2025 Theses Doctoral

Physical characterization of atmospheric boundary layer featuring roughness-induced secondary flows

Sathe, Atharva S.

Accurately modeling the atmospheric boundary layer (ABL) remains a central challenge in fluid dynamics. The ABL governs the exchange of mass, energy, and momentum between the land surface and the atmosphere, and these fluxes are the crucial drivers of the weather and climate variability. A major contributor to this modeling difficulty is the influence of complex built environments. These environments introduce organized motions beyond classical boundary layer assumptions that substantially affect surface–atmosphere exchanges.

This dissertation focuses on one such important and ubiquitous phenomenon: the generation of roughness-induced secondary flows. To investigate these effects, large-eddy simulations (LES) are used, as they resolve the dominant turbulent motions that shape the aforementioned exchange processes, offering physically grounded flow fields without heavy dependence on empirical closures.

However, because LES remain approximations of reality, careful attention is required when designing the computational setup to ensure that the physical processes of interest are represented accurately. The analysis in chapter 2 showed that the choice of the numerical domain can significantly impact the accuracy of turbulent flow statistics, potentially causing a mismatch between numerical studies and experimental data.

The study examined the influence of cross-stream aspect ratio (YAR), streamwise aspect ratio (XAR), and scale separation (SS) on first- and second-order flow statistics and turbulence topology. It is found that domains with a low YAR underestimate the velocity variance, while those with a low XAR overestimate the variance value. The study proposed a new approach based on the Buckingham Pi theorem to evaluate the effect of SS, as the existing method has major limitations for canopy flows in the presence of secondary flows. The results suggested that domains with small SS underpredict the variance value. To minimize the artificial impact of the numerical domain on turbulent flow statistics, the study recommended guidelines for future research, including a YAR of 3 or more, an XAR of 6 or more, and an SS of 12 or more.

Building on secondary flow insights from chapter 2, chapter 3 focused on uncovering the physical mechanisms governing these flows. This study addressed a central question in secondary flow research: which surface parameters determine the polarity of these flows, specifically whether low- or high-momentum fluid aligns with regions of high roughness. Through a systematic series of LES, the study identified that the spanwise gap between edge-most roughness elements of adjacent columns is a critical parameter that controls the polarity of mean secondary flows.

In addition to analyzing the time-averaged structure, the study investigated how variations in polarity affect the instantaneous dynamics of secondary flows. Here, it is observed that the regions of high- and low-momentum fluid created by the secondary flows alternate in a chaotic, non-periodic manner over time. Further analysis of the vertical velocity signal showed that variability in vertical momentum transport is a persistent and intrinsic feature of secondary flow dynamics. Taken together, these findings provided a comprehensive picture of how the geometric arrangement of roughness elements governs both the mean structure and temporal behavior of secondary flows.

Chapter 4 extended this effort by investigating additional surface parameters that are critical in governing the secondary flow polarity. Here, the aerodynamic roughness of the lower surface is found to play a decisive role in determining the polarity of secondary flows. When the lower wall is rougher, the low-momentum pathways align with the cuboids. As the wall is progressively smoothed, the polarity transitions gradually—first entering an intermediate regime with no preferred alignment, and eventually reaching a reversed state where high-momentum pathways align with the cuboids.

Analysis of the instantaneous secondary flow dynamics across the three cases suggested a cycle reminiscent of the self‐sustaining process in canonical wall-bounded turbulence, underscoring its role in shaping the surface‐layer response to spanwise heterogeneity. Here, it is proposed that spanwise variations in roughness topography repeatedly perturb the flow with a fixed directional bias, systematically reinforcing streaks of a given orientation, thereby establishing a preferred polarity. Although simplified, this framework offers a unifying perspective on the origin, persistence, and polarity of secondary flows, with the potential to reconcile diverse observations reported in the literature.