LES basic and simple questions.
 

I am trying to understand and hopefully implement LES. I read a couple of
texts and somehow understand the general idea but I have basic and simple questions on
how the spatial filtering is done.

For RANS time averaging gives Reynolds stress, but for LES spatial filtering
two more terms (leonard and cross) appear which I recall would be 0 for Reynold's (time) averaging.
Why are those terms not zero in the LES case ?

I understand that the filtering operation is a fourier transform (forward and backward) with
a cutoff frequency to remove the higher frequencies. From wiki I read that this is not usually
done in commericial codes by using the grid itself as a filter (box filter). How is that done ?
I need to decide if to use matlab incase i need to do fft or other complicated convolutions.
Do I even need to do that as it sounds the operation is implicit (variables to be filtered are
unknown before filtering ??)

If anyone has a link to simple and basic explanation of LES, that
would be much better than answering my silly questions here. I have a textbook on LES but it is
too complicated for me. What I need is something that describes LES with simplest model (Smagorinsky) and
wall functions ,from implementation perspective.


Thank you
A student