The use of velocity gradient factor as a pressure gradient parameter

This paper traces the evolution of the velocity gradient factor as the most appropriate boundary-layer laminarisation criterion as a prelude to examination of its advantages over the original and modified Pohlhausen parameters for laminar boundary-layer situations involving positive and negative pressure gradients. The velocity gradient factor, which involves only directly-measurable quantities, appears a more sensitive indicator of changes in other parameters over a wide range of flow models, including the onset of transition in the presence of significant mainstream turbulence.