In other cases, you may know exactly how a fluid behaves and which equations, models, or physics interfaces best describe it, but because the model is so complex it is difficult to reach an immediate solution. Simpler assumptions may need to be made to solve the problem, and other physics interfaces might be better to fine-tune the solution process for the more complex problem. The next section gives you an overview of each of the single-phase flow interfaces to help you choose.

The Single-Phase Flow Interface Options

Several of the interfaces vary only by one or two default settings (see Table 4-1) in the

Physical Model section, which are selected either from a check box or drop-down list. For the Single-Phase Flow branch, the Laminar Flow, Turbulent Flow k- , Turbulent Flow k- , Turbulent Flow Low Re k- , Turbulent Flow Spalart-Allmaras and Creeping Flow interfaces all have the same Interface Identifier (spf), and the differences are based on the default settings required to model that type of flow as described below. Figure 4-1 shows the Laminar Flow interface's Settings window with drop-down lists to choose the type of compressibility (incompressible or compressible at Mach numbers below 0.3) and the turbulence model (or none for laminar flow), and a check box to model Stokes flow by neglecting the inertial term.

TABLE 4-1: THE SINGLE-PHASE FLOW PHYSICAL MODEL DEFAULT SETTINGS

T H E M E C H A N I S M S F O R M O D E L I N G S I N G L E - P H A S E F L O W I N T E R F A C E S | 83

TABLE 4-1: THE SINGLE-PHASE FLOW PHYSICAL MODEL DEFAULT SETTINGS

L A M I N A R F L O W

The Laminar Flow Interface () is used primarily to model flows of comparably low Reynolds number. The interface solves the Navier-Stokes equations, and by default assumes that a flow can be compressible; that is, the density is not assumed to be constant.

Compressible flow is possible to model in this interface at speeds of less than 0.3 Mach, but you have to maintain control over the density and any of the mass balances that are deployed to help with this. You can also choose to model incompressible flow and simplify the equations to be solved.

This interface also allows for easy definition of non-Newtonian fluid flow through access to the dynamic viscosity in the Navier-Stokes equations. You can model the fluid using the power law and Carreau models or enter another expression that describes the dynamic viscosity appropriately.

You can also describe other material properties such as density by entering equations that describe this term as a function of other parameters such as material concentration, pressure, or temperature. Many materials in the material libraries use temperatureand pressure-dependent property values. If the density is affected by temperature, the Non-Isothermal Flow interface may be applicable (see Non-Isothermal Flow Branch).

84 | C H A P T E R 4 : S I N G L E - P H A S E F L O W B R A N C H

Figure 4-1: The Settings window for the Laminar Flow interface. Model compressible or non-compressible flow, laminar or turbulent flow, and Stokes flow. Combinations are also possible.

T U R B U L E N T F L O W

The Single-Phase Flow, Turbulent Flow Interfaces () model flow of high Reynolds numbers. The interface uses the Reynolds-averaged Navier-Stokes (RANS) equations and solves for the filtered velocity field and filtered pressure as well as a model for the turbulent viscosity.

There are three turbulence models available—a basic k- model, a k- model, a Low Reynolds number k- model and the Spalart-Allmaras model. Each model has its merits and weaknesses. See also the Theory for the Turbulent Flow Interfaces for more details.

T H E M E C H A N I S M S F O R M O D E L I N G S I N G L E - P H A S E F L O W I N T E R F A C E S | 85