Every journey of learning has some 'fundamental entities' typically known as 'building blocks'. They are the essential ones as they lay good foundation for learning. In CFD learning journey, simulations play vital role to understand the underlying physics or process or a phenomenon being modelled besides the theoretical concepts.
Straight to the question been asked, what are those simulations one could think of as building blocks of CFD learning. It is highly likely to see 'Lid-Driven Cavity' as the most fundamental simulation or the first and foremost simulation one setup as CFD learner while doing a course on CFD. In LDC case, one can see flow features like 're-circulation pockets' at the corners co-exist with a big flow-circulation at the center of cavity. The other aspect of LDC in terms of learning is to define moving wall as boundary condition. The Reynolds number of this LDC case is 5000 for a top wall moving at 1 m/s on a cavity with width of 1 m which is filled with air (rho=1 m^3/s; mu=2e-4 Pa-s)
The second simulation anyone would come across in the journey of learning is 'Flow over a Cylinder'. Tommy of BBC (Building Blocks CFD) says, "You are not a true CFD professional if you haven't solved 'Flow over a Cylinder' case at least once in your CFD career. That is how FOC has secured an inevitable position in CFD role.
From beginners to advanced users, FOC offers many interesting concepts of mono-physics to multi-physics of flow phenomenon.