|摘要: ||本文以直接模擬蒙地卡羅(Direct Simulation Monte Carlo,DSMC)法計算二維超極音速稀薄氣體中自由剪力層之流場狀況及速度脈動相關函數。在自由流為氮分子氣體,且上下流溫度(固定為 273K)與密度均相同之條件下,首先觀察自由流分子數密度為每立方公尺10/sup 21/個分子,高速自由流馬赫數20,速度比為0.6之流場的結構。接著分別討論高速自由流馬赫數10、速度比為0.6下,改變密度(自由流分子數密度每立方公尺 10/sup 20/、10/sup 21/、10/ sup 22/個分子)造成之流場變化及自由流密度同為每立方公尺10/sup 20/個分子,速度比相同(0.6)但速度不同(高速自由馬赫數10、15、20)造成之流場變化。其次探討自由流密度同為每立方公尺10/sup 20/個分子,高速自由流馬赫數20,速度比為0.3、0.45及0.6下自由剪力層結構之差異。最後並與連性紊流自由剪力層之結果相較。計算結果發現:1.流場密度不同時,將其尺寸對自由流之平均自由徑(mean free path)作無因次化後,流場變化之結構完全相同。2.不同流速但相同速度比之流場,其混合層之厚度及成長十分相近。3.混合層中之動量脈動相關量之分布趨勢與連性紊流混合層中者一致。但稀薄氣體之動量傳遞中,由速度脈動所形成之傳遞量約為分子黏滯性所形成之動量傳遞量的七成左右,此現象顯然比連性紊流中者為低。|
The direct simulation of Monte Carlo method was employed to calculate the flow structures, such as mean velocities, temperatures, pressures and mean quantities of various order of momentum fluctuations correlations of a free shear layer at hypersonic speed. The free stream temperature was kept as 273K for all cares of calculations. The free stream velocities (of the higher one stream) considered were M/sub .inf./=10, 15 and 20. The free stream densities considered were 10/sup 20/, 10/sup 21/ and 10/sup 22/ per m/sup 3/. The velocity ratio chosen as r=0.3, 0.45 and 0.6 for the present calculations. From the comparisons of above mentioned conditions of the rarefied parallel flow structures it is found that the effect of the changes of free stream density on the various flow properties was monotonously as the length scale nondimensionalized by the mean free path of the free stream. For different M/sub .inf./ with the same velocity ratio r the calculated shear layer growth were almost the same for M/sub .inf./. The tendency of the distributions of various order of fluctuation correlations in the mixing layer was the same as that of the continuum one. The momentum transfer due to mean fluctuation correlations was about 70% of that due to molecular viscosity. This is obviously lower than that in the case of continuum shear layer.