Free Surface Simulation

Many different phenomena  in industry or nature include a deformable interface for example the water falling from a spillway. The simulation of such events is essential to better understanding of  two-phase flow behavior, in order to optimize the design or to calculate hydrodynamic coefficients. The precise and correct modeling of interface is the key parameter in simulation of free surface. There are different interface tracking methods such as VoF (volume of fluid), LS (level set) and  FT (front tracking). Each method has its own advantages and disadvantages, however VoF is becoming the most common interface tracking method due to it’s ease of implementation and mass conservation.  OpenFOAM VoF solver is interFoam. The solver uses a technique called MULES to compress the interface and return the sharp interface. The interFoam is suitable for two-phase isothermal  incompressible system. The derived solvers form interFoam is:

  • interDyMFoam:  interFoam  added dynamic Mesh
  • multiPhaseInterFoam:  interFoam extended to simulate more than two-phases forexample: water-oil-gas system.
  • compressibleInterFoam: the compressible version of interFoam

Users also extended and adapt interFoam in recent years to include some features such as wave generation. These extensions are:

In the rest of this post, i will show some of my simulations using interFoam family.

Free Surface Flow Over Stepped Spillway


The details of  simulation procedure is available in my book. The gird for two dimensional free surface flow over spillway is created using  OpenFOAM mesh generators: blockMesh, snappyHexMesh and extrudeMesh from imported CAD geometry.  blockMesh is applied to create the base grid, then snappyHexMesh extract the final three dimensional  grid from base grid around imported CAD file. The grid is refined locally to better capturing interface. Finally, for two dimensional simulation, the grid is extruded to have just one cell thickness across the flow stream.

The grid created with OpenFOAM tools

The turbulent flow is considered with kOmegaSST  Model.

simulation of free surface flow over stepped spillway using interFoam

Free Surface Flow Over Sub Merged body


The schematic of problem is shown below. The simulation is conducted with olaFoam. The solver provides wave generation and absorption boundary condition. My simulation is done with OpenFOAM-3.0.1. However, it is worth to note the wave boundaries are added to recent version of OpenFOAM-v5, so there is no need to install olaFoam any more.

The problem description of flow over sub-merged cylinder

The grid is created with blockMesh. It contains only hex elements which is refined near interface or near submerged body.

The grid for submerged cylinder with blockMesh

closer look at grid:

The grid for submerged cylinder with blockMesh

The wave behavior is shown below, the LES model is employed for turbulence modeling.

numerical simulation of wave force on submerged cylinder

The hydrodynamic coefficient is important in this study, so lift and drag force is calculated. The normal lift coefficient is shown here.

The normal lift coefficient due to wave impact on submerged cylinder

Sloshing in Tank


It’s a part of tutorial, i used to teach in my training workshop at IHA. It’s the sloshing tank. The grid is created with blockMesh and snappyHexMesh. The tank moving is supported with dynamicMesh in the solver interDyMFoam.

simulation of sloshing using interDyMFoam

 

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