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Physics Simulation
Simcenter STAR-CCM+ can model a wide range of physics phenomena including fluid mechanics, solid mechanics, heat transfer, electromagnetism, and chemical reactions. Scenarios with multiple time scales can be solved within the same simulation.
Multiphase Flow
Multiphase flow is a term which refers to the flow and interaction of several phases within the same system where distinct interfaces exist between the phases. Simcenter STAR-CCM+ considers flow options where phases coexist as: gas bubbles in liquid, liquid droplets in gas, and/or solid particles in gas or liquid, and/or (large scale) free surface flows.
Volume Of Fluid (VOF) Multiphase
The Volume Of Fluid (VOF) Multiphase model is a simple multiphase model. It is suited to simulating flows of several immiscible fluids on numerical grids capable of resolving the interface between the phases of the mixture.
Wall Porosity
The wall porosity model lets you define phase-specific porous boundaries within a VOF Multiphase simulation. That is, it permits material that belongs to single-component phases to pass through selected wall boundaries.
Wall Porosity Model Reference
The wall porosity model uses the porous inertial resistance coefficient $α$ and the porous viscous coefficient $β$ to determine the pressure drop across a porous boundary. The ambient pressure is also used to calculate the pressure drop from which the phase extraction mass is calculated.

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Physics Simulation
Simcenter STAR-CCM+ can model a wide range of physics phenomena including fluid mechanics, solid mechanics, heat transfer, electromagnetism, and chemical reactions. Scenarios with multiple time scales can be solved within the same simulation.
- Defining Physics Workflow
  Simcenter STAR-CCM+ contains a wide range of physics models and methods for the simulation of single- and multi-phase fluid flow, heat transfer, turbulence, solid stress, dynamic fluid body interaction, aeroacoustics, and related phenomena. These physics models are all selected using a physics continuum.
- Setting Up Physics
  This section provides information on how to set up physics models in STAR-CCM+.
- Running Physics Simulations
  This part of the documentation describes preparation and procedures for running a Simcenter STAR-CCM+ simulation.
- Motion
  In general, motion can be defined as the change in position of a body with respect to a certain reference frame.
- Space
  The primary function of the Space models in Simcenter STAR-CCM+ is to provide methods for computing and accessing mesh metrics. Examples of mesh metrics include cell volume and centroid, face area and centroid, cell and face indexes, and skewness angle.
- Time
  The primary function of the time models in Simcenter STAR-CCM+ is to provide solvers that control the iteration and/or unsteady time-stepping.
- Mesh Motion and Adaption
  Many simulations that involve motion or geometry change require you to move or deform the mesh. Other simulations require localized mesh adaption in order to achieve an accurate solution.
- Materials
  Material models simulate substances, including various mixtures.
- Fluid Flow
  Many engineering design projects require you to predict the effect of flowing fluids on containing structures or immersed objects. While you can analyse simple scenarios with hand calculations, complex scenarios require you to apply numerical methods for accurate solutions.
- Viscous Flow
  Viscous Flow is a finite element approach for use with viscoelastic materials and other highly viscous non-Newtonian fluids, such as liquid plastics and rubber, dough and similar foodstuffs, molten glass, and mud. Viscoelastic materials resemble elastic materials, but also exhibit viscous effects, rebounding slowly from deformations.
- Passive Scalars
  Passive scalars are user-defined variables of arbitrary value, assigned to fluid phases or individual particles. They are passive because they do not affect the physical properties of the simulation. An intuitive way to think of passive scalars is as tracer dye in a fluid, but with numerical values instead of colors, and with no appreciable mass or volume.
- Heat Transfer
  Heat transfer is the study of energy in transit due to a temperature difference in a medium or between media. Heat transfer extends thermodynamic analysis through the study of the modes of energy transfer and through development of relations to calculate energy transfer rates.
- Species
  This chapter provides information about the chemical species models in Simcenter STAR-CCM+. A species model is activated whenever a multi-component liquid or multi-component gas is chosen from the Material model section of the Physics Model Selection dialog.
- Porous Media
  Simcenter STAR-CCM+ allows you to simulate the transport of a fluid or energy (e.g. heat, electrical charge) through porous materials using the concept that the action of the porous media can be represented using appropriate loss (or 'diffusion') coefficients.
- Adjoint
  The adjoint method is an efficient means to predict the influence of many design parameters and physical inputs on some engineering quantity of interest, that is, on the engineering objective of the simulation. In other words, it provides the sensitivity of the objective (output) with respect to the design variables (input).
- Fans and Blowers
  Simcenter STAR-CCM+ provides models for axial and radial fans where classical fan laws apply.
- Virtual Disk Model
