Why all LES simulations should be 3D?
 

Why all LES simulations should be 3D? is it because we don't assume turbulence to be isotropic?

Turbulence in nature is 3D. There are may ways to look at this, one of them is to imagine a spinning vortex, which gets stretched in the direction perpendicular to the spinning motion. This process can only be described, of course, in a 3D configuration. To assume isotropic turbulence only suggests that the statistics (not instantaneous behavior) are the same in all three directions.

However, there are 2D LES, which should be justified carefully. This really depends on the problem you are trying to attack. For example, while 3D LES would be desired, 2D LES for mixing problems can still provide useful information. It may not fully solve your problem, but it is a step towards the end goal. You should also be careful when drawing conclusions from 2D LES.