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    Title: 紊流度對分離及再接觸流流場效應探討
    Other Titles: Freestream turbulence effects on flow characteristics of separated and reattached flows
    Authors: 陳增源;Chen, Tzeng-yuan;施建宇;Shih, Jian-yu
    Contributors: 淡江大學航空太空工程學系
    Keywords: 三維分離及再接觸流;再接觸長度;紊流度;Three-dimensional separated and reattached flow;Reattachment length;Turbulence intensity
    Date: 2008-10
    Issue Date: 2010-08-09 19:28:17 (UTC+8)
    Publisher: 台南市:中國航空太空學會
    Abstract: 本研究利用背階管道及紊流器使管道背階入口爲具紊流度發展流流況,實驗探討在層流、過渡流及紊流雷諾數下(介於500至18000),背階入口流場紊流度對三維分離及再接觸流再接觸長度、再接觸長度橫向分佈及近背階壁面紊流度分佈效應;研究中以雷射測速儀作三維向穩態及擾動速度量測,利用圓柱體紊流器產生管道背階入口流場紊流度。研究結果顯示背階入口紊流度對再接觸長度效應包括(1)縮短再接觸長度,尤其在過渡流時縮小迴流區範圍可達50%,(2)明顯降低過渡流、紊流起始雷諾數,使過渡流與紊流提早發生,(3)明顯縮小過渡流雷諾數範圍;紊流度愈大效應更明顯。對再接觸長度橫向分佈效應包括(1)造成較差之對稱性,(2)造成管道中央附近再接觸長度有明顯變化,(3)最小再接觸長度往管道中央移動,(4)再接觸長度由管道中央往側壁方向先縮短再變長之趨勢更爲明顯,且雷諾數愈大,最短再接觸長度愈靠近側壁。對近背階壁面流場紊流度效應,未明顯改變其分佈趨勢,紊流時最大紊流度仍約產生在流場再接觸位置附近,層流時無明顯最大紊流度位置,分佈較無變化;但是,在紊流時於迴流區內、外則分別增加及降低近壁面流場紊流度,層流時則在迴流區內外皆增加近壁面流場紊流度。
    The research experimentally investigates the freestream turbulence effects on flow characteristics of three-dimensional separated and reattached flows. A backward-facing step duct and cylinder-type turbulators are used to generate different levels of turbulence on the inlet freestream. The Reynolds numbers range from 500 to 18000, which cover the laminar, transitional and turbulent flows. The measurements include the reattachment length, the spanwise distributions of reattachment length, and the turbulent intensity distributions near the step-wall using laser Doppler velocimetry. Results of this study show that the freestream turbulence effects on the reattachment length are to (1) shorten the reattachment length, especially in transitional flow regions, (2) reduce the Reynolds numbers for the onset of transitional and turbulent flows, i.e., cause the transitional and turbulent flows occurring early, (3) reduce the Reynolds number range of the transitional flow. The freestream turbulence effects on the spanwise distribution of reattachment length are to (1) cause some loss of the symmetry of the spanwise distribution of reattachment length with respect to the duct center, (2) cause the reattachment length varied in the duct center region, (3) move the location of minimum reattachment length toward the duct center, (4) make the spanwise distribution of reattachment length decreasing and, then, increasing more clearly approaching the sidewall. Results of this study also show that the freestream turbulence generally does not change the distributions of near-wall turbulent intensity. The maximum near-wall turbulent intensity occurs approximately at reattachment under turbulent flow conditions, while the streamwise distributions of near-wall turbulent intensity are generally constant under laminar flow conditions. However, the freestream turbulence increases and decreases the near-wall turbulent intensity inside and outside of the recirculation zone, respectively, under turbulent flow conditions, and increases the near-wall turbulent intensity both inside and outside of the recirculation zone under laminar flow conditions.
    Relation: 航空、太空及民航學刊系列 B = Journal of Aeronautics, Astronautics and Aviation, Series B 40(2), pp.85-91
    Appears in Collections:[Graduate Institute & Department of Aerospace Engineering] Journal Article

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