Transitional Flow Physics and Flow Control for Swept Wings: Experiments on Boundary-Layer Receptivity, Instability Excitation and HLF-Technology

Summary

Basic experiments on transitional flow physics for swept wings are performed at DLR-Göttingen supporting the theoretical-numerical work (see the corresponding CEAS/DragNet-contribution by Janke et al.). One common aim is to predict and to control boundary-layer transition as reliable and simple as possible. The DLR swept flat plate transition experiment provides the setup with a crossflow dominated three-dimensional boundary-layer flow, to investigate the stabilization with the aid of surface suction through holes and slots. An effective and resistant attenuation of the disturbances initiating transition can be reached. One objective of this study is to quantify the effectiveness and robustness, i.e. the flow control sensitivity on suction device imperfections and variations. Another objective is the prediction of transition for the design of Hybrid Laminar Flow (HLF) transport-aircraft wings. This attempt requires experimental informations for physical modeling of the origin of boundary-layer instability. Therefore, the impact of vortical freestream disturbances on the receptivity of three-dimensional boundary layers is evaluated. A major goal is to identify the mechanism(s) that cause the final breakdown of a swept wing’s three-dimensional boundary layer into a fully turbulent flow regime. With a controlled excitation of primary and secondary traveling disturbances, the appearance and growth of high-frequency secondary instability are investigated upstream of breakdown. Two high-frequency secondary modes are identified in agreement with theoretical predictions. Finally experiments using a wing with a thickness ratio of 15%, the damping effect of the convex surface curvature on disturbance growth is demonstrated and transition scenarios different from the DLR swept flat plate transition experiment become obvious.

Dedicated to the memory of Hans Bippes, our friend and colleague as well as long-time spiritus rector for experimental laminar-turbulent transition research at DLR in Göttingen.