Examination of changes to the spatial structure of turbulent boundary layers due to surface-wave forcing using POD

Abstract

Changes to the spatial structure of turbulent boundary layers due to the presence free-surface waves are investigated using Partial orthogonal decomposition (POD). For the current study, the wavelength of the surface waves is approximately three times the boundary layer thickness. Large particle image velocimetry (PIV) datasets are used to obtain a wide range of converged POD modes, both with and without surface wave forcing, allowing the comparison of spatial structure. A hierarchical series of inclined lowand highmomentum structures that contribute to negative turbulent shear stress, and are reminiscent of hairpin packets, are observed near to the wall in the unperturbed case. These structures are observed to be highly disrupted by the depth-varying wave perturbation, with fewer and weaker inclined near-wall structures observed in the later case. Perhaps most significantly, all of the remaining inclined attached structures have streamwise lengthscales less than the wavelength of the disturbance and have increased angle of inclination. These results suggest that surface waves significantly alter the structure of large-scale turbulent packets with streamwise lengthscales greater or equal to the wavelength but have less effect on smaller scale productive motions.