Hydroelastic Analysis of Aluminum and Composite High-Speed Planing Craft Structures During Slamming

Abstract

The design of optimal planing craft structures is a challenging process that must take into account aspects such as cost, weight, operability, maintainability, manufacturability, and structural integrity. To ensure structural integrity, an accurate determination of the extreme loads that the structure will experience during its service lifetime is essential. Even more important is the ability to determine the effects of hydrodynamic loading on the structural response and the subsequent effect of the structure deformation on the fluid loading, a phenomenon known as hydroelasticity. In the field of naval architecture, concerns arise when high-speed vessels impact the water when operating in waves, a process know as slamming. Another example in which hydroelastic analysis is needed during design is the emergency landing of an aircraft in water, known as ditching. Both problems are related since the craft impacts the dense fluid at high horizontal-to-vertical speed ratios, developing a large fluid loading which, when coupled with the structural response, becomes a complex fluid-structure interaction FSI system.

In practice, empirical and experimental models are used to calculate loads and response in FSI problems, but are inadequate, especially when considering new materials such as composites. Experimental testing campaigns use rigid scale models to mimic full-scale structural phenomena. The primary challenge is to select the adequate scaling of all the physical processes of the high-speed water entry problem between the full and scale models. Empirical approaches lack essential features such as three-dimensional effects and the FSI of the problem.

For this reason, a tightly-coupled FSI solver is developed. The FSI solver is based on Computational Fluid Dynamics (CFD) with a Volume of Fluid (VoF) approach to precisely track the complex non-linear free surface coupled with Finite Element Method (FEM) and modal decomposition, which reduces the complexity of the coupled system. The tightly-coupled approach accounts for the time-dependent wetness of structure, accurately predicting fluid loading and structure deformation through time. Furthermore, the FSI solver is capable of performing a local and global hydroelastic analysis of composite structures, while previous work examines only one or the other.

The FSI solver is validated with aluminum high-horizontal-speed flat-plate ditching experimental data, becoming the first FSI solver with a CFD method to study high forward speed problems in three dimensions. Several test conditions are analyzed that ensure that the FSI solver can capture highly localized pressure, hydrodynamic loading, jet root propagation, free-surface nonlinearities, and hydroelastic coupling. The local peak pressure is captured with an error of 0.45% for locations where enough integration points are present along the plate. The overall hydroelastic response is captured with a slight underprediction in the maximum strain due to a fully-clamped edge boundary condition used to model the flat-plate. The FSI tool is used to investigate the influence of aspect ratio on the maximum pressure distribution and water pile-up. A wide range of aspect ratio is studied, and it is shown that two-dimensional solution applies only for very large beam-to-length ratios (B/L >> 1). The FSI framework is expanded to incorporate composite structures using a modal basis coupled with CFD. The validation of the FSI tool for uniformly loaded composite plates is presented. A more complex slamming case is analyzed, highlighting the importance of time-dependent wetness and nonlinear geometric effects in the hydroelastic analysis of composite structures.