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Section: New Results

Analysis and modeling of turbulent flows

Mixing layers between a uniform flow and a shear flow

Participant : Dominique Heitz.

We have addressed the analysis and modelling of non canonical turbulent mixing layers between a uniform flow and a shear flow. From a parametric study by bidimensional direct numerical simulations two mixing layer configurations between a uniform flow and a shear flow have been selected. These two configurations share the same shear flow but have a different uniform flow.

The shear flow was obtained with curved gauze. However the theoretical shear parameter predicted by the literature is different from the value obtained by experiments. In order to study these discrepancies, the flow through a gauze was studied by particle image velocimetry. This allowed the general modeling of the uniform flow through curved wire gauze, leading to linear mean velocity profiles. From a hot-wire anemometry study of the two flow configurations it was observed that one flow behaves like a mixing layer whereas the other flow yields a wake behaviour. The mixing layer indicates an increasing turbulent kinetic energy along its longitudinal development, while the wake exhibits an asymmetry.

Hot-wire anemometry at low velocities

Participant : Dominique Heitz.

A new dynamical calibration technique has been developed for hot-wire probes. The technique permits, in a short time range, the combined calibration of velocity, temperature and direction calibration of single and multiple hot-wire probes. The calibration and measurements uncertainties were modeled, simulated and controlled, in order to reduce their estimated values. This year a patent application has been submitted.

Experimental studies for the assessment of turbulence statistical models

Participants : Patrick Héas, Dominique Heitz, Etienne Mémin.

[In collaboration with G. Artana and P. Minini (Univ. Bueno Aires)]

Selecting directly from images the most likely scaling motion priors enables the recovery of physical quantities related to the energy flux and the flow regularity. Such measurements are of major interest for turbulence studies. In particular, determining the energy flux across scales and characterizing intermittency is very important to assess the relevance of the statistical models proposed for atmospheric turbulence. Although, the measurement of flux and atmospheric flow regularity has already been obtained previously using in situ data, it required an important measurement campaign lasting several years based on sensors placed on airplanes. Therefore, the proposed motion estimation technique described above represents an attractive tool since it enables the direct estimation of these quantities from a couple of images. A paper concerning an atmospheric turbulence study using Meteosat Second Generation (MSG) images has been accepted in the journal Tellus A. Experimental studies of three-dimensional turbulence behind a grid or in the wake of a cylinder has been performed and will be submitted in the journal Experiments in fluids. New experiments for the assessment of turbulence statistical models are currently going on in collaboration with the laboratory of fluid mechanics and turbulence scientists in Argentina. They focus on two-dimensional turbulence of soap films visualized with a Schlieren imagery system. The goal of this work is to validate experimentally the theoretical model predicting non-intermittent inverse energy cascades in pure two-dimensional flows