[nusq]

CFD is improving at great speed, along with the increase of computer power, but isn't yet in the level required to replace all scale model tests and they can still be very computer intensive. I guess that nowadays the state of the art are these new Smoothed-particle hydrodynamics models. I've worked with basins like that to test harbour and breakwaters designs and I can tell that it is common and good practice to test breakwater designs in scale models before being built, there are too many variables and hydrodynamic process (like turbulence, porous media flow, wave breaking, over-topping etc..) that cannot yet be reliably modelled using CFD, specially when they cannot be simplified to a 2D domain. Nonetheless there are types of scale models that are increasingly rare these days because they were replaced by CFD modelling, like sediment transport, although they were/are pretty cool to watch. Also scale models are very useful to calibrate CFD models.

[walshemj]

And you do need to back prove your mathematical models against reality.