Flow Rate Outlet Velocity - Boundary Condition

The Flow Rate Outlet Velocity boundary condition is designed for patches where fluid exits the computational domain. Its primary purpose is to ensure that a specified mass flow rate or volumetric flow rate is maintained at the boundary. In simpler terms, it adjusts the velocity profile on the patch to match the target flow rate provided.

The boundary condition can be applied for both incompressible and compressible flows. When applied for incompressible flows, the flow rate is the volumetric expressed in [\(\frac{m^3}{s}\)]. When applied for compressible flows, the flow rate is mass-based in [\(\frac{kg}{s}\)]. The user specifies a desired total flow rate. The condition ensures that the integral of the velocity normal component over the patch area matches that prescribed rate.

Although a total flow rate is specified, the local velocity profile is not specified strictly by the user. The solver will determine how the velocity is distributed spatially across the boundary to meet the total prescribed outflow.

Flow Rate Outlet Velocity can be defined as a constant or time-dependent value. Time-dependent values are defined in the form of the table.

The Flow Rate Outlet Velocity boundary condition functions similarly to Flow Rate Inlet Velocity, but with one crucial difference: it applies the specified flow rate at the outlet rather than the inlet of the domain. While Flow Rate Inlet Velocity controls the flow rate of fluid entering the simulation, Flow Rate Outlet Velocity ensures that the specified flow rate is maintained as fluid exits the domain. This distinction becomes particularly significant when the Extrapolate Profile option is enabled, as the flow rate is adjusted based on upstream conditions (for outlet flow rate control) or downstream conditions (for inlet flow rate control).

The Flow Rate Outlet Velocity boundary condition ensures that a specified flow rate exits through an outlet patch. Instead of imposing a fixed local velocity profile, it adjusts the distribution to ensure that the total integrated mass or volumetric flow matches the target value. This approach represents physical scenarios where the total outflow—such as through a vent, exhaust duct, or downstream boundary—is known. However, the precise local velocity pattern is determined by the internal flow field and the solver’s balance of momentum and pressure.

The definition of boundary conditions in SimFlow is both simple and intuitive. To specify the Flow Rate Outlet Velocity boundary condition, the user must navigate to the Boundary Conditions panel, select the appropriate boundary, and choose the correct option from the drop-down menu.

Flow Rate Outlet Velocity can be defined as a constant value where a specified filed (scalar or vector) is not changing in time - Figure 1.

Alternatively, the User can switch to table, where time-dependent value can be defined - Figure 2.

User needs to define:
Flow Rate - either constant or time-dependent (in the form of a table) [\((kg)/s\)] or [\((m^3)/s\)]

In this section, we propose boundary conditions that are alternative to Flow Rate Outlet Velocity. While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.