Excerpt from: EUROMECH Colloquium 359 Stability and Transition of Boundary-Layer Flows,
10.--13.3.1997, Stuttgart, FRG

Experimental investigation of laminar-turbulent transition using laser Doppler anemometry (LDA)

Kruse M., Wagner S., Institut für Aero- und Gasdynamik, Uni Stuttgart, FRG

Laminar-turbulent transition in a flat-plate boundary layer was investigated by means of laser-doppler anemometry. 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 and produce so-called spikes in their tips. Flow visualizations and detailed simultaneous measurements of the main-stream velocity and the vertical velocity captured the disturbance development on the way to turbulence. The suitability of the modern non-intrusive measurement technique could be demonstrated.

As the interaction of waves plays an important role in laminar-turbulent transition, amplitudes and phases of waves had to be determined accurately. Therefore a fourier-analysis technique was developed that accounts for non-equidistant data and uses consequently the periodicity of the flow. The quality of the fourier technique was confirmed when measurements were compared with the theoretical eigenfunctions of the boundary-layer oscillations. Self-made programs, compatible with the commercial packages of the LDA, let large amounts of data be evaluated quickly.

Main topics of the measurements include disturbance introduction, boundary conditions, wave synchronization and fundamental resonance, Lambda-vortices and their formation, vorticity in the peak plane, and finally the multi-spike stages.

Detailed measurements of the initial wave development and the subsequent fourier analysis of the data clearly revealed a synchronization of the fundamental 2-D-wave with a pair of 3-D-waves and a spanwise modulation of the mean flow (fundamental resonance). After this synchronization the disturbances are strongly amplified. At the stage where the Lambda-vortices evolve, measured vortical structures in the peak plane are in good agreement with existing DNS data. The shape of the Lambda-vortices with their long tip in streamwise direction is marked best with the vertical velocity component. Measurements in the tips of these vortices detected multiple spikes with a high repetition frequency (8 to 9 times the fundamental frequency) in both velocity components. The results support a model where these multiple spikes are generated by vortical structures detaching from a high-shear layer. Due to certain irregularities in the flow at this stage of the transition process these vortical structures could not be measured directly (with our point measurement technique making use of the periodicity of the process). But the simultaneous measurements of the horizontal and the vertical velocity showed a fixed relation between these two components, so that -- together with other observed phenomena -- it can be concluded that coherent structures are shed from the boundary layer into the free stream.