed snowdrift model was applied to the prediction of snow distribution on the two-level flat-roof building placed perpendicular to the approaching wind flow. The building model used in this study was adopted from the detailed field measurements obtained in Hokkaido, Japan. In this study, the following two cases were considered: (a) lower roof located on the windward side (windward case) and (b) lower roof located on the leeward side (leeward case).</p><p> First, the velocity vectors obtained from the wind tunnel experiment for the same configurations as well as the streamlines obtained by the present CFD were compared for each case. For the windward case, two vortices, i.e., a recirculation flow resulting from separation at the upwind corner and a standing vortex near the roof step, were observed on the lower roof. For the leeward case, considerable recirculation flow on the lower roof was present. The general patterns of the flow field were very similar between the experiment and CFD. Next, the snow depth ratios obtained by a single CFD, in which the effects of snow distribution change on the flow field were not considered, were compared with the field measurements. For the windward case, the peak of the snow depth near the upwind edge of the lower roof, which was not observed in the measurements, was observed in the CFD results. This peak appeared in CFD and was attributed to the recirculation flow at the windward corner of the lower roof. The snow depth on the lower roof was generally underestimated in CFD because of the large erosion caused by the strong separation flow. Furthermore, a large undulation near the roof step, which was observed in the measurements, was not reproduced in CFD. For the leeward case, CFD also underestimated the snow depth and failed to reproduce a large undulation near the roof step. Finally, a phased CFD, in which the snowdrift calculations were conducted stepwise by considering snow depth change on the flow field, was applied to the same target. The underestimation of snow depth observed in the single CFD was clearly improved by the phased CFD. The prediction accuracy of the large undulations near the roof step was also improved, and it was confirmed that reproducing the separation flow near the upwind corner of the building correctly by considering the snow depth distribution on the roof is essential.</p>

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