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I know there's something about that in the training materials as I've been involved in writing/revising/teaching/reviewing the content at various times. However, I don't think there's anything in the formal documentation. You may want to check the Fluent course on here (look in Learning) as I'm told it's very similar to the course I am used to. 

Back flow can have an adverse effect on residuals, check it's not reaching any of the areas of interest in your model.