## 9.5.1 Overview of Swirling and Rotating Flows

Axisymmetric Flows with Swirl or Rotation

Your problem may be axisymmetric with respect to geometry and flow conditions but still include swirl or rotation. In this case, you can model the flow in 2D (i.e., solve the axisymmetric problem) and include the prediction of the circumferential (or swirl) velocity. It is important to note that while the assumption of axisymmetry implies that there are no circumferential gradients in the flow, there may still be non-zero swirl velocities.

Momentum Conservation Equation for Swirl Velocity

The tangential momentum equation for 2D swirling flows may be written as

 (9.5-1)

where is the axial coordinate, is the radial coordinate, is the axial velocity, is the radial velocity, and is the swirl velocity.

Three-Dimensional Swirling Flows

When there are geometric changes and/or flow gradients in the circumferential direction, your swirling flow prediction requires a three-dimensional model. If you are planning a 3D FLUENT model that includes swirl or rotation, you should be aware of the setup constraints listed in Section  9.5.4. In addition, you may wish to consider simplifications to the problem which might reduce it to an equivalent axisymmetric problem, especially for your initial modeling effort. Because of the complexity of swirling flows, an initial 2D study, in which you can quickly determine the effects of various modeling and design choices, can be very beneficial.

 For 3D problems involving swirl or rotation, there are no special inputs required during the problem setup and no special solution procedures. Note, however, that you may want to use the cylindrical coordinate system for defining velocity-inlet boundary condition inputs, as described in Section  7.4.1. Also, you may find the gradual increase of the rotational speed (set as a wall or inlet boundary condition) helpful during the solution process. This is described for axisymmetric swirling flows in Section  9.5.5.

Flows Requiring a Rotating Reference Frame

If your flow involves a rotating boundary which moves through the fluid (e.g., an impeller blade or a grooved or notched surface), you will need to use a rotating reference frame to model the problem. Such applications are described in detail in Section  10.2. If you have more than one rotating boundary (e.g., several impellers in a row), you can use multiple reference frames (described in Section  10.3.1) or mixing planes (described in Section  10.3.2).

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