[kaya]

In an LES simulation with a spectral solver (dealiased convective), we explicitly model SGS viscosity.

However if we were to approximate our derivatives with finite differences, we would introduce some numerical dissipation. If we say that numerical dissipation in our system is equal to the energy dissipation, we don't add an explicit filter. This happens to be implicit LES.-correct me if I am wrong.

My first question is, when should a user consider applying an explicit filter and by using which evidence? In other words, how to make such an argument like my "numerical dissipation is not enough"? By looking at the energy spectrum?

In my mind, numerical dissipation would act on the the smallest structure that the grid could resolve. If so, how correct is to assume that "the smallest structure could be resolved, is the only one that would transfer the energy to smaller scales if it was a DNS" ?