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Theory
Simcenter STAR-CCM+ simulations are built on numerical algorithms that solve relevant laws of physics according to the conditions that the simulation defines. So that you can identify the physical laws, and understand the methods by which they are solved, Simcenter STAR-CCM+ comes with a Theory Guide.
Fluid Flow
Simcenter STAR-CCM+ can simulate internal and external fluid flow across a wide range of flow regimes, and for a variety of fluid types. It solves the conservation equations for mass, momentum, and energy for general incompressible and compressible fluid flows.
Boundary Conditions
To complete the mathematical model, you specify conditions on the solution domain boundary.
Velocity Inlet
The velocity inlet boundary is usually used as an inflow condition where the distributions of velocity and fluid properties are known. At this inflow boundary, the conditions are prescribed and used to calculate the inlet volume flux as well as the fluxes of momentum and energy.

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Theory
Simcenter STAR-CCM+ simulations are built on numerical algorithms that solve relevant laws of physics according to the conditions that the simulation defines. So that you can identify the physical laws, and understand the methods by which they are solved, Simcenter STAR-CCM+ comes with a Theory Guide.
- Fundamental Equations
  Simcenter STAR-CCM+ models a range of physics phenomena including fluid mechanics, solid mechanics, heat transfer, electromagnetism, and chemical reactions. On a macroscopic scale, where the typical lengths are much greater than the inter-atomic distances, the discrete structure of matter can be neglected and materials can be modeled as continua. The mathematical models that describe the physics of continua are derived from fundamental laws that express conservation principles.
- Fluid Flow
  Simcenter STAR-CCM+ can simulate internal and external fluid flow across a wide range of flow regimes, and for a variety of fluid types. It solves the conservation equations for mass, momentum, and energy for general incompressible and compressible fluid flows.
  - Axisymmetric Flow
    For fluid flows that are symmetric about a central axis in terms of geometry and flow conditions, Simcenter STAR-CCM+ can model the flow on a 2D solution domain.
  - Equations of State
    The equations of state are constitutive relations that describe the relation between the density and the internal energy to the two basic thermodynamic variables pressure and temperature.
  - Newtonian Fluids
    A Newtonian fluid is described by an explicit constitutive equation that relates the viscous stress tensor $T$ to the velocity field through a constant viscosity. The relation between the shear stress and the shear rate is linear.
  - Non-Newtonian Fluids
    For many industrial fluids, it is not sufficient to assume a linearly viscous fluid behavior due to their complex microstructure. These fluids exhibit non-Newtonian behavior.
  - Boundary Conditions
    To complete the mathematical model, you specify conditions on the solution domain boundary.
    - Stagnation Inlet
      The stagnation inlet boundary is an inflow condition at which you prescribe the total values for pressure and temperature as well as the flow direction. The stagnation conditions refer to the conditions in an imaginary plenum, far upstream, in which the flow is completely at rest.
    - Velocity Inlet
      The velocity inlet boundary is usually used as an inflow condition where the distributions of velocity and fluid properties are known. At this inflow boundary, the conditions are prescribed and used to calculate the inlet volume flux as well as the fluxes of momentum and energy.
    - Mass Flow Inlet
      You can use the mass flow inlet boundary to specify the known mass flow rate or the mass flux (mass flow rate per unit area) at a boundary. This boundary condition is designed primarily for specifying mass flow inward across the boundary, but also works for specifying mass flowing outward. Mass flow inlet is available for both compressible and incompressible flow regimes.
    - Pressure Outlet
      The pressure outlet boundary is an outflow condition which imposes the working pressure. This boundary pressure can be considered as the static pressure of the environment into which the fluid enters.
    - Outlet
      The outlet boundary represents an outflow condition where no backflow occurs. You apply the outlet boundary at locations of the computational domain where the flow is directed outwards only. As such, the outflow conditions are not prescribed, but they are determined by the flow upstream of the outlet boundary. You can place more than one outlet boundary in your computational domain.
    - Symmetry Plane
      A symmetry plane boundary represents an imaginary plane of symmetry in the simulation. You can use the symmetry plane boundary condition to reduce the extent of the computational domain in locations where the geometry and the flow are symmetric. The symmetry plane boundary can also be used as a planar "slip wall" with the shear stress being zero.
    - Wall
      A wall boundary represents an impermeable surface that confines fluid or solid regions.
    - Free-Stream
      The free-stream boundary allows you to model free-stream compressible flow conditions at a far-field boundary.
  - Gravity
    Forces due to gravity often play an important role in the momentum balance for fluids and solids. The body forces due to gravity are added to the momentum equations as a source term.
  - Vorticity Confinement
    Many areas of engineering today require tracking isolated vortices accurately over long distances.
  - Volume Partitioning
    When simulating two-way coupled Lagrangian multiphase flow or Dispersed Multiphase (DMP) flow, the actual volume that is occupied by the continuous fluid and the actual area over which fluxes are computed are reduced by the presence of the dispersed phase. For Fluid Film simulations, the available volume for the fluid neighbouring the fluid film is smaller than the cell volume. Similarly, in porous media modeling using the physical velocity formulation, a fraction of the cell volume is occupied by a solid phase, thus reducing the available volume for the fluid. For two-dimensional models, the mesh is assumed to have a unit depth (in SI units).
  - Fluid Flow Bibliography
- Numerical Flow Solution
  In the finite volume method, the solution domain is subdivided into a finite number of small control volumes, corresponding to the cells of a computational grid.
