Abstracts by Speaker > Delache Alexandre

In rotating-stratified flows, two characteristic time scales, namely the BV frequency $N$ and the Coriolis parameter $f$, are driving the scale of transition from isotropy to anisotropy in homogeneous turbulence. Introducing the turbulent dissipation rate $\epsilon$, two length scales can be obtained, namely the Ozmidov length scale [1] $L_o=(\epsilon/N^3)^{1/2}$ and the Hopfinger length scale [2] $L_h=(\epsilon/f^3)^{1/2}$. Note that the Hopfinger length scale is often called the Zeman length scale [3], but was first introduced by Mory and Hopfinger in 1985 [2]. $L_o$ and $L_h$ compare the relative effects of inertia and of the buoyancy force or of the Coriolis force respectively, and thus quantify the rise of anisotropy in different scale ranges: at large scales (larger than $L_o$ or $L_h$) the anisotropy due to strong stratification or strong rotation is dominant, whereas at small scales (smaller than $L_o$ or $L_h$), universal 3D isotropic characteristics of turbulence appear to be restored. \\

To confirm directly the role of these two scales, we performed numerical simulations at high resolution ($2048^3$ points) in freely decaying turbulence at four different stratification rates and six rotating rates. We confirm the role played by $L_o$ and $L_h$ by considering the angular energy spectra. Moreover the two scales are associated to a change of behavior of the poloidal/toroidal components of velocity, linked to Riley's decomposition in wave/vortex mode [4], revisited using its spectral counterpart given by the Craya/Herring frame of reference in Delache et al.\ [5]. In addition, the latter paper has shown the evidence by DNS of a non-monotonic scale-by-scale distribution of directional anisotropy, from the smallest wave vectors (larger scales) to the Hopfinger threshold wavenumber.

\begin{itemize}\frenchspacing\item[{[1]}] R.V. Ozmidov,``On turbulent exchange in stable stratified ocean.'', Izvestia Acad. Sci. USSR, Atmosphere and Ocean Physics, 1965,N 8\item[{[2]}] Mory, M. \& Hopfinger, Rotating turbulence evolving freely from an initial quasi 2D state, Macroscopic Modelling of Turbulent Flows: Proceedings of a Workshop Held at INRIA, Sophia-Antipolis, France, December 10--14, 1984, Springer Berlin Heidelberg, E. Frisch, U.; Keller, J. B.; Papanicolaou, G. C. \& Pironneau, O. (Eds.), 1985, 218-236\item[{[3]}] O. Zeman,``A note on the spectra and decay of rotating homogeneous turbulence,'' Phys. Fluids 6, 3221 (1994).\item[{[4]}] Riley, J. J., Metcalfe, R. W., \& Weissman, M. A. (1981, December). Direct numerical simulations of homogeneous turbulence in density‐stratified fluids. In Nonlinear Properties of Internal Waves: La Jolla Institute, 1981 (Vol. 76, No. 1, pp. 79-112). AIP Publishing.\item[{[5]}] Delache, A., Cambon, C., \& Godeferd, F. (2014). Scale by scale anisotropy in freely decaying rotating turbulence. Physics of Fluids, 26(2).\end{itemize}