CFX12 04 Solver

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Transcript CFX12 04 Solver

Chapter 4 Solver Settings

Introduction to CFX

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Solver Settings

Overview

•

Initialization

•

Solver Control

•

Output Control

•

Solver Manager

Training Manual

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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Solver Settings

Initialization

•

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

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The initial values can be set in 3 ways: 1.

Solver automatically calculates the initial values 2.

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

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Solver Settings

Initialization – Setting Initial Values

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

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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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Solver Settings

Initialization – Setting Initial Values

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

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Will be based on boundary condition values and domain settings

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

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Can use a constant value or an expression

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Solver Settings

Initialization – Using a Previous Solution

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

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You can provide multiple initial values files

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

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Usually each file would correspond to a separate region of space

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

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Solver Settings

Solver Control – Editing

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Edit the Solver Control object in the Outline tree

Training Manual

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Solver Settings

Solver Control – Options

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

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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

Training Manual

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Solver Settings

Solver Control – Advection Scheme

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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 Unsteady Advection Diffusion Generation

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

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Discussed in more detail next

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

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Solver Settings

Solver Control – Advection Scheme Theory

Training Manual

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

•

The upstream nodal values (

up ) are interpolated to the integration points (

) on the control volume faces using:

– f

= f

+  

= f

+ 

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

can have values between 0 and 1 …

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Solver Settings

Solver Control – Advection Scheme Theory

= f

+  

Flow is misaligned with mesh

•

= 0 we get the Upwind advection scheme, i.e. no correction

– –

This is robust but only first order accurate Sometimes useful for initial runs, but usually not necessary

Upwind Scheme •

The Specified Blend scheme allows you to specify

between 0 and 1 (i.e. between no correction up to full correction)

–

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

b =1.00

Training Manual

Theory

1 0

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The High Resolution scheme maximizes

throughout the flow domain while keeping the solution bounded

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Solver Settings

Solver Control – Turbulence Numerics

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

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Usually this is sufficient

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The High Resolution scheme can be selected for additional accuracy

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Can give better accuracy in boundary layers on unstructured meshes

Training Manual

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Solver Settings

Solver Control – Convergence Control

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

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

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

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When the Solver finishes you should always check why it finished

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

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Solver Settings

Solver Control – Timescale Background

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ANSYS CFX employs the so called False Transient Algorithm

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

Training Manual

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

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

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Converged solution is independent of the timescale used

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Solver Settings

Solver Control – Timescale Selection

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

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

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

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Solver Settings

Solver Control – Timescale Selection

Training Manual

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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,…..

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For rotating machines, 1/



(



in rad/s) is a good choice

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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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Solver Settings

Solver Control – Timescale Control

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

Factor

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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

Training Manual

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Solver Settings

Solver Control – Timescale Control

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Auto Timescale

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

Conservative

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

upper limit to provide an – Tends to produce a conservative timescale – Timescale factor (default = 1) is a multiplier which can be changed to adjust the automatically calculated timescale ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Solver Settings

Solver Control – Timescale Control

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Local Timescale Factor

– Timescale varies throughout the domain

Local Timescale = Local Mesh Length Scale Local Velocity Scale Training Manual

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Smaller Timescale in high velocity and/or fine mesh regions

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 ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Solver Settings

Solver Control – Convergence Criteria

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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!

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Solver Settings

Solver Control – Residuals Theory

Training Manual

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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

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If the equation were solved exactly we would have: [A] [ Φ] - [b] = [0]

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The residual vector [R] is the error in the numerical solution: [A] [ Φ] - [b] = [R]

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

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Solver Settings

Solver Control – Residuals

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Residual Type

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MAX: Convergence based on maximum residual anywhere

– –

RMS: Convergence based on average residual from all control volumes Root Mean Square =



i R i

2 n •

Residual Target

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

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The targets dependent on the accuracy needed

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Lower values may be needed for greater accuracy

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Solver Settings

Solver Control – Conservation Target

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The Conservation Target sets a target for the global imbalances

% Imbalance  Flux In  Flux Out Maximum Flux •

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

•

Clearly in a converged solution Flux In should equal Flux Out

Training Manual

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It’s good practice to set a Conservation Target and/or monitor the imbalances during the run

