Core Development

OpenFOAM provides an ideal development environment for high-end physics modelling, relying on the underlying classes providing discretisation of space and time, FVM and FEM solvers, mesh handling, I/O and database functionality. One of the most demanding jobs in the project is working on the core classes, allowing for various and ease of implementation. The list below summarises the collaborative work-in-progress led by Dr. sc. Hrvoje Jasak of Wikki Ltd., author and chief architect of OpenFOAM, in collaboration with various University research groups and commercial companies.

Fluid Structure Interaction

Doing fluid-structure interaction simulations in OpenFOAM is considerably easier than using other solvers: both fluid and structure are simulated in the same software side by side. Mapping, coupling and contact detection are built as a toolkit and can be configured to fit the coupling requirements. Run-time selection in fluid-structure interaction code allows the user to dynamically choose the physical model on either side. For example, fluid-structure interaction is algorithmically identical for incompressible laminar flow, multi-phase, free surface etc: this is mirrored in the coupling algorithm.

The example below shows oscillation of an elastic pipe under a flow pulse (please click the image for animation). Note how the flow pattern in the pipe changes as a result of combined pulsing and straightening of the pipe wall. For more details on implementation and capabilities, please contact Wikki Ltd.

Automatic mesh motion is the final part of the puzzle. On the structures side, a large deformation stress analysis solver naturally includes mesh motion. On the fluids side, only boundary motion is given. Here, the automatic mesh motion solver with polyhedral mesh support available in OpenFOAM provides a solution. Boundary motion is used in a FEM-type solver to calculate point motion for all mesh points, while preserving mesh spacing and quality.

Engine simulation in OpenFOAM

For realistic combustion simulations in internal combustion engines, it is necessary to simulate the flow during the exhaust and intake stroke. Intake conditions control the level and distribution of turbulence, as well as potential fuel-air mixing, with critical impact on the combustion phase. CFD modelling of intake and exhaust stroke requires handling the valve action, including opening and closing and at the same time preserving sufficient mesh quality in the critical valve region.

Example of topological changes in action during the exhaust stroke of a Diesel engine show how a combination of topological modifiers work together in a 3-D geometry with a flow solution. Away from top dead centre, the mesh is modified using mesh layering. At the end of exhaust or during combustion, increased mesh resolution is achieved by deforming the mesh.

Valve action

A combination of topology modifiers recently implemented in OpenFOAM allows me to model the valve action using a combination of several layer addition/removal surfaces and sliding interfaces for each valve. The first series of pictures shows the mesh modifiers in action during the exhaust and intake strokes. Note a change in the number of cells in the cylinder and above and below the valves in motion.

In-cylinder flow

A finer mesh has been chosen to simulate the flow field during the exhaust and intake stroke. The work continues in collaboration with Dr. Gianluca d'Errico and Tommaso Lucchini of the Internal Combustion Engine Group, Dipartimento di Energetica, Politecnico di Milano, Italy.

Free surface flow modelling

• Droplet impact studies. Droplet-wall interaction is an important subject in fluid dynamics and the newly developed free surface capturing techniques allow us to perform very detailed droplet-wall interaction simulations. The main objective is to provide a deeper insight into the physics in order to provide a baseline for other modelling approaches, e.g. Lagrangian particle tracking for Diesel spray and spray-to-wall interaction through wall film modelling. For more details on the free surface capturing solver in OpenFOAM, see the Free surface capturing presentations.
  
• Bubbles, bubbles The work presented here is done by my student, Dr. Sc. Zeljko Tukovic at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Croatia. The figures show the unstable behaviour of a 1 mm air bubble in water in 2-D bubble. 3-D simulations are ongoing and some preliminary results are shown. For more details on the methodology and the automatic mesh motion solver, see the presentations page. Simulations are performed using the surface tracking technique, where the shape of the computational domain is adjusted for the motion of the free surface.

Topological changes in OpenFOAM

• Raw dynamic mesh changes Completed. Primitive operations are:
  • Add, modify and remove a point
  • Add, modify and remove a face
  • Add, modify and remove a cell
• Pre-implemented mesh modifiers
  • Arbitrary sliding interface.
    • Example: Mixer vessel. Internal part of the mixer moves relative to the external part and the mesh is dynamically connected and updated on topological changes.
      

  • Cell layer addition/removal.

    • Example: Moving body inside of a box. The example has got one layer addition/removal modifier in front of the body and one behind it. Notice the change in the number of layers in front and behind the body.
      

  • Arbitrary attach/detach boundary.

    Modifies a pre-defined set of internal faces to make them boundary faces and vice-versa.
    • Example: attaching a detaching a T-junction
      

Numerics and OpenFOAM

• Finite Area Method

  When a two-phase system with a free surface is not chemically clean, surfactant chemicals concentrate on the free surface, modify local surface tension properties and significantly influence the behaviour of the system.

  In simulations or air bubbles in water, this effect needs to be taken into account. The transport of surfactant chemicals is two-fold: they are transported through the volume of water and once they reach the free surface they move along it until equilibrium is reached. The concentration on the free surface acts as a boundary condition for the volume transport; the volume, on the other hand provides "area sources and sinks" for ths surface simulation, depending the the ratio of local concentration.

  The figure shows a 1 mm 3-D air bubble in water and the surrounding velocity field. The surface is coloured by the surfactant concentration; as the surfactants concentrate at the bottom of the bubble, we are looking at the bottom of the bubble.

  OpenFOAM provides a Finite Area discretisation, which allows us to simulate the transport phenomena on a curved surface in 3-D. This tool is useful for many other applications as well, e.g. wall films in Diesel engines (created when the injected Diesel fuel hits the wall).