Rearrangement of Secondary Flows over Multi-Column Roughness Configurations*

Turbulent flows over spatially inhomogeneous rough surfaces are crucial in various applications, such as wind farms, urban landscapes, and fluvial processes. When these rough surfaces exhibit cross-stream heterogeneity comparable to the boundary layer height, they generate secondary flows. Over the past decade, significant efforts have focused on understanding secondary flows due to their critical role in modulating flow statistics. A key interest in these flows is identifying the parameters that govern their polarity, which dictates whether fast-moving or slow-moving fluid aligns with the roughness elements. In this study, we investigate how the cross-stream gap between the roughness elements in localized high-roughness patches affects the polarity of secondary flows. We will demonstrate how this parameter alters the preferential fluid pathways in these patches, thereby affecting the polarity globally. Our analysis includes three simulations with varying cross-stream gaps: two showing clear polarity reversal and one intermediate case. The insights will be based on first and second-order velocity statistics and TKE budget analysis. Overall, this research aims to deepen our understanding of the characteristics of secondary flows over complex arrangements of roughness elements.