Introduction to CFX

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Chapter 4


Solver Settings

Introduction to CFX


Solver Settings

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Overview



Initialization



Solver Control



Output Control



Solver Manager



Note: This chapter considers solver settings for steady
-
state simulations.
Settings specific to transient simulation are discussed in a later chapter.

Solver Settings

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Iterative solution procedures require that all solution variables are
assigned initial values before calculating a solution



A good initial guess can reduce the solution time



In some cases a poor initial guess may cause the solver to fail
during the first few iterations



The initial values can be set in 3 ways:

1.
Solver
automatically calculates the initial values

2.
Initial values are entered by the user

3.
Initial
values are obtained from a previous solution



Initial values can be set on a per
-
domain basis or globally for all
domains

Initialization

Solver Settings

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Initialization



Setting Initial Values


Insert
Global Initialisation
from the toolbar or by right
-
clicking on
Flow Analysis 1






Edit each Domain to set initial
values on a per
-
domain basis


When both are defined the
domain settings take
precedence


Solid domain must have
initial conditions set on a per
-
domain basis


Solver Settings

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Initialization



Setting Initial Values


The
Automatic

option means that the
CFX
-
Solver will calculate an initial value
for the solved variable unless a previous
results file is provided


Will be based on boundary condition
values and domain settings



The
Automatic with Value

option means
that the specified value will be used
unless a previous results file is provided


Can use a constant value or an expression

Solver Settings

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Initialization



Using a Previous Solution


To use a previous solution as the
initial guess enable the
Initial Values
Specification
toggle when launching
the Solver


You can provide multiple initial values
files


When simulating a system you can
provide previous solutions for each
component of the system as the initial
guess


Usually each file would correspond to a
separate region of space


It is best if domains in the Solver Input
File do not overlap with multiple initial
values files


Solver Settings

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Edit the Solver Control object in the Outline tree

Solver Control


Editing

Solver Settings

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The Solver Control panel contains
various controls that influence the
behavior of the solver



These controls are important for the
accuracy of the solution, the stability of
the solver and the length of time it takes
to obtain a solution

Solver Control


Options

Solver Settings

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Solver Control


Advection Scheme


The Advection Scheme refers to the way the
advection term in the transport equations is
modeled numerically


i.e. the term that accounts for bulk fluid motion


Often the dominant term







Three schemes are available,
High
Resolution
,
Upwind
and
Specified Blend


Discussed in more detail next



There is rarely any reason to change from the
default High Resolution scheme

Unsteady

Advection

Diffusion

Generation

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Solver Control


Advection Scheme Theory


Solution data is stored at nodes, but variable values are required at
the control volume faces to calculate fluxes



The upstream nodal values (
f
u
p
) are interpolated to the integration
points (
f
ip
) on the control volume faces using:




Where is the variable gradient and is the vector between the
upstream node and the integration point


In other words, the
ip

value is equal to the upstream value plus a
correction due to the gradient


b

can have values between 0 and 1 …

f
i
p
f
u
p
b
f


r

+
=

f
f
i
p
f
u
p
b
f


r

+
=
Solver Settings

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Solver Control


Advection Scheme Theory


If
b

㴠=⁷攠来琠瑨攠
Upwind

advection
scheme, i.e. no correction


This is robust but only first order accurate


Sometimes useful for initial runs, but
usually not necessary



The
Specified Blend

scheme allows you to
specify
b

扥瑷敥渠〠0湤ㄠ⡩1攮e扥瑷敥渠湯
捯牲散瑩t渠異瑯⁦畬t捯牲散瑩t温


But this is not guaranteed to be bounded,
meaning that when the correction is
included it can overshoot or undershoot
what is physically possible



The
High Resolution

scheme maximizes
b

throughout the flow domain while keeping
the solution bounded

f
i
p
f
u
p
b
f


r

+
=
Theory

High Resolution

Scheme

Upwind Scheme

b
=1.00

Flow is misaligned
with mesh

0

1

Solver Settings

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Solver Control


Turbulence Numerics


Regardless of the Advection Scheme
selection, the Turbulence equations
default to the First Order (Upwind)
scheme


