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Lecture R1: Day 1 Review & Tips
Introduction to ANSYS CFX
1 © 2016 ANSYS, Inc. May 23, 2016
Introduction
Lecture Theme:
• The focus of Day 1 has been to cover topics (defining material properties, defining
boundary conditions and physical models, running the simulation, post-processing the
results) which include operations that are common to any CFD analysis in CFX.
• Many analyses will require additional inputs, such as turbulence models or system
rotation, which will be the focus of Day 2.
Learning Aims – you have learned:
• Boundary conditions, physical models and material properties
• Post-processing
• Solver settings
Learning Objectives:
• After Day 1 you will be able to set up, run and post-process your own CFD simulation
Introduction Problem Definition Running Simulation Post-processing Summary
2 © 2016 ANSYS, Inc. May 23, 2016
Before You Start CFX
Define your modeling goals
Identify the computational domain
• Simplify if possible
• Think about where boundary conditions can be set
• Avoid placing boundaries in potential recirculation areas when possible
Create / import the geometry
• Consider meshing requirements when creating the geometry
• Do not include unnecessary detail
Create a suitable mesh
• Resolve expected gradients in the solution variables
• Check mesh quality metrics
Introduction Problem Definition Running Simulation Post-processing Summary
3 © 2016 ANSYS, Inc. May 23, 2016
Working with Workbench
• Save your Project to set the working
directory
• Create the workflow by dragging and
dropping Analysis and Component
Systems on to the Project Schematic
‒ Systems can share or transfer data by
dropping on to an appropriate cell
• Configure Tools > Options to suit
your needs
Introduction Problem Definition Running Simulation Post-processing Summary
4 © 2016 ANSYS, Inc. May 23, 2016
Domains
Domains define a region of consistent materials and physical models
Use different domains for:
• Different reference frames, e.g. rotating, stationary
• Different domain types – fluid, solid, porous
• Different materials
– Fluid domains that are connected should use consistent physics – might require
multiphase
All regions that have the same physics can be grouped into a single domain
• Regions do not have to be connected
• Mesh does not have to be continuous
The Reference Pressure should be set to the operating pressure of the
device
Introduction Problem Definition Running Simulation Post-processing Summary
5 © 2016 ANSYS, Inc. May 23, 2016
Boundary Conditions
It is important to consider the accuracy of the boundary conditions
• E.g. a uniform velocity profile is usually not realistic, but can be used if placed a suitable distance
upstream
Avoid setting boundary conditions in recirculation zones if possible
Use well posed boundary conditions
• Mass Flow or Velocity Inlet, Static Pressure Outlet
– Will give a uniform inlet velocity profile
• Total Pressure Inlet, Mass Flow Outlet
– Will allow an inlet velocity profile to develop
• Total Pressure Inlet, Static Pressure Outlet
– Will allow an inlet velocity profile to develop
Introduction Problem Definition Running Simulation Post-processing Summary
6 © 2016 ANSYS, Inc. May 23, 2016
Solver Settings
A good initial guess will assist with Solver stability during the first few
iterations
The timestep is an important solver control
• Smaller Timestep = More Stable, but slower convergence
• Larger Timestep = Faster convergence, but too large will cause the solver to fail
When the solver finishes check:
• Residuals are converged to less than RMS 1e-4 (or even lower target)
• Imbalance are below 1% (or even <0.01%)
• Monitor Points for quantities of interest have reached steady values
Introduction Problem Definition Running Simulation Post-processing Summary
7 © 2016 ANSYS, Inc. May 23, 2016
Post-Processing
Automate post-processing through Session files, State files and Report
templates
Make use of Expressions and User Variables to extract engineering data
Compare solutions using the Multi-file mode and the Case Comparison
tools
Save images in the 3D CFX Viewer format to provide management or your
customers with a better understanding of the flow
Introduction Problem Definition Running Simulation Post-processing Summary
8 © 2016 ANSYS, Inc. May 23, 2016
Summary
Remember to first think about what the aims of the simulation are prior to
creating the geometry and mesh
All CFD simulations involve some common operations
• Problem definition
• Defining boundary conditions and cell zone conditions
• Defining material properties
• Post-processing the results
Use residual monitors, flux balances and solution monitors to judge
convergence
• Unconverged results can be misleading
What Next:
• Day 2 will focus on physical models and best practices
Introduction Problem Definition Running Simulation Post-processing Summary
9 © 2016 ANSYS, Inc. May 23, 2016
