[Muzz]

The question in the title is something that I've been trying to grasp at a reasonably high level to answer some general modelling questions.

I've been doing some atmospheric boundary layer modelling for urban flow problems and I've been using RKE with good success. For simple building configurations (single/twin building setups, where the buildings are just cuboids), RKE residuals decrease by five orders of magnitude and the relevant engineering quantities settle.

For realistic urban environments (modelling real cities), I get residual decrease of 9 orders of magnitude and the relevant engineering quantities settle.

So far, so good. However, when I run a URANS simulation (I initialise the URANS with RANS result), the simple building configurations exhibit large scale vortex shedding and is clearly unsteady (which is fine). However, for the realistic urban environment, there is almost no temporal variation in the results.

Of course, considering the physical nature of the problem, transient behaviour is surely expected. Why is there a difference? My time step in each case is relatively large (0.5 s) but the inner iterations demonstrate the same, if not improved, levels of convergence. Could it be that the time step must be reduced? However, wouldn't a larger time step impact residual convergence or does it only impact the solution?

To this end, I'm not entirely sure how URANS is making my simulation unsteady.