本研究以風洞實驗探討穩態風場和陣風風場中，三維矩柱、平板和模型樹所受風力之特性，實驗所使用的力平衡儀可以量測物體所受到的縱向力和側向力的瞬時變化。由實驗結果可以計算得物體的平均風力係數、風力係數之均方根和史徹荷數，矩柱的渦漩逸散頻率可由側向力資料經由以跨零點的方式計算得。本研究並採用希爾伯特-黃(HHT)的經驗模態分解法(Empirical Mode Decomposition, EMD)找出渦漩逸散的內建模態函數(Intrinsic Mode Function, IMF)分量，其結果和傅立葉頻譜所找出的渦散頻率比較，發現以上方法求得的矩柱在窄迎風面狀況下，史徹荷數都相當接近0.08，符合前人研究之穩態均勻流況的結果。陣風風場的實驗結果：阻力係數較穩態風場中為小，顯示紊流強度增強，阻力係數減小；矩柱在寬迎風面與窄迎風面狀況下之史徹荷數與穩態風場中接近。本研究之結果可以瞭解穩態風場與陣風風場中物體所受風力的影響，可提供相關工程參考。 This study experimentally investigates the wind force on three-dimensional rectangular prism, plate and tree model in a steady flow and periodically varying flows. The experiments were carried out in an atmospheric boundary layer wind tunnel. This wind tunnel is equipped with a gust generator that can generate periodically varying flows of adjustable frequency. The along wind force and cross wind force were measured by a high frequency force balance. Based on the force measurement, the mean drag, mean lift, rms drag, rms lift coefficients were calculated. The frequency of vortex shedding was determined by the method of cross-zero, Fourier spectrum and Hilbert-Huang Transform (HHT). The results agree with each other and previous studies in a steady flow. The drag coefficient decreases, the rms drag and lift increase under gusty flow. The Strouhal number is closed to those in a steady flow.
