Basic Idea

The stength of Wildkatze lies in elaborated details on solvers and framework side, which cannot be found in the present commercial CFD softwares. We designed our software such, that it contains robust parallelised features, high modelling flexibility, strong user coding capability and many innovative ideas which are described below in more detail.

Wildkatze allows combinations of solver, physics and user models in very flexible manner. It´s framework and available models can be used by customers to even create their own solvers. This is very novel idea which is not provided in other CFD tools.





Some Examples

Interacting with Wildkatze

Wildkatze is designed such that the large data is kept in different files than the other frequently needed information files. After setting up a simulation, user will have 3 files: .info.bmsh (mesh region information), .bmsh (heavy binary mesh data) and .stree (simulation set up tree structure). This separation in three files is a very big advantage, since it saves much time and offers lots of flexibility when interacting with Wildkatze software.

This feature allows to simply edit the region/boundary names in .info.bmsh using an editor and use the same mesh with it. This is leading to very short saving times after regions/boundaries were edited by user.

Wildkatze restart data which contains field variables, is saved in different file. User can also use the same .stree files for many different meshes. This will allow user to save the simulation set up in much less space compared to a setup file that contains also field variables.

User can set up one simulation file (.stree) and run it with various meshes in batch mode, just by changing the mesh name. This is very useful feature, when solver is used with optimization programs.

Different Physics Models on Different Regions

It starts with a very general question how to perform a multi-physics simulation involving one or more phases on/in a multi-region grid/mesh. The question is further expanded into asking how to perform some physics on some of the regions and some other physics on entirely different set of regions.

Natural answer to these questions is that we define sets of phases and sets of regions and we define physics model to be solved over these phase sets and region sets. This allows us to apply a physics model Physics1 on Phase-Set1 and Region-Set1, while a model Physics2 could be applied on Phase-set2 and Region-Set2. Here Phase-Set1 and Region-set1 could have common phases and common regions with Physics-Set2 and Region-Set2.

Because of the design based on Region-sets and Phase-sets Wildkatze is very flexible. For example one could define SST-K Omega turbulence model on region1 and Spalart Almaras turbulence model on region2, while same time solving flow model on set of region1 and region2. This in itself does not sound much interesting, but it implies that some challenging problems in CFD as Conjugate Heat Transfer, are defined trivially in this framework where heat transfer is calculated on bigger set of regions while flow is solved only in flow regions.

Different Time-Stepping & Gradient Methods

Other available CFD solvers do not allow user to changing time stepping method for individual Physics Models. What it means is that if user has selected the time stepping method as Euler scheme, all the Physics models like flow and turbulence are forced to run with same scheme. Wildkatze however allow user to set different time stepping method for each model.

Wildkatze allows also to set different gradient method for each model.

Automatic & Polynomial Smoother Option

For extremely difficult and degenerate meshes where convergence is very difficult, Wildkatze provides user with Auto option for smoother. This option albeit slower than standard option, will check for system consistency and convergence at coarse level of matrix system. If the check fails then it would switch to much safer option (but slower) that shall converge in most cases. This option is useful in the case the diagonal generated is negative or zero and thus system is not solvable.

There are calculations that show convergence degradation when very large numbers of processors are used. For such simulations, polynomial smoother option is available that is not affected by number of processors used and shall produce consistent results.

Several interface tracking schemes are provided for VOF

In Wildkatze solver, we provide following interface tracking schemes: HRIC, CICSAM, Upwind Cell Framework (for solver to access upwind cell value and use its exact value whenever possible), HRIC U2 (HRIC implementation using the upwind cell values from upwind mesh cell), CICSAM SHARPER (CICSAM implementation using the upwind cell values from upwind mesh cell), THINC (scheme for very sharp interfaces). Solver can blend between the flux computed by THINC scheme and the flux computed by one of the other mentioned base schemes.

Time stepping & Hybrid time stepping for VOF

In Wildkatze solver, Explicit Euler, Second Order TVD Runge Kutta and Third Order TVD Runge Kutta time stepping schemes are provided. For fast transient multi-phase simulations, Wildkatze provides a Hybrid Time Stepping method with combined implicit and explicit methods. This can reduce the computation time more than two or three times in most cases.

Level Distance Coupled VOF

Wildkatze solver provides newly developed Level Distance Coupled VOF model which is based on similar ideas to Level Set Coupled VOF model. The major difference with standard Level Set method is that in this method a transport equation for distance function is solved, where the iso-surface of volume fraction = 0.5 is assumed to be 0 distance, whereas in standard Level Set method signed distance function is solved.

Following the ideas from standard Level Set method, the density, viscosity and surface tension forces are computed from the level distance function.

Reactive Mold Filling

We offer the capability to do reactive molding simulations, where reactive mold is tracked by an interface tracking method like Volume of Fluid (VOF), while flow is solved by SIMPLE method. Flow properties are obtained based on reaction state of the mold (for example amount of gas produced and mold temperature). Energy is solved by using simplified energy equation. Fluid properties are computed based on state of reaction components.

Polyurethane expansion is an example of reactive mold filling that is directly available to user as a physics model. Here the mold filling reactions could be mainly divided into two groups: gelling and blowing reaction.