The core of a nuclear reactor is made of hundreds of fuel rods. The spacing between this rods is very small. Simulating in detail the flow at this scale around all the individual rods would be, in the one hand, much too long and, on the other hand, useless in most cases because the flow is often simpler than in the surrounding free regions. That is why it is interesting to use averaged models to describe the flow in the nuclear core.
The flow in the free regions is more complex and it is important to capture all the scales of this flow.
In some cases, the main modeling difficulty comes from the coupling between the two scales of description of the flow. The figure aside illustrates an application of such a coupling in a configuration that is similar to that of a nuclear reactor. It can be seen in particular that the main characteristics of the flow are well reproduced by the averaged model, within the charged medium as well as in the free regions.