- Turbulence
- 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 Treatment
  Walls are a source of vorticity in most flow problems of practical importance. Therefore, an accurate prediction of the flow across the wall boundary layer is essential.
- 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.
- Heat Transfer
  Heat transfer is the exchange of thermal energy between media at different temperatures. Heat transfers from locations of high temperature to locations of low temperature in order to reach an equilibrium state. The three main mechanisms of heat transfer are: conduction, convection, and radiation.
- Porous Media
  Porous media are continua that contain both fluid and fine-scale solid structures, for example: packed-bed chemical reactors, filters, radiators, honeycomb structures, or fibrous materials. The solid geometrical structures are too fine to be individually meshed and fully resolved by a computational grid.
- Species and Passive Scalars
  Simcenter STAR-CCM+ models the transport, the mixing, and the chemical reactions of multi-component liquids or multi-component gases by solving conservation equations for scalar variables that represent the mass fraction of each species in the mixture. This conservation equation allows for convection, diffusion, and optional source effects, and is solved in addition to the global mass continuity equation.
- Multiphase—Eulerian Approach
  Multiphase flows, where several fluids flow in the domain of interest, play an important role in variety of industrial applications. In general, we associate phases with gases, liquids or solids and as such some simple examples of multiphase flows are: air bubbles rising in a glass of water, sand particles carried by wind, rain drops in air. The definition of phase can be generalized and applied to other fluid characteristics such as size, shape, density, and temperature.
- Multiphase—Lagrangian Approach
  Multiphase flows are found in a wide variety of industrial processes, some examples of which are internal combustion engines, liquid, or solid fueled combustors, spray driers, cyclone dust separators, and chemical reactors. Multiphase in this context refers to one thermodynamic phase, be it a solid, a liquid, or a gas, interacting with another distinct phase.
- Reacting Flow
  In reacting flows, chemical species mix with each other and react when conditions allow. To model these flows, Simcenter STAR-CCM+ couples species and energy transport equations with the chemistry solvers that compute the source terms.
- Internal Combustion Engines
  Numerical modeling of internal combustion engines plays an increasingly important role in improving engine design. CFD simulation of such engines can give a comprehensive insight into the wide-ranging physics of the device, such as turbulent mixing between fuel and air, the ignition, and combustion chemistry.
- Electrochemistry
  Electrochemistry is the discipline investigating relationships between electrical currents and chemical composition change in general.
- 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.
- Electromagnetism
  Many engineering applications, such as electric motors, electric switches, and transformers, involve electromagnetic phenomena. Electromagnetic phenomena can be modeled based on the classical theory of Electromagnetism, which describes the interaction between electrically charged particles in terms of electric fields, magnetic fields, and their mutual interaction.
- Batteries
- Solid Mechanics
  Solid Mechanics describes the behavior of a solid continuum in response to applied loads. Applied loads include body forces, surface loads, point forces, or thermal loads that result from changes in the solid temperature. Applied loads induce a stress field in the structure and can cause displacement of the structure—from an initial undeformed configuration to a deformed configuration.
- Aeroacoustics
  Computational aeroacoustics (CAA) is a branch of multiphysics modeling and simulation that involves identifying noise sources that are induced by fluid flow and propagation of the sound waves then generated.
- Finite Element Method
  The Finite Element method is a powerful tool for finding approximate solutions to continuous problems. The methodology is similar to other numerical techniques that approximate continuous partial differential equations with discrete algebraic equations.
- Mesh Motion
  In transient simulations, mesh motion is a numerical technique that allows you to update the position of the computational domain while the solvers run.
- 6-DOF Rigid Body Motion
  Simcenter STAR-CCM+ allows you to model the motion of a rigid body in response to applied forces and moments. In a rigid body, the relative distance between internal points does not change. Therefore, it is sufficient to solve the equations of motion for the center of mass of the body.
- Rotating Flow
  Rotating flow usually occurs in rotating machinery such as pumps, propellers, fans, wind turbines, and so on.
- Harmonic Balance
  Harmonic Balance equations describe periodic unsteady flows where the unsteady frequencies are known beforehand. They are suited to modeling periodically repeating flow fields that typically occur in turbomachinery such as compressors, turbines, and fans.
- Adjoint
  The adjoint method is an efficient means to predict the influence of many input parameters on some engineering quantities of interest in a simulation.
- Design Manager
  Design Manager provides an automated approach within Simcenter STAR-CCM+ to run design exploration studies. Design exploration covers both performance assessment and optimization.
- Scalars, Vectors, and Tensors
  Most physical quantities in Simcenter STAR-CCM+ are either scalars, vectors, or 2nd-order tensors.
- Solution Data Interpolation
  Many operations require interpolation of solution data between different sets of mesh points. For example, remeshing operations require interpolation of existing solution data onto a new mesh. Data interpolation also occurs at the interface between regions, where contacting boundaries exchange data. Additionally, configurable data mappers allow you to interpolate field functions and tabular data to specified meshes.
- Idealizations
  In many simulations, it is common practice to reduce the size of the computational domain using planes of symmetry, axes of rotation, periodicity, or by reducing the 3D domain to a 2D domain. However, when you calculate physical quantities using reports you generally want to account for the whole domain. You can obtain report values for the full model using idealizations.

