[MUSIC] Hello. Welcome to the first week of our course on Fluid-Solid Interactions. We hope that you will enjoy it. And that most of all, you will learn with us how to deal with this fascinating field of mechanics. Together we shall go steps by steps. As a first step, I would like to try and define what we mean by fluid-solid interactions. Certainly, if you choose to attend this course, you have some idea of it. But let's try to define the kind of phenomena that we shall talk about. First of all, some words about fluid mechanics in the presence of solids. When you learned fluid mechanics, you have been interested in the effect of boundary conditions on the flow. For instance, here is the effect of a cylinder that deviates a uniform flow. In fluid mechanics, we consider solids as boundary conditions only, and not in terms of what they are made of. Here the boundary condition is just that the flow velocity shall not enter the body for instance. Let us talk now about solid mechanics in the presence of a fluid. When you learned solid mechanics, you have solved problems where you tried to compute the deformation of a solid under some external loading. Some of this loadings may be due to the presence of a fluid. For instance, the deformation of a submarine under external pressure, or of a building in a wind. In solid mechanics, we usually consider fluids just as the cause of a loading at the boundary, a force type boundary condition. These two approaches are very useful, and I extensively used in engineering. For instance, in civil engineering you may find engineers that compute wind loads on a bridge. And then send them to other engineers that will check that this is acceptable in terms of the solid mechanics of the bridge. This is often quite sufficient. Now, what do we mean here by fluid-solid interactions? We mean situations where you cannot solve these two problems independently. Here schematically, the cylinder is deformed by the flow, which itself is modified by the deformation of the cylinder. This is coupled fluid and solid mechanics. [MUSIC] Now, when you think of it, this question of coupling of models is actually quite fundamental. You have learned to use models of individual physical phenomenon. This is one of the basis of modern science. You learned maybe acoustics, chemistry, thermal science and many other things. But how do you combine, articulate, these models when there is an interaction between them? How do you build a model combining several models? So you see beyond fluid solid interactions there's a fundamental issue in learning how to deal with coupled problems. Of course, this is not only a fundamental issue. Actually fluid-solid interactions developed over the course of years, mostly driven by applications, in many different fields, aerospace engineering, nuclear engineer, civil engineering, bioengineering to cite just a few. And the development is still very much driven by applications in sensors, locomotion, food engineering and so on. We shall see plenty of examples in the course. Now that I have given some general ideas, let us see some examples of what we're talking about. [MUSIC] This video originally from NASA, shows a Comanche plane where the tail starts to vibrate in reaction to the air around it. This is often referred to as flutter, though this term is a bit vague. But we shall learn more about this later. At this stage let us just say that apparently we cannot understand this by treating independently the fluid mechanics and solid mechanics of it. So this seems to fall in the domain of what I called fluid-solid interactions. Another case, that of the famous dolphin skin effect. In the 70s, people interested in the design of submarines have devoted considerable work on the question of whether the flexibility of the dolphin skin is beneficial in reducing the friction. In other terms, does a dolphin go faster because it has a soft skin? The question behind it was actually the role of skin flexibility in delaying the instabilities in the boundary layer. Again, fluid and solid mechanics here seem fully coupled. And this falls in the domain of fluid-solid interactions. Yet, you may see that this is very different from the previous case of plane tail flutter. For instance, the fluid is much heavier here, and its velocity is lower. Look now at this field of alfalfa moving in the wind. You see waves, and that is certainly due to an interaction of some kind between the fluid and the solid. But I would say that this has nothing to do with the two previous cases. Let us move now to the behavior of a hard disk drive head. The reading head, in a conventional hard disk drive, actually floats above the surface of the disk, interacting with the air entrained by the disk. Predicting the dynamics of the head requires that the interaction with the flow is properly taken into account. This is also a fullycoupled case. Yet, clearly different from those I showed you before. Now what you see here is an inflatable dam. And this is a rather flexible structure in contact with the water that it is holding. What happens when there is an earthquake and the dam starts to oscillate? How does the water move then? Again, fluid-solid interactions and again, quite different. Finally, the famous collapse of the Tacoma Narrows Bridge. It is famous because we have a footage of what happened. And also because it was not easily understood. One of the most famous example of fluid-solid interactions. So, i all these cases which I recall below there is some solid mechanics, some fluid mechanics and in practice the two cannot reasonably be treated independently. Fluids and solids talk to each other here. They are coupled systems. The problem here is that when you see all these cases, they all belong to fluid-solid interactions, but there are huge difference between them. Is it possible to develop a general approach to model them? Well, this is the purpose of the present course. So, here are the objectives of our course. First, find a way to classify all these couplings. Why? Because the variety seems so large that it does not seem feasible to find a model that is applicable to all of them. The second objective, once we have classified them is to try and build relevant models for these classes. So, to summarize. You now understand that there is a clear issue of methodology in trying to articulate fluid mechanics and solid mechanics to solve coupled problems. And this is precisely what we are going to try to do with you. [MUSIC]