What does 'Printing Primal Residual Statistics' mean?

[wndzjl]

Professor He,
1、I would like to ask you about the residuals output by "Printing Primal Residual Statistics". I noticed that the residuals that converge at each time step are smaller than those in "Printing Primal Residual Statistics". The residuals for the time steps in the cylinder case differ more from those in "Printing Primal Residual Statistics".
2、I noticed that in the V4 version, the "nIters" has been added. What settings control this "nIters"?

Time = 400

U0 initRes: 1.30580394532331e-08 finalRes: 9.653247823672512e-10 nIters: 6
U1 initRes: 1.049441084910801e-08 finalRes: 7.688953732314135e-10 nIters: 6
U2 initRes: 4.909225141136889e-08 finalRes: 4.838154121574446e-09 nIters: 5
p initRes: 3.285497305023291e-08 finalRes: 2.269675195331427e-09 nIters: 2
Time step continuity errors : sum local = 5.773334273530405e-11
global = -1.026365184702799e-12
cumulative = -0.000283165490945873
nuTilda initRes: 5.749856661572803e-10 finalRes: 4.280804400068552e-11 nIters: 4
Bounding nuTilda>1e-16
CD: 0.02091052894554104 final: 0.02091052894554104
CL: 0.4987649965352308 final: 0.4987649965352308
yPlus min: 14.24953310517374 max: 101.0851042174774 mean: 59.95687652750413
ExecutionTime = 6.9 s ClockTime = 10 s

Time = 435
Minimal residual 9.716836625388712e-09 satisfied the prescribed tolerance 1e-08

CD: 0.02091050986768742 final: 0.02091050986768742
CL: 0.4987652660392374 final: 0.4987652660392374
End

Printing Primal Residual Statistics.
U Residual Norm2: (0.0002100417560633355 0.0002334620581360085 1.122999306626796e-12)
U Residual Mean: (1.58404180396192e-06 6.407957533576595e-07 8.463967167449291e-15)
U Residual Max: (5.420109755505109e-05 0.0001013754535961198 1.685677808097118e-13)
p Residual Norm2: 7.15098536250842e-05
p Residual Mean: 1.027839796917936e-07
p Residual Max: 3.261838942890636e-05
nuTilda Residual Norm2: 8.963377556924731e-08
nuTilda Residual Mean: 3.073518428093193e-10
nuTilda Residual Max: 1.683857052427017e-08
phi Residual Norm2: 3.228322394661262e-07
phi Residual Mean: 8.078188146733666e-10
phi Residual Max: 8.149133647690638e-08
Total Residual Norm2: 0.0003220804919575545

[friedenhe]

"Printing Primal Residual Statistics" is the residual that the adjoint solver will use. It differs from OpenFOAM's CFD residual because they use different normalization methods. nIters is the number of linear equation solution iterations needed to converge relTol for that variable. You don't explicitly prescribe nIters, instead, you can prescribe the relTol in system/fvSolution.