Transition control in three-dimensional flows by means of distributed Micron-Sized Roughness-elements (MSR) has been under focus during the last decade. The successful experiments of Prof. Saric and his research group have created lots of attention and interest. In the reported work by Saric et al. always a significant positive effect of MSR on delay of transition has been observed. Similar attempts to control transition in three-dimensional flows by means of MSR have not been successful in the same degree. It has argued that the differences in the outcome of the experiments may be due to small differences in the level of noise (acoustic and free-stream turbulence) in the wind tunnels used in the experiments.

To our knowledge there are no investigations that really address the sensitivity and robustness of the MSR for transition control in three-dimensional flows. The objective of the proposed activities was to, numerically and experimentally address these issues.

The carefully performed numerical simulations allowed us to characterise the effects of acoustic and vortical perturbations separately or simultaneously. A controlled variation of level of ‘noise’ made it possible to understand the limitations of the MSR approach. The direct numerical simulations were accompanied with the non-linear stability calculations using the Parabolised Stability Equations (PSE). These calculations were much faster and cheaper in terms of computational costs. This made it possible to do a wide variation of parameters.

Carefully performed experimental investigations in a low-disturbance environment also generated valuable information and data. Experiments with controlled acoustic perturbations and turbulence level completed the direct numerical simulations and were used to validate the numerical results.