| 1. |
- Augier, Pierre, et al.
(författare)
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A New Formulation of the Spectral Energy Budget of the Atmosphere, with Application to Two High-Resolution General Circulation Models
- 2013
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 70:7, s. 2293-2308
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Tidskriftsartikel (refereegranskat)abstract
- A new formulation of the spectral energy budget of kinetic and available potential energies of the atmosphere is derived, with spherical harmonics as base functions. Compared to previous formulations, there are three main improvements: (i) the topography is taken into account, (ii) the exact three-dimensional advection terms are considered, and (iii) the vertical flux is separated from the energy transfer between different spherical harmonics. Using this formulation, results from two different high-resolution GCMs are analyzed: the Atmospheric GCM for the Earth Simulator (AFES) T639L24 and the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) T1279L91. The spectral fluxes show that the AFES, which reproduces quite realistic horizontal spectra with a k(-5/3) inertial range at the mesoscales, simulates a strong downscale energy cascade. In contrast, neither the k(-5/3) vertically integrated spectra nor the downscale energy cascade are produced by the ECMWF IFS.
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| 2. |
- Augier, Pierre
(författare)
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A new formulation of the spectral energy budget of the atmosphere, with application to two high-resolution general circulation models
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- In order to investigate the dynamics of atmospheric turbulence, we have derived a new formulation of the spectral energy budget of kinetic and available potential energies of the atmosphere, with spherical harmonics as base functions. Compared to previous formulations, there are three main improvements: (i) the topography is taken into account, (ii) the exact three-dimensional advection terms are considered and (iii) the vertical flux is separated from the energy transfer between different spherical harmonics. Using this formulation, results from two different high resolution GCMs are analyzed: the AFES T639L24 and the ECMWF IFS T1279L91. The spectral fluxes show that the AFES, which reproduces realistic horizontal spectra with a k−5/3 inertial range at the mesoscales, simulates a strong downscale energy cascade. In contrast, neither the k−5/3 vertically integrated spectra nor the downscale energy cascade are produced by the ECMWF IFS.
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| 3. |
- Augier, Pierre, et al.
(författare)
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FluidDyn: A Python Open-Source Framework for Research and Teaching in Fluid Dynamics by Simulations, Experiments and Data Processing
- 2019
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Ingår i: Journal of Open Research Software. - United Kingdom : Ubiquity Press, Ltd.. - 2049-9647. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- FluidDyn is a project to foster open-science and open-source in the fluid dynamics community. It is thought of as a research project to channel open-source dynamics, methods and tools to do science. We propose a set of Python packages forming a framework to study fluid dynamics with different methods, in particular laboratory experiments (package fluidlab), simulations (packages fluidfft, fluidsim and fluidfoam) and data processing (package fluidimage). In the present article, we give an overview of the specialized packages of the project and then focus on the base package called fluiddyn, which contains common code used in the specialized packages. Packages fluidfft and fluidsim are described with greater detail in two companion papers [4, 5]. With the project FluidDyn, we demonstrate that specialized scientific code can be written with methods and good practices of the open-source community. The Mercurial repositories are available in Bitbucket (https://bitbucket.org/fluiddyn/). All codes are documented using Sphinx and Read the Docs, and tested with continuous integration run on Bitbucket Pipelines and Travis. To improve the reuse potential, the codes are as modular as possible, leveraging the simple object-oriented programming model of Python. All codes are also written to be highly efficient, using C++, Cython and Pythran to speedup the performance of critical functions.
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| 4. |
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| 5. |
- Augier, Pierre, et al.
(författare)
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Kolmogorov laws for stratified turbulence
- 2012
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 709, s. 659-670
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Tidskriftsartikel (refereegranskat)abstract
- Following the Kolmogorov technique, an exact relation for a vector third-order moment J is derived for three-dimensional incompressible stably stratified turbulence under the Boussinesq approximation. In the limit of a small Brunt-Vaisala frequency, isotropy may be assumed which allows us to find a generalized 4/3-law. For strong stratification, we make the ansatz that J is directed along axisymmetric surfaces parameterized by a scaling law relating horizontal and vertical coordinates. An integration of the exact relation under this hypothesis leads to a generalized Kolmogorov law which depends on the intensity of anisotropy parameterized by a single coefficient. By using a scaling relation between large horizontal and vertical length scales we fix this coefficient and propose a unique law.
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| 6. |
- Augier, Pierre, et al.
(författare)
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Reducing the ecological impact of computing through education and Python compilers
- 2021
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Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 5:4, s. 334-335
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Tidskriftsartikel (refereegranskat)abstract
- We read with interest the Comment by Portegies Zwart on the ecological impact of high-performance computing in astrophysics. We fully agree with its take-home message: scientists should be mindful of their carbon footprint. One of the proposed solutions, however, is to avoid the Python programming language. We contend that this would be counterproductive and that scientific programs written in Python can be efficient and energy-friendly. We argue that human factors, such as education, and the advancement of compiler technology are much more important than choice of language.
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| 7. |
- Augier, Pierre, et al.
