BREAKDOWN OF A STREAK VIA DEVELOPMENT OF VARICOSE SECONDARY MODE ON THE STRAIGHT WING WITH PRESSURE GRADIENT

Abstract

The latest stages of the laminar flow breakdown on a wing are associated with a formation of longitudinal structures of lowand high-speed fluid streaks, which later on make the flow susceptible to secondary instabilities. In natural case such disturbances can be initiated by some surface irregularities such as uniform surface roughness or humps. On a swept wing, secondary instability of the cross flow plays an important role in the formation of streaks as it was shown in our previous studies in the frame of this project [1]. At that work it was found that the swept wing flow with the longitudinal vortices or streaks became secondary unstable to periodical disturbances. In the present investigation it is also shown that similar processes lead to the final flow breakdown at very late stages of transition. Additionally, for the first time it is demonstrated that the external pressure gradient have very strong influence on the development of secondary instabilities on the straight wing and hence the applicability of pressure gradient variation for the control purposes is proven in this case.