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Chapter 4

              Solver Settings




              Introduction to CFX


ANSYS, Inc. Proprietary                  ...
Solver Settings
Overview                                                                 Training Manual




 • Initializa...
Solver Settings
Initialization                                                        Training Manual


  •       Iterativ...
Solver Settings
Initialization – Setting Initial Values                     Training Manual




  •       Insert Global In...
Solver Settings
Initialization – Setting Initial Values                Training Manual


 • The Automatic option means tha...
Solver Settings
Initialization – Using a Previous Solution                     Training Manual


 • To use a previous solu...
Solver Settings
Solver Control – Editing                                   Training Manual



    • Edit the Solver Contro...
Solver Settings
Solver Control – Options                        Training Manual


 • The Solver Control panel contains
   ...
Solver Settings
Solver Control – Advection Scheme                              Training Manual




                       ...
Solver Settings
Solver Control – Advection Scheme Theory                                 Training Manual


 • Solution dat...
Solver Settings
Solver Control – Advection Scheme Theory                                        Training Manual

         ...
Solver Settings
Solver Control – Turbulence Numerics              Training Manual


 • Regardless of the Advection Scheme
...
Solver Settings
Solver Control – Convergence Control                         Training Manual


 • The Solver will finish w...
Solver Settings
Solver Control – Timescale Background                                                Training Manual



  ...
Solver Settings
Solver Control – Timescale Selection                Training Manual



  • For obtaining successful
    co...
Solver Settings
Solver Control – Timescale Selection                                          Training Manual



 • For ad...
Solver Settings
Solver Control – Timescale Control                        Training Manual


 • Timescale Control can be Au...
Solver Settings
Solver Control – Timescale Control                             Training Manual



 • Auto Timescale
      ...
Solver Settings
Solver Control – Timescale Control                                                             Training Ma...
Solver Settings
Solver Control – Convergence Criteria                              Training Manual


 • Convergence Criter...
Solver Settings
Solver Control – Residuals Theory                                           Training Manual


  • The cont...
Solver Settings
Solver Control – Residuals                                         Training Manual


     • Residual Type
...
Solver Settings
Solver Control – Conservation Target                                    Training Manual

 • The Conservati...
Solver Settings
Solver Control – Elapsed Time and Interrupt Control                                  Training Manual



 •...
Solver Settings
Solver Control – Solid Timescale Control                                      Training Manual



 • This o...
Solver Settings
Solver Control – Equation Class Settings             Training Manual


  • The Equation Class Settings tab...
Solver Settings
Output Controls – Results                                            Training Manual


 • The Output Contr...
Solver Settings
Output Controls – Backup                                                          Training Manual


 • The...
Solver Settings
Output Controls – Monitor                                   Training Manual


   • The Monitor tab allows ...
Solver Settings
Solver Manager                                                Training Manual


 • The CFX-Solver Manager ...
Solver Settings
Solver Manager – Defining a Run                                      Training Manual


 • Define a new Sol...
Solver Settings
Solver Manager – Defining a Parallel Run                           Training Manual


 • By default the Sol...
Solver Settings
Solver Manager – Defining a Parallel Run                                      Training Manual



 • Serial...
Solver Settings
Solver Manager – Define Run Advanced Controls                             Training Manual



 • The Show A...
Solver Settings
Solver Manager – Define Run Advanced Controls                                     Training Manual


 • On ...
Solver Settings
Solver Manager – Interactive Solver Control                                    Training Manual


 • During...
Solver Settings
Solver Manager – Additional Solution Monitors                           Training Manual


 • By default mo...
Solver Settings
Solver Manager – Additional Icons                                         Training Manual


