---

Flow regime in dam's bottom outlet is divided in pressurized flow and free surface flow by the gate located for discharge control. Down stream tunnel involves high velocity Multi component Air –water flow studied by mathematical model. In this research work, we used Finite volume mixture two phase flow model. Because of high Reynolds number, standard two equations k-e turbulence model was used. Model was verified by backward-facing step flow and results have been compared with experiments founded by Durst and Schmitt. Air demand ratio has been determined as function of Froude number at contracted section. Flow patterns have been compared at two categories of slug & stratified flows, Air mean concentration profile has been obtained at down stream tunnel. Comparison of flow pattern at two case with and without of aeration was investigated. Pressure drop behind of the gate and formation of vortex flow after the gate section have been discussed. Measurement of flow discharge and determination of contraction coefficient of the gate was outlined.

---

This paper compares the hydraulics and 3D flow features of the ordinary rectangular and trapezoidal plan view piano key weirs (PKW) using two phase RANS numerical simulations. The main aim is to investigate effects of the inlet key area and side walls angle on discharge capacity of the PKW, while keeping the developing length of the crest intact. The numerical model has been used to carry out a sensitivity analysis for geometrical parameters and hydrodynamics of the rectangular and trapezoidal weirs have been compared for wide range of water head on the weir. Results show that the trapezoidal weir has higher efficiency than the ordinary rectangular PKW. This is partly related to the inlet key flow conditions. The trapezoidal geometry increases the inlet flow area resulting in reduction of the velocity along the key axis. Consequently, flow is distributed more uniformly over the side weirs. The gradual transition of the inlet key limits the development of the recirculation zones along the side walls and it also limits the formation of the critical section along the inlet key. These phenomena result in increase of the effective weir length and thus discharge capacity increases in trapezoidal weir compared with rectangular one. Discretization of the discharge along the crest of the tested weirs clearly confirms these findings.