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

摘要

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.