CFX13 04 Solver
Transcript of CFX13 04 Solver
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Chapter 4
Solver Settings
Introduction to CFX
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Training ManualOverview
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.
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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. Solverautomatically calculates the initial values
2. Initial values are entered by the user3. Initial values are obtained from a previous solution
Initial values can be set on a per-domain basis or globally for all
domains
Initialization
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Training ManualInitialization Setting Initial Values
Insert Global Init ial isationfrom the toolbar or by right-
clicking on Flow Analys is 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
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Training ManualInitialization Setting Initial Values
The Automat icoption 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 Automat ic wi th Valueoption means
that the specified value will be used
unless a previous results file is provided
Can use a constant value or an expression
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Training ManualInitialization Using a Previous Solution
To use a previous solution as the
initial guess enable the Initial Values
Specif icat iontoggle when launching
the Solver
You can provide multiple initial values
files
When simulating a system you can
provide previous solutions for eachcomponent 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 initialvalues files
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Edit the Solver Control object in the Outline tree
Solver Control Editing
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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 takesto obtain a solution
Solver Control Options
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Training ManualSolver 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
Resolut ion, Upwindand Speci f ied B lend
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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Training ManualSolver 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 (fup) are interpolated to the integrationpoints (fip) 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 ipvalue is equal to the upstream value plus a
correction due to the gradient
bcan have values between 0 and 1
fip
fup
b f r+=
f r
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Training ManualSolver Control Advection Scheme Theory
Ifb = 0 we get the Upwindadvectionscheme, i.e. no correction
This is robust but only first order accurate
Sometimes useful for initial runs, but
usually not necessary
The Speci f ied Blendscheme allows you to
specify b between 0 and 1 (i.e. between nocorrection up to full correction)
But this is not guaranteed to be bounded,
meaning that when the correction is
included it can overshoot or undershootwhat is physically possible
The High Resolu t ionscheme maximizes bthroughout the flow domain while keeping
the solution bounded
fip fup b f r+=Theory
High Resolution
Scheme
Upwind Scheme
b=1.00
Flow is misaligned
with mesh
0
1
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Training ManualSolver 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 beselected for additional accuracy
Can give better accuracy in boundary
layers on unstructured meshes
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Training ManualSolver Control Convergence Control
The Solver will finish when it reaches Max.
Iterationsunless convergence is achieved
sooner IfMax. Iteration sis reached you may not have
a converged solution
Can be useful to set Max. Iteration sto a large
number
When the Solver finishes you should always
check whyit finished
Fluid Timescale Control sets the timescale in
a steady-statesimulation
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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 theinitial 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
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For obtaining successful
convergence, the selection of thetimescale plays an important role
If the timescale is too large, the
convergence becomes bouncy or
may even lead to the failure of theSolver
If the timescale is too small, the
convergence will be very slow and
the solution may not be fully
accurate
Solver Control Timescale Selection
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Training ManualSolver Control Timescale Selection
For advection dominated flow, a fraction of the fluid residence time is
often a good estimate for the timescale A timescale of1/3 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/ ( in rad/s) is a good choice For buoyancy driven flows, the timescale should be based on a
function of gravity, thermal expansivity, temperature difference and
length scale (see documentation)
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Timescale Control can be Auto Timescale,Physical TimescaleorLoc al 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 AutoTimescale would produce fasterconvergence
Solver Control Timescale Control
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Training ManualSolver Control Timescale Control
Auto Timescale
The Solver calculates a timescale based onboundary / initial conditions or current solutionand domain length scale
Use a Conservative orAggressive estimate forthe domain length scale, or a specified value
Timescale is re-calculated and updated everyfew iterations as the flow field changes
Can set a Maximum Timescale to provide anupper limit
Tends to produce a conservative timescale
Timescale factor (default = 1) is a multiplierwhich can be changed to adjust theautomatically calculated timescale
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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
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Training ManualSolver 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 neverget 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 truesystem!
