怎样判断结果是否收敛了？

sjfyy

判断结果是否收敛可以从那几个方面判断，哪位大虾知道望告知，很困惑！

moitoi

看看前面的贴子吧

sjfyy

谢谢斑竹的回答，前面的贴子我看过了，对于误差还是不太明白，run earth 后，图形显示窗口中的error是误差值还是误差/精度？

phoenics

Q1. Question: - How do I assess Convergence?
Answer:
A number of devices are provided in PHOENICS to monitor and control the convergence of the solution.
RESIDUALS
The residuals are the quantities used in PHOENICS to monitor the convergence procedure. These are the imbalances in the FVE, defined as:
ep = ap(f p) - S (ai(f i-f p)) - sf
  
i=W,E,S,N,H,L,T
During the computation, the residuals are printed by Earth to the VDU, or are displayed graphically.
The frequency of display update is controlled by the PIL variable TSTSWP, which is set in the VR-editor Main menu / Output / Monitor update frequency panel. This panel also determines whether the data is shown graphically or in ASCII.
The quantities printed out are, in fact,
S |e p| / RESREF (f )
where the sum is extended to the current slab (in PARABolic cases) or to the whole field (in elliptic cases).
When the sum of errors divided by RESREF falls below 1.0, solution for that variable stops. However, the residual continues to be computed for the variable, and should it rise above 1.0, the variable re-enters the solution cycle. When the residuals for all variables fall below 1.0, sweeping stops.
RESREF
EARTH can calculate values for RESREF, based on the net sum of fluxes for a variable, if the PIL variable SELREF=T. Cases set through the VR-Editor have this as a default, but it is not set in all library cases.
The supplementary variable RESFAC acts as a tolerance - a setting of 0.01 means that sweeping stops when the errors are 1% of the reference fluxes.
These two variables can be set from the VR-Editor Main menu / Output / Relaxation control panel.
When SELREF is set to T, the value of RESREF calculated by EARTH is
RESREF(f) = RESFAC*TOTFLO(f)/(NX*NY*NZ)
TOTFLO(f) is the summation of all sources for variable phi, including sources associated with convection and diffusion transport. It contains terms of the form:
SOURCES: SUM1[ SUM2[ Cf (Vf -f p) ]]
sum1 is over all cells, sum2 is for all sources
Cf is the coefficient
Vf the value
f p is the in-cell value of f
CONVECTION: SUMi[ (max(0,(Mdot*f I)) + time-flux)]
sumi is for all directions
Mdot is the mass flow rate
time-flux for steady cases is zero, for transient is
(abs(Mdot*f )T - abs(Mdot* f )Told)/DT
DIFFUSION: SUMi[ (max(0,(G f *A*(f i - f p)/D )))]
sumi is for all directions
Gf is the diffusion coefficient
D the internodal distance
A is the cell-face area
For pressure TOTFLO(P1) is SUMi[ MAX(0,(Mdot))], sumi is for all directions.
RESREF is updated at the end of each sweep, when IZSTEP=NZ. For W1 and W2 resref would be updated at the end of the sweep when IZSTEP=NZ-1.
If SELREF=F, the user is free to set their own values for RESREF for each variable. The default value of 1E-8 is small enough to ensure that sweeping does not stop before LSWEEP. RESREF values can be set from the VR-Editor Main menu / Numerics / Iteration control panel (but only if SELREF=F).
A suitable value for RESREF is often the inflow source for that variable. This can either be calculated in Q1, or obtained from the 'net sources' printout in RESULT from an earlier run.
RESREF(f) would then be set to tolerance*inflow-flux, where tolerance can be say 0.01. Solution for the variable would stop when the sum of errors is less than 0.01 of the inflow flux of that variable.
The units of RESREF are always those of the equation in question. For the velocities, they are Newtons, for H1 or TEM1 they are Watts. The only exception is P1, where the error printed is a volume error, so RESREF(P1) has units of m^3/s
Q2. Question: - How do I choose my Relaxation settings?
Answer:
The false time step relaxation works by adding the term
r * Vol / D tf * (f p* - f p)
to the equation. f p* in the in-store value from the previous sweep, and f p is the value which is about to be calculated.
If the D tf is very big, then the term is negligible. If D tf is very small, then the term becomes dominant and the value cannot change. In between, there is a large range of D tf values, which make the term approximately the same order of magnitude as the other terms in the equation. The term then acts to damp down violent sweep-to-sweep changes, which may lead to divergence.
The question now arises - how to estimate values for D tf?
It is usually best to base it on some characteristic time-scale of the process. One time scale is based on convection - (minimum distance)/(maximum velocity), another is based on diffusion - (minimum distance)2/(kinematic viscosity).
Other time scales can be based on KE/EP or buoyancy. Normally the smallest of these will be used, multiplied by some factor.
A typical starting point is the domain length divided by the inlet velocity divided by the number of cells in that direction - i.e. typical cell residence time.
The values appearing in the Relaxation Control panel of the VR 2D menu, the RELAX command in Q1, and seen from the Reset option of the graphical convergence monitor, are actual D tf. Their size depends on the problem being solved. There are no limits on 'largeness' or 'smallness'.
From Version 2.2.0 onwards, Earth has been able to calculate average time-scales for each variable, much in the same way as it can calculate residual normalising factors if SELREF=T. It does this if the PIL variable SARAH > 0. The actual false time step used for each variable is then:
D tf = SARAH * Internally_calculated_value.
Typical values of SARAH seem to be in the range 0.1 - 0.001, but can be greater or smaller. The value of SARAH can be changed during run-time from the Earth graphics monitor.
False-time step relaxation can be applied to all SOLVEd variables, except P1, R1, R2 and RS. These, and all STOREd variables, can use linear relaxation. Relaxation for pressure is rarely needed, except for highly compressible flows, and most BFC cases.
Linear relaxation is also now recommended for KE and EP, in conjunction with the source-term linearisation KELIN=3. Typical relaxation factors are LINRLX 0.4 for both KE and EP.
At the end of the day, the only correct relaxation is one which allows the case to converge in a reasonable time. Once you have achieved such a value, you will probably waste more time in trying to tune it just that bit better, than you will eventually save.