Thanks Rob for the direction and followup.Â
Btw, I am also encountering convergence issues when computing adjoint residuals, see if you or everyone could give me some insights please!
I checked my mesh quality - the orthogonality minimum > 0.3 and aspect ratio maximum ~ 25. But the skewness maximum reaches 0.77 in the sharp bifurcated point.
 
The flow (primal) residual converged smoothly at ~500th iteration.
I am wondering if the convergence issues of adjoint residual relates to mesh quality or adjoint solving setup.Â
Sometimes there are errors about AMG.
  
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Console:
Computing the adjoint for observable flowrate_vent
Building matrix
adj-iter continuity x-velocity y-velocity flow-rate Time/Iter
1 7.2318e-01 2.2030e-02 2.2030e-02 1.0849e-01 00:00:00 1000
2 1.4271e+00 1.2319e+02 1.4418e+02 5.4420e-03 00:00:00 999
3 1.9062e+00 1.2993e+01 1.6403e+01 4.2350e-03 00:00:00 998
4 7.7279e-01 2.4223e+00 2.7424e+00 4.5459e-03 00:00:00 997
5 4.4907e-01 7.4641e-01 7.2582e-01 6.8885e-03 00:00:00 996
6 5.2455e-01 3.4912e-01 3.2358e-01 1.0935e-02 00:00:00 995
7 8.1358e-01 2.2345e-01 1.9349e-01 1.6073e-02 00:00:00 994
8 1.2856e+00 1.7354e-01 1.4127e-01 2.1458e-02 00:00:00 993
9 1.9739e+00 1.4778e-01 1.1828e-01 2.5596e-02 00:00:00 992
10 2.9258e+00 1.3446e-01 1.0747e-01 2.8035e-02 00:00:00 991
11 4.1593e+00 1.2670e-01 1.0160e-01 2.9023e-02 00:00:00 990
12 5.6364e+00 1.2173e-01 9.7927e-02 2.9159e-02 00:00:00 989
13 7.2619e+00 1.1829e-01 9.5377e-02 2.8913e-02 00:00:00 988
14 8.9073e+00 1.1580e-01 9.3494e-02 2.8543e-02 00:00:00 987
15 1.0448e+01 1.1394e-01 9.2067e-02 2.8168e-02 00:00:00 986
16 1.1798e+01 1.1255e-01 9.0979e-02 2.7834e-02 00:00:00 985
17 1.2915e+01 1.1151e-01 9.0150e-02 2.7556e-02 00:00:00 984
18 1.3800e+01 1.1072e-01 8.9523e-02 2.7331e-02 00:00:00 983
19 1.4478e+01 1.1013e-01 8.9052e-02 2.7154e-02 00:00:00 982
20 1.4984e+01 1.0969e-01 8.8700e-02 2.7017e-02 00:00:00 981
adj-iter continuity x-velocity y-velocity flow-rate Time/Iter
21 1.5356e+01 1.0936e-01 8.8438e-02 2.6911e-02 00:00:00 980
22 1.5625e+01 1.0912e-01 8.8245e-02 2.6832e-02 00:00:00 979
23 1.5818e+01 1.0894e-01 8.8102e-02 2.6771e-02 00:00:00 978
24 1.5955e+01 1.0880e-01 8.7997e-02 2.6726e-02 00:00:00 977
25 1.6053e+01 1.0871e-01 8.7921e-02 2.6693e-02 00:00:00 976
26 1.6123e+01 1.0864e-01 8.7865e-02 2.6668e-02 00:00:00 975
27 1.6172e+01 1.0859e-01 8.7824e-02 2.6650e-02 00:00:00 974
28 1.6207e+01 1.0855e-01 8.7794e-02 2.6637e-02 00:00:00 973
29 1.6232e+01 1.0852e-01 8.7772e-02 2.6627e-02 00:00:00 972
30 1.6250e+01 1.0850e-01 8.7756e-02 2.6620e-02 00:00:00 971
31 1.6263e+01 1.0849e-01 8.7745e-02 2.6615e-02 00:00:00 970
32 1.6272e+01 1.0848e-01 8.7736e-02 2.6611e-02 00:00:00 969
33 1.6278e+01 1.0847e-01 8.7730e-02 2.6608e-02 00:00:00 968
34 1.6283e+01 1.0846e-01 8.7726e-02 2.6606e-02 00:00:00 967
35 1.6286e+01 1.0846e-01 8.7723e-02 2.6605e-02 00:00:00 966
36 1.6288e+01 1.0846e-01 8.7720e-02 2.6604e-02 00:00:00 965
37 1.6290e+01 1.0845e-01 8.7719e-02 2.6603e-02 00:00:00 964
Detected: slow convergence!
Residual Minimization scheme is enabled.
38 1.6291e+01 1.0845e-01 8.7717e-02 2.6603e-02 00:00:00 963
Initial step of residual minization scheme (40,10).