        • By d...
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  1. 1. Chapter 4 Solver Settings Introduction to CFX ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-1 Inventory #002598
  2. 2. Solver Settings Overview Training Manual • 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. ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-2 Inventory #002598
  3. 3. 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 • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-3 Inventory #002598
  4. 4. Solver Settings Initialization – Setting Initial Values Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-4 Inventory #002598
  5. 5. Solver Settings Initialization – Setting Initial Values Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-5 Inventory #002598
  6. 6. Solver Settings Initialization – Using a Previous Solution Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-6 Inventory #002598
  7. 7. Solver Settings Solver Control – Editing Training Manual • Edit the Solver Control object in the Outline tree ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-7 Inventory #002598
  8. 8. Solver Settings Solver Control – Options Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-8 Inventory #002598
  9. 9. Solver Settings Solver Control – Advection Scheme Training Manual Generation • 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 Diffusion – Often the dominant term Advection • Three schemes are available, High Resolution, Upwind and Specified Blend – Discussed in more detail next Unsteady • There is rarely any reason to change from the default High Resolution scheme ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-9 Inventory #002598
  10. 10. Solver Settings Solver Control – Advection Scheme Theory Training Manual • Solution data is stored at nodes, but variable values are required at the control volume faces to calculate fluxes • The upstream nodal values (φ up) are interpolated to the integration points (φ ip) on the control volume faces using: φip = φup + β ∇φ ⋅ ∆r – Where ∇φ is the variable gradient ∇φ ⋅ ∆r is the vector between the φip = φup + β and 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 – β can have values between 0 and 1 … ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-10 Inventory #002598
  11. 11. Solver Settings Solver Control – Advection Scheme Theory Training Manual Flow is misaligned φip = φup + β ∇φ ⋅ ∆r with mesh Theory 1 • If β = 0 we get the Upwind advection scheme, i.e. no correction 0 – 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 β between 0 and 1 (i.e. between no correction up to full correction) – But this is not guaranteed to be bounded, β=1.00 meaning that when the correction is included it can overshoot or undershoot what is physically possible • The High Resolution scheme maximizes β High Resolution throughout the flow domain while keeping Scheme the solution bounded ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-11 Inventory #002598
  12. 12. Solver Settings Solver Control – Turbulence Numerics Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-12 Inventory #002598
  13. 13. Solver Settings Solver Control – Convergence Control Training Manual • 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 … ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-13 Inventory #002598
  14. 14. Solver Settings Solver Control – Timescale Background Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-14 Inventory #002598
  15. 15. Solver Settings Solver Control – Timescale Selection Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-15 Inventory #002598
  16. 16. Solver Settings Solver Control – Timescale Selection Training Manual • 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/ω (ω 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) ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-16 Inventory #002598
  17. 17. Solver Settings Solver Control – Timescale Control Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-17 Inventory #002598
  18. 18. Solver Settings Solver Control – Timescale Control Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-18 Inventory #002598
  19. 19. Solver Settings Solver Control – Timescale Control Training Manual • Local Timescale Factor – Timescale varies throughout the domain Local Mesh Length Scale Local Timescale = Local Velocity Scale 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; always finish off with a constant timescale ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-19 Inventory #002598
  20. 20. Solver Settings Solver Control – Convergence Criteria Training Manual • 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) ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-20 Inventory #002598
  21. 21. Solver Settings Solver Control – Residuals Theory 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] [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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-21 Inventory #002598
  22. 22. Solver Settings Solver Control – Residuals Training Manual • Residual Type – MAX: Convergence based on maximum residual anywhere – RMS: Convergence based on average residual from all control volumes – Root Mean Square = ∑R i i 2 n • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-22 Inventory #002598
  23. 23. Solver Settings Solver Control – Conservation Target Training Manual • The Conservation Target sets a target for the global imbalances Flux In − Flux Out % Imbalance = 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 • 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% ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-23 Inventory #002598
  24. 24. Solver Settings Solver Control – Elapsed Time and Interrupt Control Training Manual • 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 – 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-24 Inventory #002598
  25. 25. Solver Settings Solver Control – Solid Timescale Control Training Manual • 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 • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-25 Inventory #002598
  26. 26. Solver Settings Solver Control – Equation Class Settings Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-26 Inventory #002598
  27. 27. Solver Settings Output Controls – Results Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-27 Inventory #002598
  28. 28. Solver Settings Output Controls – Backup Training Manual • 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 Frequency of output can be adjusted ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-28 Inventory #002598
  29. 29. Solver Settings Output Controls – Monitor Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-29 Inventory #002598
  30. 30. Solver Settings Solver Manager Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-30 Inventory #002598
  31. 31. Solver Settings Solver Manager – Defining a Run Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-31 Inventory #002598
  32. 32. Solver Settings Solver Manager – Defining a Parallel Run Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-32 Inventory #002598
  33. 33. Solver Settings Solver Manager – Defining a Parallel Run Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-33 Inventory #002598
  34. 34. Solver Settings Solver Manager – Define Run Advanced Controls Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-34 Inventory #002598
  35. 35. Solver Settings Solver Manager – Define Run Advanced Controls Training Manual • 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 • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-35 Inventory #002598
  36. 36. Solver Settings Solver Manager – Interactive Solver Control Training Manual • 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 ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-36 Inventory #002598
  37. 37. Solver Settings Solver Manager – Additional Solution Monitors Training Manual • By default monitor plots New Monitor 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) Right-click • Normal (pressure) – Source terms … Monitor Plot .out file ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-37 Inventory #002598
  38. 38. Solver Settings Solver Manager – Additional Icons Training Manual • By dragging the cursor over any icon, the feature description will appear Start a new Switch Simulation Residual Plot between Monitor Stop Current RMS and Finished Run Run MAX Monitor Run Save Current in Progress Run ANSYS, Inc. Proprietary April 28, 2009 © 2009 ANSYS, Inc. All rights reserved. 4-38 Inventory #002598
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