As discussed in Section 8.4.1, you can solve a 2D axisymmetric problem that includes the prediction of the circumferential or swirl velocity. The assumption of axisymmetry implies that there are no circumferential gradients in the flow, but that there may be non-zero circumferential velocities. Examples of axisymmetric flows involving swirl or rotation are depicted in Figures 8.4.3 and 8.4.4.
Problem Setup for Axisymmetric Swirling Flows
For axisymmetric problems, you will need to perform the following steps during the problem setup procedure. (Only those steps relevant specifically to the setup of axisymmetric swirl/rotation are listed here. You will need to set up the rest of the problem as usual.)
Define Models Solver...
Define Boundary Conditions...
!!Remember to use the axis boundary type for the axis of rotation.
The procedures for input of rotational velocities at inlets and at walls are described in detail in Sections 6.4.1 and 6.13.1.
Solution Strategies for Axisymmetric Swirling Flows
The difficulties associated with solving swirling and rotating flows are a result of the high degree of coupling between the momentum equations, which is introduced when the influence of the rotational terms is large. A high level of rotation introduces a large radial pressure gradient which drives the flow in the axial and radial directions. This, in turn, determines the distribution of the swirl or rotation in the field. This coupling may lead to instabilities in the solution process, and you may require special solution techniques in order to obtain a converged solution. Solution techniques that may be beneficial in swirling or rotating flow calculations include the following:
See Chapter 24 for details on the procedures used to make these changes to the solution parameters. More details on the step-by-step procedure and on the gradual increase of the rotational speed are provided below.
Step-By-Step Solution Procedures for Axisymmetric Swirling Flows
Often, flows with a high degree of swirl or rotation will be easier to solve if you use the following step-by-step solution procedure, in which only selected equations are left active in each step. This approach allows you to establish the field of angular momentum, then leave it fixed while you update the velocity field, and then finally to couple the two fields by solving all equations simultaneously.
!!Since the coupled solvers solve all the flow equations simultaneously, the following procedure applies only to the segregated solver.
In this procedure, you will use the Equations list in the Solution Controls panel to turn individual transport equations on and off between calculations.
In addition to the steps above, you may want to simplify your calculation by solving isothermal flow before adding heat transfer or by solving laminar flow before adding a turbulence model. These two methods can be used for any of the solvers (i.e., segregated or coupled).
Gradual Increase of the Rotational or Swirl Speed to Improve Solution Stability
Because the rotation or swirl defined by the boundary conditions can lead to large complex forces in the flow, your FLUENT calculations will be less stable as the speed of rotation or degree of swirl increases. Hence, one of the most effective controls you can apply to the solution is to solve your rotating flow problem starting with a low rotational speed or swirl velocity and then slowly increase the magnitude up to the desired level. The procedure for accomplishing this is as follows:
Postprocessing for Axisymmetric Swirling Flows
Reporting of results for axisymmetric swirling flows is the same as for other flows. The following additional variables are available for postprocessing when axisymmetric swirl is active: