IFIBA   22255
INSTITUTO DE FISICA DE BUENOS AIRES
Unidad Ejecutora - UE
artículos
Título:
Helicity dynamics in stratified turbulence in the absence of forcing
Autor/es:
C. RORAI; D. ROSENBERG; A. POUQUET; P. D. MININNI
Revista:
PHYSICAL REVIEW E
Editorial:
AMER PHYSICAL SOC
Referencias:
Lugar: New York; Año: 2013 vol. 87 p. 6300701 - 6300713
ISSN:
1539-3755
Resumen:
A numerical study of decaying stably stratified flows is performed. Relatively high stratification (Froude number ≈10−2?10−1) and moderate Reynolds (Re) numbers (Re≈ 3?6×103)
are considered and a particular emphasis is placed on the role of
helicity (velocity-vorticity correlations), which is not an invariant of
the nondissipative equations. The problem is tackled by integrating the
Boussinesq equations in a periodic cubical domain using different
initial conditions: a nonhelical Taylor-Green (TG) flow, a fully helical
Beltrami [Arnold-Beltrami-Childress (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. No
production of helicity is observed, contrary to the case of rotating and
stratified flows. 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 nonhelical
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,
as observed in the planetary boundary layer.