The Pipe Flow (pfl) interface (), found under the Single-Phase Flow branch () when adding a physics interface, is used to compute the velocity and pressure fields in pipes and channels of different shapes. It approximates the pipe flow profiles flow by 1D assumptions in curve segments, or lines. These lines can be drawn in 2D or 3D and represent simplifications of hollow tubes. The interface is available in 3D on edges, and in 2D on boundaries.

When this physics interface is added, these default nodes are also added to the Model Builder — Fluid Properties, Pipe Properties, Pressure, Lossless Fitting and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and volume forces. You can also right-click Pipe Flow to select physics features from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions 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 Name field. The first character must be a letter.

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

Select a Fluid model — Newtonian (the default), Power Law, or Bingham.

For Newtonian select Single phase (the default), Gas-Liquid, friction factor multiplier, or Gas-Liquid, effective Reynolds number.

When Gas-Liquid, friction factor multiplier or Gas-Liquid, effective Reynolds number are selected, the Phase Fractions node is automatically added as a default node. Selecting these options also enables the choice of Liquid material and Gas material for the Fluid Properties node.

For more information about the fluid models, see the section Theory for the Pipe Flow Interface.

To display this section, click the Show button () and select Discretization.

The Value type when using splitting of complex variables setting should in most transport problems be set to Real which is the default. It makes sure that the dependent variable does not get affected by small imaginary contributions, which can occur, for example, when combining a Time Dependent or Stationary study with a frequency-domain study. For more information, see Splitting Complex-Valued Variables in the COMSOL Multiphysics Reference Manual.