Visualization and laser Doppler measurements of the development of Lambda vortices in laminar-turbulent transition

Abstract

Laminar-turbulent transition in a flat-plate boundary layer was investigated by means of laser Doppler anemometry and flow visualizations in water. In order to generate a controlled fundamental-type breakdown with peak-valley splitting, well defined waves were introduced continuously into the boundary layer. Aligned Lambda vortices appear in a typical pattern for a fundamental-type transition. These vortices were visualized in detail, which also helped us to choose the experimental set-up and the disturbance parameters for best regularity of the transition process. In combination with the visualizations, the development of the disturbance waves, the fundamental-type resonance and the evolving vortices were captured with laser Doppler measurements. The suitability of the modern non-intrusive measurement technique for transition experiments is demonstrated. Additionally to this paper, excerpts from the recorded visualizations are available as video files.