MODELS FOR UNSTEADY WAKE-INDUCED TRANSITION IN AXIAL TURBOMACHINES

In turbomachinery, a considerable proportion of the blade surface area can be covered by transitional boundary layers. This may occur partic ularly when transition is periodically induced by the passing of wakes shed by upstream blade rows. Accurate prediction of the profile loss and heat transfer depends on the successful modelling of these transit ional boundary layers. This paper considers the effects of wake intera ctions on the transition process. As a consequence of experimental obs ervations, a physical model of unsteady transition is put forward. Bas ed on the fact that the intermittency of steady boundary layers can be calculated from a knowledge of the behaviour of turbulent spots, a th eoretical treatment of the unsteady problem is presented. Two models, each based on different hypotheses concerning the origin of the turbul ent spots are described. Numerical integration of the resulting equati ons provides results for comparison with experimental data. In die cas e of high Reynolds numbers, the results are consistent with diose obta ined using a much simplified model which shows that a functional relat ionship exists between the time-averaged behaviour of the boundary lay er and a new form of reduced frequency. Favourable comparisons are mad e with the measured intermittency and integral boundary layer paramete rs and the equivalent predicted values for a number of test configurat ions and operating conditions. It is shown that the spot-based transit ion models successfully predict the boundary layer development over a wide range of circumstances providing that an adequate description of the unsteady location of spot-formation is available.