An experimental investigation of unsteady effects in a high-lift system

A fundamental investigation of the confluent boundary layer formed by the interaction of the slat wake with the main element boundary layer of a multi-element airfoil was reported in Thomas, Liu and Nelson. One unanticipated result of that study was the observation of significant unsteady high lift flow behavior for certain slat positions. These unsteady motions can have a strong effect on the aerodynamic performance of the high lift system. This appears related to the observation that the unsteady flow caused more aggressive mixing between the slat wake and main wing boundary layer. This results in a decrease in high lift performance and an increase in airframe noise. This unsteady aspect of the high lift flow field forms the focus of this paper. Experimental evidence of unsteady flow field behavior is presented first. These results suggest that the unsteady flow behavior is a manifestation of unsteady vortex shedding and separated shear layer reattachment associated with the slat cove separation region. A discussion of the wavelet analysis technique is presented and this is shown to be a powerful tool in the characterization of transient flow phenomena. Some representative wavelet transform results obtained in the slat wake are presented and discussed. This work represents part of an ongoing study and the paper concludes by describing our current research effort to understand the unsteady aspects of high-lift flows.