Velocity Inlet

The velocity inlet boundary is usually used as an inflow condition where the distributions of velocity and fluid properties are known. At this inflow boundary, the conditions are prescribed and used to calculate the inlet volume flux as well as the fluxes of momentum and energy.

Boundary Inputs

For a velocity inlet boundary, you specify the following variables:


Inputs	Incompressible Equation of State	Compressible Equation of State (use only for subsonic compressible flow)
velocity $v_{s p e c}$	✓	✓
inflow direction $θ_{s p e c}$	✓	✓
static temperature $T_{s, s p e c}$	✓	✓

The velocity

v_{s p e c}

is specified with respect to a reference frame, which can be the laboratory, the region, or the local reference frame. The flow direction is either given as normal to the boundary, as individual angle components, or directly as flow direction angles.

Computed Values

The velocity inlet boundary condition is valid for incompressible and (subsonic) compressible flows. At the boundary, Simcenter STAR-CCM+ imposes or computes the following values, depending on the flow situation:

velocity $v$
static pressure $P_{s}$
static temperature $T_{s}$

Incompressible and Compressible Equation of State

The boundary face velocity is assigned the specified velocity at the boundary:

Figure 1. EQUATION_DISPLAY

v = v_{s p e c} \cdot θ_{s p e c}

(780)

The static pressure $P_{s}$ is extrapolated from the static pressure at the interior of the domain:

Figure 2. EQUATION_DISPLAY

P_{s} = P_{s}^{e x t}

(781)

For non-isothermal cases, the temperature at the boundary face is computed as follows:

Inflow: Under inflow conditions, the face value of static temperature is assigned the specified static temperature:; Figure 3. EQUATION_DISPLAY

$T_{s} = T_{s, s p e c}$
(782)
Outflow: Under outflow conditions, the face value of static temperature is extrapolated from the adjacent cell:; Figure 4. EQUATION_DISPLAY

$\begin{matrix} T_{s} \end{matrix} = T_{s}^{e x t}$
(783)

The density $ρ$ and the total enthalpy $H_{t}$ are calculated as:

Figure 5. EQUATION_DISPLAY

\begin{matrix} ρ & = & ρ (P_{s}, T_{s}) \\ H_{t} & = & H_{s} (P_{s}, T_{s}) + \frac{v^{2}}{2} \end{matrix}

(784)