# Physic models in STAR-CCM+ (Part II)

We’ve already discussed space, time and motion models, which represent just a small portion of the physic models available in STAR-CCM+. This is Part II.

## Material

Most —if not all— simulations will involve modelling a substance such as air, water, aluminium or a mixtures of gases and fluids.

The material model is responsible for managing the material, i.e., substance or substances, being simulated in the continuum. The material, in turn, is responsible for managing the various thermodynamic and transport properties relevant to that material and to the physical processes being modelled in the continuum.

There are three general types of material models available in STAR-CCM+: single-component, multi-component, and multiphase mixture.

### Single-component material

The single-component material models are for modelling pure substances in of their fundamental states, namely gas, liquid and solid. There is a fourth state, plasma, but there’s no option for it in STAR-CCM+.

The difference between a solid and a fluid is that the latter deforms continuously when subjected to a shear stress, no matter how small that hear stress may be. And CFD is all about fluids —that’s what the F stands for—, and although the term is often used as a synonym for liquid, it includes both liquids and gases. For those fluids, we obtain numerical approximations to the solution of the governing fluid flow equations.

Cases where we may use single-component materials are those related to external aerodynamics of aircrafts and vehicles, where the material used is air.

### Multi-component material

The multi-component material models are single-phase models for simulating fully miscible mixtures of two or more pure substances in the same phase. These models can be either reacting of non-reacting.

For instance, when modeling reactions in multi-component gases —like during a combustion—, the user has to define the proportions in which chemical species combine with one another. The stoichiometric equation of a chemical reaction, which defines the proportions, is a statement of the relative number of molecules or moles of reactants and products that participate in the reaction. For example, the stoichiometric equation for the combustion of propane is

$$\ce{C3H8 + 3/2O2 -> 3CO + 4H2}\\ \ce{CO + 1/2O2 -> CO2}\\ \ce{H2 + 1/2O2 -> H2O}$$

For other complex chemistry definitions, specific formats can be used, such as the Chemkin format.

### Multiphase mixture material

The multiphase mixture material model is for simulating two or more immiscible phases, where each phase is composed of a pure gas or liquid substance, such as air or water. When several of those phases exist in equilibrium together, the thermodynamic condition of equilibrium is described in terms of a system property g, or Gibbs functions,

$$g = h – Ts$$

which is minimized at thermodynamic equilibrium.

See the following video to see what can be achieved modeling such a mixture —red is for water and blue represents air.

## References

[1] User Guide STAR-CCM+ Version 8.06. 2013.
[2] Ganić, E., Hicks, T. and Predko, M. 2003. McGraw-Hill’s Engineering Companion. New York: McGraw-Hill.