  The virtual disk model is based upon the principle of representing propellers, turbines, rotors, fans, and so on, as an actuator disk. The actuator disk treatment is practical when you are concerned about the influence of the rotor/propeller behavior on the flow rather than knowing about the detailed interactions between the flow and the blades of the rotating device.
- Turbulence
  Most fluid flows of engineering interest are characterized by irregularly fluctuating flow quantities.
- Transition
  The term transition refers to the phenomenon of laminar to turbulence transition in boundary layers. A transition model in combination with a turbulence model predicts the onset of transition in a turbulent boundary layer.
- Wall Distance
  Wall distance is a parameter that represents the distance from a cell centroid to the nearest wall face with a non-slip boundary condition. Various physical models require this parameter to account for near-wall effects.
- Radiation
  The Radiation Model is the gateway, or entry point to all of the radiation modeling capabilities of Simcenter STAR-CCM+. This section describes Simcenter STAR-CCM+’s radiation modeling.
- Aeroacoustics
  Aeroacoustics investigates the aerodynamic generation of sound.
- Reacting Flow
  Simcenter STAR-CCM+ provides a selection of models that you can use to simulate a wide range of reacting flow applications.
- Internal Combustion Engines
  In an internal combustion engine (ICE), for example a gasoline engine, the process of combustion takes place in a cylinder (or cylinders) within the engine. The working fluid is a fuel and oxidizer mixture (usually air), which reacts to form combustion products.
- Multiphase Flow
  Multiphase flow is a term which refers to the flow and interaction of several phases within the same system where distinct interfaces exist between the phases. Simcenter STAR-CCM+ considers flow options where phases coexist as: gas bubbles in liquid, liquid droplets in gas, and/or solid particles in gas or liquid, and/or (large scale) free surface flows.
  - Multiphase Framework
    In mathematical terms, there are two main frameworks in the formulation of the multiphase models in Simcenter STAR-CCM+, the Eulerian and Lagrangian modeling strategies.
  - Multiphase Simulation Concepts
    This section presents the key concepts you come across when using the Simcenter STAR-CCM+ multiphase models.
  - Modeling Multiphase Flows
    Simcenter STAR-CCM+ supports several types of multiphase models, each with their own workflow. Use this section to help identify the choice of models that are appropriate for the type of multiphase analysis that you intend to simulate.
  - Defining Eulerian Phases
    A Eulerian phase in Simcenter STAR-CCM+ represents a distinct physical substance – the phase material – that is modeled in a Eulerian framework. Two materials constitute two different Eulerian phases. The definition of a Eulerian phase includes the set of models that apply to the phase material and the relevant material properties.
  - Defining Phase Interactions
    A phase interaction is an object that defines the choice of models that are used to predict the influence of one phase upon another phase within a multiphase flow simulation. Define a phase interaction for every possible significant coupling.
  - Discrete Element Method (DEM)
    The Discrete Element Method (DEM) is an engineering numerical method to simulate motion of many interacting discrete objects that are typically solid particles. Although DEM modeling demands significant computing power, it provides detailed resolution that other methods cannot achieve. In Simcenter STAR-CCM+, DEM can be applied to regions with or without a volume mesh.
  - Dispersed Multiphase (DMP)
    The Dispersed Multiphase (DMP) model simulates dispersed phases in a Eulerian manner. The Dispersed Multiphase model combines aspects of both the Lagrangian Multiphase (LMP) model and the Eulerian Multiphase (EMP) models.
  - Eulerian Multiphase (EMP)
    The Eulerian Multiphase (EMP) model in Simcenter STAR-CCM+ is based on an Eulerian-Eulerian formulation where each distinct phase has its own set of conservation equations.
  - Fluid Film
    Problems where a thin film of fluid exists on solid boundaries are common in engineering practice. Examples include fluid film on the internal surfaces of automotive engines, gas turbines, spray-cooling systems and ink-jet printers.
  - Lagrangian Multiphase (LMP)
    A wide variety of flow processes involve the transport of solid particles, liquid droplets, or gas bubbles—known as dispersed phases—by a gaseous or liquid continuous phase.
  - Mixture Multiphase (MMP)
    In the Mixture Multiphase (MMP) model, mass, momentum, and energy are treated as mixture quantities rather than phase quantities. Simcenter STAR-CCM+ solves transport equations for the mixture as a whole, and not for each phase separately. The model is computationally more efficient than models that simulate each phase separately.
  - Resolved Transition
    Simcenter STAR-CCM+ provides the Resolved Transition model as a phase interaction model, which lets you capture the breakup of pools of liquid to form droplets emerging/stripping from a free surface or small bubbles stripped from a large bubble. This model is meant to be used as part of a hybrid multiphase approach where the Volume of Fluid (VOF) model or Mixture Multiphase (MMP) model is used alongside the Lagrangian Multiphase (LMP) model.
  - Smoothed-Particle Hydrodynamics (SPH)
