Abstract
Up until now it has been believed that there is a control section at the crest of a weir even in an unsteady flow.
In this study, an attempt is made to analyze theoretically various concepts of a control section operative (present) in hydraulic systems under unsteady conditions.
Verification of the results were made by numerical simulation using the characteristic method. In this analysis, however, for simplisity, it was assumed that the flow is one dimensional and non viscous. Also in an unsteady flow the critical section (where u=√gh) at the crest is called the “control section”.
The following results were obtained:
(1) When a negative small surge runs over the crest, the control section moves accordingly downstream near the crest. In the case of a positive surge, the section moves upstreamnear the crest. These relationships are represented by Eq.(25).
(2) In an unsteady flow, the position of the control section is these different from that of the minimum specific energy section.(see Table 1)
(3) A small disturbance occurring near the downstream part of the control section can be transferred in the upstream direction as shown in Fig. 6. In other words, the state of the flow between points xD and xC in Fig. 3 influences the upper flow profile.
Based on these findings, control section concepts in steady hydraulic systems do not seem particularly significant in unsteady hydraulics.
Based on the results of this study, the so-called “control section” under steady conditions cannot be applied directly to an unsteady flow.
