PIV measurement of turbulent flow within a street canyon: Detection of coherent motion

Turbulent flow inside a street canyon was investigated in a wind-channel. Velocity measurements were performed in vertical planes by means of particle image velocimetry (PIV) at the repetition rate 500 Hz. Two geometries were used for comparison purposes: buildings with pitched roofs and with flat roofs. Both induce different dynamic regimes in the street, so that the generated turbulent flows are of different categories. Velocity data were analysed by proper orthogonal decomposition (POD). POD identifies the most dominant modes with high coherency in the flow and also evaluates the relative contributions of each mode to the overall kinetic energy of turbulence. Rigorous analysis of correctness of the physical interpretation of such decomposition was carried out. POD reconstruction of the original vector field was performed and the accuracy of the method was evaluated. Wavelet analysis was applied to the time-series of the POD expansion coefficients in order to reveal the dynamics of the modes. Vorticity, calculated from the original velocity data, was decomposed by POD as well. Finally, the correlation between the vorticity and other methods for vortex detection was assessed.