Examination of the changes in spatial structure of the turbulent boundary layer under wave forcing using Proper Orthogonal Decomposition

Thesis (Master's)--University of Washington, 2018The interaction between bottom turbulent boundary layers and surface waves has previously been characterized using point-wise, statistical methods suggesting significant non-linear changes to turbulent boundary layer structure. To examine changes to these structures, a detailed set of experiments were conducted using a combination of wave-height sensors and particle image velocimetry (PIV) to examine the bottom turbulent boundary layer under quasi-sinusoidal waves of differing amplitude. Spatial structures were examined using proper orthogonal decomposition (POD), which has been demonstrated to reveal near-wall attached structures that have been seen to contribute to the majority of negative turbulent shear stress and positive turbulent kinetic energy production in conventional boundary layer flows. Through an examination of streamwise and wall-normal modes it is demonstrated that POD is able to reveal these structures in the present dataset despite the presence of the free surface and high freestream turbulence of the current facility. Under surface wave forcing, the streamwise structures were seen to exhibit a higher inclination than in the unperturbed boundary layer, and wall-normal modes that are detached from the wall. In addition, there is loss of anti-correlation between streamwise and wall-normal modes resulting in stronger positive and weaker negative shear stress events that are clustered further from the wall suggesting significant disruption to turbulent production mechanisms