在車輛行進中，側向強風可能造成行進中車輛翻覆及打滑的危險，台灣及世界各國均曾發生行進中車輛受側風吹襲打滑及翻覆的意外事故等。本研究使用三維的大渦流模式來計算高架橋上行進中車體周遭之流場及車體所受之側向風力與升力，並探討橋梁兩側護欄上裝設透風式防風牆對行車安全的影響。車體之風力係數模擬結果與Baker et al. (1990)、Cheli et al. (2011)之實驗結果及Bettle (2003)之數值結果比較，擋風牆後方的模擬風場則與風洞實驗之數據驗證，以增加數值模式的可信度。 本研究利用數值模式探討橋梁兩側護欄上設置透風式擋風牆對於行進中車輛的保護效果，研究流況包括三種不同孔隙率(0, 0.233, 0.485)的擋風牆及三種不同高度(1 m, 2 m, 3 m)與無擋風牆，並比較車體在迎風車道與背風車道受側向強風之影響，配合車輛動力學與行車安全分析，評估不致產生打滑及翻覆的臨界風速以供道路設計參考。結果顯示沒有擋風牆的狀況下，迎風車道與背風車道車輛打滑之臨界風速小於30 m/s，車輛翻覆之臨界風速則為43.1 m/s，表示車輛打滑的機率較大，在中度颱風(32.7 m/s ~ 50.9 m/s)便可能使車輛有打滑。在比較迎風車道與背風車道的模擬結果發現，由於迎風車道受不透風水泥護欄保護效果較大，使得迎風車道上車輛的側向力係數較背風車道上的車輛為小。比較擋風牆高度為2 m及3 m，車體的側向力係數相近，表示當擋風牆高度大於2m時，車輛已受到相當的保護效果。在擋風牆高度大於等於2 m，孔隙率小於等於0.485的情況下，可提升車輛翻覆之臨界風速至69.3 m/s以上，表示車輛翻覆的風險大幅減小。在車輛打滑方面，在擋風牆高度較低( 1 m)或是擋風牆孔隙率較高( 0.485)的情況下，打滑臨界風速小於30 m/s，顯示會有打滑的危險，建議將擋風牆高度提升至2 m以上，且使用孔隙率較低( 0.485)的擋風牆。This study used a three-dimensional Large Eddy Simulation (LES) model to investigate the crosswind effect on a moving vehicle. The model prediction of the side force and lift coefficients compared favorably with the experimental results of Baker et al. (1990) and Cheli et al. (2011) for vehicles on the ground. The simulation results of the wake flows behind porous windbreaks were verified by the results of wind tunnel experiments. Then the validated numerical model was used to investigate the effect of porous windbreaks for the protection of moving vehicles on a bridge. The flow conditions include three different windbreak heights (1 m, 2 m, 3 m) and three different porosity (0, 0.233, 0.485). The numerical results show that the side force coefficient for vehicle on the windward lane is smaller than that on the leeward lane, because of the impermeable barrier on the bridge. The predicted side force and lift coefficients of the vehicle were used to evaluate the critical wind speed for vehicle on the windward and leeward lanes to slide and to rollover. The results revealed that the critical wind speed for vehicle to slide and to rollover is less than 30 m/s and 43.1 m/s without windbreak. This means the risk of vehicle to slide is higher than vehicle to rollover. When the porous windbreak (porosity 0.485) is higher than 2 m, the critical wind speed VWc to rollover reaches 69.3 m/s. Furthermore, the critical wind speed VWs for vehicle to slide will be larger than 30 m/s when the windbreak height is higher than 2 m, and there is a minor difference between the 2 m and 3 m windbreak.
