TRANSITION LENGTH PREDICTION FOR FLOWS WITH RAPIDLY CHANGING PRESSURE-GRADIENTS

A new method for calculating intermittency in trasitional boundary lay ers with changing pressure gradients is proposed and tested against st andard turbomachinery flow cases. It is based on recent experimental s tudies, which show the local pressure gradient parameter to have a sig nificant effect on turbulent spot spreading angles and propagation vel ocities (and hence transition length). This can be very important for some turbomachinery flows. On a turbine blade suction surface, for exa mple, it is possible for transition to start in a legion of favorable pressure gradient and finish in a region of adverse pressure gradient. Calculation methods that estimate the transition length from the loca l pressure gradient parameter at the start of transition will seriousl y overestimate the transition length under these conditions. Conventio nal methods based on correlation overestimate pressure gradient transi tion data are similarly inaccurate. The new calculation method continu ously adjusts the spot growth parameters in response to changes in the local pressure gradient through transition wing correlations based on data given in the companion paper by Gostelow et al. (1996). Recent e xperimental correlations of Gostelow et al. (1994a) are used to estima te the turbulent spot generation rate at the start of transition. The method has been incorporated in a linear combination integral computat ion and tested with good results on cases that report both the intermi ttency and surface pressure distribution data. It has resulted in a mu ch reduced sensitivity to errors in predicting the start of the transi tion zone, and can be recommended for engineering use in calculating b oundary layer development on axial turbomachine blades.