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Partitioning Waves and Eddies in Stably Stratified Turbulence

Abstract : We consider the separation of motion related to internal gravity waves and eddy dynamics in stably stratified flows obtained by direct numerical simulations. The waves' dispersion relation links their angle of propagation to the vertical θ, to their frequency ω, so that two methods are used for characterizing wave-related motion: (a) the concentration of kinetic energy density in the (θ, ω) map along the dispersion relation curve; and (b) a direct computation of two-point two-time velocity correlations via a four-dimensional Fourier transform, permitting to extract wave-related space-time coherence. The second method is more computationally demanding than the first. In canonical flows with linear kinematics produced by space-localized harmonic forcing, we observe the pattern of the waves in physical space and the corresponding concentration curve of energy in the (θ, ω) plane. We show from a simple laminar flow that the curve characterizing the presence of waves is distorted differently in the presence of a background convective mean velocity, either uniform or varying in space, and also when the forcing source is moving. By generalizing the observation from laminar flow to turbulent flow, this permits categorizing the energy concentration pattern of the waves in complex flows, thus enabling the identification of wave-related motion in a general turbulent flow with stable stratification. The advanced method (b) is finally used to compute the wave-eddy partition in the velocity-buoyancy fields of direct numerical simulations of stably stratified turbulence. In particular, we use this splitting in statistics as varied as horizontal and vertical kinetic energy, as well as two-point velocity and buoyancy spectra.
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Henri Lam, Alexandre Delache, Fabien Godeferd. Partitioning Waves and Eddies in Stably Stratified Turbulence. Atmosphere, MDPI 2020, 11 (4), pp.420. ⟨10.3390/atmos11040420⟩. ⟨hal-02906451⟩

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