10.8 Solution Strategies for a Single Rotating Reference Frame

The difficulties associated with solving flows in rotating reference frames are similar to those discussed in Section  9.3.4. The primary issue you must confront is the high degree of coupling between the momentum equations when the influence of the rotational terms is large. A high degree of rotation introduces a large radial pressure gradient which drives the flow in the axial and radial directions, thereby setting up a distribution of the swirl or rotation in the field. This coupling may lead to instabilities in the solution process, and hence require special solution techniques to obtain a converged solution. Some techniques that may be beneficial include the following:

• (Pressure-based solver only) Consider switching the frame in which velocities are solved by changing the velocity formulation setting in the General task page. (See
  Section  10.7.1 for details.)

• (Pressure-based segregated solver only) Use the PRESTO! scheme (enabled in the Solution Methods task page), which is well-suited for the steep pressure gradients involved in rotating flows.

• Ensure that the mesh is sufficiently refined to resolve large gradients in pressure and swirl velocity.

• (Pressure-based solver only) Reduce the under-relaxation factors for the velocities, perhaps to 0.3-0.5 or lower, if necessary.

• Begin the calculations using a low rotational speed, increasing the rotational speed gradually in order to reach the final desired operating condition (see below).

See Chapter  26 for details on the procedures used to make these changes to the solution parameters.