Preface

Fluid-Structure Interactions in Naval Engineering

Jean-François SIGRIST1 and Cédric LEBLOND2

1 DGA, Tours, France

2 Naval Group, Bouguenais, France

Published in 1870, in the century that saw the formalization of the equations governing fluid flows (Navier 1823; Cauchy 1829; Reynolds 1883; Stokes 1845), Twenty Thousand Leagues Under the Sea is one of the most well-known works by the French writer Jules Verne (1828-1905). Today, it is also among the 20 best- selling books in the world; it gave rise to many cinema, television and comic book adaptations. Professor Aronnax, an eminent expert from the Natural History Museum in Paris, his servant Conseil and Ned Land, an experienced sailor and seasoned harpooner, embark on the Abraham Lincoln to find a sea monster. The extraordinary beast is in reality a machine made from steel and electricity: the Nautilus, a formidable engine imagined, built and ordered by Captain Nemo, to reign supreme in the world under the sea. Over the course of their long stay on the submersible, the novel's three heroes discover magnificent countries and experience incredible adventures. They measure the vastness of the ocean, its resources and its riches. This odyssey, which took them more than 20 thousand leagues under the sea, was a voyage of dreams and discoveries for Professor Aronnax, but a gilded cage for Ned Land, enamored of the freedom that sailing on the sea's surface affords. Verne gives Captain Nemo these words:

If danger threatens one of your vessels on the ocean, the first impression is the feeling of an abyss above and below. On the Nautilus men's hearts never fail them. No defects to be afraid of, for the double shell is as iron; no rigging to attend to; no sails for the wind to carry away; no boilers to burst; no fire to fear; for the vessel is made of iron, not of wood; no coal to run short, for electricity is the only mechanical agent, no collision to fear, for it alone swims in deep water; no tempest to brave, for when it dives below the water, it reaches absolute tranquillity. There sir! That is the perfection of vessels! And if it is true that the engineer has more confidence in the vessel than the builder, and the builder than the captain himself, you understand the trust I repose in my Nautilus; for I am at once captain, builder, and engineer. (Verne 1870)

How do we realize Captain Nemo's dream? How do we design a ship as complex as a submarine (Laisney 2012; Bovis 2016) and guarantee that it will allow its crew safe sailing, in difficult sea conditions or during delicate operations?

Figure P.1. The submarine, here the SNLE "Le Triomphant", is one of the most complex mechanical constructions

(source: Lebourdais (2019))

In the 21st century, engineers can enjoy the experience and expertise of those who preceded them, their physical good sense and the sum of their technical knowledge (Griset 2017). Other tools are also available, those of numerical simulation in particular.

Based on the postulate that it is possible to represent physical phenomena - or other phenomena (biological, economic, demographic, physiological, etc.) - by means of mathematical models, numerical simulation is necessary today in the industrial world and in many scientific disciplines (Sigrist 2019). It contributes greatly to innovation in this sector, fulfilling two main objectives:

• - mastery of technical risks: it enables the formation of regulatory documentation, the demonstration of safety and reliability, the development of environmental impact studies, etc.;
• - economic performance: it contributes to the optimization of products, to demonstrating their robustness, predicting their performances or reducing the cost of their manufacturing and use.

Although numerical simulations in structural and fluid mechanics have long accompanied construction programs for military or civilian ships (Besnier 2006), the modeling of fluid-structure interactions (FSI) in naval engineering is, for its part, more recent. It is moreover well-formalized and documented in various reference works (Morand and Ohayon 1998; Sigrist 2015), which involve numerous applications, from nuclear to civilian engineering (Paidoussis 2004; Axisa 2007; Paidoussis et al. 2011), via construction in aeronautics, space or car-making (Ohayon and Soize 1998; Bazilevs et al. 2012; Souli and Benson 2013; Ohayon and Soize 2014) or in biomechanics (Murea 2017).

The aim of this book is to contribute to the expansion of knowledge of models allowing simulation of fluid-structure interactions for applications useful to the naval domain. These models thus cover three domains:

• - the acoustic radiation of marine platforms;
• - the hydrodynamics of marine platforms;
• - the vibration behavior of submerged structures in direct or indirect response to the effects of a submarine explosion.

This book is formed of eleven chapters, which tackle various models (numerical, semi-analytical, empirical), calculating methods (finite elements, boundary elements, finite volumes) and numerical approaches (reduced-order models, coupling strategy, etc.).

Chapter 1 offers an introduction to the theme of fluid-structure interactions from a historical angle by briefly discussing the main stages of constructing hydrodynamic models.

Chapters 2 to 4 tackle the theme of "vibro-acoustics":

- Chapter 2 recalls the principles of finite element modeling, allowing the "low frequency" behavior of submerged structures to be calculated;

- Chapter 3 is an extension of Chapter 2 and presents the bases for "hybrid" vibro-acoustic modeling, allowing for the extension of vibration calculations into the domain of "medium frequencies";

- Chapter 4 completes these two presentations by covering emerging "vibro- acoustic" techniques, aiming, for example, to consider complex loads as well as uncertainties on the design data and the environment variables - these techniques are based in particular on reduced-order models.

Chapters 5 and 6 pursue the exploration of model reduction techniques or calculation time reduction techniques:

- Chapter 5 presents the theoretical bases for proper orthogonal decomposition (POD) models and the lattice Boltzmann method (LBM) formulated in the case of FSI for hydrodynamic problems and the movements of structures under flow;

- Chapter 6 offers a summary of IFS modeling in the vibration behavior of tube bundles (present in the propulsion systems of nuclear submarines) containing fluids, with homogenization techniques.

Chapters 7 to 9 address the modeling of hydrodynamic loads on submerged structures and the dynamic response of the latter:

- Chapter 7 tackles the modeling of pressure spectra engendered by the turbulence of flows in contact with structures (such as hulls, hull appendages, propeller blades etc.). The proposed modeling links theoretical models, empirical data and numerical calculation;

- Chapter 8 focuses on calculating the mechanical behavior of defomable structures (such as lifting profiles formed of supple material in composite form, for example) under the effect of the hydrodynamic load. This modeling relies on techniques known as "co-simulation", exploiting the pairing between a structural dynamic code and the fluid dynamic code;

- Chapter 9 tackles the problems of platforms' sea-keeping: it presents the modeling of their behavior in response to hydrodynamic wave loads.

Chapters 10 and 11 concern the response of submerged structures to the effects of an underwater explosion:

- Chapter 10 focuses on analytical modeling of the coupled problem for submarines. This approach, although it simplifies reality - in particular the geometry of ships - makes it possible to very quickly carry out parametric studies, which proves very useful to designers and architects;

- Chapter 11 tackles the modeling of submerged composite structures and their behavior under the effect of load due to the explosion; the proposed modeling links analytical and numerical approaches, adapted to the difficulties that the representation of composites poses.

Written by a team of confirmed experts and young researchers from the naval sector, this book is addressed first to their peers, student engineers as well as research or design engineers, who would like an overview of the various technical themes (hydrodynamics, seakeeping, sound, vibrations). The overview, which aims to be both fluid and concise, shows the reader, via different examples, how numerical simulation contributes to modeling and understanding fluid-structure interactions, with the aim of designing and optimizing the ships of tomorrow - in the words of Jules Verne: "Everything that a man can imagine, other men can carry out!".

September 2022

References

1. Axisa, F. (2007). Modelling of Mechanical Systems - Fluid-Structure Interaction. Butterworth-Heinemann, Oxford.
2. Bazilevs, Y., Takizawa, K., Tezduyar, T.-E. (2012). Computational Fluid-Structure Interaction: Methods and Applications. John...