Usually this is sufficient



The High Resolution scheme can be
selected for additional accuracy


Can give better accuracy in boundary
layers on unstructured meshes

Solver Settings

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Solver Control


Convergence Control


The Solver will finish when it reaches
Max.
Iterations
unless convergence is achieved
sooner


If
Max. Iterations
is reached you may not have
a converged solution


Can be useful to set
Max. Iterations
to a large
number



When the Solver finishes you should always
check
why

it finished



Fluid Timescale Control sets the timescale in
a
steady
-
state
simulation …

Solver Settings

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ANSYS CFX employs the so called False Transient Algorithm


A timescale is used to move the solution towards the final answer



In a steady
-
state simulation the timescale provides relaxation of the
equation non
-
linearities



A steady
-
state simulation is a “transient” evolution of the flow from the
initial guess to the steady
-
state conditions


Converged solution is independent of the timescale used

Initial Guess

50 iterations

100 iterations

150 iterations

Final Solution

Solver Control


Timescale Background

Solver Settings

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For obtaining successful
convergence, the selection of the
timescale plays an important role



If the timescale is too large, the
convergence becomes bouncy or
may even lead to the failure of the
Solver



If the timescale is too small, the
convergence will be very slow and
the solution may not be fully
accurate


Solver Control


Timescale Selection

Solver Settings

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Solver Control


Timescale Selection


For advection dominated flow, a fraction of the fluid residence time is
often a good estimate for the timescale


A timescale of
1
/
3

of (Length Scale / Velocity Scale) is often optimal


May need a smaller timescale for the first few iterations and for complex
physics, transonic flow,…..



For rotating machines, 1/


(


楮i牡搯猩r楳i愠杯潤a捨潩捥



For buoyancy driven flows, the timescale should be based on a
function of gravity, thermal expansivity, temperature difference and
length scale (see documentation)

Solver Settings

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Timescale Control can be
Auto Timescale
,
Physical Timescale

or
Local Timescale
Factor



Physical Timescale



Specify the timescale. Usually a constant but
can also be variable via an expression



Can often set a better timescale than Auto
Timescale would produce


faster
convergence

Solver Control


Timescale Control

Solver Settings

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Solver Control


Timescale Control


Auto Timescale


The Solver calculates a timescale based on
boundary / initial conditions or current solution
and domain length scale



Use a
Conservative

or
Aggressive

estimate for
the domain length scale, or a specified value



Timescale is re
-
calculated and updated every
few iterations as the flow field changes



Can set a
Maximum Timescale

to provide an
upper limit



Tends to produce a conservative timescale



Timescale factor (default = 1) is a multiplier
which can be changed to adjust the
automatically calculated timescale


Solver Settings

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Local Timescale Factor


Timescale varies throughout the domain











Can accelerate convergence when vastly different local velocity scales exist


E.g. a jet entering a plenum


Best used on fairly uniform meshes, since small element will have a small
timescale which can slow convergence


Local Timescale Factor is a multiplier of the local timescale


Never use as final solution
; always finish off with a constant timescale

Local Timescale =

Local Mesh Length Scale

Local Velocity Scale

Smaller Timescale in high
velocity and/or fine mesh regions

Solver Control


Timescale Control

Solver Settings

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Solver Control


Convergence Criteria


Convergence Criteria settings determine
when the solution is considered converged
and hence when the Solver will stop


Assuming
Max. Iterations
is not reached



Residuals are a measure of how accurately
the set of equations have been solved


Since we are iterating towards a solution, we never
get the exact solution to the equations


Lower residuals mean a more accurate solution to
the set of equations (more on the next slide)


Do not confuse accurately solving the equations
with overall solution accuracy


the equations may
or may not be a good representation of the true
system!


