Prace Tier-0 Project 2011050773 (2012-2013)

Fluid turbulence: self and passive scalar diffusion. Application to stably stratified flows

Experiments show that turbulent diffusion is complex and that discrete structures or processes, spatially localized within the system, may exist. To obtain a better handling of fundamental issues, we propose an approach where the turbulence self-diffusion is modelled by the interaction between two different isotropic turbulent fields. This simplifies the main mechanisms. In fact, it does not include the nonlinear production of turbulent energy. However, it retains two of the most important features present in real flows: inhomogeneity and anisotropy.

Recent simulations in our group revealed the generation of small-scale anisotropy in turbulence self-diffusion. A long-term interaction must be active to transfer to small scale the information on the anisotropy of the initial and boundary conditions (PRL 2011). Data from direct numerical simulations show that there is a departure of the longitudinal velocity derivative moments from the values found in HIT and that the anisotropy induced by the presence of a kinetic energy gradient has a different pattern from the one generated by an homogeneous shear. Other results concern the relationship between the correlation length and intermittency. A variation of the correlation length is not necessary to depart from Gaussianity (PHYSD 2012, PRE 2008). However, if the correlation length variation is concurrent with that of the energy, the mixing is enhanced, if is opposite, the mixing is decreased (JFM 2006). The transport of a passive scalar or a stable stratification added to the system highlight other phenomenology. The dimensionality of the system is in particular of great relevance for some aspects (temporal mixing growth and vorticity suppression).

We propose here to carry out a number of simulations to account for: a) the variation over two orders of magnitude of the parameters associated to the presence of a stable density stratification, b) the passive scalar transport across an interface, and c) an increase of the amount of statistics for the turbulence self-diffusion already studied. The results, including raw data, will be made available to the community by giving access to the web disks of our group and by posting the data to HPC repositories (e.g. i-cfd at CINECA, or web sites of international cooperations, like ICTR

The impact of this research is expected in all the fields where turbulence does matter and where a manifestation of the nonuniversal behavior of small scales is closely related to small-scale anisotropy. In this concern, the project is fully interdisciplinary. Potential applications of this research span from atmospheric science to astrophysics, but are ubiquitous also in engineering systems.

Resource awarded: 2.440.500 core hours on CURIE FN (GENCI@CEA, France) and 536.850 core hours on CURIE TN (GENCI@CEA, France)

Passive scalar transport: two intermittency fronts

Self-diffusion: one intermittency front

Scheme of the flow configuration. (a) Initial distribution of the turbulent energy and passive scalar. (b) total and fluctuating density distributions in the periodic box at the initial time in presence of a constant density gradient. (c-f) turbulent energy self-diffusion in 3D [Phys.Rev.E 77 (2008), Physica D 241 (2011)] and 2D, (d-g) preliminary results on 3D and 2D passive scalar transport. (e-g) vorticity self diffusion in presence of a stable density stratification in both 3D (e) and 2D (g) formulations. The vertical dashed lines indicate the position of the intermittency front, the dotted lines indicate the centre of the mixing layer.