Transition to turbulence in swept wing boundary layers has been the subject of much research over the last years. Several experiments have already highlighted the importance of distributed micro-sized roughness (DMSR) elements, showing that very small surface imperfections can produce a prominent effect on the transition location. However, researchers' efforts to successfully control transition with these elements have not been met with success because of small differences in the level of noise in the wind tunnels used in current experiments. EU-funded scientists working on the project RODTRAC (Robustness of distributed micron-sized roughness-element for transition control) employed numerical modelling techniques and conducted wind tunnel experiments to test the receptivity of the boundary layer to external disturbances such as freestream turbulence and acoustic waves. Receptivity is the mechanism by which freestream disturbances enter the boundary layer and create the initial conditions for unstable waves. Detailed numerical simulations enabled careful evaluation and explanation of the effects of acoustic and vortical perturbations either in conjunction or separately. Simulations were accompanied with stability calculations and receptivity analyses. Results showed that independently of the freestream turbulence level, stationary crossflow vortices are the dominant instability mechanisms when MSR elements are present. In addition, the interaction of acoustic waves and roughness elements excited unsteady crossflow vortices. Experiments with controlled acoustic perturbations and turbulence complemented the numerical work and were used to validate results. In low-turbulence levels, DMSR elements stabilised boundary layer flow and displaced the transition farther downstream. In such cases, acoustic fields in certain frequency ranges were found to further destabilise the transition location. High-turbulence levels proved not to delay the laminar-turbulent transition. RODTRAC enhanced knowledge about the interaction of different sources of perturbations on transition in swept wing boundary layers. Project results and findings will be used to improve predictions regarding the performance of aircraft with laminar wings, facilitating the design of advanced aircraft.
Laminar-turbulent transition, swept wings, distributed micro-sized roughness, RODTRAC, freestream turbulence