Influence of Surface Roughness and of Suction on the Receptivity of a Swept Wing Boundary Layer

Abstract

It is well known today that for the three dimensional flow, an important factor for the receptivity mechanisms is the surface polishing close to the leading edge. Several experiments [1], [2] highlighted the importance of micron-sized roughness elements by showing that very small surface imperfections can produce a noticeable effect on the transition location. The surface polishing of the leading edge region is thus a very important parameter for laminar flow control, mainly for the small size models designed for wind tunnel testing. Moreover, in order to maintain laminar flow conditions on aircraft wing, boundary layer suction through perforated walls in the leading edge region is often used. The question arises of the analogy between roughness elements and suction holes in the receptivity process. The present study is devoted to the analysis of the effect of micron-sized roughness elements and of perforated walls on the initial amplitude of the stationary modes developing in a three dimensional boundary layer. The first part of the paper describes experiments carried out on a swept model in the F2 wind-tunnel at the Fauga-Mauzac ONERA centre. Typical results are presented to illustrate the effect of “positive” and “negative” roughness elements on the disturbances developing in the laminar boundary layer. In the second part stability computations were performed in order to analyse these experimental results with linear and non linear approaches.