This picture shows very thin structures that remain clearly observable and was obtained with the recommended setting of Fluent for the VOF method. The mesh has a 3mm step size. The result can be compared with the picture below where the default setting was used. The spiraling vortex is fully developed with  3 rolls as smaller flow rates were used for this computation. Structures are irregular because of the setting of the perturbation.


This picture was obtained with the default setting. The step size is 3 mm. This shows that while the results are still "correct", the vortex is being "sliced" by the too large step size.
 


This picture shows "spontaneous" billows that may appear while using the VOF recommended setting. The flat zone on the left is the perturbed zone, at a very high frequency, not leading to instabilities. All the instabilities are the result of "numerical" perturbation. On the right side the effect of the too large step size can still be seen despite the better setting.

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