A numerical study of decaying stably-stratified flows is performed.
Relatively high stratification and moderate Reynolds numbers are considered,
and a particular emphasis is placed on the role of helicity (velocity-vorticity
correlations). The problem is tackled by integrating the Boussinesq equations
in a periodic cubical domain using different initial conditions: a non-helical
Taylor-Green (TG) flow, a fully helical Beltrami (ABC) flow, and random flows
with a tunable helicity. We show that for stratified ABC flows helicity
undergoes a substantially slower decay than for unstratified ABC flows. This
fact is likely associated to the combined effect of stratification and large
scale coherent structures. Indeed, when the latter are missing, as in random
flows, helicity is rapidly destroyed by the onset of gravitational waves. A
type of large-scale dissipative "cyclostrophic" balance can be invoked to
explain this behavior. When helicity survives in the system it strongly affects
the temporal energy decay and the energy distribution among Fourier modes. We
discover in fact that the decay rate of energy for stratified helical flows is
much slower than for stratified non-helical flows and can be considered with a
phenomenological model in a way similar to what is done for unstratified
rotating flows. We also show that helicity, when strong, has a measurable
effect on the Fourier spectra, in particular at scales larger than the buoyancy
scale for which it displays a rather flat scaling associated with vertical
shear