(författare)
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Shallow water wave turbulence
- 2019
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 874, s. 1169-1196
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of irrotational shallow water wave turbulence forced in large scales and dissipated at small scales is investigated. First, we derive the shallow water analogue of the `four-fifths law' of Kolmogorov turbulence for a third order structure function involving velocity and displacement increments. Using this relation and assuming that the flow is dominated by shocks we develop a simple model predicting that the shock amplitude scales as (ϵd)1/3, where ϵ is the mean dissipation rate and d the mean distance between the shocks, and that the pth order displacement and velocity structure functions scale as (ϵd)p/3r/d, where r is the separation. Then we carry out a series of forced simulations with resolutions up to 76802, varying the Froude number, Ff=ϵ1/3/ckf1/3, where kf is the forcing wave number and c is the wave speed. In all simulations a stationary state is reached in which there is a constant spectral energy flux and equipartition between kinetic and potential energy in the constant flux range. The third order structure function relation is satisfied with a high degree of accuracy. Mean energy is found to scale as E∼√(ϵc/kf), and is also dependent on resolution, indicating that shallow water wave turbulence does not fit into the paradigm of a Richardson-Kolmogorov cascade. In all simulations shocks develop, displayed as long thin bands of negative divergence in flow visualisations. The mean distance between the shocks is found to scale as d∼Ff1/2/kf. Structure functions of second and higher order are found to scale in good agreement with the model. We conclude that in the weak limit, Ff→0, shocks will become denser and weaker and finally disappear for a finite Reynolds number. On the other hand, for a given Ff, no matter how small, shocks will prevail if the Reynolds number is sufficiently large.
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| 8. |
- Augier, Pierre, et al.
(författare)
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Spectral analysis of the transition to turbulence from a dipole in stratified fluid
- 2012
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 713, s. 86-108
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the spectral properties of the turbulence generated during the nonlinear evolution of a Lamb-Chaplygin dipole in a stratified fluid for a high Reynolds number Re = 28 000 and a wide range of horizontal Froude number F-h epsilon [0.0225 0.135] and buoyancy Reynolds number R = ReFh2 epsilon [14 510]. The numerical simulations use a weak hyperviscosity and are therefore almost direct numerical simulations (DNS). After the nonlinear development of the zigzag instability, both shear and gravitational instabilities develop and lead to a transition to small scales. A spectral analysis shows that this transition is dominated by two kinds of transfer: first, the shear instability induces a direct non-local transfer toward horizontal wavelengths of the order of the buoyancy scale L-b = U/N, where U is the characteristic horizontal velocity of the dipole and N the Brunt-Vaisala frequency; second, the destabilization of the Kelvin-Helmholtz billows and the gravitational instability lead to small-scale weakly stratified turbulence. The horizontal spectrum of kinetic energy exhibits epsilon(2/3)(K)k(h)(-5/3) power law (where k(h) is the horizontal wavenumber and epsilon(K) is the dissipation rate of kinetic energy) from k(b) = 2 pi/L-b to the dissipative scales, with an energy deficit between the integral scale and k(b) and an excess around k(b). The vertical spectrum of kinetic energy can be expressed as E(k(z)) = C(N)N(2)k(z)(-3) + C epsilon(2/3)(K)k(z)(-5/3) where C-N and C are two constants of order unity and k(z) is the vertical wavenumber. It is therefore very steep near the buoyancy scale with an N(2)k(z)(-3) shape and approaches the epsilon(2/3)(K)k(z)(-5/3) spectrum for k(z) > k(o), k(o) being the Ozmidov wavenumber, which is the cross-over between the two scaling laws. A decomposition of the vertical spectra depending on the horizontal wavenumber value shows that the N(2)k(z)(-3) spectrum is associated with large horizontal scales vertical bar k(h)vertical bar < k(b) and the epsilon(2/3)(K)k(z)(-5/3) spectrum with the scales vertical bar k(h)vertical bar > k(b).
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| 9. |
- Campagne, Antoine, et al.
(författare)
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First report of the MILESTONE experiment: strongly stratified turbulence and mixing efficiency in the Coriolis platform
- 2016
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Ingår i: <em>VIIIth International Symposium on Stratified Flows (ISSF)</em>, 2016.
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Konferensbidrag (refereegranskat)abstract
- Strongly stratified turbulence is a possible interpretation of oceanic and atmospheric mea-surements. However, this regime has never been produced in a laboratory experiment be-cause of the two conditions of very small horizontal Froude number Fh and large buoyancy Reynolds number R which require a verily large experimental facility. We present a new attempt to study strongly stratified turbulence experimentally in the Coriolis platform.The flow is forced by a slow periodic movement of an array of six vertical cylinders of 25 cm diameter with a mesh of 75 cm. Five cameras are used for 3D-2C scanned horizontalparticles image velocimetry (PIV) and stereo 2D vertical PIV. Five density-temperatureprobes are used to measure vertical and horizontal profiles and signals at fixed positions.The first preliminary results indicate that we manage to produce strongly stratified tur-bulence at very small Fh and large R in a laboratory experiment.
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| 10. |
- Chomaz, Jean Marc, et al.
(författare)
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Spectral analysis of the transition to turbulence from a dipole in stratified fluid
- 2013
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
-
Konferensbidrag (refereegranskat)abstract
- We investigate through numerical simulations the spectral properties of the turbulence generated during the nonlinear evolution of a Lamb-Chaplygin dipole in a stratified fluid for a high Reynolds number Re = 28000 and a wide range of horizontal Froude number Fh ∈ [0.0225 0.135] and buoyancy Reynolds number R = ReFh2 ∈ [14 510]. A spectral analysis shows that this transition is dominated by two kinds of transfers: first, the shear instability induces a direct non-local transfer toward horizontal wavelengths of the order of the buoyancy scale Lb = U/N, where U is the characteristic horizontal velocity of the dipole and N the Brunt-Väisälä frequency; second, the destabilization of the Kelvin-Helmholtz billows and the gravitational instability lead to small-scale weakly stratified turbulence. We show that the anisotropic spectra at the maximum of dissipation share many characteristics with those obtained from numerical simulations of forced stratified turbulence and from measurements in the atmosphere and in the ocean. The article presenting this study [2] is the subject of a Focus on Fluids article [10].
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