Residuals are just one measure of accuracy and
should be combined with other measures:


Monitor Points (ch. 8) and Imbalances (below)

Solver Settings

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The continuous governing equations are discretized into a set of linear
equations that can be solved. The set of linear equations can be written in
the form:


[A] [
Φ
] = [b]


where [A] is the coefficient matrix and [
Φ
] is the solution variable



If the equation were solved exactly we would have:





[A] [
Φ
]
-

[b] = [0]



The residual vector [R] is the error in the numerical solution:





[A] [
Φ
]
-

[b] = [R]



Since each control volume has a residual we usually look at the RMS
average or the maximum normalized residual

Solver Control


Residuals Theory

Solver Settings

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Residual Type


MAX: Convergence based on maximum
residual anywhere


RMS: Convergence based on average
residual from all control volumes



Root Mean Square =






Residual Target


For reasonable convergence MAX residuals
should be 1.0E
-
3, RMS should be at least
1.0E
-
4


The targets dependent on the accuracy
needed


Lower values may be needed for greater
accuracy

n
2

i
i
R
Solver Control


Residuals

Solver Settings

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Solver Control


Conservation Target


The
Conservation Target

sets a target for the
global imbalances





The imbalances measure the overall
conservation of a quantity (mass, momentum,
energy) in the entire flow domain

Flux

Maximum
Out
Flux
In
Flux
Imbalance

%



Clearly in a converged solution Flux In should equal Flux Out



It’s good practice to set a
Conservation Target

and/or monitor the
imbalances during the run



When set, the Solver must meet both the
Residual
and
Conservation Target

before stopping (assuming
Max. Iterations

is not reached)



Set a target of 0.01 (1%) or less


Flux In


Flux Out < 1%

Solver Settings

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Elapsed Time Control


Can specify the maximum wall clock time
for a run


Solver will stop after this amount of time
regardless of whether it has converged



Interrupt Control


Can specify other criteria for stopping
the Solver based on logical CEL
expressions


When the expression returns
true

the
solver will stop


Any value >= 0.5 is true

Solver Control


Elapsed Time and Interrupt Control


Examples


If temperature exceeds a specified value

if(
areaAve
(T)@wall>200[C],1,0)


If mesh quality drops below a specified value in a moving mesh case


More on logical expressions in the CEL lecture


Solver Settings

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This option is only available when a solid
domain is included in the simulation



The
Solid Timescale
should be selected such
that it is MUCH larger than the fluid timescale
(100 times larger is typical)


the energy equation is usually very stable in
the solid zone


solid timescales are typically much larger than
fluid timescales

Solver Control


Solid Timescale Control


The fluid timescale is estimated using Length Scale / Velocity Scale



The solid timescale is automatically calculated as function of the length
scale, thermal conductivity, density and specific heat capacity


Or you can choose the Physical Timescale option and provide a timescale
directly

Solver Settings

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The
Equation Class Settings

tab is an
advanced option that can be used to
set Solver controls on an equation
specific basis


Not usually needed


Will override the controls set on
Basic
Settings
for the selected equation



Advanced Options


Advanced solver control options


Rarely needed

Solver Control


Equation Class Settings

Solver Settings

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Output Controls


Results


The
Output Control
settings control the output
produced by the Solver


The
Trn Results
,
Trn Stats
and
Export
tab only apply to
transient simulations and are covered in the Transient
chapter



The
Results

tab controls the final .res file


Generally do not use the
Selected Variables
(or
None!)
option since it probably won’t contain enough
information to restart the run later


Output Equation Residuals
is useful if you need to
check where convergence problems are occurring


Extra Output Variables List

contains variables that are not

written to the standard results

file


E.g. Vorticity

Solver Settings

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Frequency of output can be adjusted

Output Controls


Backup


The
Backup

tab controls if and when
backup results files are automatically
written by the Solver



Recommend for long Solver runs in case
of power failure, network interruptions, etc



Option:


Standard: Like a full results file


Essential: Allows a clean solver restart


Smallest: Can restart the solver, but
there’ll be a jump in the residuals


Selected Variables: Not recommended



Can also manually request a backup file
from the Solver Manager at any time

Solver Settings

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The
Monitor

tab allows you to create
Monitor
Points


These are used to track values of interest as
the Solver runs



The
Cartesian Coordinates Option

is used to
track the value of a variable at a specific X, Y,
Z location



The
Expression Option

is used to monitor the
values of a CEL expression


E.g. Calculate the area average of
Cp

at the
inlet boundary:
areaAve(Cp)@inlet


E.g. Mass flow of particular fluid through an
outlet:
oil.massFlow()@outlet



In steady
-
state simulations you should create
monitor points for quantities of interest


