LESによる高密度市街地の建物高さの非一様性が風環境に及ぼす影響の分析(その１)：市街地抗力及び主流方向の運動量輸送量の定量的評価

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  • LES ANALYSIS OF IMPACT OF NON-UNIFORMITY OF BUILDING HEIGHT IN HIGHLY DENSE URBAN DISTRICT ON WIND ENVIRONMENT (PART 1): QUANTITATIVE EVALUATION OF DRAG FORCE OF URBAN DISTRICT AND STREAMWISE MOMENTUM TRANSPORT
  • LES ニ ヨル コウミツド シガイチ ノ タテモノ タカサ ノ ヒイチヨウセイ ガ カゼ カンキョウ ニ オヨボス エイキョウ ノ ブンセキ(ソノ 1)シガイチ コウリョク オヨビ シュリュウ ホウコウ ノ ウンドウリョウ ユソウリョウ ノ テイリョウテキ ヒョウカ

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<p> In recent years, high dense cities such as Tokyo and Hong Kong are facing serious problems regarding thermal comfort and air quality of outdoor space caused by the poor ventilation in cities. Urban ventilation has been regarded as an effective countermeasure against such problems in many studies. The majority of previous attempts for improving urban ventilation have focused on the methods to lead the horizontal wind inside the urban area by reducing building coverage ratio, that is, to secure “horizontal ventilation paths”. However, it is almost impossible to considerably reduce building coverage ratio in already developed high dense urban districts in cities such as Tokyo and Hong Kong. In the urban districts along the coastal areas, it is more realistic to allow sea breeze above the urban canopy layer to blow down to the ground level, that is, to secure “vertical ventilation paths”. In recent years, the non-uniformity of building height has been attracting attention because of its effect to create “vertical ventilation paths”. Thus, the non-uniformity of building height has the effect of improving the quality of outdoor environment by increasing the wind velocity at the pedestrian level within a focused urban district. However, the urban district with non-uniform building height causes more momentum loss of streamwise wind than an urban district with uniform height. Consequently, this leads to decreased wind velocity in leeward areas of the focused urban district. In this study, Large-eddy Simulations were applied to flowfields in two urban district models consisting of rectangular buildings with uniform and non-uniform heights.</p><p> The result of the case with uniform building heights (Fig. 6(1)) showed that the incoming airflow was moving along the top of the buildings and only little airflow was going downward to the area near the ground. On the other hand, the result of the case with non-uniform building heights (Fig. 6(2)) indicated that circulating flow regions were formed above the low-rise and mid-rise buildings, and the airflow moved down to the area near the ground in front of the high-rise buildings along the circulating flows. In section 3.3, the total drag force of buildings of two cases was evaluated as a summation of form drag caused by the pressure difference between windward and leeward sides of the buildings (Eq.(7)) and friction caused by the shear stress on the building and ground surfaces (Eq.(9)). It was confirmed that the total drag force of urban district was drastically increased by the effect of the non-uniformity of building height. This was mainly caused by the increase of the form drag on upper parts of high-rise buildings. In section 3.4, the streamwise momentum balance equation was derived to evaluate the streamwise momentum transport and the contribution of each term of momentum transport equation to the total streamwise momentum transport. From these results, the influence of non-uniformity of building height on the streamwise momentum transport was quantitatively evaluated. As shown in Fig. 15, streamwise momentum transport decreased by the effect of drag force of buildings, and the decrease of streamwise momentum transport in the case with non-uniform building height was about 3.75 times of that in the case with uniform building height, in this study.</p>

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