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[FAQ]

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 that 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: 1. (Pressure-based segregated solver only) Use the PRESTO! scheme (enabled in the Pressure list for Spatial Discretization in the Solution Methods Task Page), which is well-suited for the steep pressure gradients involved in swirling flows. 2. Ensure that the mesh is sufficiently refined to resolve large gradients in pressure and swirl velocity. 3. (Pressure-based solver only) Change the under-relaxation parameters on the velocities, perhaps to 0.3–0.5 for the radial and axial velocities and 0.8–1.0 for swirl. 4. (Pressure-based solver only) Use a sequential or step-by-step solution procedure, in which some equations are temporarily left inactive (see below). 5. If necessary, start the calculations using a low rotational speed or inlet swirl velocity, increasing the rotation or swirl gradually in order to reach the final desired operating condition (see below). For more details, visit the help documentation: help/flu_ug/flu_ug_uns_sec_swirl.html