Wake dynamics of flow past a curved circular cross-section body under cross-flow vibration

Abstract

The principal objective of this thesis was to investigate the fundamental mechanism of vortex shedding past curved oscillating bluff bodies of circular cross-section exposed to an external uniform flow. Two different geometrical configurations have been studied: in both cases the main component of the body is a circular cylinder, whose centreline axis is prescribed by a quarter ring and the plane of curvature is aligned to the free-stream direction. According to whether the inflow is directed towards the outside or the inside of the quarter ring, two different configurations are identified, named respectively “convex” and “concave”. Fully three dimensional and a sectional two-dimensional simulations were performed to assess the validity of strip theory for complex non-straight bodies. Each geometry was tested in forced and free cross-flow vibration and the outcome of these simulations was compared to the fixed body case. Forced vibration simulations rely on the assumptions of sinusoidal motion, rigid body and locked-in wake dynamics and represent a simplified approach to the study of Vortex-Induced Vibration: imposing a cross-flow displacement to the body can provide valuable information on the forces on the structure if frequencies and amplitude are set to match those of free vibrations. In the convex configuration, the forced translation generated in-phase shedding with vortices bent according to the body curvature. The sectional simulations could approximate the force distribution, but failed to capture correctly the wake topology, especially when vortex dislocations appeared in the near-wake. The lower part of the body, nearly aligned to the inflow direction, gave rise to a strong hydrodynamic damping: free vibration simulations achieved a maximum amplitude one order of magnitude smaller than the values found for a straight cylinder under the same conditions. For this reason, an oscillatory roll motion about the horizontal extension axis was imposed as an alternative: the damping effect in the lower part of the body was inhibited and the resulting energy transfer was positive, making this type of motion a good basis for free vibration simulations. In the absence of motion the concave geometry was found to suppress vortex shedding; the controlled oscillation disrupted the stabilising mechanism triggered by curvature and gave rise to two different vortex topologies, a wide and a narrow wake. None of these features could be reproduced with two-dimensional simulations, demonstrating the intrinsic limitation of the two-dimensional approach for curved geometries. The present work highlights suggests that a redefinition of the lock-in boundaries for non-straight geometries should be undertaken to understand the combined influence of motion and curvature on the vortex shedding.