Solver Control

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4-1ANSYS, Inc. Proprietary

2009 ANSYS, Inc. All rights reserved.April 28, 2009

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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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Training Manual

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 Initialisationfrom 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


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Training ManualInitialization Setting Initial Values

The Automatic option means that theCFX-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 meansthat 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 ValuesSpecification toggle when launchingthe 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, HighResolution, 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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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 ip value is equal to the upstream value plus acorrection 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 Upwind advectionscheme, 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 tospecify 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 Resolution scheme 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.Iterations unless convergence is achieved

sooner IfMax. Iterations is reached you may not have

a converged solution

Can be useful to set Max. Iterations to a largenumber

When the Solver finishes you should always

check why it finished

Fluid Timescale Control sets the timescale in

a steady-state simulation


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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 Timescale orLocal 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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Training Manual

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 Conservation Target sets 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 Target and/or monitor theimbalances during the run

When set, the Solver must meet both the Residual and Conservation Targetbefore stopping (assuming Max. Iterations is 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 true thesolver 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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Training Manual

This option is only available when a soliddomain is included in the simulation

The Solid Timescale should 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

Solver Settings


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The Equation Class Settings tab 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 BasicSettings for the selected equation

Advanced Options

Advanced solver control options

Rarely needed

Solver Control Equation Class Settings

Solver Settings


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Training ManualOutput Controls Results

The Output Control settings control the outputproduced by the Solver

TheTrn Results,Trn Stats and Export tab only apply totransient simulations and are covered in the Transient

chapter

The Results tab 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 Equation Residuals is useful if you need tocheck where convergence problems are occurring

Extra Output Variables Listcontains 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 Backup tab controls if and whenbackup 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 Monitor tab allows you to create MonitorPoints

These are used to track values of interest asthe Solver runs

The Cartesian Coordinates Option is used totrack the value of a variable at a specific X, Y,Z location

The Expression Option is used to monitor thevalues of a CEL expression

E.g. Calculate the area average ofCp at 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

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 .deffile 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 .deffile and provide it as the SolverInput 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 themeshes are identical can use either option,otherwise:

If you use the Solver Input File mesh, the InitialValues solution is interpolated onto the input file

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

Continue History From carriers over convergencehistory and iteration counters

Solver Manager Defining a Run

Solver Settings


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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 Mode to one of the parallel optionsto make use of multiple cores/processors

Requires parallel licenses

Allows you to divide a large CFD problem intosmallerpartitions

Faster solution times

Solve larger problems by making use of memory

(RAM) on multiple machines

The Local Parallel options should be usedwhen running on a single machine

The Distributed Parallel options should beused 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 Advanced Control toggle enables the

Partitioner, Solver and Interpolator tabs

On the Partitioner tab 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

Solver Settings


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On the Solver tab you can select the DoublePrecision option

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 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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Training ManualSolver 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, BCs, etc can

Solver Settings


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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

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