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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-vorticitycorrelations). The problem is tackled by integrating the Boussinesq equationsin a periodic cubical domain using different initial conditions: a non-helicalTaylor-Green (TG) flow, a fully helical Beltrami (ABC) flow, and random flowswith a tunable helicity. We show that for stratified ABC flows helicityundergoes a substantially slower decay than for unstratified ABC flows. Thisfact is likely associated to the combined effect of stratification and largescale coherent structures. Indeed, when the latter are missing, as in randomflows, helicity is rapidly destroyed by the onset of gravitational waves. Atype of large-scale dissipative "cyclostrophic" balance can be invoked toexplain this behavior. When helicity survives in the system it strongly affectsthe temporal energy decay and the energy distribution among Fourier modes. Wediscover in fact that the decay rate of energy for stratified helical flows ismuch slower than for stratified non-helical flows and can be considered with aphenomenological model in a way similar to what is done for unstratifiedrotating flows. We also show that helicity, when strong, has a measurableeffect on the Fourier spectra, in particular at scales larger than the buoyancyscale for which it displays a rather flat scaling associated with verticalshear.

Helicity dynamics in stratified turbulence in the absence of forcing