Flow field decomposition applied to experimental data obtained for a transitional boundary layer

Using experimental data from Particle Image Velocimetry (PIV) measurements, coherent structures of a transitional spatially developing boundary layer are determined. The coherent structures are determined utilizing the Proper Orthogonal Decomposition (POD), which is based on an expansion of the flow field variables into a set of eigenfunctions or modes. For having constant and reproducible flow field conditions, the flow is artificially excited by means of periodic velocity fluctuations. The used excitation device allows the generation of different transition scenarios, where this paper focuses on the case of thefundamental transition. Phase locked excitation signals allow the recording of instantaneous velocity fields of the flow field at certain instants of time. It can be shown that PIV is a suitable technique to provide experimental data for POD. The results of the POD show that already a small number of modes cover most of the kinetic energy of the flow.