Air entrainment by spillway aerators

Aerators on spillways may be regarded as an effective protection against cavitational erosion. The air entrainment of aerators is governed by a number of independent parameters, that include the slopes of the spillway and the aerator, the heights of ramp and offset, the approaching flow depth and Froude number, and the stagnation pressure in the nappe cavity. Based on model observations, two approaches are presented for the air entrainment coefficient β. The first, direct approach is based on the analysis of each effect on β. An expression is presented that should be tested for other geometries. The second, indirect approach is based on the length of jet for zero cavity subpressure. The resulting expression for the maximum air entrainment coefficient may then be corrected for the effect of cavity subpressure. The present approach introduces developing and developed aerator flow conditions. Further, the cavity subpressure is accounted for by a difference Euler number. The final system of equations for β is compared to model and prototype data, and a fair agreement is noted.