Scenarios of Swept-wing Boundary-layer Transition in Presence of Various Kinds of Freestream Turbulence and Surface Roughnesses

This paper is devoted to a detailed experimental investigation of scenarios of the laminar-turbulent transition occurred in 3D boundary layers developing on models of 25- and 35-degree swept wings in presence of various kinds of freestream turbulence and distributed surface roughnesses. The study is performed in a low-turbulence wind tunnel at both low turbulence level (LTL) and enhanced turbulence levels (ETL) of the incident flow. Several scenarios of the transition phenomenon are presented with detailed qualitative visualization of fields of mean and fluctuating components of velocity disturbances measured by means of hot-wire anemometry. The laminar-flow breakdown is found to start in all cases with “sudden” appearance of a kind of local high-frequency secondary instability of the primary flow. Meanwhile, properties and conditions of origin of these secondary instabilities depend significantly on the parameters of external perturbations and the base-flow velocity.