具凸塊障礙物之矩形渠道紊流結構之實驗探討

An Experimental Investigation of the Turbulence Structure of a Block-Mounted Rectangular Channel Flow

本文利用雷射都卜勒測速儀研究具凸塊障礙物之完全擴展渠道流在Re=11500，BR=0.5，及W/H=2 狀況下之紊流結構。相關流場結構之主要擾動參數包括軸向平均速度、紊流強度、紊流動能及雷諾剪應力等均詳加探討。本研究採用折射率配合之流體以使近壁處之雷射光束不致扭曲。實驗結果顯示共有四個特徵迴流包出現在凸塊四周。最大迴流包出現在凸塊後方之主迴流區，此迴流包始於凸塊後緣，附著於渠道底板上，附著長度為6.6H。主迴流區內軸向平均速度不遵循對數律之速度分佈；然而在近壁之層流次層區域，U+=Y+ 之關係仍然成立。附著點之後，近壁流體之軸向平均速度漸趨向對數律之速度分佈。在主迴流區內，摩擦係數CfN 與雷諾術數ReN 有強烈相關性；CfN 在0.01~0.04 之間變化，較一般完全擴展紊流之邊界層者大。另一方面，軸向紊流強度之分佈在凸塊周圍差異甚大；此意味著常於紊流數值計算上所假設之紊流等向性用之於本研究或類似之流場並不適當。全域之紊流強度、流動能及雷諾剪應力之極大值發生於凸塊頂部約X/H=0.5~1.2 之截面；而次極大值則發生於主迴流區附著點前約2~3H 之位置。 This study examines in detail the turbulence structure in a fully-developed turbulent channel flow with a block mounted on one principal wall via laser-Doppler velocimetry (LDV) at Re=1.15× 104, BR=0.5, and W / H=2. The variations in major fluctuating parameters throughout the flow, including the axial mean velocity, turbulence intensity, turbulent kinetic energy and Reynolds stress, are examined. A refractive-index-matched fluid is adopted during the test to permit access to the near-wall region without distorting the laser beam. The experimental results indicate that four circulating bubbles exist around the block, thereby characterizing the flow structure. The largest circulating bubble is located behind the block (the main recirculation region); the bubble begins at the trailing edge of the block and reattaches itself onto the bottom wall of the channel with a reattachment length of 6.6H. In the main recirculation region, the axial mean velocity distributions do not obey the log-law. However, the viscous-sublayer linear relationship U+=Y+ is maintained when the fluid is sufficiently near the channel wall. After departing from the reattachment point, the axial mean velocity of the fluid gradually approaches the log-law distribution while in the near-wall region. In the main recirculation region, there is a strong correlation between CfN and ReN, where CfN varies in the range of 0.01~0.04, being higher than that in a normal turbulent boundary layer. The axial turbulence intensity of the fluid is distributed differently around the block. This implies that the assumption of isotropic flow in the analysis of the turbulent flow over a protruding block(s), although widely used in numerical computations for turbulent flows, is no longer suitable. The global maximums for turbulence intensity, turbulent kinetic energy, and Reynolds stress all occur in the top region of the block at stations about X / H=0.5~1.2; the second maximums for these parameters all appear at stations about 2~3H before the main reattachment point.