摘 要

　　本文探討平板混合紊流場的控制參數(r,Reθ)對流場平均及擾動特性的影響,藉由調整兩側高低速度差可分別顯現出各參數的效應。於低速度比的上游截面中,仍存在一大尺度渦流結構流場主導之層流區域,於此區域時的流場特性與呈紊流狀態之下游截面,有截然不同的特性。亦由於此顯著的差異造成不同控制參數(r,Reθ),其側向擴散率以及往下游發展的趨勢會隨之改變。
　　利用FFT(Fast Fourier Transform)與自相關性(Autocorrelation)函數的分佈趨勢來比較不同控制參數(r,Reθ)在不同截面上渦流結構所受到的影響。本文以Frenkiel 經驗式與Altinsoy et al.(2002)提出之修正式來近似自相關性函數,觀察於不同流場條件時最佳近似之循環參數m及m*值。雖然Frekiel經驗式在tau=0的位置不合乎物理現象(斜率不為零),但相較於Altinsoy et al.提出之修正式,更能代表自相關性函數的整體趨勢。對照本文與黃士軒(20 -02)使用Frenkiel經驗式整理出之自相關性函數結果,發現在剪力流區的最佳遞迴參數m值,應介於1~1.5之間。
　　當流場經過一定之下游距離會發展到自我保持的狀態(self-preserving),到達此狀態時,流場特性經適當無因次化後即不會隨下游而變化,此發展完全之距離也會因為控制參數(r, Reθ)的不同而有所差異。於自我保持狀態,Townsend(1976)提出一經驗公式可近似最大雷諾應力值,由本文結果可知,經驗式中的實驗係數會隨控制參數(r,Reθ)而變。故使用此經驗式時要先找出實驗係數與控制參數間相對關係,才能得到較好的近似結果 。

Abstract

　　Influences of two controlling parameters (r,Reθ) on the flow structure of a planar mixing layer are investigated. It is found that there exists a large-scale vortex structure in the very upstream regions under the conditions of low r values. Their flow characteristics in these regions are obviously different form what are observed in the downstream regions. Therefore, the lateral expansion and devel -oping tendency along the flow direction would vary significantly with adjusting the controlling parame -ters.
　　Influences of the controlling parameters on the flow structure are analysed using the Fast Fourier Transform (FFT) and the auto-correlation distribution function. Two empirical functions of the autocorrelation distribution including the Frenkiel func -tion and the one currently proposed by Altinsoy et al. (2002) are used to fit the measurements.Although the Frenkiel function cannot meet the symmetric condition at tau=0, i.e. the slope at tau =0 must be equal to zero, it can represent the overall auto-correlation distribution better than what was proposed by Altinsoy et al. It is found that the loop parameter, m, used in the Frenkiel function varies between 1 and 1.5 in the shear layer, which is consistent what were observed in the previous work done by S. H. Huang (2002).
　　Flow would reach its self-preserving condition after traveling a sufficiently distance. However, the distance required to reach the self-preserving condition is dependent on the parameters of r and Reθ. Townsend (1976) argued that the maximum Reynolds stress under the self-preserving condition can be well represented by an empirical formulae. However, this study found that the coefficient appeared in the empirical formulae is dependent on the parameters of r and Reθ too.

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