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

Abstract

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=1.15×10^4, 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(superscript +)=Y(superscript +) 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 C(subscript fN) and Re(subscript N), where C(subscript fN) 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.