The effects of turbulence intensities and scales on vortex shedding from a square cylinder in homogeneous turbulent flows

本文探討逼進亂流場中，亂流強度（u'/U）與亂流長度尺度（Lx/D）對二維方柱尾流渦散發展之影響。實驗所需之亂流強度（ 1 ％∼ 18 ％）與亂流尺度（ .88 ∼ 4.42 ），係使用不同尺寸之柵板，配合不同模型斷面所達成。 實驗風速之對應雷諾數為 2.7 × 1 �捸C 本實驗以方柱體側風面前緣之壓力孔，量取其表面之擾動壓力。同時以方柱體後並列之兩根熱膜探針， 量取其尾流速度場。 從量測點求得之頻譜（ powerspectral density ）和頻相關（ coherence ）中顯示出二維方柱尾流的發展主要是受逼進流場之亂流強度與尺度的影響。亂流強度增加，消弱渦漩結構並使渦漩形成區在更下游處形成；亂流尺度對渦漩結構有消弱之作用，但渦漩形成區並無移動之現象產生。This paper is devoted to the study of the development of the vortex shedding behind a square cylinder placed in homogenous turbulent flows with different intensities (u'/U:1%-18%) and length scales (Lx/D:.88-4.42). The Reynolds number corresponding to the mean approaching velocity and the width of the cylinder was 2.7 × 1 ��. The pressure fluctuations located on the lateral side near the sharp front edge of the cylinder were measured through a pressure tap. The velocity fluctuations were simultaneously detected by a pair of side by side hot film probes in the wake of the cylinder. The power spectral density and coherence of the pressure and velocity fluctuations revealed that the development of the vortex shedding of the square cylinder was mostly affected by the turbulence intensity and length scale of the approaching flow. When the turbulence intensity increases, the strength of the vortex is reduced and the formation of the vortex occurs further downstream. The turbulence scales also reduces the vortex strength. However, the location of the formation of the vortex remains unchange.紙