    Smoothed-Particle Hydrodynamics (SPH) is a numerical method that overcomes the volume meshing-related constraints occurring with mesh-based models. SPH represents the fluid as a collection of particles with material properties that move with the fluid, making it well-suited for applications with highly dynamic free-surface flows.
  - Two-Phase Thermodynamic Equilibrium
    The Two-Phase Thermodynamic Equilibrium is a multiphase mixture approach that is restricted to modeling two phases. With this model Simcenter STAR-CCM+ solves the mass, momentum, and energy as mixture quantities rather than phase quantities. Therefore, the computational efforts are reduced by assuming the suspension to be a homogeneous single-phase system.
  - Volume Of Fluid (VOF) Multiphase
    The Volume Of Fluid (VOF) Multiphase model is a simple multiphase model. It is suited to simulating flows of several immiscible fluids on numerical grids capable of resolving the interface between the phases of the mixture.
    - Modeling Volume of Fluid Flow
      The Volume of Fluid (VOF) Multiphase model is used in cases when each phase constitutes a large structure, with a relatively small total contact area between phases. This model is used to solve problems involving immiscible fluid mixtures, free surfaces, and phase contact time.
    - Volume Of Fluid (VOF) Multiphase Model Reference
      Volume Of Fluid (VOF) is a multiphase model that is suited to simulating flows of several immiscible fluids on numerical grids capable of resolving the interface between the phases of the mixture.
    - Segregated VOF Solver Reference
      The Segregated VOF Solver controls the solution update for the phase volume fractions. More specifically, it solves the discretized volume-fraction conservation equation for each phase that is present in the flow.
    - Segregated Multiphase Temperature Model Reference
      The Segregated Multiphase Temperature model lets you control thermal effects in your simulation.
    - Setting Up Free Surface Mesh Refinement
      Free Surface Mesh Refinement is an Adaptive Mesh Refinement (AMR) criteria designed for VOF multiphase simulations. Adaptive Mesh Refinement refines or coarsens cells in the volume mesh based on specified criteria, which means that you use a fine mesh only in the areas where the simulation requires it. Solution quantities are automatically interpolated to the adapted mesh.
    - Replacing VOF Phases
      In some VOF simulations, under-resolution of the mesh, time-step, or other numerical issues can cause numerical ventilation to occur. The VOF Phase Replacement model lets you reduce or eliminate numerical ventilation by replacing the unwanted phase (typically gas) with the appropriate phase (typically liquid) in specified regions. If appropriate, you can specify a maximum volume fraction in the phase replacement.
    - Modeling Surface Tension
      Surface tension effects are included in a VOF simulation when the Surface Tension Force model is activated in a phase interaction. You specify the surface tension coefficient $σ$ for each phase interaction as a material property. You also specify the surface tension contact angle at each wall boundary.
    - Slip Velocity Model
      The phases in a phase interaction can move at different velocities, which can be both different magnitudes and different directions. Many important phenomena (for example, phase separation) require the movement of the gas phase relative to the liquid phase. To model differences in velocity between two phases, use the Slip Velocity model.
    - Volume Fraction Reinitialization
      The Volume Fraction Reinitialization model allows for an on demand modification of the free surface by sharpening or smearing the interface, based on user-defined criteria for reinitializing the volume fraction field.
    - Contact Time Model Reference
      The Contact Time phase interaction model estimates how long two VOF phases have been in contact with each other. A typical application is estimating the probability of surface reactions such as oxidation. The transported quantity is the contact area multiplied by the contact time.
    - Interface Turbulence Damping Model Reference
      At a liquid-gas interface with a high relative velocity between the phases, the large gradients in the boundary layer near the free surface can cause significant turbulence fluctuations. The Interface Turbulence Damping model accounts for this in VOF multiphase simulations. This model is available for K-Epsilon, K-Omega, and Reynolds Stress Transport Turbulence.
    - Using the Interface Momentum Dissipation Model
      The Interface Momentum Dissipation model adds extra momentum dissipation in the proximity of the free surface to dissipate parasitic currents. The momentum dissipation is proportional to the velocity gradients and the interface artificial viscosity.
    - Mass Error Rate Model Reference
      The Mass Error Rate model triggers the creation of the Relative Mass Error Rate field function. You can use this field function to create a stopping criterion for inner iterations.
    - Mass Imbalance Correction Model Reference
      The Mass Imbalance Correction model enforces mass conservation of a particular Eulerian phase in a VOF simulation. You can activate this model in one (at most) Eulerian phase in a VOF simulation.
    - Wall Porosity
      The wall porosity model lets you define phase-specific porous boundaries within a VOF Multiphase simulation. That is, it permits material that belongs to single-component phases to pass through selected wall boundaries.
