Swirl Inlet - Boundary Condition

Swirl Inlet is a "Fixed Value" type velocity boundary condition that describes a swirling inflow, by specifying a central axis, central point, axial, radial, and tangential velocity profiles. The three components of swirl flow (axial, radial, and tangential) can be constant or time-dependent functions.

Mathematically, Swirl Inlet constructs the inlet velocity vector in the following way:

\(\vec U = U_z + U_r + U_{\phi} \)

where:
\(U_z\) - axial component,
\(U_r\) - radial component,
\(U_{\phi}\) - tangential component.

Each component is calculated in the following way:

\(U_z = U_{axial} \cdot \vec a\),
\(U_r = U_{radial} \cdot \vec r\),
\(U_\phi = U_{tangential} (\vec a \otimes \vec r)\),

where:
\(\vec a\) - unit-length vector in an axial direction,
\(\vec r\) - unit-length vector in a radial direction.

Axial (\(U_{axial}\)), radial (\(U_{radial}\)) and tangential velocity (\(U_{tangential}\)) components are taken directly from the User input.

Swirl Inlet is similar to Cilindrical Inlet, however, the key difference is the definition of tangential velocity components. The first one (Swirl Inlet) allows user to define a real velocity profile, while the latter define revolutions per minute which are later internally transformed to rotational speed \(\omega\). Additionally, when a Table option is used (see Swirl Inlet in SimFlow section), all velocity components are radially dependent, and not time-dependent like for Cilindrical Inlet.

Swirl Inlet describes an inflow naturally expressed in cylindrical coordinates. It is perfectly suited for CFD simulations of cyclone separators, swirl burners, annular sonic nozzles or rotating-mesh mixers.

To define Swirl Inlet on the domain’s inlet, the proper option must be selected from the drop-down menu for velocity in Boundary Conditions tab - Figure 1

The following parameters need to be defined:
Axis - axis of the coordinate system,
Origin - origin of the coordinate system,
Axial - axial velocity,
Radial - radial velocity,
Tangential - rotational speed used to define the tangential velocity component.

Axial, radial and RPM can be defined as a constant values or as time-dependent values, which is presented in Figure 2 and Figure 3.

When table definition is used, the radial-dependency is introduced.

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