An analysis of procedures in current use for prediction of transition onset location shows that they are generally in poor agreement with data obtained in test facilities at low freestream turbulence levels. It has been shown elsewhere that under such conditions transition is driven by residual nonturbulent disturbances in the facility. A method is developed for taking such disturbances into account by defining an equivalent free-stream turbulence intensity; values for this parameter are derived for each facility from which onset data are available. A new correlation incorporating this effect is shown to be in good agreement with all available data on two-dimensional flows with pressure gradient. The correlation suggests that the onset Reynolds number (based on boundary-layer thickness) depends inversely on the total disturbance level when the latter is low.

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