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Fluid-Structure Interaction

Have you ever wondered how the inside of a petrol tanker looks like? Why are there more than one opening at the top? If you think that the tanker is continuous from inside and petrol is filled in the entire tanker as a continuous fluid, you are absolutely incorrect. Matter of fact, petrol tankers have vertical compartments or baffles inside to prevent it from sloshing about. Why is this done? Adding compartments add to the cost of the fluid carrier; be it on road or at sea. However, it is a safety requirement from design engineers. Tankers also face potholes if the roads are not maintained properly, they need to deal with sharp turns, and need to brake suddenly in case of an emergency. On winding mountain roads, it is a real challenge to start and stop for even a passenger car. Imagine how difficult it would be to ply a fully loaded fluid tanker in such conditions, with the fluid sloshing about. If the tanker is not designed properly, it would result in accidents as moving fluid changes the centre of gravity of the vehicle rapidly. In harsh weather, especially in countries like India, fuel expands and contracts, and poses additional challenges. What is true of fluid tankers is also true of very large cargo ships that carry crude oil and other fluids. Just imagine the consequences if a behemoth that is carrying tons and tons of crude oil encounters rough sea. If it is not designed properly, it will simply topple and sink. These are but two examples of the difficulty engineers encounter while dealing with various aspects of fluid movement, and specially in conditions where the fluid encounters a solid structure. The branch of engineering that deals with the flow of fluid (it can be liquid, gas or even molten metal) that encounter solid structures is called Fluid-Structure interactions. It is a fascinating and challenging job to design for systems where fluid meets or flows about solid structures. Other examples of fluid-structure interaction are large bridges built at sea, chemical agitators, aircraft carriers (which need to face vortex wakes), etc.

Fluid-structure interactions play an important role especially where large structures are involved. Take the case of a Very Large Floating Structures (VLFS). Also called as mega-floats, these structures are gaining in popularity because they are cost effective, environment friendly, and easy to assemble and dismantle. VLFS are used in creating floating airports, artificial islands, mobile offshore bases, etc. VLFS are of two types, semi-submersible and pontoon type. Designing semi-submersible type of VLFs is more challenging as they need to face waves and still obtain structural stability. They face significant elastic deformation in addition to the rigid body motion under wave actions. Apart from fluid-structural considerations, hydro-elastic analysis too comes into picture while designing VLFS.

Defining Fluid-Structure Interactions
In simple words, fluid-structure interaction is the study of what happens when a moving fluid encounters a structure and the laws of physics that govern this flow. It is thus a combination of fluid dynamics and structural mechanics. The structure in question may be stationary or itself moving. The challenges of fluid-structure interaction are manifold when the fluid flow encounters a structure that itself is moving (for example, an aeroplane flying through air turbulence) or can be deformed by the fluid and change boundary of the fluid flow (for example, fluid flow inside a butterfly valve or a speaker cone that vibrates due to air pressure). In such cases, the laws of physics that govern stress, strain, and structural deformation come into play. It really is an engineering challenge to develop products or systems that are able to handle all these conditions simultaneously. Fluid-structural interaction also plays a crucial role in aviation and automobile industry. Consider the passenger compartments in a vehicle or an aeroplane.  The increased use of light-weight materials in both of them usually makes it complicated to achieve good passenger comfort in terms of low level of interior noise. When the weight of the compartment structure is reduced, the vibrations increase and lead to higher noise levels. When the weight is increased, fuel efficiency suffers, and is unacceptable especially in the aviation industry. Design engineers need to seek an optimal solution where both passenger comfort and fuel considerations are matched, all the time keeping in mind the safety aspect.

Tools of the Trade
Applying methods of fluid-structural optimization aims at defining light yet sturdy structures that withstand the loads and stresses induced by the surrounding fluid. It should be able to withstand flow-induced deformations and vibrations. Every meaningful large fluid-structural analysis is an exercise in abstraction about a structural system in the real world, so its viability and safety is intricately associated with the methodology underpinning such abstractions. In fluid-structural analysis, abstraction is in the form of geometric modelling of its material body, its relevant support conditions and its imposed loads as well as its material properties.  Fluid-structural interaction simulation is therefore best performed combining fluid dynamics, structural mechanics and numerical analysis.  

Each fluid flow is different and presents a unique design challenge. For example, aero-elastic phenomenon in an aircraft and its structural components involve a complex nonlinear fluid flow with shock waves, vortices, flow separations, and aerodynamic heating computations into picture. On the other hand, the designing of a VLFS or a very large crude carrier present different engineering challenges that involve water pressure, hull-form and higher coefficient of buoyancy. Very large bridge structures need to deal with water current, eddies, structural stability of the bed, etc. Fluid flow that involve deformation of the structure around which it flows present yet another set of deign challenges. Aeroplane wings are an example of such a flow. There are various simulation models that are used in fluid-structural interaction, depending on what needs to be studied. In some cases, the use of computational fluid dynamics (CFD) is justified while in other cases, computational structural dynamics using finite-element methods is appropriate.

In general though, software that uses finite element analysis and numerical simulation (for example, Altair AcuSolve and Inspire) are used for fluid-structure interaction analysis.

Fluid-Structure Interaction Services
As seen, designing for systems where fluids and structure interact with each other is very challenging task from an engineering perspective. It requires a multi-physics approach, and a combination of more than one software tools. It takes years of engineering experience to devise a solution, especially since there is no unique approach to fluid-structure interaction analysis. When it comes to large structures especially, there is no room for error. A single error in calculation compounds design faults, and can result in total failure of the system. Not many companies have the expertise to simulate fluid-structure interactions successfully for small systems, let alone very large scale projects. It is a critical task, and it is better to outsource it to an experienced team that specializes in providing high end engineering services. More so since the results of simulation are not in black and white; they need to be interpreted correctly. In India, DesignTech Systems is one such trusted and reliable company that provides fluid-structure interaction services, especially to large scale projects.