Numerical Investigation of the Effect of Entrained Air in Navigation Locks

Many navigation locks in Germany with lifting heights lower than 10 m are filled through valves in the upstream gates. Downstream of the valves a free falling jet evolves which plunges into the chambers water body and entrains a significant amount of air. Due to buoyancy the air bubbles introduce a momentum towards the water surface. As a result the longitudinal forces on a ship in the lock chamber can increase. To guarantee a safe filling process the ship forces are limited when designing ship lock filling systems.

In Froude scaled models, which are often used for the optimisation of the filling process, scale effects regarding the entrained air cannot be avoided. But also the widely applied numerical investigation approaches based on the Volume-of-Fluid (VoF) method are not able to physically capture the effects of the air entrainment and transport processes with feasible computational resources. A satisfying examination of ship forces is therefore not possible with the existing methods.

To enable the investigation of the effect of air transport processes on the ship forces, a new numerical approach was developed and implemented into the open source CFD-library OpenFOAM®. For capturing the large scale water air interface in the nearfield of the hydraulic structures, the new solver uses a VoF method. As entrained air bubbles cannot be resolved with reasonable mesh sizes, the approach is extended to a mixture approach so that the momentum transfer between the phases is physically captured. The implementation is optimized for large hydraulic engineering applications including complex geometries and highly turbulent flows.

This study aims to explain the background of the developed approach and shows an example application. In particular, the effects of air transport and detrainment processes within an existing lock are investigated, where strong air entrainment was observed during the commissioning of the prototype. The numerical model examines the sensitivity of the ship forces to different air contents.