Author(s): Fawaz Alzabari; Pablo Ouro; Catherine Wilson

Linked Author(s):

Keywords: Free surface; Single cylinder; Level set method; Submergence ratio; Wake structures

Abstract: The flow around a submerged horizontally-oriented circular cylinder influenced by the presence of the free-surface is of special relevance in practical applications such as sub-sea cables in offshore environments or hydraulic structures for the construction of woody debris dams for natural flood management. Whilst the wake dynamics of unbounded cylinder flows has been largely investigated, the flow around cylinders located underneath a free-surface layer and over a bottom wall has been less studied despite the notable impacts of these two constrains in the generated flow structures. When the cylinder is placed near to the free surface, the dynamics of the vortex shedding is altered as a result of a limited vertical expansion. The vortical structures produced by the cylinder interact with the deforming free surface modifying the vortical instabilities in the wake, especially if the relative submergence is relatively small, i.e., there is a reduced water level overtopping the cylinder. Further insights into the wake coherence and shedding patterns need to be studied to determine under which range of conditions the cylinder flow is affected by the proximity to the free-surface. Here large-eddy simulations that adopt the level-set method to resolve the air-water interface are performed to analyse the turbulent flow behind a circular cylinder under four different submergence configurations. Four submergence ratios (h/D = 2.1, 1.5, 0.9 and 0.5 with h being the distance between the water surface and the top of the cylinder and D the cylinder’s diameter) are considered with a Froude number in the range of 0.53-0.26 and constant Reynolds number of 13,333 based on the cylinder’s diameter. The computed statistics have been validated with experimental measurements for the case with h/D = 1.5 showing good agreement of first- and second-order statistics. The instantaneous wake dynamics are analysed using Proper Orthogonal Decomposition (POD) applied to the LES-computed velocity field, to investigate the spatial coherence of the turbulent structures developed according to the submergence ratio. Based on the vertical velocity fluctuations, the first two POD modes contained the most associated modal energy accounting for approx. 50-60%. However, this contribution reduced to approximately 30% found for the shallowest case with Fr = 0.53, which is linked to the rapid loss of coherence from the von-Karman vortices in the near wake due to the impact of the free surface. The remaining modal energy is mostly shared between the next four modes.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022760

Year: 2022