On the prediction of laminarisation

Abstract

In rapid accelerations it is known that an originally turbulent boundary layer may undergo a partial or complete decay to laminar; a phenomenon known as laminarisation, The report distinguishes between 'moderate and 'severe' accelerations. For the former, the sublayer of the boundary layer undergoes considerable change in structure but the boundary layer remains essentially turbulent. For the latter, a complete degeneration to laminar flow will take place if the acceleration continues over sufficient distance. Two simple models have been proposed for the variation of turbulent shear stress, according to whether the acceleration is 'moderate' or 'severe'. These models have been incorporated into the finite difference prediction procedure of Patankar and Spalding and comparison made with a limited number of experiments. Agreement with experiment is reasonably good and progress to date has heen sufficiently encouraging to suggest that the accurate prediction of laminarisation is now an attainable objective.