It is widely recognized that the water flow in pipes can be affected by trapped air pockets. However, the underlying air–water interactions of free flow with non-jacking downstream of the pipe are rarely investigated. There are no studies at this time that clearly elucidate the differences in pipe flow between the vacuum and the presence of air in the free flow case. To this end, the smoothed particle hydrodynamics (SPH) method is applied to study the air–water hydraulic dynamics in the pipe under free flow. We perform SPH simulations in several different scenarios and find that (i) the SPH method is competent to simulate both single- and two-phase flows in the pipe due to its outstanding advantages in capturing complex interfaces; (ii) only when the upstream water level is higher than the top of the pipe inlet, and the water level in the pipe rises to the top of the pipe due to air resistance, can a full pipe flow be formed; and (iii) the presence of air can cause the water to form a full pipe flow, causing a siphon-like effect that promotes pipe drainage. These results provide insights into the underlying complex air–water hydraulic properties in pipe flows.

  • The smoothed particle hydrodynamics method is performed for air–water simulations of pipe drainage.

  • The air–water hydraulic dynamics in the pipe under free flow conditions are systematically explored by analyzing changes in the flow rate and flow state.

  • The results show that, contrary to the commonly recognized blocking effect of trapped air in submerged flow, air in free flow can actually facilitate pipe draining.

This content is only available as a PDF.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).