Please use this identifier to cite or link to this item:
Experimental and Numerical Studies of Flow Fields in Tandem Arranged Square Cylinders
|關鍵字:||tandem square cylinders;縱列雙方柱;reattachment flow;hysteresis;cavtiy flow;critical spacing;jump flow;再回附流場;遲滯;穴蝕流場;臨界間距;跳躍流場||出版社:||機械工程學系||摘要:||
本文以方柱體為實驗模型，針對不同攻角的單一方柱及縱列雙方柱體流場進行實驗研究與數值模擬的流場分析。實驗研究主要是量測尾流場頻率響應、柱體表面壓力、阻力計算、流場可視化觀測、縱列雙方柱遲滯(hysteresis)流場的探討。數值模擬使用大尺度渦度模擬(Large-Eddy Simulation)的紊流模型來求解奈維爾-史托克方程式(Navier-Stokes equations)，研究的焦點在於探討縱列雙方柱流場模態轉變時的動態特性。
實驗在開放式風洞中進行，在單一方柱的流場實驗中，著重在低雷諾數的範圍(Re = 2000-21000)。對於Re > 5300，史卓赫數(Strouhal number)隨攻角的增加呈現相似的趨勢，並且在攻角約13o，史卓赫數出現突增的情形，代表完全分離的流場轉變為再回附流場。當再回附流場發生時，方柱下表面將產生強大的壓力回復現象。對於Re = 2000-3300，史卓赫數的最大值約在17o。在這個攻角附近，由壓力的量測得知其存在著較弱的壓力回復現象。另外，再回附流場的特性更進一步經由對方柱下表面的壓力頻譜分析，得知其為低頻低能階的特性。
在縱列雙方柱流場實驗方面，雷諾數範圍為2000-16000，間距範圍為L/D = 1.5-9.0，其中L為上下游方柱形心的距離，D為方柱邊長。當以增加間距及減少間距兩種方式改變L/D時，發現存在路徑相依的現象伴隨著兩個不連續的跳躍點，分別出現在增加及減少間距的路徑，不連續跳躍點的出現則代表流場型態的改變。在流場發生轉變之後，並不會如同雙穩定流場般再回復到前一流場的型態產生流場模態的交替現象。遲滯的特性，在本研究中以阻力、史卓赫數、表面壓力及流場可視化來進行，並利用尾流場的速度訊號的時序結果及頻譜來說明。
數值模擬中，針對縱列雙方柱的流場以二維不可壓縮流場的假設進行，雷諾數為Re = 100、500及2700。另外，單一方柱在研究中用來測試使用方法的試用性，經由與實驗結果及過去文獻的比較，具有相當高的流場準確性。在Re = 500間距L/D = 3.0，由渦度等值的時序圖發現了類穴蝕流場的存在，而在間距L/D = 3.5時則為臨界間距。研究中進一步使用速度相位圖來探討流場的動態特性，結果發現流場的轉變與“糾結”現象有關。本研究中並未發現雙穩定流場的存在，並且由研究結果得知，在三維的流場中可能將會有更複雜相位圖產生而需更進一步的研究。
The flow characteristics around a single and two square cylinders in a tandem arrangement were examined experimentally and numerically. The vortex shedding frequency, unsteady surface pressures, drag coefficients, flow visualizations and the hysteresis phenomenon were carefully investigated in the experiments. In the numerical simulations, a general numerical package, Fluent, was employed with LES (Large-Eddy Simulation) to solve the continuity and Navier-Stokes equations. The numerical simulation is focused on the dynamic characteristics of flow, in particular, the change of the flow pattern with the variation of spacing between the two square cylinders in a tandem arrangement.
The experiments were performed in an open type wind tunnel with a square test section of 305㎜×305㎜ and 1100㎜in length. In the study of flow around a single square cylinder, the Reynolds number ranges between 2000-21000. For Reynolds numbers greater than 5300, the Strouhal number shows a similar trend with changing angle of incidence; that is a rapid rise in Strouhal number occurs at an angle of around 13o. The jump in Strouhal number was found to be associated with onset of the flow reattachment, bringing in a strong pressure recovery on the lower side face of the cylinder. For lower Reynolds number 2000 to 3300, the maximum Strouhal number occurs at a relatively higher angle of 17o. Around this angle, the pressure measurements exhibit a rather weak pressure recovery, suggesting a less firm shear-layer reattachment to the side face of the cylinder. The nature of the reattachment flow was further examined by spectral analysis of the fluctuating pressure coefficients measured on the lower side face of the cylinder.
The flow around two square cylinders in a tandem arrangement was examined experimentally for the Reynolds number ranging from 2.0×103 to 1.6×104 and the spacing L/D = 1.5-9.0, where L is the distance between the centre of the cylinders and D is the dimension of the cylinder. Hysteresis with two discontinuous jumps occurred for all of the Reynolds numbers when the spacing was varied in two different ways, one is being a progressive increased and the other a progressive decreased. The hysteresis is associated with two different flow patterns, and the discontinuity is attributed to a sudden change of the flow pattern. In the hysteresis regime, only one flow pattern is stable, differing from the bistable phenomenon that involves intermittent switching between two stable flow patterns as reported for higher Reynolds numbers by previous studies. The time history and spectra of velocity as well as the flow visualization corresponding to the change of flow pattern in the hysteresis regime are presented and discussed in detail. The effects caused by hysteresis and Reynolds number on drag forces, mean and fluctuating pressures, and Strouhal numbers of the cylinders are examined.
The numerical simulation was carried out for two-dimensional, incompressible flow for Re = 100, 500 and 2700. Prior to the simulation of two cylinders in a tandem arrangement, the flow over a single cylinder was examined and checked with the present experimental results as well as previous numerical simulations. In the study of tandem cylinders, the flow field of cavity mode was observed at Re = 500 as the spacing was set at L/D = 3.0 slightly below the critical value. At the critical spacing L/D = 3.5 for the same Reynolds number, the flow changes from reattachment to jump flow. Moreover, the velocity phase diagrams are presented to describe the dynamic response of the wake with variation of spacing. It is found that the change of the flow pattern is related to the “knot” phenomenon. The dynamic behavior reported in the present study suggests that a three-dimensional flow may lead to much richer variations in phase diagram. Furthermore, bistable phenomenon was not found in the present simulation.
|Appears in Collections:||機械工程學系所|
Show full item record
TAIR Related Article
Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.