Chapter 6

Day 1 Review and Tips

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Introduction to CFX

Review and Tips

Training ManualBefore 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

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• Create / Import the Geometry– Consider meshing requirements when creating the geo metry– Do not include unnecessary detail

• Create a suitable mesh– Resolve expected gradients in the solution variable s– Check mesh quality metrics

Review and Tips

Training ManualWorking 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

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– E.g. Enable Named Selectionsand blank the Filter to alwayspass Named Selections fromthe Geometry to the Mesh

Review and Tips

Training ManualDomains

• Domains define a region of consistent materials and physical models

• Use different domains for:– Different reference frames, e.g. rotating, stationa ry– Different domain types – fluid, solid, porous– Different materials, e.g. oil, copper and water

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• Fluid domains that are connected should use consist ent physics

• All regions that have the same physics can be group ed 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

Review and Tips

Training ManualBoundary Conditions

• It is important to consider the accuracy of the bou ndary conditions– E.g. a uniform velocity profile is usually not real istic, 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

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– 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

Review and Tips

Training ManualSolver Settings

• A good initial guess will assist with Solver stabil ity during the first few iterations

• The timestep is an important solver control– Smaller Timestep = More Stable, but slower converge nce– Larger Timestep = Faster convergence, but too large will cause the solver

to fail

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• 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 reac hed steady values

Review and Tips

Training ManualPost processing

• Automate post-processing through Session files, Sta te files and Report templates

• Make use of Expressions and User Variables to extra ct engineering data

• Compare solutions using the Multi-file mode and the Case Comparison tools

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Comparison tools

• Save images in the 3D CFX Viewer format to provide management or your customers with a better understanding of the f low