Date of Award
Spring 2026
Abstract
The ability to predict ground-level scalar concentrations inobstacle-influenced boundary-layer flows is critical to a wide range of engineering and environmental applications, including pollutant dispersion, industrial safety, and leak detection. While scalar transport in canonical, unobstructed turbulent boundary layers is well understood, far less is known about non-canonical configurations in which fluid--structure interactions fundamentally alter plume behavior. This dissertation investigates scalar dispersion in such a configuration: a passive scalar released from the free end of a wall-mounted cylindrical obstacle immersed in a high-Reynolds-number turbulent boundary layer, with emphasis on the transitions between three distinct plume regimes that arise from this coupled interaction: the elevated plume (EP), the ground-level plume (GLP), and the ground-level source (GLS). A comprehensive experimental campaign was conducted in the Flow Physics Facility at the University of New Hampshire, generating a unique dataset of concurrent, co-located velocity and scalar measurements for four cylinder geometries spanning aspect ratios $AR = 0.70$--$11.47$ at friction Reynolds numbers $\delta^{+} \simeq 2{,}826$--$5{,}489$. The measurements resolve both the structure of the cylinder-induced wake and the resulting scalar field, providing direct insight into the coupled dynamics governing plume evolution and the downstream development of the mean scalar field. The EP, GLP, and GLS regimes introduced above provide the unified interpretive framework used throughout this work. The experiments demonstrate that plume behavior is governed primarily by cylinder aspect ratio, which controls the underlying vortex dynamics and therefore the mechanism of scalar transport. Following the wake-topology classification established in prior studies of wall-mounted finite cylinders, a critical aspect ratio of $AR \approx 3$ separates two qualitatively distinct regimes: high-aspect-ratio geometries ($AR \gtrsim 3$) produce gradual EP--GLP--GLS transitions driven by aerodynamic downwash, whereas low-aspect-ratio geometries ($AR \lesssim 3$) exhibit rapid transition to GLS behavior through wake-cavity entrainment. The present velocity measurements reveal that, despite large local perturbations, the induced turbulence is weakly coupled to the mean shear, resulting in limited turbulence production, rapid decay of the disturbance, and a correspondingly fast relaxation of the boundary layer toward its canonical zero-pressure-gradient state. This energetic decoupling provides a physical explanation for both the observed plume-regime transitions and the suppression of vertical plume growth in obstacle-influenced flows. Motivated by these findings, a data-informed predictive framework is developed that extends classical Gaussian dispersion modeling by explicitly incorporating plume-regime transitions through the evolution of the normalized plume centroid $z_c/\delta_{cz}$. The model captures the coupled effects of geometry, velocity ratio, and downstream development on plume structure and provides accurate predictions of concentration profiles and ground-level concentrations across the measured parameter space. Overall, this work establishes a physics-based framework linking vortex dynamics, turbulent transport, and scalar dispersion in non-canonical boundary-layer flows. The results provide both a high-fidelity experimental benchmark for model validation and a practical predictive tool for estimating ground-level concentrations in obstacle-influenced environments.
Document Type
Dissertation
First Advisor
Christopher White
Second Advisor
Yves Dubief
Third Advisor
Lukasz Zielinski
Department or Program
Mechanical Engineering
Degree Name
Doctor of Philosophy
Recommended Citation
Amankwah, Kofi Agyemang, "Scalar Dispersion from Wall-Mounted Cylinders at Large Reynolds Number: Plume Transitions and Regime Classification" (2026). Doctoral Dissertations. 3010.
https://scholars.unh.edu/dissertation/3010