One measure of convergence is when these
values are no longer changing

Output Controls


Monitor

Solver Settings

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The CFX
-
Solver Manager is a graphical user interface used to:


Define a run


Control the CFX
-
Solver interactively


View information about the emerging solution


Export data


Solver Manager

Solver Settings

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Define a new Solver run



Solver Input File

should be the
.def

file


Can also pick
.res
,
.bak

or
_full.trn

files to restart a
previous incomplete run



To make a physics change and restart a solution,
create a new
.def

file and provide it as the
Solver
Input File

then select the
.res
,
.bak

or
_full.trn

file
in the
Initial Values Specification

section


If both files have the same physics, this is the same
as picking the
.res/.bak/_full.trn

file as the input file



Use Mesh From

selects which mesh to use. If the
meshes are identical can use either option,
otherwise:


If you use the
Solver Input File

mesh, the
Initial
Values

solution is interpolated onto the input file


If you use the
Initial Values
mesh only the physics
from the
Solver Input File
is used



Continue History From
carriers over convergence
history and iteration counters

Solver Manager



Defining a Run

Solver Settings

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Solver Manager



Defining a Parallel Run


By default the Solver will run in serial


A single solver process runs on the local
machine



Set the
Run Mode
to one of the parallel options
to make use of multiple cores/processors


Requires parallel licenses


Allows you to divide a large CFD problem into
smaller
partitions


Faster solution times


Solve larger problems by making use of memory
(RAM) on multiple machines



The
Local Parallel

options should be used
when running on a single machine



The
Distributed Parallel

options should be
used when running across multiple machines

Solver Settings

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Serial





Local Parallel






Distributed Parallel





Different communication methods are available (MPICH2, HP MPI, PVM)


See documentation “When To Use MPI or PVM” for more details, but HP MPI is
recommended in most cases

Solver Manager



Defining a Parallel Run

Solver Settings

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The
Show Advanced Control

toggle enables the
Partitioner
,
Solver

and
Interpolator

tabs



On the
Partitioner

tab you can pick different
partitioning algorithms


Partitioning is always a serial process


Can be a problem for v.large cases since you
cannot distribute the memory load across multiple
machines


The default MeTiS algorithm uses more memory
than others, so if you run out of memory use a
different method (see documentation for details)



Multidomain Option:


Independent Partitioning: Each domain is
partitioned into n partitions


Coupled Partitioning: All domains are combined
and then partitioned into n partitions


There’s a specific option for Transient Rotor Stator
cases

Solver Manager



Define Run Advanced Controls

Solver Settings

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On the
Solver

tab you can select the
Double
Precision
option


The solver will use more significant figures in its
calculations


Doubles solver memory requirements


Use when round
-
off error could be a problem


if
‘small’ variations in a variable are important,
where ‘small’ is relative to the global range of
that variable, e.g:


Many Mesh Motion cases, since the motion is often
small relative to the size of the domain


Most CHT cases, since thermal conductivity is
vastly different in the fluid and solid


If you have a wide pressure range, but small
pressure changes are important


Small values by themselves do not need DP

Solver Manager



Define Run Advanced Controls


The Solver estimates its memory requirements upfront


Memory Alloc Factor

is a multiplier for this estimate


Use when the solver stops with an “
Insufficient Memory Allocated
” error

Solver Settings

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Solver Manager



Interactive Solver Control


During a solution
Edit Run in Progress

lets you make changes on the fly


Models generally cannot be changed, but timescales, BC’s, etc can

Solver Settings

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.out file

Monitor Plot

Solver Manager



Additional Solution Monitors

Right
-
click


By default monitor plots
are created showing the
RMS residuals for each
equation solved, plus one
plot for any monitor points


Right
-
click to switch
between RMS and MAX


Additional monitors can be
selected showing:


Imbalances


Boundary fluxes (FLOW)


Boundary forces


Tangential (viscous)


Normal (pressure)


Source terms …


New Monitor

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Start a new
Simulation

Monitor Run
in Progress

Monitor
Finished Run

Stop Current
Run

Save Current
Run

Switch
Residual Plot
between
RMS and
MAX


By dragging the cursor over any icon, the feature
description will appear

Solver Manager



Additional Icons