The bottom outlet in question comprises a bulkhead gate, a vertical shaft, a horizontal tunnel and a submerged exit in the tailwater. Field tests show that the gate outflow at partial openings entrains air into the tunnel which leads to blowouts at the exit. CFD modeling is performed, with the purpose to understand the phenomenon and estimate the amount of trapped air. The gate is instantaneously opened to a designated position. Three gate opening heights, 0.45, 0.80 and 5.00 m (full opening), are modelled. The simulations show that, due to the impact of the gate outflow, surge oscillations occur in the shaft-tunnel-tailwater system; the surge amplitude descends with time. The air entrained in the shaft enters the tunnel and forms air pockets that follow the tunnel flow downstream. The air pockets travel along the roof of the tunnel and may coalesce and break up. At the the0.45-m opening, the blowouts in the model exhibit almost the same frequency (28‒29 s) as observed in the field tests (~30 s). At the 0.80-m opening, more air is entrained in the shaft; after the first major blowout, the air is released in form of small blowouts, mostly at a 7‒12 s frequency. The blowouts are found to be associated with small gate openings. At the full opening, air ceases to enter the tunnel and water is discharged without air entrainment and blowouts.