Effects of Floating Rafts as Anti- Vortex Devices at Single and Multiple- Horizontal Intake Structures Having Air- Entraining Vortices

In modern era, water resources are started to dry up and humans are faced with a serious waste water problem, so it is important to use water resources effectively and carefully. For this reason, more efficent design criteria should be applied for water intake structures to minimize cost and operational problems. One of these problems is air- entraining vortices which are created by swirling flows on intake. Occurence of air- entraining vortices causes serious problems such as; increasing loss of hydraulic load and loss of discharge at water intake structures, loss of efficiency, operational problems, caviation and vibration problems in hydraulic machines. The position of intake should be justified for the most critical scenario which is in the case when the reservoir is at dead or at minimum storage level, water should be sufficently above from the intake to provide a vortex free flow. By the way, the intake has to be located close to the water surface so as the decrease cost of construction. Although, intakes are designed by considering vortices, it cannot be prevented due to approach flow conditions, submergence and financial problems. To prevent vortices, some structural changes can be considered. One of these methods is using some anti- vortex devices to prevent asymmetric flow conditions. In this experimental study floating rafts at different sizes were tested as anti- vortex devices to prevent the formation of air- entraining vortices at single and multiple- horizontal intake structures under symmetrical and asymmetrical approach flow conditions. In this study, three identical pipes of diameter D_i= 0.265 m were tested at a wide range of discharge with varying side wall clearances under symmetrical and asymmetrical approach flow conditions. For single and double water intake structures, W_raft= 10 cm width; for triple water intake structure, W_raft= 20 cm width were successful for vortex prevention. Moreover W_rafts/D_i values which gave successful results are 0.38 for single and double intake structures and 0.75 for triple intake structure.

Key Words: Open channel flow, Horizontal intakes, Multiple intakes, Air- entraining vortices, Anti- vortex devices.