Cylindrical Inlet - Boundary Conditions

Cylindrical Inlet is a "Fixed Value" type velocity boundary condition that describes a swirling inflow by giving the axial and radial components of \(\vec U\) in a cylindrical coordinate system. This boundary condition is typically applied when the inflow has a cylindrical pattern — for example, flow entering through a pipe or nozzle — and the velocity profile is non-uniform across the inlet cross-section.

Cylindrical Inlet calculates axial, radial and tangential components of the velocity, based on the provided input data (center of rotation, rotational axis, axial and radial velocity and rotational speed).

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

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

Each component is calculated in the following way:

\(U_\phi = \omega \cdot (\vec a \otimes \vec d)\)
\(U_z = U_{"axial"} \cdot \vec a\)
\(U_r = U_{radial}\frac{d}{|d|}\)

where:
\(d\) - radial direction
\(\vec a\) - unit-length vector that points in the direction of the cylindrical axis
\(\omega\) - angular speed in [rad/s] - \(\omega = \frac{2pi}{60}\cdot RPM\)

The boundary condition is essential when the inlet’s flow physics is complex and needs an accurate representation of the velocity profile entering the computational domain. It helps in capturing the development of the flow correctly, especially in the regions close to the inlet where flow development occurs. By setting this condition, the user instructs the solver to apply the specified velocity components at the boundary face, influencing how the fluid particles enter the simulation domain, thereby affecting the solution’s accuracy and stability.

Cylindrical 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 Cylindrical 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 needs to be defined:
Centre - centre of the coordinate system
Axis - axis of the coordinate system
Axial Velocity - axial velocity
Radial Velocity - radial velocity
RPM - rotational speed used to define tangential velocity

Axial, radial and RPM can be defined as constant values or as time-dependent values.

In this section, we propose boundary conditions that are alternative to Cylindrical 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.