本研究以數值模式來探討二維方柱體位於非均勻流中的渦流逸散現象與其所受之升力與阻力，研究的目的在探討剪力效應、柱體至邊牆的間隙與雷諾數等參數對渦漩逸散現象、升力與阻力的影響。非均勻流況包括邊界層流與定剪力流，定剪力流雖然在自然界中極少發生，但透過剪力流場之探討可進一步釐清剪應力對於柱體尾流之影響，因此本研究以定剪力流來探討剪應力與渦散之關係。同時揚棄傳統的時間平均分析法，改以相位平均法來研究柱體尾流之紊流速度場，希冀量化出渦流逸散之時變特性，探討速度梯度對渦流逸散的影響。 本研究亦結合數值模式與風洞實驗以相互驗證，其結果可使吾人對柱體尾流有定量之分析，進而增加對非均勻流中渦流逸散現象的瞭解，並可提供正確的升力和阻力係數供工程設計之用，以降低天然災害對工程設施的危害。計算結果顯示：(1) 當迫近流為均勻流時，流場呈上下對稱，但有剪力效應時，柱體上下方的流場會受到迫近流速度梯度的影響而有不對稱的現象，且剪力參數越大，不對稱現象越明顯；(2) 隨著迫近流的剪力參數愈大，柱體上下方渦峰值呈非線性的增加；(3) 剪力參數b值增加，史徹荷數St、阻力係數Cd及升力係數之均方根值CL¢有遞減的趨勢；(4) 當雷諾數Re>104，流場不再受雷諾數的影響，但史徹荷數因剪力參數增加而有些微下降的趨勢。 The wake formation behind a bluff body has been widely studied for over a century. However, most previous studies concentrated on flow conditions without side walls and with uniform velocity and low turbulence intensities. But some practical flow situations may involve objects located close to a wall and velocity distribution which are non-uniform. In this way, the bluff body is under the influence of a shear flow and proximity effect of a solid wall. This phenomenon has not been thoroughly studied. This study employed Large Eddy Simulation to investigate the wake flow around a two-dimensional square cylinder placed in constant shear flows. The influences of velocity gradient and gap ratio on the vortex shedding, drag and lift coefficients were studied in this research. The experimental results revealed that the turbulence intensity, turbulent kinetic energy and vorticity all have asymmetric distribution in the near-wake region. Also, the difference between the upper and lower vorticity is larger than the upstream vorticity. Furthermore, the peak vorticity has a periodic variation, but decays as the downstream distance increases.
