POD Analysis of the Near-Wall Region of a Rough Wall Turbulent Boundary Layer

A proper orthogonal decomposition (POD) analysis is carried out with particle image velocity (PIV) measurements of a turbulent boundary layer over a rough wall made of transverse square bars. Two bar spacings were investigated (w/k = 3 and 7, w is the bar spacing and k the bar diameter). This ratio affects the energy distribution amongst the POD modes throughout the boundary layer, the higher modes, i.e. the relatively small-scale and less energetic motions, being the most perturbed. The first few near-wall POD modes were also affected by the spacing.