Amplified Laminar Boundary Layer Oscillations and Transition at the Front Attachment Line of a 45° Swept Flat-Nosed Wing with and without Boundary Layer Suction

Abstract

Transition was observed in the Northrop 7 × 10 ft tunnel at the attachment line of a 45° swept flat-nosed wing with and without suction through closely spaced chordwise slots, starting with an undisturbed laminar initial boundary layer at the apex stagnation point. Wind tunnel screen turbulence induced regular amplified attachment line boundary layer oscillations. Their frequencies correlated with theoretical values for the most strongly amplified attachment line boundary layer oscillations. They were generally modulated and superimposed over random low frequency oscillations induced presumably by free-stream larger scale eddies. Slightly off the attachment line the high frequency boundary layer velocity fluctuations grew to several percent, causing finally turbulent spots in combination with the random low frequency oscil-ations. Without suction, transition started at Re θa.l.T = 240 and 310 at (WZ/v) = 5.5 × 106 and 3.5 × 106, respectively. Boundary layer suction rapidly increased these transition Reynolds numbers. They are substantially higher than in the presence of large disturbances which induce turbulent spots directly. However, Re θa.l.T was much lower than on flat plates, explainable by the destabilizing influence of transverse disturbance vortex stretching in the diverging flow field of the attachment line region and the large growth factor of attachment line boundary layer oscillations, resulting from the relatively large Ci and α values Open image in new window