摘要
論文題目：以三維大尺度渦流模擬法模擬平板混合紊流場
研究生：劉育政
指導老師：張克勤

本文以大尺度渦流模擬(LES)計算平板混合紊流場，並以熱線測速儀量測之資料作為比對，同時亦以非線性RANS模式Gatski model之模擬結果置入結果中相互比較，評估兩者適用性。
首先以實驗值之資料為入口條件，使用(1)dynamic Smagorinsky model (2) dynamic kinetic transport model作為本文模擬之兩個紊流模式，以模擬流場由紊流狀態發展至自我保存(self-preserving)之流場行為。結果發現雷諾應力(uu、uv)於近入口段仍有大尺度運動之截面處，LES模擬結果與實驗結果仍有差距，由於本文以等向性渦流法去推估入口瞬時速度，但在上游仍有大尺度運動；因而造成假設與實際物理狀況不符，因此找出非等向性之入口邊界條件則變得重要；但在其他截面上之速度統計量、紊流動能、混合長度以及雷諾應力分佈在LES模擬結果有不錯的符合度。本文也歸納出格點尺寸必須比流場中最小泰勒尺度小一個量級，以確保達到格點獨立。雖然LES演算法較RANS耗時，但是LES模式於混合紊流場之求解是較佳之紊流模擬方法。 關鍵字：大尺度渦流模擬法、平板混合紊流場

ABSTRACT

Subject: Numerical Studies of the Mixing-layer flow using 3D Large Eddy Simulation
Student: Yu-Cheng Liu
Advisor : Keh-Chin Chang

Turbulent mixing-layer flow is simulated using the large eddy simulation (LES) model, and their prediction are compared with both the available corresponding experimental measurements, which were performed with the hotwire anemometry , and the predictions obtained with a nonlinear RANS model. Two versions of (1)dynamic Smagorinsky and (2)dynamic kinetic transport sub-grid models are investigated in association with the measured inlet conditions. It is found that the performances made with LES, as compared to the experimental data, are not satisfactory, particularly in the developing regions. It is speculated that the assumption of vrms=wrms set in FLUENT code may not work for the cases of mixing layer. It is also concluded the order magnitude of grid size used for LES has to be smaller than that of the minimum Taylor length scale in the flow field. Overall, although the computational expenditure made with LES is higher than that with RANS, LES has a better potential to make more satisfactory flow predictions in the flows inherent to large scale motion such as the mixing layer.

Keywords：large eddy simulation , mixing layer

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FLUENT：http://hpce.iitm.ac.in/Manuals/Fluent_6.3/fluent6.3/help/html/ug/main_pre.htm