具有兩倍直徑比與不同間距之錯列圓柱下游流場特性之研究

Abstract

本論文利用質點影像測速系統(PIV)、雷射都普勒量測系統(LDV)、流場可視化、快速傅立葉轉換(FFT)等進行分析，探討當雷諾數為1000，入流速度5cm/s，具有兩倍直徑比(D/d=2)之錯列圓柱在不同垂直間距(T/D=2、1.5、1)及水平間距(-3.5≦L/D≦3)下游流場結構變化。研究結果顯示:流場結構大致可分為兩大類:當間隙流偏向大圓柱時，此流場定義為Mode1；反之，當間隙流偏向小圓柱時，此類流場定義為Mode2。兩大類流場依照不同流場特性可再細分如下:在Mode1流場中，當間距為T/D=2，-3.5≦L/D≦-1.5，及T/D=1.5，-3.5≦L/D≦-2.5時，此流場結構呈現寬、窄域尾流各自脫離。當間距為T/D=1，-3.5≦L/D≦-1，因間隙流偏向大圓柱，小圓柱的寬域尾流C渦漩與窄域A渦漩結合為A+C渦漩渦旋並與B渦漩往下游流場移動；其中在-3.5≦L/D≦-3時，小圓柱的位置剛好位於大圓柱度擾動最大處，使的小圓柱尾流完全被抑制。當間距為T/D=1.5，-2.5≦L/D≦-1時，會有大圓柱尾流牽引小圓柱尾流脫離的情況發生。當間距為T/D=1，-2.5≦L/D≦1時，在近域區內會有寬域尾流切斷窄域尾流的情況發生。在T/D=1，L/D=0處為Mode1及Mode2轉換處，會有兩種不同方向的切斷方式，其一為小圓柱後方寬域尾流切斷大圓柱下游的窄域尾流，屬於Mode1流場；其二為大圓柱寬域尾流切斷小圓柱下游的窄域流尾，此為Mode2流場。在Mode2流場中，當間距為T/D=2，-1≦L/D≦0，及T/D=1.5，-0.5≦L/D≦1時，在大圓柱的下剪力層會有Bn渦漩結合的情況發生。當間距為T/D=2，0≦L/D≦3時，寬、窄域尾流各自脫離；當間距為T/D=1.5、1， 1≦L/D≦3時，隨著垂直間距縮小，間隙流偏向角度增加使得間隙流推擠窄域尾流的情況更明顯。

關鍵字:錯列、鈍體、間隙流、雙圓柱、不等直徑

Abstract This study investigates the flow characteristics using Particle Image Velocimetry (PIV), Laser Doppler measurement system (LDV), flow visualization and fast Fourier transform (FFT). The Reynolds number is 1000, the inflow velocity is 5cm/s, and the diameter ratio of staggered cylinders is 2.0. The staggered cylinders are arranged at three vertical spacing (T/D=2, 1.5, 1) with the horizontal spacing varying within -3.5 ≤ L/D ≤ 3. The flow structures behind the staggered cylinders can be divided into two main categories. First of all, as the gap flow biases toward the large cylinder, it is defined as Mode 1; while the gap flow biases toward the small cylinder, the flow is defined as Mode 2. According to the spacing arranged, the flow can be divided into several kinds. For the Mode 1, while T/D=2, L/D=-3.5 ≤ L/D ≤ -1.5 and T/D=1.5, -3.5 ≤ L/D ≤ -2.5, the wide and narrow wakes of each cylinder shed nearly independently. For T/D=1, -3.5 ≤ L/D ≤ -1, the C vortex of small cylinder (wide wake) and the A vortex of large cylinder (narrow wake) merges and move downstream with B vortex. For -3.5 ≦ L / D ≦ -3, the wake behind small cylinder is completely suppressed. For T/D=1.5, -2.5 ≤ L/D ≤ -1, the wake flow of large cylindrical interacts strongly with that of small cylinder. For T/D=1, -2.5 ≤ L/D ≤ 1, the narrow wake is terminated by the wide wake. In case of Mode 1, the narrow wake of large cylinder is truncated by the wide wake of small cylinder; and vice versa in the case of Mode 2. Further, in cases of Mode 2, for T/D=2, -1 ≤ L/D ≤ 0, and T/D=1.5, -0.5 ≤ L/D ≤ 1, the inner shear layer of large cylinder rolls up a series of vortex Bn, induced by the formation of the vortices C and D of the narrow of small cylinder. For T/D=2, 0 ≤ L/D ≤ 3, the wide and the narrow wakes shed nearly independently. In cases of T/D=1.5, 1, 1 ≤ L/D ≤ 3, the biased angle of gap flow increases and the degree of squeezing of the narrow wake becomes more severely.

Keywords: staggered, blunt, tip clearance flow, double cylinder, unequal diameter