Selecting the Physics Models

• Three Dimensional
• Two Dimensional
• Axisymmetric—select this model for two-dimensional axisymmetric flow domains.

For more information, see Modeling Space.

• Gas
• Liquid

For more information, see Modeling a Multi-Component Mixture.

• Segregated Flow—this model invokes the segregated flow solver, which solves the governing equations in a sequential, uncoupled manner. For most incompressible or mildly compressible flow applications, you choose this solution method. It is robust and memory efficient.
• Coupled Flow—this model invokes the coupled flow solver, which solves the governing equations in a coupled manner. You choose this solution method if you want to simulate highly compressible flows such as transonic, supersonic, or hypersonic flows, or rotational flows as in turbomachinery. In addition to compressible flows, choose this model for natural convection problems and flows with large body forces or energy sources.

For more information, see Flow Models Reference.

• Contant Density—select this model for fluid flows where the density is invariant throughout the continuum.
• Ideal Gas (only for gases)—select this model to express density as a function of temperature and pressure using the ideal gas equation.
• Thermal Non Equilibrium Ideal Gas (only for gases and coupled flow)—select this model for flows at high temperatures and low densities, where the vibrational/electronic energy modes become active yet the density is low enough that equilibration does not occur.
• Real Gas (only for gases)—select this model to account for non-ideal behavior such as compressibility effects, variable specific heat, van der Waals forces, and non-equilibrium thermodynamic effects. The real gas model is suitable for modeling complex chemistry combustion applications at high pressures and low temperatures in transcritical and supercritical environments, such as in liquid rocket engines.
• Polynomial Density—select this model to describe the density as a polynomial function of temperature.
• IAPWS-IF97 (Water) (only for liquids)—select this model for liquid water. The material properties follow the IAPWS-IF97 specification.
• User Defined EOS—select this model when the other Equation of State models do not adequately describe your working fluid. This model allows you to specify the density and density derivatives using user-defined expressions or tables of property data.

For more information, see Equation of State Models Reference.

All Equation of State models—apart from Contant Density and User Defined EOS—require the selection of an Energy model. For more information, see Heat Transfer.

• For a steady-state simulation, select Steady.
• For a simulation that considers unsteady effects and a time-dependent behaviour of the flow, select one of the following models depending on the selected Flow model:
  • For the Segregated Flow model, one of:
    • Implicit Unsteady—this model invokes the SIMPLE algorithm.
    • PISO Unsteady—this model invokes the PISO algorithm.
    The PISO algorithm is suited for transient cases where the convective Courant number is small. Therefore, you use SIMPLE for cases where large time-step sizes can be used, such as in achieving a steady state solution through time-marching. Use PISO where the problem demands use of small time-step size and greater temporal accuracy.
  • For the Coupled Flow model, select one of:
    • Implicit Unsteady—this model is appropriate if the time scales of the phenomena of interest are either of the following:
      • The same order as the convection and/or diffusion processes (for example, vortex shedding)
      • Related to some relatively low frequency external excitation (for example, time-varying boundary conditions or boundary motion)
    • Explicit Unsteady (not for constant density or polynomial density flows)—this model is the proper choice if the unsteady time scales are of the order of the acoustic processes (for example, shock front tracking). The Explicit Unsteady model is unsuited for incompressible flow simulations.

For more information, see Modeling Time.

• Inviscid—this model neglects the viscous effects in simulating the equations of motion. The solution of the resulting Euler equations (as opposed to the Navier-Stokes equations) generally results in significant savings of computer resources. Boundary layers and other viscous effects are not resolved. This approximation is only valid for certain physical situations, such as high-Reynolds number compressible aerodynamics.
• Laminar—select this model for well-ordered fluid flow, free of macroscopic, non-repeating fluctuations. Laminar flows occur in nature when the Reynolds number (the ratio of viscous to inertial forces) is low enough that transition to turbulence does not occur.
• Turbulent—select this model for fluid flow that is in a state of continuous instability, exhibiting irregular, small-scale, high-frequency fluctuations in both space and time. It is strictly possible to simulate turbulent flow directly by resolving all the scales of the flow (termed direct numerical simulation). However, the compute resources that are required are too large for practical flow simulations. Therefore, a suitable turbulence modeling approach must be selected. For more information, see Turbulence.

For more information, see Viscous Regime Models Reference.

• If you want to account for the effect of gravitational acceleration, select one of:
  • Gravity—select this model when the density is temperature-dependent.
  • Boussinesq Model (only for constant density flows)—select this model to enhance convergence of natural convection simulations when there are only small variations of density due to temperature variations.

  For more information, see Gravitational Flow Models Reference.

• For laminar flows of gases, if you want to model partial slip at the wall, select Maxwell Slip. It is usually used for situations where the slip or the no-slip wall boundary condition do not apply. Such situations include rarefied flows where the Knudsen number is between 0.01 and 0.1. To model a temperature slip at the wall, also select von Smoluchowski Slip. For more information, see Partial Slip Models Reference.
• For an axisymmetric simulation, if you want to model swirling or rotating flow about the central axis, select Axisymmetric Swirl. You can use this model when your simulation includes a swirling flow from an inlet. For more information, see Axisymmetric Swirl Model Reference.
• If you want to track isolated vortices over long distances, select the Vorticity Confinement Model. This model adds a force term to the momentum equations that prevents the rapid dissipation of vortices. For more information, see Vorticity Confinement Model Reference.
• If you want to trace the fluid flow with variables of arbitrary value, select Passive Scalar. Passive scalars have no appreciable mass or volume and do not affect the physical properties of the simulation. For more information, see Passive Scalars.
• For an unsteady simulation, if you want to adjust the time-step automatically during the run to attain a specified temporal resolution, select Adaptive Time-Step. Adaptive time-stepping can improve the stability and run-time of your simulation and the accuracy of the results and is particularly useful for cases with large variations of flow topology or large varying time scales of the physics. For more information, see Adaptive Time-Stepping.