How could you set a fixed laminar to turbulent transition location?
 

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I would like to set the laminar to turbulent transition location at the point where the grit is used in the experiment to compare with data. How would you do this in CFX? Thanks.

Use the SST turbulence model with the turbulent transition model. The default turbulent transition model is Gamma Theta, but if you change that to Specified Intermittency you can define where turbulent transition occurs with a CEL expression.

See Solver Modelling Guide, section 4.1.11

Okay thanks. I thought you would use the specified intermittency option, just not sure what the CEL expressions would look like. Where can I find the Solver modelling guide?

The Solver Modelling Guide is one of the CFX documentation manuals. You can access it through the help menu.

The CEL function will need to be a 3D function (even 4D if it changes with time) which defines whether each point in the domain is laminar or turbulent.