Air Entrainment Mechanism in the Vortex Structure: Experimental Study

Vortex drop structure is used to convey water through underground conduits in urban sewers and drainage systems. During the plunge, a large volume of air is entrained into the water and then is released from the drop shaft downstream. Basically, volume of entrained air is comparatively hard to measure. In this research, a physical model was constructed to understand the mechanism of air circulation through vortex structure. In fact, experiments were tested to investigate effects of variables on the air circulation. Through the experiments, results of dimensional analysis results indicated that the approach flow Froude number ($F_r$), drop total height to shaft diameter ratio ($L/D$), and sump depth to shaft diameter ratio ($H_s/D$) had significant influences on the relative air discharge ($\beta$). To express the role of each independent variable on relative air discharge ($\beta$) in terms of regression analysis, response surface methodology, based on central composite face-entered design (RSM-CCFD) was examined. Hence, a regression-based-equation in form of quadratic polynomial was proposed to estimate $\beta$ variable. Additionally, experimental design was to investigate simultaneous effects of $F_r$, $L/D$, and $H_s/D$ on the $\beta$. Results of experimental study indicated that $\beta$ variable had upward trends with an increase in $F_r$ variable and $L/D$ ratio. Analysis of variance for the proposed regression model demonstrated that simultaneous effect of $L/D$ and $H_s/D$ on $\beta$ variable remained statistically significant, whereas other interaction effects of variables were insignificant. Ultimately, the optimum location for installation of air vent pipe ranged from $D$ to $2.25D$ in a way that air vent pipe had the most satisfying level of air outlet flow performance.