[fivos]

Hi to all,

I have been simulating a case in 3D unsteady, with standard RANS model (k-ω) and a hybrid RANS/LES model.

The case is shown in both RANS_result.png and in LES_result.png. The geometry simulated is axis-symmetric, with the axis of symmetry indicated by the dashed line. Dimensions are in mm and max. velocity is ~70m/s (fluid water at 20C). Both simulations are 3D unsteady and a slice is shown on a symmetry plane. Unfortunately there are no experimental flow data yet; though we may obtain them in the following months.

The RANS results show a single recirculation zone (R1) and with a smooth velocity jet formed on the wall at the right side. On the other hand, the RANS/LES hybrid detects several vortices in two main zones (shown as R1, R2) and also show an instability downstream at Y~25mm; the jet detaches and strikes the opposite side (left side). The same happens further downstream with the jet changing side once more. The position of these instabilities changes over time - they move, fade out and re-generate.

Now, I understand that RANS generally oversmooths the solution, due to the inherent time-averaging, so such instabilities are hidden. However, has anyone experienced a similar phenomenon, i.e. a fluid jet/stream moving parallel to the wall and then suddenly detaching and hitting the other wall, especially in a tight space (note here that the space between the walls is 2.5mm, so the flow is rather constrained)?

If this effect is true and not a computational artifact, does it have a specific name (like the Koanda effect)? I was unable to find something like that in literature.

I am going to check with a finer mesh and see how it will behave and maybe with pure LES (I haven't done that already because my mesh is not that fine especially upstream.). The existing mesh consists only of hexahedra and has an inflation layer with y+~1.

Any comments/ideas are welcome
Thank you in advance.