Wave – Current– Floating Structure Interaction Study: Uniform Flow Model Extension and Its Comparison with Fully Nonlinear Simulations
Abstract
The wave-current (or wave-forward speed) interaction study is an effort to incorporate essential coupling effects between the unsteady waves problem and the steady flow problem into the floating structure’s dynamic. The linear solution that only considers the coupling effect with the uniform flow is still considered the most practical way of incorporating the interaction effect. This so-called Uniform Flow (UF) approximation model is valid for a slender body under relatively low current speed. By considering the UF approximation, the interaction effect can be reduced to the solution of a wave-only problem (i.e., zero speed solution) and its correction factors. This approximation is implemented into the existing in-house 3-D BEM code in the frequency domain. Noting the UF approximation model’s versatility and simplicity, we extend the existing model to cover a broader range of applications.
First, a practical approach to incorporate forward speed and hydro-elasticity effect in the frequency domain was developed. By utilizing the discrete-module-beam (DMB) method, flexible structures are partitioned into multiple rigid bodies connected by beam elements. The forward speed effect is taken into consideration in the multi-body hydrodynamics through the UF-based simplified method. The present numerical results compare well with published experimental and FEM-BEM coupling method. Several parametric studies were also conducted to quantify the forward speed effect on the structure’s elastic behavior.
Then, a formulation for the second-order difference-frequency wave load in the presence of a steady uniform current is presented. The free-surface integral is not included in evaluating the difference-frequency force quadratic transfer function (QTF) considering its relatively negligible contribution compared to other terms, significantly reducing the computational cost. The importance of properly incorporating the uniform flow effects into both the boundary value problem and the force formulation was underscored even for the small uniform flow speed.
Finally, all of the results except for the hydro elasticity study were compared against the completely nonlinear CFD method. The CFD results show several phenomena not captured by the BEM, such as the Kelvin ship’s wave, nonlinear body motion, and breaking waves. However, due to its lower computation requirements, BEM is still preferred in the majority of cases.
Subject
Wave-current interactionNeumann-Kelvin
forward speed
uniform flow
hydroelasticity
DMB (discrete module beam)
multi-body hydrodynamics
second-order force, added resistance, quadratic transfer function, CFD
Citation
Bakti, Farid Putra (2020). Wave – Current– Floating Structure Interaction Study: Uniform Flow Model Extension and Its Comparison with Fully Nonlinear Simulations. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /193025.