Laminar-turbulent transition prediction in three-dimensional flows

Accurate laminar-turbulent transition prediction is needed for many practical problems. This paper presents an overview of the methods which can be used today to estimate the location of the transition onset. The applications are restricted to subsonic flows (incompressible and transonic) developing on swept wings of infinite span. It is assumed that the environmental disturbances are small, so that the transition process is governed by the spatial growth of unstable eigenmodes. Therefore the main theoretical tools are stability theories,linear and nonlinear. The most popular prediction method is the e(N) method which is based on linear theory. This method was initially developed for two-dimensional flows and then extended to more complex problems. The paper describes various approaches currently available to compute the N factor; examples of application are given for wind tunnel and free flight experiments. More sophisticated prediction methods make use of weakly nonlinear theories, the governing equations being solved by a PSE (Parabolized Stability Equations) approach. In this case, the problem of choosing a critical Value of the N factor at transition is replaced by the problems of defining the most interesting nonlinear scenario and of imposing adequate initial conditions. The examples of application presented in the paper illustrate the need for a better understanding of the receptivity mechanisms. (C) 2000 Elsevier Science Ltd. All rights reserved.