The Effects of Freestream Turbulence on Bluff Body Aerodynamics in Separated and Reattaching Flows

Abstract

Large-scale turbulence is prevalent in the atmospheric boundary layer. However, due to technical limitations in replicating large-scale turbulent structures in wind tunnel tests, accurately assessing its influence on wind loads has long been a challenge, potentially compromising structural safety. By integrating Large Eddy Simulations (LES), wind tunnel pressure measurements, and Particle Image Velocimetry (PIV), this thesis systematically investigates the generation mechanisms of extreme wind loads and the effects of freestream conditions on the aerodynamic characteristics of bluff bodies, with particular emphasis on effects of large-scale freestream turbulence.

First, a multiscale vortex-dynamics-based framework for generation of negative peak pressures is established. In separated flows, such pressures typically occur near the trailing edges and are induced by the interaction between large-scale vortex merging events and small-scale vortex convection motions. The spatial and temporal characteristics of these multiscale interactions are quantitatively characterized. In separated-reattaching flows, negative peak pressures predominantly emerge near the leading edges, driven by the combined effects of large-scale shear-layer flapping and the passage of small-scale vortices. The spatial extent, convection speed, and passage duration of these vortices relative to extreme pressure generation are quantitatively analyzed.

Second, the effects and mechanisms of sinusoidal inflow disturbances on separated flow fields around bluff bodies and on their aerodynamic characteristics are systematically revealed. Under streamwise sinusoidal inflows, flow-regime transitions from classical separated flows to separated-reattaching flows are observed, accompanied by an attenuation of the Kármán vortex street. Under transverse sinusoidal inflows, flow resonance phenomena are identified, and their onset conditions and wake structures are characterized. The effective ranges of inflow frequencies and oscillation intensities for these flow evolutions are determined, and the aerodynamic consequences of such transitions are clarified.

Finally, the effects of freestream turbulence on bluff body aerodynamics are investigated, with special attention paid to turbulence scale effects and the associated flow mechanisms. The critical integral length scale of freestream turbulence beyond which turbulence scale effects converge is revealed. In separated flows, increased turbulence intensity and integral scale enhance shear-layer flapping and promote intermittent flow reattachment, leading to both intermittently weakened vortex shedding and elevated surface pressure fluctuations. In separated-reattaching flows, the present study covers the broadest range of freestream turbulence scales to date (up to 40D), revealing the convergence behavior of turbulence scale effects. The momentum convection mechanism through which large-scale freestream turbulence amplifies surface pressure fluctuations is clarified.

These findings advance the fundamental understanding of bluff body aerodynamics under realistic atmospheric conditions. By elucidating the mechanisms underlying extreme wind load generation and the scale effects of both turbulent and sinusoidal inflows, this research provides essential theoretical support for wind-resistant design and offers valuable guidance for enhancing structural safety in engineering practice.

Date of Award	21 Oct 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Qiusheng LI (Supervisor)