Detailed Analysis of Experimental Investigations on Boundary Layer Transition in Wake Disturbed Flow

In the framework of a co-operation between the University of Genoa and the Darmstadt University of Technology measurement data of a former investigation at Darmstadt, comprising measurements with surface-mounted hot-film sensors on the boundary layer transition in wake disturbed flow, were transferred to Genoa, then re-evaluated and in great detail analyzed, much further than the original data evaluation. In these experimental investigations at Darmstadt, the boundary layer transition with and without transitional separation bubbles was studied on a circular cylinder in cross flow. The comparison of hot-wire traverses with the surface-mounted hotfilm distributions clearly indicated that the surface-mounted hot-film technique is a very suitable measurement technique to obtain reliable information on transition and separation phenomena with both high spatial and temporal resolution. The new data evaluation techniques applied to these data at Genoa further enhanced the insight into the details of the boundary layer transition and separation process. The surface-mounted hot-film data were evaluated by means of time-space diagrams for the first three statistical moments (mean, RMS and skewness), with which the origin and the extent of unsteady separation bubbles clearly could be seen. The results obtained from these data analyses on the one hand yield a considerable enhancement of the understanding of the periodically unsteady boundary layer transition process and on the other hand they form the basis for the application of surface-mounted hot-film sensors in more complex flow situations like e.g. in cold flow multistage turbine or compressor test rigs or even in the hostile environment of real aero engine compressors or turbines.