Bodies that are surround by a fluid are called immersed bodies. Most objects on earth are surrounded by air or water making them an immersed body. However, the main interest of an immersed body is how fluid will flow around it. Airplanes are examples of objects that would have air flowing around it that we would be interested in. On the other hand, submarines are examples of objects that would have water flowing around it
Determining the aerodynamics of immersed bodies is a key interest. Without aerodynamics we wouldn’t be able to determine if an airplane could fly. Also, without aerodynamics engineers wouldn’t be able design fuel efficient cars. As result, the main areas of interest are how fluid forces (drag and lift) interact with the surface of the immersed body.
There are also cases when an object does have a fluid completely surrounding it. An example of this would be a building. For example, knowing how air flows around a building is important since it will provide information on if the building will be able to withstand a certain wind shear before suffering damage.
You can use various methods when studying flow around an immersed body. First, you could use theoretical methods. Theoretical methods will provide you with fundamental information. However, you must realize that theoretical methods have their limitations. You can overcome these limitations by using numerical and experimental methods. For an immersed body you can use a wind tunnel to obtain experimental data. Finally, as the complexity of the problem increases, a CFD Analysis can be conducted.
In the image below engineers are testing a Rimac C_Two inside a wind tunnel. You can see this experiment being performed in this YouTube video.