This study investigates the influence of wall roughness on large-scale structures within turbulent boundary layers, using wall-parallel stereoscopic Particle Image Velocimetry (PIV) at a Reynolds number of 1.3 × 106m−1. The data was collected at a distance of = 2.8 from the wall under adverse = 0.98, small = −0.04, and favorable = −0.60 pressure conditions. It was observed that coherent structures larger than the field of view (FOV) modify their length scales, streamwise orientation and spanwise distribution in response to changes in surface roughness and pressure gradients. The study also examines implications wall similarity hypothesis by comparing the development of these large-scale structures over smooth and rough wall flows under small pressure gradient and identical test conditions. Notably, the results reveal that existing models may not accurately capture the observed dynamics, as evidenced by discrepancies with previous studies, thereby enhancing our understanding of turbulent flow dynamics in non-equilibrium conditions. / Master of Science / Understanding the behavior of turbulent air and water flows around objects is crucial for improving engineering designs and various environmental applications. This study investigates how surface roughness, such as on ship hulls or aircraft wings, affects the movement of large-scale structures in turbulent flows. Advanced flow visualization techniques were employed to observe these turbulent structures in detail over homogeneous roughness and varying pressure gradients. The findings demonstrate that rough surfaces significantly alter the movement and interaction of these large-scale turbulent structures. Specifically, favorable pressure gradients result in organized flow structures, whereas adverse pressure gradients cause reorganization, altering the size and distribution of these structures within the flow. By comparing smooth and rough surfaces under identical conditions, the study aims to evaluate how well existing hypotheses predict these changes. The results highlight that rough surfaces and pressure variations critically impact turbulent flow characteristics, emphasizing the need for more sophisticated criteria to improve predictions and designs in real-world applications. This research provides valuable insights into the dynamics of turbulent flows over rough surfaces and stresses the importance of refining current models for better accuracy in practical engineering applications.