Interaction of a laminar boundary layer with roughness elements in the close vicinity of an airfoil leading edge

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Roughness induced transition is of sustained interest, especially with regard to the design of highly efficient wind turbine blades, since the airfoil leading edge can easily be contaminated. As a consequence boundary layer transition is shifted upstream leading to a significant performance degradation. The present experimental investigations provide an increased understanding of the transition process downstream of a single, three-dimensional roughness element, which is located in an airfoil leading edge region. The experiment is closely adapted to operational conditions of large wind turbines (Re = 5-6 million). In the first period of the project, it was focused on the disturbance evolution downstream of medium height roughness elements. This case turned out to be advantageous with respect to a clear physical understanding of the influence of key parameters, that is the disturbance amplitude and frequency, roughness height and pressure gradient. Moreover, based on a controlled disturbance excitation upstream of the roughness, it was possible to reveal the dominant instability mechanism, which prevails for low amplitude disturbance levels at the roughness position. In the second period of the project, the investigations in the medium roughness height range will be complemented by varying the roughness geometry and the natural disturbance level at the roughness position. A second focus of the experimental investigations will be on the investigations of spatially and temporally well-resolved flow fields in the wake of high (supercritical) roughness elements, for which a different transition scenario is expected. Therefore, a three-component PIV measurement setup has been build up, adapted and validated in the first period of the project. Hence, mean flow characteristics obtained with the PIV measurement setup and unsteady disturbance fields gained with the hot-wire measurement technique can be linked to shed further light on the instability mechanism in the wake of high roughness elements. The experimental investigations will be accompanied by Direct Numerical Simulations (DNS), which are performed at the Laboratory of High Performance Computations, University of Sao Paolo. Numerical simulations (DNS) will also be carried out in the group Transition and Turbulence at the IAG in Stuttgart for an enhanced interpretation of the experimental results and the underlying physical process.