Unsteady flow conditions at dam bottom outlet works due to air entrainment during gate closure : Berg River dam model

[摘要] ENGLISH ABSTRACT: A trial closure of the emergency gate of the Berg River Dam was undertaken by the Trans-Caledon Tunnel Authority (TCTA) on 12 June 2008. The air vent downstream of the emergencygate was designed to introduce air to mitigate the negative pressures that were expected in theconduit during emergency gate operations. The emergency gate has to close when the radialgate at the downstream end of the outlet conduit fails. Contrary to the theoretical design, themeasured air vent velocities in the field indicated that, while the emergency gate was closing,very large volumes of air were apparently continuously being released from the air vent,commencing when the gate was about 30% closed (i.e. 70% open). This is in contrast to whatthe design intended, namely that air should have been drawn into the vent.This thesis is concerned with the testing of a 1:14.066 physical model representing the outletworks and air vent of the Berg River Dam as a means to determine the reasons for the releaseof large volumes of air from the air vent during the trial closure in 2008. It also seeks solutions tomitigate the excessive airflow from the air vent. It was concluded that the air velocity in the airvent was independent of the rate of closure of the emergency gate, but to increase withincreasing water head. The problem at the Berg River Dam was determined to be one of airblowback.Modifications were made to the configuration of the model in order to determine whether theconfiguration of the outlet works caused air to be released from the air vent. It was determinedthat the downward sloping roof at the outlet of the conduit, used to accommodate the radial gatechamber, was the cause of the air blowback phenomenon. An additional air vent was fitteddirectly onto the conduit at the constriction was found to be ineffective in reducing the airblowback. It was concluded that there are no rational structural change that can prevent orinhibit a recurrence of the blowback phenomenon in the Berg River Dam outlet conduit.The recommendation follows that the outlet conduit should not be constricted by any structuralor mechanism further downstream in the conduit.

[发布机构] Stellenbosch University