Bottom outlets play a vital role in dam safety. They are key elements to control the water surface elevation in the reservoir. Since the early twentieth century it has been found that an aeration system is necessary in the downstream side of the gate to achieve an appropriate performance of the structure. The system is placed downstream of the sealing device to prevent occurrence of negative pressures. Otherwise, damages due to cavitation and vibration are frequent.

Due to the great difficulty to collect data or to study the phenomenon either in situ or in full scaled experimental facilities, the analysis has traditionally been carried out from laboratory tests in small-scale physical models. There are several types of valves which have showed an acceptable behavior and are supported by experience. However, some specific problems have been occasionally found, so that some modifications seem to be convenient. In these situations, manufacturers face the problem of inexistence of means to validate their designs further than full scale tests, with the consequent costs of manufacturing and testing. The tests are usually limited to the valve''s watertightness verification because they are the only tests economically affordable.

Figure 1.Typical layout of a dam bottom system: two parallel conducts and two serial valves (safety and operation) in each conduit. Observe the aeration devices.

The project aims to develop a numerical tool useful in the design of this type of valves. It will be based on the Particle Finite Element Method (PFEM), and needs to be able to consider the effects of aeration.

 
Figure 2. Preliminary results. Gate opening: 50%.

The application of advanced simulating techniques to model both air and water flows in bottom outlets is unprecedented and would allow a detailed understanding of the structural and hydraulic behavior of the valves. The objective is the optimization in design of both the valve and the aeration system, so to prevent operational problems and thus to improve the safety of the facility.

  
Figure 3. Numerical simulation of the bottom outlets of Susqueda Dam, left water flow, right air flow.