The Laminar Flow Interface

The interface (

) is used to compute the velocity and pressure fields for the flow of a single-phase fluid in the laminar flow regime. A flow remains laminar as long as the Reynolds number is below a certain critical value. At higher Reynolds numbers, disturbances have a tendency to grow and cause transition to turbulence. This critical Reynolds number depends on the model, but a classical example is pipe flow where the critical Reynolds number is known to be approximately 2000.

The physics interface supports incompressible flow, weakly compressible flow (the density depends on temperature but not pressure) and compressible flow at low Mach numbers (typically less than 0.3). It also supports flow of non-Newtonian fluids.

The equations solved by the Laminar Flow interface are the Navier-Stokes equations for conservation of momentum and the continuity equation for conservation of mass.

The Laminar Flow interface can be used for stationary and time-dependent analyses. Time-dependent studies should be used in the high-Reynolds number regime as these flows tend to become inherently unsteady.

When the Laminar Flow interface is added, the following default nodes are also added in the : , (the default boundary condition is ), and . Other nodes, that implement, for example, boundary conditions and volume forces, can be added from the toolbar or from the context menu displayed when right-clicking .

The is the default physics interface name.

The is used primarily as a scope prefix for variables defined by the physics interface. Physics interface variables can be referred to using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the field. The first character must be a letter.

The default (for the first physics interface in the model) is spf.

Depending of the fluid properties and the flow regime, three options are available for the option. In general the computational complexity increases from to to but the underlying hypotheses are increasingly more restrictive in the opposite direction.

When the option (default) is selected, the incompressible form of the Navier-Stokes and continuity equations is applied. In addition, the fluid density is evaluated at the and at the defined in . The fluid dynamic viscosity is evaluated at the .

The option models compressible flow when the pressure dependency of the density can be neglected. When selected, the compressible form of the Navier-Stokes and continuity equations is applied. In addition, the fluid density is evaluated at the defined in .

When the option is selected, the compressible form of the Navier-Stokes and continuity equations is applied. Ma < 0.3 indicates that the inlet and outlet conditions, as well as the stabilization, may not be suitable for transonic and supersonic flow. For more information, see The Mach Number Limit.

With the addition of various modules, the check box is available. Selecting this option, a node, a node, and a subnode are added to the physics interface. These are described for the interface in the respective module’s documentation. The can be applied on all domains or on a subset of the domains.

Reference values are global quantities used to evaluate the density and viscosity of the fluid when the or the option is selected.

There are generally two ways to include the pressure in fluid flow computations: either to use the absolute pressure pA=p+pref, or the gauge pressure p. When pref is nonzero, the physics interface solves for the gauge pressure whereas material properties are evaluated using the absolute pressure. The reference pressure level is also .

The reference temperature is 293.15 K.

When is selected, the reference position can be defined. It corresponds to the location where the total pressure (that includes the hydrostatic pressure) is equal to the

The following dependent variables (fields) are defined for this physics interface—the u and its components, and the p.

If required, the names of the field, component, and dependent variable may be edited. Editing the name of a scalar dependent variable changes both its field name and the dependent variable name. If a new field name coincides with the name of another field of the same type, the fields share degrees of freedom and dependent variable names. A new field name must not coincide with the name of a field of another type or with a component name belonging to some other field. Component names must be unique within a model except when two fields share a common field name.

To display this section, click the button (

) and select . Normally these settings do not need to be changed.

The option adds pseudo time derivatives to the equation when the form is used in order to speed up convergence. When selected, a should also be defined. For the default option, the local CFL number (from the Courant–Friedrichs–Lewy condition) is determined by a PID regulator.