Swept-wing boundary-layer transition at various external perturbations: Scenarios, criteria, and problems of prediction

This experimental work is devoted to a parametric study of characteristics of the laminar-turbulent transition occurred in a model of a crossflow-dominated swept-wing boundary layer. The experiments are performed in 39 different regimes with low and elevated freestream turbulence levels both in the absence and in the presence of steady freestream vortices of various spanwise scales. The measurements are performed at several values of freestream velocity for two types of distributed surface roughness. Transition scenarios and criteria of turbulence onset are investigated and the problem of transition prediction is examined. It is shown that in all studied cases, the turbulence onset starts with the appearance of local high-frequency secondary instability of the base flow perturbed by primary instability modes. It is found that the 30% level of the threshold zero-to-peak amplitude of combined boundary-layer disturbances (i.e., the steady ones plus unsteady ones) can be used in all studied cases as a simplified turbulence-onset criterion, which was found in previous experiments at other flow and disturbance conditions. A very important, often decisive role of extremely weak steady free-steam disturbances produced by turbulence generating grids is investigated. A generalized combined variable N-factor method of transition prediction is suggested. Published by AIP Publishing.