Small-scale dissipation

[mtbrolly]

Hi everyone,

I've just started playing with GeophysicalFlows.jl and have quite a naive question.

It looks like there is no explicit small-scale dissipation specified in the 2D decaying example; n\nu = 1, but \nu = 0. With zero viscosity I would expect the simulation to become unstable, but that isn't the case. Is there some magic going on in FourierFlows.jl that I should know about?

Thanks in advance.

[mtbrolly]

I've realised there is an exponential filter applied via stepper="FilteredRK4". I think it deserves a comment in the documentation.

[glwagner]

I agree! A comment here:

GeophysicalFlows.jl/examples/twodnavierstokes_decaying.jl

Lines 45 to 48 in e25b664

	# ## Problem setup
	# We initialize a `Problem` by providing a set of keyword arguments. The
	# `stepper` keyword defines the time-stepper to be used.
	prob = TwoDNavierStokes.Problem(dev; nx=n, Lx=L, ny=n, Ly=L, dt=dt, stepper="FilteredRK4")

would be useful.

[navidcy]

It's written in some other examples. I'll open a PR to add a remark like this in all of them.