      - Setting up Wall Porosity
        You can model porous boundaries at walls and contact interfaces. The wall porosity is phase-specific and applies only to single-component gases and liquids.
      - Wall Porosity Model Reference
        The wall porosity model uses the porous inertial resistance coefficient $α$ and the porous viscous coefficient $β$ to determine the pressure drop across a porous boundary. The ambient pressure is also used to calculate the pressure drop from which the phase extraction mass is calculated.
    - Modeling Evaporation and Condensation
      The particular phase-change type that is described in this section concerns evaporation and condensation.
    - Modeling Boiling
      Boiling is a rapid vaporization of a liquid. It typically takes place when a liquid is heated to the boiling point (saturation temperature of the liquid $T_{s a t}$ ). Its saturation vapor pressure then becomes equal to or larger than the pressure of the surrounding liquid.
    - Modeling VOF with Radiation
      You can use radiation and VOF models together in many cases involving solidification, boiling, evaporation, and melting, wherever the effects of radiative heat transfer are significant—for example, the case of glass melting in a furnace.
    - Modeling Gas Dissolution
      This section describes the particular phase-change type that concerns gas dissolution.
    - Modeling Cavitation
      When the static pressure at a particular location within a liquid falls below the saturation vapor pressure of the liquid, the liquid undergoes a phase change called cavitation. This phase change creates cavitation bubbles that are filled with the vapor of the liquid.
    - Modeling Melting and Solidification
      Melting is the process which changes the state of matter from solid to liquid. It takes place when a solid substance is heated to the melting temperature. The opposite process is called solidification.
    - VOF Guidelines
      This section describes the set-up and analysis procedures applicable to Volume Of Fluid (VOF) simulations.
  - Modeling VOF Waves
    The VOF Waves model is used to simulate surface gravity waves on the interface between a light fluid and a heavy fluid. This model is typically used with the 6-DOF Motion model for marine applications.
- Dynamic Fluid Body Interaction
  Dynamic Fluid Body Interaction (DFBI) in Simcenter STAR-CCM+ allows you to simulate the motion of a 6-DOF body with the displacement and rotation resulting from the defined mechanical and multiphysics interaction (flow, DEM, solid stress, EMAG).
- Harmonic Balance
  Some unsteady flows have a regularly repeating flow pattern, that is, they are time-periodic. Consider the flow from a fan blade passing across the entrance to a duct. Measurements of the instantaneous flow just within the duct would show a regularly repeating pattern. If the flow disturbances are sufficiently large, and propagate to the end of the duct, measurements of the unsteady flow at any point within the duct show repeating patterns. Such time-periodic patterns can be expressed using Fourier series.
- Solid Stress
  Simcenter STAR-CCM+ allows you to model the response of a solid continuum to applied loads, including mechanical loads and thermal loads that result from changes in the solid temperature.
- Electromagnetism
  Simcenter STAR-CCM+ allows you to model engineering applications involving electromagnetic phenomena. Example applications are electric motors, electric switches, and transformers, which can be modeled based on the classical theory of Electromagnetism.
- Electrochemistry
  Electrochemistry is the study of chemical reactions which occur due to an imposed electrical charge, or a difference in electrical potential at a boundary between a conductor–such as a metal–and an electrolyte. Simcenter STAR-CCM+ provides models that allow you to simulate batteries, corrosion, etching, and other electrochemical reactions.
- Electric Circuits
  Electrical circuits are conducting loops of interconnected electrical components, such as batteries, power sources, resistors, and inductors.
- Plasma
  Plasma is a state of matter similar to a gas that is composed partially or completely of charged particles such as ions and electrons which are not bound to each other.
- Batteries
  You can simulate batteries in Simcenter STAR-CCM+ using battery cells and battery cycling procedures that are defined either directly in Simcenter STAR-CCM+, or in the external software package Simcenter Battery Design Studio.
- Casting
  Casting simulations are performed using transient multiphase simulations using the VOF model with solidification. Conjugate heat transfer is applied between the solidifying melt and the solid mold.
- Region Sources
  This section provides some guidelines on how to use region sources for some common problems.
- Cell Quality Remediation
  The Cell Quality Remediation model helps you get solutions on a poor-quality mesh. This model identifies poor-quality cells, using a set of predefined criteria, such as Skewness Angle exceeding a certain threshold. Once these cells and their neighbors have been marked, the computed gradients in these cells are modified in such a way as to improve the robustness of the solution.
- Guidelines for Applying Simcenter STAR-CCM+
  This part of the documentation provides guidelines on applying Simcenter STAR-CCM+ models to your applications.
- Common Solvers Reference
  In Simcenter STAR-CCM+, solvers compute the solution during the simulation run.

