Coupled fluid structure interaction analysis on a cylinder exposed to ocean wave loading

Abstract

This thesis work deals with fluid structure interaction (FSI), one of the emerging areas of numerical simulation and calculation. FSI occurs when the flow of fluid influences the properties of a structure or vice versa. It is a great challenge to deal with such problems due to its complexity in defining the geometries, nature of interaction between a fluid and solid, multi-physics facts and requirements of computational resources. This kind of interaction occurs in a wide spectrum of engineering problems and as such remains a main attraction of engineering profession. In this thesis, the FSI analysis has been conducted on a typical slender cylindrical structural member used either as a brace or leg of an offshore truss structure. The supporting members of the structure (near to the free surface of the water) are subjected, more dominantly, to wave induced loads. Gradually, this interaction leads to the fatigue of existing structures. In 2009, a numerical study was conducted to predict the wave-in-deck load due to extreme waves on jacket platform near to free surface of the water [1]. The recent developments in numerical abilities provide easier ways to do this analysis. Here, the analysis has been approached using the partitioned method in which two ways of coupling (one-way and two-way) have been applied to simulate this cylindrical member subjected to ocean wave loads. The fluid and structural model have been created with appropriate dimensions. ANSA [3] is used as a pre-processing tool for creating the whole computational domain and volume mesh. In order to model the non-linear high amplitude ocean wave, fifth order Stokes wave theory is used in ANSYS Fluent [4]. For the structural model, ANSYS Mechanical (transient structural) is used to determine the dynamic response of a structure under unsteady wave loads. The free surface of the water phase is tracked by using the volume of fluid (VOF) technique in Fluent. The two solvers (ANSYS Fluent & Mechanical) are coupled (exchange of data) using system coupling in ANSYS Workbench. In order to understand the dynamics of a structural member, modal analysis has been conducted to determine the natural frequencies and its respective mode shapes. The differences in fluid forces acting on a structural member for both the ways of coupling have been analyzed. Also, an investigation has been done on the modes of deformation of a structural member in response to wave loads. Numerical results of wave force have been compared to results using Morison equation and theoretical slamming forces. Comparison work between two methods of coupling on combustion system has been taken as one of the references for the assessment of the two approaches [2].