This is my private limited point of view on CFD
CFD is a very specific field with its problems and solutions, its large community, and very wide applications. It is a young multidisciplinary field that brings together mathematicians, physicists, programmers and engineers - workers, scientists, students and hobbyists.
A lot of CFD community members use some old-style forum/wiki systems to exchange informations. Probably a lot of them never heard about stack-overflow-type sites - so it will take time for them to get used to.
Currently - as you indicated in the comments (in main question) - someone who has a question in the field of CFD must refer to non-cfd-specialized sources based on stack-overflow engine. CFD is not easy - and this situation not helps too much.
I think that if CFD-Overflow will start, we will attract specialists here over time and everyone will benefit from it - both people with CFD-problems and the entire community. But I aware that this need time
I will cite this journal to give you more info about CFD "size/complexity" :
CFD is now considered an indispensable analysis/design tool in an ever-increasing range of industrial applications. Practical flow problems are often so complex that a high level of ingenuity is required. Thus, besides the development work in CFD, innovative CFD applications are also encouraged. PCFD's ultimate goal is to provide a common platform for model/software developers and users by balanced international/interdisciplinary contributions, disseminating information relating to development/refinement of mathematical and numerical models, software tools and their innovative applications in CFD.
- Two-phase flows
- Heat transfer
- Chemical reactions and combustion
- Unsteady flows
- Fluid-solid interaction
- Navier-Stokes solution techniques for incompressible and compressible flows
- Discretisation methods and schemes
- Convergence acceleration procedures
- Grid generation and adaptation techniques
- Mesh-free methods
(and much more)