An Experimental Study of the Effect of a Baffle on the Flow Structure in a Rectangular Open Channel Using UVP

摘要

A flow structure around an intermediate standing baffle in a rectangular open channel has been investigated experimentally. The instantaneous vertical velocity components were successfully measured using an Ultrasonic Velocity Profiler (UVP). Various spatial distributions such as profiles of the vertical time-averaged velocity and relative turbulent intensities at various vertical measuring lines around the baffle indicate how the flow structure changes from up- to downstream of the baffle. At the baffle's upstream they indicate the flow structure of the uprising flow. But behind the baffle indications of vortex shedding and flow separation such as the prominent peak values in the relative turbulent intensity profiles is observed. Also, spatio-temporal distributions of the vertical velocity at up- and downstream sections confirm the existence of periodic change of flow direction near the edge of the baffle at its downstream which can be attributed to the vortex shedding from the baffle edge. In addition, space-dependent power spectra indicate the existence of some peak structures near the baffle edge height at its downstream. For these sections existence of peak values in the space distribution of two frequency modes could be confirmed corresponding to the vortex shedding due to the existence of the baffle. Furthermore, by using multi-line method and multiplexer the peak of the absolute value of the normalized two-point cross-correlation coefficients between vertical velocity fluctuations could be obtained to evaluate the effect of the baffle on the degree of correlation between vertical velocity fluctuations at upstream points with that of downstream ones. It has been found quantitatively that a baffle acts as a barrier wall and causes the degree of correlations to be decreased significantly from the vicinity of its edge height to the channel bed. Also existence of a local peak region in between the baffle edge height and free surface was found in color maps of degree of correlation. It was found that the degree of correlation decreases gradually from the peak region to the proximity of the baffle edge height. Also the decrease in the degree of correlation from that peak to the free surface was captured which can be attributed to the effect of free surface. Thus, flow around the baffle can be characterized into two regions with very different characteristics.