Choked Flows through Short Contractions

This paper presents a study on choked flows through short, lateral contractions in subcritical open channels. A new discharge equation is developed based on the conservation of energy and experimental data with a wide range of opening ratios (ratio of contracted width-to-total width). The theoretical derivation accounts for the critical flow conditions that take place in the contraction under choking conditions. The effects of opening ratio sigma, encroachment structure shape, inlet angle alpha , and relative contraction length L* on discharge coefficients are considered. The coefficients of the newly developed discharge equation are determined using a total of 186 sets of choking experiments conducted by various researchers. The analysis shows that the discharge coefficient is mainly affected by sigma and may vary between 0.29 and 0.54 for the range of sigma values used in the study (0.12-0.88). It is shown that the discharge coefficient (correspondingly the discharge) can vary up to 16% for encroachment-structure shape changing from sharp corner to rounded corner conditions-up to 11% for alpha going from 30° to 90° and up to 6% for L* ranging from 0 to 1.33. This study extends the traditional choking analysis to severe contractions. The proposed equation fits previous experimental data well and can be used to predict either the water discharge through a contraction for a given upstream depth or the water depth upstream from a contraction corresponding to a given discharge under choking conditions.