Prediction of the transition location on a hybrid laminar flow engine nacelle using the e^N method of linear boundary layer stability theory and validation by flight testing

Abstract

The prediction of the location of the transition from laminar to turbulent boundary layer flow on a hybrid laminar flow(HLF) engine nacelle flight tested on the DLR VFW614/ATTAS is carried out using the e^N method of the local linear stability theory. The value of the limiting N factor N_LIM required for the transition prediction has been derived from an analysis of flight tests with a natural laminar flow engine nacelle previously flight tested on the same aircraft at the same flight and engine operating conditions. In contrast to the flight trials with the natural laminar flow nacelle, for the HLF nacelle flight tests there existed an additional receptivity component due to the waviness of the forebody surface of the manufactured HLF nacelle fan cowl as result of the aerodynamic surface quality not complying with the manufacturing specification. For the validation of the transition prediction, results obtained using the infrared thermography as well as the external surface temperature distribution on the HLF nacelle fan cowl have been drawn upon. It is found that the transition location prediction by the e^N method underestimates the longitudinal extent of the laminar boundary layer flow on the HLF nacelle fan cowl by about the same amount by which the experimentally determined transition location differs between the infrared thermography and the surface temperature measurement results.