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When set, the Solver must meet both the Residual and Conservation Target before stopping (assuming Max. Iterations is not reached)

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Set a target of 0.01 (1%) or less

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Flux In – Flux Out < 1%

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Solver Settings

Solver Control – Elapsed Time and Interrupt Control

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Elapsed Time Control

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Can specify the maximum wall clock time for a run

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

Training Manual

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Interrupt Control

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

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When the expression returns true the solver will stop

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Any value >= 0.5 is true

– –

Examples

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If temperature exceeds a specified value if(areaAve(T)@wall>200[C],1,0)

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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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Solver Settings

Solver Control – Solid Timescale Control

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

Training Manual

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

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the energy equation is usually very stable in the solid zone

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solid timescales are typically much larger than fluid timescales

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The fluid timescale is estimated using Length Scale / Velocity Scale

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

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

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Solver Settings

Solver Control – Equation Class 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

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Advanced Options

– –

Advanced solver control options Rarely needed

Training Manual

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Solver Settings

Output Controls – Results

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The Output Control settings control the output produced by the Solver

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

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The Results tab controls the final .res file

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

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

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Extra Output Variables List

contains variables that are not written to the standard results file

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E.g. Vorticity

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Solver Settings

Output Controls – Backup

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

Training Manual

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

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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

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Selected Variables: Not recommended

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Can also manually request a backup file from the Solver Manager at any time Frequency of output can be adjusted

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Solver Settings

Output Controls – Monitor

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The Monitor tab allows you to create Monitor

Points

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

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

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The Expression Option is used to monitor the values of a CEL expression

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E.g. Calculate the area average of Cp at the inlet boundary:

areaAve(Cp)@inlet

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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

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Solver Settings

Solver Manager

Training Manual

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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

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Solver Settings

Solver Manager – Defining a Run

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Define a new Solver run

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Solver Input File should be the .def file

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Can also pick .res, .bak or _full.trn files to restart a previous incomplete run

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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

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

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

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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

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Solver Settings

Solver Manager – Defining a Parallel Run

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By default the Solver will run in serial

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A single solver process runs on the local machine

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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

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Solver Settings

Solver Manager – Defining a Parallel Run

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Serial

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Local Parallel

Training Manual

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Distributed Parallel

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

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

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Solver Settings

Solver Manager – Define Run Advanced Controls

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The Show Advanced Control toggle enables the Partitioner, Solver and Interpolator tabs

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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

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

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There’s a specific option for Transient Rotor Stator cases

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Solver Settings

Solver Manager – Define Run Advanced Controls

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On the Solver tab you can select the Double Precision option

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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

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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

Training Manual

• •

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

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Solver Settings

Solver Manager – Interactive Solver Control

Training Manual

•

During a solution Edit Run in Progress lets you make changes on the fly

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

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Solver Settings

Solver Manager – 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 … New Monitor

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Solver Settings

Solver Manager – Additional Icons

•

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

Start a new Simulation Monitor Finished Run Stop Current Run

Training Manual

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CFX12 04 Solver

Transcript CFX12 04 Solver

Chapter 4 Solver Settings

Introduction to CFX

Overview

Initialization

Solver Control

Output Control

Solver Manager

Initialization

Initialization – Setting Initial Values

Initialization – Setting Initial Values

Initialization – Using a Previous Solution

Solver Control – Editing

Solver Control – Options

Solver Control – Advection Scheme

Solver Control – Advection Scheme Theory

Solver Control – Advection Scheme Theory

Solver Control – Turbulence Numerics

Solver Control – Convergence Control

Solver Control – Timescale Background

Solver Control – Timescale Selection

Solver Control – Timescale Selection

Solver Control – Timescale Control

Solver Control – Timescale Control

Solver Control – Timescale Control

Solver Control – Convergence Criteria

Solver Control – Residuals Theory

Solver Control – Residuals

Solver Control – Conservation Target

Solver Control – Solid Timescale Control

Solver Control – Equation Class Settings

Output Controls – Results

Output Controls – Backup

Output Controls – Monitor

Solver Manager

Solver Manager – Defining a Run

Solver Manager – Defining a Parallel Run

Solver Manager – Defining a Parallel Run

Solver Manager – Interactive Solver Control

Solver Manager – Additional Icons

Directory