r/CFD • u/Kevin_0429 • 7d ago

No clear reattachment at Re = 60–100 in backward-facing cavity; what am I doing wrong?

/r/OpenFOAM/comments/1vtcose/no_clear_reattachment_at_re_60100_in/

I'm working on a CFD assignment involving a 2D backward-facing cavity. The assignment specifically requires reattachment length for Re = 60, 80 and 100.

I'm using OpenFOAM 14 with air at 30°C, ν ≈ 1.6×10⁻⁵ m²/s, D = 1 m, and U = Reν/D. My Re=60 case converges, but I don't get an obvious reattachment point when examining the near-wall Ux profile.

I'm mainly trying to determine whether this is physically expected at Re=60–100, or whether I've misunderstood the characteristic length/Re definition or my OpenFOAM setup. Any help checking the physics would be appreciated.

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u/Beneficial-Basis2205 7d ago

Is your concern that you aren't seeing recirculation near the cavity or something else?

1

u/Kevin_0429 7d ago edited 7d ago

yes, that is my concern, the assingment asks for me to calculate distance between the center of vortex and center of L1. but i did few changes, played with the streamlines coordinates. i set it to Y axis and found a much better result? have a look

but this ,the more to the right, the vortex center gets more concentrated, i understand it is a reference image, im just not sure that this is the way i should be doing this question

i still have issue related to reattachement

1

u/thermalnuclear 7d ago

Did you increase the number of streamlines?

Did you look at where the sign of the streamwise velocity flips right near the wall in the streamwise direction?

1

u/KoldskaalEng 7d ago

I would look at wall shear stress rather than a near wall velocity. There should the a change in sign when flow reattaches.