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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Water engineering : Hydraulic analysis and cavitation phenomenon in physical and numerical models of the bottom outlet i...
Hydraulic analysis and cavitation phenomenon in physical and numerical models of the bottom outlet in rudbar dam, lorestan province, iran.
Author : SATTAR KHAZAEI(1), REZA ELLAHI(2) , EHSAN NIKNAMI(3) & KOOROSH SHAHVERDY(4)
ABSTRACT
Bottom outlets are one of the main components in dams that are prone to numerous hydraulic problems such as
variations in pressure that can affect the structure¡¯s performance if not controlled. Rudbar Dam in Lorestan with a height
of 158 meters is one of the highest earth dams under construction in Iran. In operation, this dam uses bottom outlets for
flood discharge and controlling the water level in the reservoir. Due to the fact that the upstream pressure head in the
discharge tunnel and at the control gate is high at all times (Normal head is 127.2m), the flow in the tunnel is under
pressure and past the control gate it becomes a free surface flow, by observing and studying the physical model, two
major Hydraulic problems, were clearly detected. The first problem was the occurrence of low pressure (near zero) at the
final ring of the transition structure Piezometers, installed in the downstream of the valves at different reservoir water
levels and different gate openings. In a way that cavitation index calculated based on the piezometer readings in this
domain was less than standard level and which means there is a higher chance of cavitation occurring at the transition
section. But the second problem observed in the physical model was the improper flow of output jet stream from the
outlet conduit in downstream of the transition structure (the steel liner), Due to the poor geometry of the transition
structure and the high head of the reservoir. Even when gate are 100% open, flow continues in a pressurized state to the
end of the transition section and the outlet jet from the end of this section hits the floor of the steel liner and reflects the
roof of the tunnel (creating a wing-shaped flow). Obviously, at different levels of the reservoir, the impact position of the
outlet jet and the reflection type is varies and as the flow is non-uniform, at times blocking was observed in the tunnel.
Because of the problems observed in the preliminary physical model of the bottom outlet, the outlet was designed using
computational fluid dynamics (CFD) and the flow was simulated with numerical models using Flow 3D and ANSYS
(Fluent) software products to eliminate the problem and to avoid redoing the physical modeling of the outlet and to also
limit the variations in it as much as possible. Using the numerical model, it was observed in early stages of the study
that past the gate, flow has a similar consistency with the ones obtained from it. After calibrating the numerical model
with the initial results of the physical model, several changes were applied to the geometry of downstream tunnel and
after analyzing in both numerical models (Fluent, Flow 3D), the conditions were improved. By analyzing the
phenomenon of cavitation and taking the necessary steps, it was prevented in bottom outlets.
File Size : 350,158 bytes
File Type : Adobe Acrobat Document
Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
Article : Water engineering
Date Published : 13/08/2015
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