Air–water interaction in a low-head plunging-type dropshaft

The mechanisms of air–water interaction motion in a dropshaft with a height of 0.95 m were investigated by means of both physical modeling and computational fluid dynamics (CFD) simulations. The detailed air entrainment rate of the specific dropshaft was measured and analyzed. Three stages of internal air circulation in the dropshaft were proposed, revealing the patterns of air movement within the shaft through visualization using CFD simulations. Detailed measurements of air content in the bottom pool under various flow conditions were conducted, and the concept of bubble retention time (BRT) was introduced as the ratio of the total air bubble volume in the bottom pool and the air inflow rate. The BRT value represents the duration for which the intake air flow stays in the pool before entering the downstream pipe. The results show that the BRT decreases as the dimensionless flow rate increases, while a higher downstream filling ratio leads to a higher BRT. The internal factors of water flow dragging air in low dropshafts are further elucidated. Based on the momentum transfer of the water flow, the drag coefficient of water on air was calculated. It was found that the drag coefficient initially increased and then decreased with increasing water flow rate mainly due to the location of the impingement of the water flow in the pool.