cfd_pro_14.5_l11_solversettings_outfile

2013 ANSYS, Inc. December 12, 2013 1 Release 14.5

14. 5 Release

Introduction to ANSYS CFD Professional

Lecture 11 Solver Settings and Output File


Introduction CFX requires inputs (solver settings) which tell it how to calculate the

solution. By introducing the concepts of accuracy, stability and convergence, the purpose of each setting can be understood. Emphasis will be placed on convergence, which is critical for the CFD simulation

We will cover: How to select the timestep and the convergence criteria Solution Initialization Using the Solver Manager How to monitor and judge the solution convergence and accuracy Specifying the output options for the run Using the Output file


Initialization Initial values must be provided for all solution variables

A good initial guess can reduce the solution time

A poor initial guess may lead to a failure in the first few iterations

The initial values can be set in 3 ways: Solver automatically calculates the initial values Initial values are entered by the user Initial values are obtained from a previous solution

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


Initialization Setting Initial Values Automatic: the CFX-Solver calculates an

initial value for the solved variable unless a previous results file is provided

Automatic with Value: the specified value will be used unless a previous results file is provided

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


Solver Control

The Solver Control panel contains various controls that influence the behaviour 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 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

Rarely need to change from the default High Resolution scheme as it keeps the solution as close to 2nd order as possible without going unbounded

Unsteady Advection Diffusion Generation 0

1

Theory Upwind, diffusive

Blend Factor = 1, unbounded High Resolution


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

0

1


Timescale Background ANSYS CFX employs the so-called False Transient

A timescale is used to move the solution towards the final answer Converged solution is independent of the timescale used

0

1

Choice of timescale is important in obtaining convergence

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

Timescale too large: Convergence becomes bouncy. Solver might fail

Timescale too small: Convergence will be very slow


Timescale Control

Physical Timescale Specify the timescale. Usually a constant but can also be

variable via an expression

Auto Timescale The Solver calculates a timescale based on boundary /

initial conditions or current solution and domain length scale

Tends to be conservative

Local Timescale Factor Multiplies the local timescale (local mesh

lengthscale/local velocity scale

Useful when vastly different local velocity scales exist Never use as final solution; always finish with constant

timescale

0

1 Timescale Control can be Auto Timescale, Physical Timescale or Local Timescale Factor


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

Lower residuals mean a more accurate solution to the set of equations

Residuals are just one measure of accuracy and should be combined with monitor points and imbalances

Residual Target For reasonable convergence MAX residuals

should be below 1.0E-3 and RMS should be less than 1.0E-4. In fact target RMS values might need to be 1.0E-5 or 1.0E-6.

0

1


Conservation Target

The Conservation Target sets a target for the global imbalances

Its good practice to set a Conservation Target and/or monitor the imbalances during the run

Set a target of 0.01 (1%) or less Flux In Flux Out < 1% ( for some applications < 0.01%)

Flux Maximum

OutFlux InFlux Imbalance %


Solid Timescale Control

Timescale for solution in a solid domain

The Solid Timescale should be 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

The solid timescale is automatically calculated as a 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


Output Control Results The Output Control settings control the

output produced by the Solver

The Results tab controls the final .res file Generally do not use the Selected Variables (or

None) option since it probably wont 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


Output Control Backup The Backup tab controls if and when

backup results files are automatically written by the Solver

Recommended 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 will

be a jump in the residuals

Selected Variables: Not recommended

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

Frequency of output can be adjusted


Output Control Monitor The Monitor tab allows you to create the 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

You should create monitor points for quantities of interest

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


Solver Manager

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


Defining a Run Define a new Solver run

Solver Input File should be the .def file Can pick a .res or .bak file to continue a previous run

To make a physics change and restart a solution, create a new .def file and select the .res or .bak file in the Initial Values Specification section

To interpolate solution in initial values file on to the mesh in the def file, set Use Mesh From to Solver Input File

Double Precision The solver uses more significant figures

Reduce round-off error when small variations in a variable are important, where small is relative to the global range of that variable

Increases solver memory & CPU requirements


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 are used when running on a single machine

The Distributed Parallel options are used when running across multiple machines


Advanced Controls

On the Solver tab you adjust Solver Memory settings

The Solver estimates its memory requirements Memory Alloc Factor is a multiplier for this estimate

Use when the solver stops with an Insufficient Memory Allocated error

Can provide individual factors for each stack


Interactive Solver Control During a solution, Edit Run in Progress lets you make changes on the fly

Models generally cannot be changed, but numeric values can


Additional Solution Monitors 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 .out file Monitor Plot

Right-click

New

Monitor


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

Start a new

Simulation

Monitor Run

in Progress

Monitor

Finished Run Stop Current

Run

Back up

Current Run

Switch

Residual Plot

between

RMS and

MAX


Solver Output File Produced by the ANSYS CFX-Solver and contains information about your

simulation:

Model setup The state of the solution during the run Job statistics for the particular run

Now lets take a look at an out file..

CFX-Solver

CFX-Pre

CFX-Post

.def

.res

.cfx .cfx, .def, .res

.out

.gtm, .def


Model Setup

Physics definition

CFX Command Language (CCL)


Solver

Mesh quality

checks, mesh

statistics and GGI

intersection details

are shown here

188 MB of

RAM

allocated

The start of the Solver

process

Memory requirements

shown here are for

the Solver. If memory

errors occur check if

its the Interpolator, Partitioner or Solver

process


State of the Solution Diagnostics shown as the solver iterates towards a solution

Upper case OK is good. Lower case ok means youre on the limit. F means failed to solve the equation. Reducing the

timestep may help avoid ok and F. The first few iterations may show ok and F and can be ignored as long as they go away


Final Imbalances When the solution finishes, the Imbalances are shown

Equation Imbalance in domain


Summary This section has covered the following important points:

Initialization is key to providing a stable solution, especially for complex physics or flows with high solution gradients

Convergence is one important part of judging solution progress. Dont forget that the solver will terminate when the Max Iterations has been

reached, regardless of solution convergence levels

Output Control: Always monitor imbalances to ensure conservation Use backups so that data can be rescued if the solver/hardware fails Monitor quantities of interest as an additional aid when judging steady-state

behaviour

The output file gives important information about the physics, resource usage and solution progress

cfd_pro_14.5_l11_solversettings_outfile

Documents

initial values initial

solution convergence

initialization initial

solution time

converged solution

initial values automatic

solver control advection

solver manager