POD of turbulent flow above surface with different roughness classes

The boundary layer dynamics above surfaces of a various roughness is measured in the wind-channel by 2D 2C TR-PIV. Five types of the surfaces are covered by five different types of elements in order to provide a roughness belonging into slightly rough, moderately rough, rough and very rough categories according to guideline VDI (2000). For each surface two measurements are conducted. Firstly, the measurement with low sampling frequency of 100 Hz but lasting for 20 s carries out the statistics of each ensemble. Then, the short measurement with high sampling rate of 2000 Hz helps to understand the dynamics of the flow. Statistics of basic turbulent characteristics are compared as they depend on the given class of the roughness. The spatial correlation and time-lagged spatial correlation, followed by the temporal correlation is evaluated in order to confirm the validity of the Taylor hypothesis of 'frozen turbulence' in a very turbulent flow. Proper orthogonal decomposition (POD) is performed to reveal the dynamical modes containing significant amount of turbulent kinetic energy (TKE) for each category. Data are decomposed also with Quadrant analysis, so we can detect the events as sweep and ejection. These events are energetically dominant, since they contain up to 80% of turbulent energy, they contribute markedly to the momentum flux in the boundary layer (occasionally 80-90%) and moreover, they are spatially compact.