Transient SST initialization fails on refined mesh / higher Reynolds number

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Hello,

I am modelling 3D flow around an inclined circular cylinder in water. The cylinder axis is aligned with the z-direction and periodic conditions are used in the spanwise direction.

Instead of rotating the geometry, I decompose the towing velocity into:

Un = Usin(alpha) Ua = Ucos(alpha)

where alpha is the angle between the cylinder axis and the towing direction.

I impose Un as the cross-flow velocity, and I account for the axial component using a reference-frame/Galilean transformation by applying a cylinder moving-wall velocity of approximately -Ua.

The final goal is to run sweeps over approximately:

U = 1-25 knots alpha = 1-40 deg D = 20-25 mm

The attached model contains a stationary k-epsilon study followed by a transient SST study, with the SST initialized from the k-epsilon solution.

On a coarse mesh this works well and vortex shedding develops correctly. However, when I refine the mesh or increase the Reynolds number, the transient SST often fails right at startup.

Typical errors are:

"Failed to find consistent initial values"

or the segregated solver repeatedly reduces the first timestep and is not able to converge.

The problem seems to be related to the initialization/nonlinear coupling rather than the linear solver. I have also tested PARDISO.

A stationary SST solution is not a good workaround in this case because the flow is strongly unsteady and the stationary SST problem itself does not reliably converge.

I would appreciate advice on the most robust way to initialize the transient SST solution, especially regarding:

  • Starting SST from a stationary k-epsilon solution.
  • Transferring a developed transient SST solution from a coarse mesh to a finer mesh.
  • Correctly handling k, omega, and the SST wall-distance variable during mesh transfer.
  • Whether consistent initialization should be modified or disabled in this situation.
  • Recommended startup settings for timestep, BDF order, segregated damping, Jacobian updates, and maximum segregated iterations.
  • A robust continuation strategy for future sweeps in U, alpha, and D.
  • My main objective is to find an initialization procedure that remains stable when the mesh is refined and when the operating conditions change.

Any suggestions would be greatly appreciated.



0 Replies Last Post 2026/09/22 3:03 GMT-5
COMSOL Moderator

Hello Oriol Pelegrí

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