Introduction to CFX -Day 1 Review and Tips

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

Working with Workbench

Save your Project to set the working directory

Create the workflow by dragging and dropping Analysis and Component Systems onto the Project Schematic

-Systems can share or transfer data by dropping onto an appropriate cell

Configure Tools > Options to suit your needs

-E.g. Enable Named Selections and blank the Filter to always pass Named Selections from the Geometry to the Mesh

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, e.g. oil, copper and water

Fluid domains that are connected should use consistent physics

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

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

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 at least RMS 1e-4

-Imbalance are below 1%

-Monitor Points for quantities of interest have reached steady values

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

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