Subiter 0 : residual 5.19566e+03
Subiter 1 : residual 3.49130e+03
Subiter 2 : residual 2.18049e+03
Subiter 3 : residual 1.59751e+03
Subiter 4 : residual 1.22762e+03
Subiter 5 : residual 8.73199e+02
Subiter 6 : residual 6.90256e+02
Subiter 7 : residual 5.49072e+02
Subiter 8 : residual 4.15565e+02
Subiter 9 : residual 2.96731e+02
Subiter 10 : residual 2.58569e+02
Subiter 11 : residual 2.33898e+02
Subiter 12 : residual 2.18858e+02
Subiter 13 : residual 2.05658e+02
Subiter 14 : residual 1.63541e+02
Subiter 15 : residual 1.14036e+02
Subiter 16 : residual 8.59763e+01
Subiter 17 : residual 6.80927e+01
Subiter 18 : residual 5.05588e+01
Subiter 19 : residual 3.92616e+01
Subiter 20 : residual 3.24915e+01
Subiter 21 : residual 2.74532e+01
Subiter 22 : residual 2.26129e+01
Subiter 23 : residual 1.92560e+01
Subiter 24 : residual 1.56194e+01
Subiter 25 : residual 1.17093e+01
Subiter 26 : residual 9.45615e+00
Subiter 27 : residual 7.70489e+00
Subiter 28 : residual 7.06570e+00
Subiter 29 : residual 6.46197e+00
Subiter 30 : residual 6.05601e+00
Subiter 31 : residual 5.84756e+00
Subiter 32 : residual 5.63515e+00
Subiter 33 : residual 5.03297e+00
Subiter 34 : residual 4.46038e+00
Subiter 35 : residual 3.93606e+00
Subiter 36 : residual 3.40827e+00
Subiter 37 : residual 3.05956e+00
Subiter 38 : residual 2.83304e+00
Subiter 39 : residual 2.70420e+00
Subiter 40 : residual 2.50963e+00
(Ref: Subiter 0 : residual 5.19566e+03)
39 1.1310e-01 2.0475e-02 2.1688e-02 1.8579e-04 00:03:12 962
Subiter 0 : residual 2.50963e+00
Subiter 1 : residual 2.47081e+00
Subiter 2 : residual 2.45322e+00
Subiter 3 : residual 2.33711e+00
Subiter 4 : residual 2.09191e+00
Subiter 5 : residual 1.79734e+00
Subiter 6 : residual 1.65007e+00
Subiter 7 : residual 1.53513e+00
Subiter 8 : residual 1.44179e+00
Subiter 9 : residual 1.41316e+00
Subiter 10 : residual 1.37896e+00
Subiter 11 : residual 1.20338e+00
Subiter 12 : residual 8.64229e-01
Subiter 13 : residual 5.58113e-01
Subiter 14 : residual 4.23508e-01
Subiter 15 : residual 3.25290e-01
Subiter 16 : residual 2.37135e-01
Subiter 17 : residual 1.68159e-01
Subiter 18 : residual 1.12768e-01
Subiter 19 : residual 8.10711e-02
Subiter 20 : residual 5.72218e-02
Subiter 21 : residual 4.44752e-02
Subiter 22 : residual 3.54293e-02
Subiter 23 : residual 2.44222e-02
Subiter 24 : residual 1.54195e-02
Subiter 25 : residual 1.01831e-02
Subiter 26 : residual 6.23093e-03
Subiter 27 : residual 3.67927e-03
Subiter 28 : residual 2.23154e-03
Subiter 29 : residual 1.47669e-03
Subiter 30 : residual 9.85336e-04
(Ref: Subiter 0 : residual 2.50963e+00)
40 1.4761e-02 2.5215e-03 2.7998e-03 1.8157e-05 00:02:33 961
Subiter 0 : residual 9.85336e-04
Subiter 1 : residual 7.90151e-04
Subiter 2 : residual 6.79519e-04
Subiter 3 : residual 5.39879e-04
Subiter 4 : residual 4.83322e-04
Subiter 5 : residual 3.78174e-04
Subiter 6 : residual 2.73488e-04
Subiter 7 : residual 2.10568e-04
Subiter 8 : residual 1.33842e-04
Subiter 9 : residual 7.92288e-05
Subiter 10 : residual 5.22424e-05
Subiter 11 : residual 3.74638e-05
Subiter 12 : residual 2.76896e-05
Subiter 13 : residual 2.07209e-05
Subiter 14 : residual 1.50188e-05
Subiter 15 : residual 9.85177e-06
Subiter 16 : residual 6.04442e-06
Subiter 17 : residual 3.97632e-06
Subiter 18 : residual 2.49694e-06
Subiter 19 : residual 1.65920e-06
Subiter 20 : residual 1.12416e-06
Subiter 21 : residual 8.02176e-07
Subiter 22 : residual 5.62510e-07
Subiter 23 : residual 3.42694e-07
Subiter 24 : residual 2.17740e-07
Subiter 25 : residual 1.51421e-07
Subiter 26 : residual 1.02756e-07
Subiter 27 : residual 6.98898e-08
Subiter 28 : residual 4.87897e-08
Subiter 29 : residual 3.53074e-08
Subiter 30 : residual 2.38567e-08
(Ref: Subiter 0 : residual 9.85336e-04)
adj-iter continuity x-velocity y-velocity flow-rate Time/Iter
41 3.7885e-07 5.7990e-08 5.7229e-08 8.1460e-10 00:02:02 960
! Adjoint solution is converged.
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