Pressure Fluctuations and Cavitation Index in the Slot of Lift Bottom Outlet Gate, by Using a Physical Model and Introduction of Normal Standard Distribution formula

The high velocity flow of water along with the severe head loss causes several turbulences and may lead to occurrence of cavitation phenomenon in hydraulic structures and severe destruction in some parts of them. Flow separation within the slots make eddies and vortices, which can also create cavitation. This study deals with the pressure fluctuations which are product of current severe turbulence, their processes and cavitation phenomenon at the gate slot region (with length/wide ratios of 2 and a 45° angle to the edge of the vertical-gate) of physical model of bottom outlet of Gotvand Olya storage dam. The study was conducted at the gate openings of 10, 30, 50, 70, and 100% and in different 2, 3 and 4 meter-heads and at 0.1 % probability levels. Data collected using a transducer, were then transferred to an amplifier, and finally recorded in a computer. The results obtained indicated that at the gate openings of 30%, 50% and 70%, risk of cavitation occurrence is the maximum and at 10% and 100% it is the minimum. Also the piezometer, closest to the gate edge, is subjected to more fluctuations and critical point of pressure fluctuations are transferred to higher points upon with increase of gate opening. Coefficient of pressure fluctuations (C'p) is the maximum at 10 % opening that indicates compaction and pressure beats to the structure in this opening. Therefore control hydraulic conditions and geometric changes of the gate slot decreasing cavitation occurrence to the minimum would be possible.