Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/2104
標題: 軸對稱突張或突縮流場之數值模擬分析
Numerical Analysis of an Axisymmetric Sudden Expansion or Sudden Contraction
作者: 劉彥宏
Liu, Yen-Hung
關鍵字: Turbulence Model
紊流模式
Numerical Dissipation
Limit Cycle
Pulsating
Sudden Expansion
Sudden Contraction
Cavity
數值消散
極限環
脈動流
突張
突縮
空穴
出版社: 機械工程學系
摘要: 摘 要 本文利用Standard Model或RNG-derived- Model兩種紊流模式分析突張流場之速度分佈並與實驗數據作比較。 配合標準壁函數或阻尼函數(Wall Damping Function)在突張近壁速度分佈預測比較。本文計算流體力學軟體-PHOENICS,它在數值方法採用混合體系(Hybrid Scheme)、全隱性體系(Fully-Implicit Scheme)及SIMPLEST求解程序來求得流場的數值解。經由數值結果比較得知RNG-derived- Model或阻尼函數較能符合Dellenback et al.[30]實驗的軸向速度分佈曲線。這是由於RNG-derived- Model利用控制ε的產生量來提供較佳的軸向速度分佈結果。阻尼函數因阻尼函數公式調整得當而能得到近壁較佳的速度分佈預測。本文又利用突張流場的阻抗係數證明雙方程式所使用的μt在大突張比(ER>2.0)之下會產生數值消耗(Numerical Dissipation)的現象。 又本文利用RNG-derived- Model模擬突張具有間歇性(Intermittent)入口速度( )、突縮紊流流場及消音器(Silencer)的流場分析。本文在突張具有間歇性入口速度時,所得數值結果利用極限環說明準週期震盪的現象。從極限環相圖中可以發現到簡單吸引子(Simple Attractor)的存在,並從圖中可證實突張具有間歇性入口速度的流場系統為一保守結構。 在突縮比為2.0~4.0下,突縮流場之能損隨著突縮比的增大而增大。突縮流場在紊流與層流下,軸心速度有不同的分佈趨勢。在紊流時,開始進入突縮流場時的軸心速度變化不大,當經過突縮斷面後因為產生紊流擾動的關係使得軸心速度有變小的趨勢。在層流時因壁面摩擦的關係所以軸心速度呈現上升的趨勢,經過突縮斷面後則因無紊流擾動的關係,所以軸心速度仍呈現上升趨勢。當在雷諾數為 、突縮比為2.0時,可發現到流場有過衝(Overshooting)的流場現象。這是由於流場流經突縮斷面產生壓力降,為了滿足質能守恒定律,所以在迴流區產生過度反應現象。 消音器(Silencer)流場在穩態模擬時假設入口速度為固定大小的均勻流場。因為壓力分佈不平均造成類似突張或突縮流場的迴流現象,所以較無法表現出空穴所具有捲出渦漩的流場現象。所以本文利用暫態模擬來表現出消音器具有脈動性入口速度( )時之流場,因流體流經膨脹室受剪力層與主流擴散或被擠壓後會造成類似於空穴流場的流場結構。 關鍵字:紊流模式、數值消散、極限環、脈動流、突張、突縮、空穴
Abstract This study was utilized two turbulence models and two near wall models to analyze an axisymmetric of sudden expansion. Two turbulence models are standard k-epsilon model and re-normalization group analysis method derived- model. The standard wall function and wall damping function were used to treat wall boundary. From the numerical results, RNG and wall damping function are agreed with the experiment data of Dellenback et al. [30] because RNG controls the production rates of epsilon to get better axial velocity profiles. Wall damping function was adjusted empiric formulations to get better agreement of experiment data than standard wall function, but it cost much time to calculate the physical properties of flow field. Then we use the resistance coefficient of abrupt expansion to proof turbulent viscosity may cause numerical dissipation phenomena in a large expansion ratio (ER>2.0). This study also was used re-normalization group analysis method derived- model to analyze abrupt expansion had intermittent inlet velocity, sudden contraction in the turbulent situation and silencer flow filed. We showed the limit cycle phase diagrams in the sudden expansion had intermittent inlet velocity ( ) and explained pseudo-period oscillation. From the phase diagrams, we could find simple attractor and illustrate the sudden expansion was coincided to conservative structure. In contraction ratio was from 2.0 to 4.0, the energy loss was increased when contraction ratio became large. Sudden contraction in turbulent and laminar flow situations had different axis velocity profiles. In turbulent situation, axis velocity had unchanged when fluid entered the sudden contraction channel; but the axial velocity became smaller when fluid flowed across abrupt contraction section because the effect of turbulent fluctuating produce energy loss. At Reynolds number was and C=2.0, overshooting behavior was show in sudden contraction case when the fluid flow it across contractive section. In the steady state situation, the recirculation zones of expansive type silencer similar to the recirculation zones of abrupt expansion or contraction due to the disequilibrium of pressure distribution. In the case of unsteady situations, it assumed that the pulsating inlet velocity of the expansive silencer was because the fluid entered expansive silencer chamber and the flow filed caused dividing streamline and irregular pressure distribution. From the present numerical results, it illustrated that the vortexes of the expansive silencer similar when its fluid flow across the cavity and rolled up vortexes. Keywords: Turbulence Model, Numerical Dissipation, Limit Cycle, Pulsating, Sudden Expansion, Sudden Contraction, Cavity.
URI: http://hdl.handle.net/11455/2104
Appears in Collections:機械工程學系所

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