Bubble entrainment and distribution in a model spillway with application to total dissolved gas minimization

This paper focuses on an experimental study of the two-phase flow downstream of a laboratory model fish bypass. Experiments were performed on a 1:24 scale laboratory model of a fish bypass under consideration for construction at Wanapum Dam, on the Columbia River in Washington. The model was operated at the design condition of skimming flow regime and at the possible off-design plunging and surface jump regimes. Void fraction data were collected using an optical phase detection probe on a three-dimensional grid, and the phase indicator function was recorded at selected locations. It was found that in the laboratory model, the skimming flow regime effectively prevents bubbles from reaching deep into the tailrace, resulting in a considerably lower void fraction than plunging and surface jump regimes. For this geometry, the surface jump regime entrains air deeper than the plunging regime. To observe trends, the instantaneous source of total dissolved gas was estimated for the three regimes using the model data and several simplifying assumptions. Time distributions of the indicator function are also reported. 2008 ASCE.