Flow Rate Inlet Velocity - Boundary Conditions

Flow Rate Inlet Velocity is the velocity boundary condition derived from the flux (either volumetric or mass-based) applied in the direction normal to the patch. The condition adjusts the velocity in such a way to match the specified volumetric or mass flow rate.

Flow Rate Inlet Velocity can also extrapolate the velocity profile from the interior of the domain, which is useful in the scenarios when downstream the flow is developed. It should be considered as an extension to Fixed Value, which enforces specified value at the boundary.

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 [\(\frac{kg}{s}\)].

Flow Rate Inlet Velocity is implemented as an extension to the Fixed Value boundary condition. The velocity value at the boundary is fixed. The enforced vector value is calculated based on the specified flow rate, the surface area of the patch, the direction of the normal vector to the patch, and the fluid density in the case of incompressible flow.

Flow Rate Inlet Velocity can be defined as a constant or time-dependent value. Time-dependent values are defined in the form of the table. The value is positive into the domain (as an inlet).

By default, the prescribed flow rate is uniform at the inlet, but it is possible to adjust the profile to the downstream cells. To do this, the Extrapolate Profile option must be activated. Then, the solver calculates the velocity profile at the inlet by extrapolating from the interior field values.

This means it uses the velocity field inside the domain to estimate the velocity at the inlet boundary. Without the Extrapolate Profile option, the boundary condition typically assumes a uniform velocity profile across the inlet. However, with the extrapolation, it allows for non-uniform velocity profiles. This can be more realistic in scenarios where the flow entering the domain might have a developed or varying profile.

It should be noted that the extrapolation can lead to numerical instabilities and may require more iterations to converge. This is because the solver iteratively adjusts the velocity field to match the specified flow rate and the extrapolated profile to the upstream results.

The Flow Rate Inlet Velocity boundary condition is used to specify an inlet boundary condition where the velocity of the fluid entering the domain is defined based on a specified volumetric or mass-based flow rate. This is useful when the exact velocity profile is not known, but the total flow rate is specified.

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

Flow Rate Inlet Velocity can be defined as a constant value where a specified filed is not changing in time - Figure 2. Alternatively, the User can switch to the table, where a time-dependent value can be defined.

The required input will define either the mass-based flux [\((kg)/s\)] or the volumetric flux [\(m^3/s\)] depending on the solver.

The User needs to define:
Flow Rate - either constant or time-dependent (in the form of a table) [\((kg)/s\)] or [\((m^3)/s\)]
Extrapolate Profile - profile extrapolation option, by default, it is turned off (see description above)

In this section, we propose boundary conditions that are alternative to Flow Rate Inlet 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.