The Laminar Bubbly Flow (bf) interface (), found under the Multiphase Flow>Bubbly Flow branch () when adding a physics interface, is used to model the flow of liquids with dispersed bubbles at low and moderate Reynolds numbers.

When this physics interface is added, the following default physics nodes are also added in the Model Builder — Laminar Bubbly Flow, Fluid Properties, Wall (the default boundary types are No slip for the liquid and No gas flux for the gas), 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 Laminar Bubbly 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 bf.

The Low gas concentration check box is selected by default. This approximation is valid if the gas volume fraction is low ( less than a few percent) and its density does not have any significant effects on the continuity equation. It is then generally valid to replace the continuity equation in The Bubbly Flow Equations; see Equation 6-31 with Equation 6-34).

To add a transport equation for the bubble density in order to determine the interfacial area, select the Solve for interfacial area check box.

Reference values are global quantities used to evaluate the density of the liquid and the absolute pressure pA.

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 used to define the density of the liquid. The default Reference pressure level pref (SI unit: Pa) is 1[atm].

The reference temperature is used to define the density of the liquid. The default Reference temperature Tref (SI unit: K) is 293.15[K].

For 2D axisymmetric models, select the Swirl flow check box to include the swirl velocity component — that is, the velocity component in the azimuthal direction. While can be nonzero, there can be no gradients in the  direction. Also see General Single-Phase Flow Theory in the CFD Module User’s Guide.

The dependent variables (field variables) are the Velocity field, liquid phase u (SI unit: m/s), the Pressure p (SI unit: Pa), the Effective gas density rhogeff (SI unit: kg/m3), and the Number density, gas phase nd (SI unit: 1/m3). The names can be changed but the names of fields and dependent variables must be unique within a component.

To display this section, click the Show More Options button () and select Stabilization in the Show More Options dialog box. This section contains the settings for stabilization of the momentum transport (the fluid flow) in the Momentum transport area and stabilization of the equation for the dispersed phase in the Gas phase transport area.

To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. Normally these settings do not need to be changed.

Select the Use pseudo time stepping for stationary equation form check box to add pseudo time derivatives to the equation when the Stationary equation form is used. When selected, also choose a CFL number expression — Automatic (the default) or Manual. Automatic sets the local CFL number (from the Courant–Friedrichs–Lewy condition) to the built-in variable CFLCMP which in turn triggers a PID regulator for the CFL number. For Manual enter a Local CFL number CFLloc (dimensionless).