Wall Porosity Model Reference

The wall porosity model uses the porous inertial resistance coefficient $α$ and the porous viscous coefficient $β$ to determine the pressure drop across a porous boundary. The ambient pressure is also used to calculate the pressure drop from which the phase extraction mass is calculated.

Table 1. Wall Porosity Model Reference
Theory	See Wall Porosity.
Provided By	[physics continuum] > Models > Multiphase > Eulerian Phases > [phase] > Models > Optional Models
Example Node Path	[physics continuum] > Models > Multiphase > Eulerian Phases > [phase] > Models > Wall Porosity
Requires	Physics continuum selections: Material: Multiphase (Automatically activates Multiphase Interaction) Multiphase Model: Volume of Fluid (VOF) (Automatically activates Segregated Flow, Gradients) A Eulerian phase, with the following selections: Material: Gas or Liquid Optional Models: Wall Porosity
Properties	Key property is: Max. Mass Extraction Factor. See Wall Porosity Properties.
Activates	Boundary Inputs	See Boundary Settings.
	Field Functions	See Field Functions.

Wall Porosity Properties

Max. Mass Extraction Factor

Specifies the maximum mass fraction that can be removed from a cell. The default is 0.5.

This property prevents the model from extracting more mass from a cell than is available in that particular cell.

Do not change the value of this property unless you are thoroughly familiar with the discretization techniques used in Simcenter STAR-CCM+.

Boundary Settings

Wall Boundaries

Wall Porosity Specification

Method	Corresponding Physics Value Nodes
None	None
Specified	Ambient Pressure The ambient pressure is specified relative to the reference pressure. It is not an absolute value. Porous Inertial Resistance The porous inertial resistance coefficient $α$ in Eqn. (2641). Porous Viscous Resistance The porous viscous coefficient $β$ in Eqn. (2641).

Field Functions

These field functions are available for every Eulerian single-component gas phase that has the Wall Porosity model selected.

VOF Porous Wall Flux: The actual mass flow per unit area through the porous wall (kg/m²s).
VOF Porous Wall Target Normal Velocity: The target velocity normal to the porous wall $v_{n}$ calculated according to (Eqn. (2641)) and the pressure drop $Δ p$ .
This function is not the actual normal velocity, which is equal to VOF Porous Wall Flux divided by the phase density.