On the Roughness Reynolds Number Transition Criterion

Abstract

The theoretically validated Fage’s transition criterion for predicting transition in flow over a single roughness element is used to show that in the widely used roughness Reynolds number (Rek ) transition criterion, the critical value of Rek depends strongly on the length of the roughness element. At roughness length where the criterion is valid, the variation of the roughness height with free-stream Reynolds number is quantified.