Residuals are just one measure of accuracy and
should be combined with other measures:
Monitor Points (ch. 8) and Imbalances (below)
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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] [] = [b]
where [A] is the coefficient matrix and [] is the solution variable
If the equation were solved exactly we would have:
[A] [] - [b] = [0]
The residual vector [R] is the error in the numerical solution:
[A] [] - [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
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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 residualsshould 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 greateraccuracy
n
2
i
iR
Solver Control Residuals
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Training ManualSolver Control Conservation Target
The Conservat ion Targetsets a target for theglobal imbalances
The imbalances measure the overallconservation of a quantity (mass, momentum,energy) in the entire flow domain
FluxMaximum
OutFluxInFluxImbalance%
Clearly in a converged solution Flux In should equal Flux Out
Its good practice to set a Conservation Targetand/or monitor theimbalances during the run
When set, the Solver must meet both the Residualand Conservat ion Targetbefore stopping (assuming Max. Iterationsis not reached)
Set a target of 0.01 (1%) or less
Flux In Flux Out < 1%
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Elapsed Time Control
Can specify the maximum wall clock timefor a run
Solver will stop after this amount of timeregardless of whether it has converged
Interrupt Control
Can specify other criteria for stoppingthe Solver based on logical CELexpressions
When the expression returns t ruethesolver will stop
Any value >= 0.5 is true
Solver Control Elapsed Time and Interrupt Control
Examples
If temperature exceeds a specified valueif(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
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This option is only available when a soliddomain is included in the simulation
The Sol id Timescaleshould be selected suchthat it is MUCH larger than the fluid timescale(100 times larger is typical)
the energy equation is usually very stable inthe solid zone
solid timescales are typically much larger thanfluid 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 lengthscale, thermal conductivity, density and specific heat capacity
Or you can choose the Physical Timescale option and provide a timescaledirectly
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The Equation Class Sett ingstab is an
advanced option that can be used toset Solver controls on an equation
specific basis
Not usually needed
Will override the controls set on Basic
Sett ingsfor the selected equation
Advanced Options
Advanced solver control options
Rarely needed
Solver Control Equation Class Settings
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Training ManualOutput Controls Results
The Output Contro lsettings control the output
produced by the Solver
The Trn Results, Trn Statsand Expor ttab only apply totransient simulations and are covered in the Transient
chapter
The Resul tstab controls the final .res file
Generally do not use the Selected Variables(orNone!)
option since it probably wont contain enough
information to restart the run later
Output Equ at ion Residualsis useful if you need to
check where convergence problems are occurring
Extra Output Var iables L ist
contains variables that are not
written to the standard resultsfile
E.g. Vorticity
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Frequency of output can be adjusted
Output Controls Backup
The Backuptab 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
therell be a jump in the residuals
Selected Variables: Not recommended
Can also manually request a backup file
from the Solver Manager at any time
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The Moni tortab allows you to create Moni torPoints
These are used to track values of interest asthe Solver runs
The Cartesian Coord inates Op tionis used totrack the value of a variable at a specific X, Y,Z location
The Express ion Opt ionis used to monitor thevalues of a CEL expression
E.g. Calculate the area average ofCpat theinlet boundary: areaAve(Cp)@inlet
E.g. Mass flow of particular fluid through an
outlet: oil.massFlow()@outlet
In steady-state simulations you should createmonitor points for quantities of interest
One measure of convergence is when thesevalues are no longer changing
Output Controls Monitor
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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
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Define a new Solver run
Solver Inp ut Fi leshould be the .deffile Can also pick .res, .bakor_fu ll .t rnfiles to restart a
previous incomplete run
To make a physics change and restart a solution,create a new .deffile and provide it as the SolverInput Fi lethen select the .res, .bakor_fu l l.t rnfile
in the Ini t ial Values Specif icationsection If both files have the same physics, this is the same
as picking the .res/.bak/_full.trnfile as the input file
Use Mesh Fromselects which mesh to use. If themeshes are identical can use either option,otherwise:
If you use the Solver Input Filemesh, the Init ialValuessolution is interpolated onto the input file
If you use the Initial Valuesmesh only the physicsfrom the Solver Input Fileis used
Continu e History Fromcarriers over convergencehistory and iteration counters
Solver Manager Defining a Run
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Training ManualSolver 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 Modeto one of the parallel options
to make use of multiple cores/processors
Requires parallel licenses
Allows you to divide a large CFD problem intosmallerpart i t ions
Faster solution times
Solve larger problems by making use of memory
(RAM) on multiple machines
The Lo cal Paral leloptions should be usedwhen running on a single machine
The Distr ibuted Paral leloptions should be
used when running across multiple machines
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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
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The Show Adv anced Contro ltoggle enables the
Parti t ioner, Solverand Interpolatortabs
On the Parti t ionertab you can pick differentpartitioning algorithms
Partitioning is always a serial process
Can be a problem for v.large cases since you
cannot distribute the memory load across multiplemachines
The default MeTiS algorithm uses more memorythan others, so if you run out of memory use adifferent method (see documentation for details)
Multidomain Option:
Independent Partitioning: Each domain ispartitioned into n partitions
Coupled Partitioning: All domains are combinedand then partitioned into n partitions
Theres a specific option for Transient Rotor Statorcases
Solver Manager Define Run Advanced Controls
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On the Solvertab you can select the Double
Precis ionoption
The solver will use more significant figures in itscalculations
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 Al loc Factoris a multiplier for this estimate
Use when the solver stops with an Insuff ic ient Memo ry A l located error
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Training ManualSolver Manager Interactive Solver Control
During a solution Edit Run in Progresslets you make changes on the fly
Models generally cannot be changed, but timescales, BCs, etc can
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.out fileMonitor Plot
Solver Manager Additional Solution Monitors
Right-click
By default monitor plots
are created showing the
RMS residuals for eachequation solved, plus one
plot for any monitor points
Right-click to switch
between RMS and MAX
Additional monitors can beselected 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
MonitorFinished Run Stop CurrentRun
Save Current
Run
Switch
Residual Plot
between
RMS andMAX
By dragging the cursor over any icon, the feature
description will appear
Solver Manager Additional Icons