Baffle Drop structures are used to convey urban water flows into tunnels, usually underground. Water cascades down the baffles to a waterlogged area below which drains into a tunnel. The structure has been used successfully in urban drainage projects. However, its hydraulic characteristics pose a challenge at the design and engineering stage.
In the following image, obtained from an interesting article about this type of structure, we can see a diagram of these elements.
CFD simulation is a very useful tool for these projects. CFD analyses make it possible to check the evolution of the flow, assessing the drainage capacity and the height that the water would reach in each deflector. This makes it easier at the design stage to locate the various control and inspection elements of these structures.
In this post we will see some tips and examples of these simulations performed by ICEMM as a company with extensive experience in the analysis and optimisation through CFD simulation.
CFD model preparation
At the simulation level, the most widespread technique for modelling these free surface problems is known as VOF (Volume of Fluid). FLOW3D Hydro or MSC Cradle software, for example, use this technique.
Also in the open source alternative OpenFOAM, the interFoam solver allows the VOF method to be used.
In this type of transient simulations it is important to regulate the time step adaptively, ensuring that the Courant number stays within the limits that the solver can handle, usually below unity. This also makes the meshing very important and greatly influences the computational cost of the simulation, so the mesh size must be chosen carefully.
In the case of OpenFOAM, the appropriate discretisation and resolution schemes must be selected for each of the equations and terms. Also when imposing the boundary conditions, attention must be paid to the pressure, as the interFOAM solver solves the pressure variable without the hydrostatic term by default.
The following image shows a detail of a mesh structure in OpenFOAM with the snappyHexMesh tool.
In commercial software such as Flow3D or Cradle the case configuration is somewhat simpler, but the quality of the results for a similar mesh size is very similar.
CFD simulation results
Below is an example of a CFD simulation of this type of scenario. We have carried out a study of a model structure of about 30m in height using OpenFOAM software.
In this first video we show a perspective view of the structure where you can appreciate the overall behaviour.
In this one, we show a side view of the assembly, which allows us to see the levels reached by the water in each deflector. It would also be possible to define cuts to see only the level at the coordinates of the control or inspection elements.
As we have seen, this type of free-surface simulation can be used in preliminary design stages where rapid iteration capability is of interest. It can also be used to validate an already completed design in pre-construction stages or to explore possible optimisations.
