Experimental Investigation of the Dip Effect in Rectangular Channel

Abstract

Two different data sets (LDA, propeller) were used to observe the occurrence of the dip effectin rectangular open channel flow for aspect ratios 1.5  B/H  12. The measured andcalculated velocity distributions are compared and three different situations are detected,depending on the magnitude of the aspect ratio (B/H). When B/H  5 no dip effect wasobserved and the measured distributions along the cross section were in very good agreementalong whole depth with the distributions as calculated by the entropy equation. When B/H < 2the calculated distributions diverge from the measured ones for all vertical lines along thecross section due to the effects of free surface and sidewall. The dip effect for 2≤ B/H <5 overthe cross section is found only for verticals near the sidewall, where equation applicability isrestricted. Herein, empirical correction for the entropy velocity distribution is proposed toaccount for dip effect. For validation propeller measurements were used. Relative velocityerrors by adjusted equation were found to be only 32% of that by the original entropyequation for the near-surface (z/H  0.75) region.