On the influence of the free surface on a stationary circular cylinder with a flexible splitter plate in laminar regime

Fluid Structure Interaction (FSI) problems typically found in offshore systems and other oceanic applications are generically affected by the proximity to the free surface. The aim of this paper is to evaluate to what extent the immersion depth affects the dynamics of a simplified fluid-structure interaction model. We have selected the research conducted by Turek et al. (2010), in which a deformable splitter plate is attached to the base of a stationary circular cylinder, and a uniform flow is imposed to the system, in laminar regime. We validate our numerical techniques by reproducing the results obtained by other authors when simulating this problem, as in a benchmark exercise. After the initial validation, the structure is submerged on a liquid phase that is separated by a free surface from the gas phase placed on the top. The structure is submerged at different depths and the dependence on the Cauchy, Reynolds and Froude numbers are studied. The deformation and the drag forces acting on the structure are analysed in detail in order to understand the effects that the free surface has on the problem. It was observed that the amplitude of the tip of the splitter plate grows as the Reynolds, Froude and Cauchy numbers are increased. However, as the depth is increased, and the structure moves away from the interface, all measurements tend to saturate. For lower depths, the free surface works as a damper for the plate dynamics, reducing the oscillations. The physical mechanism of deformation based on the pressure difference at both sides of the plate is studied, monitoring how the pressure difference decreases as the structure approaches the free surface. The drag coefficient of the global structure (cylinder and plate) increases linearly with depth, but decreases when the viscosity of the fluid is reduced. (C) 2019 Elsevier Ltd. All rights reserved.