Onset of Underflow in Slightly Diverging Channels

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Abstract
Experiments are conducted in a horizontal laboratory channel with slightly diverging walls to simulate buoyancy driven plunging flow. A temperature difference between inflow and downstream ambient provides the density differential that initiates the underflow. Temperature profiles and flow visualization are used to determine flow conditions in the plunge region. Channel divergence angles (half‐angles) from 1–15° are investigated. Volumetric flow rates in the sinking flow increase from 0–45% during the plunging process. Maximum entrainment rates are nearly linearly related to divergence angle. Depths of the underflow downstream from the plunge line are from 0.65–0.9 times the depth at the plunge line. Densimetric Froude numbers from 0.56–0.89 occur at the plunge line, depending on the channel divergence angle and the discharge densimetric Froude number. A flow analysis that explains and supports the experimental data is presented. Channel expansion affects the plunging process significantly, particularly entrainment rates and densimetric Froude numbers at plunging.

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