INFORMATIVO ABCM Nº 026/08 – Post Doctoral Position in Fluid Mechanics FAST –CNRS (Orsay, France)

Post Doctoral position in Fluid Mechanics
FAST –CNRS (Orsay, France)

Title: CONTROL OF CONVECTIVE INSTABILITIES IN A RAYLEIGH-BÉNARD-POISEUILLE FLOW
Duration of the contract: 24 MONTHS
Expected starting date: september 2008 or october 2008

Scientific Context: The study is concerned with thermoconvective hydrodynamic instabilities of open flows, and more precisely with the experimental characterization of the convection patterns that develop in a channel flow heated from below.

The long term objective is the improvement of CVD processes such as those encountered in glass coatings or electronic chips. In such processes a mixture of reacting and inert gases is introduced in a reactor where the lower heated surface constitutes the substrate. The corresponding configuration can be viewed as combining a Poiseuille flow subjected to heating from below, hence possibly giving rise to a Rayleigh-Benard-Poiseuille type instability. In the range of parameters usually encountered in such processes, the longitudinal pressure gradient coupled to the unstable temperature gradient results in the formation of stationary convective rolls aligned with the main flow direction. These longitudinal rolls may result in an inhomogeneity of the deposit, thereby impairing the quality of the process.

An experimental set-up has been developed in our laboratory in order to study the great variety of flow structures likely to be found in such configurations. It consists of a 3 meter long rectangular channel heated from below and cooled from above. The working fluid is air. Great attention was paid to the boundary conditions, thereby allowing us to observe fully established longitudinal rolls for a large range of Reynolds and Rayleigh numbers.

An ongoing study has already allowed us to characterize the longitudinal rolls. We have shown that they can give rise to oscillating structures, the wavy instability, provided one constantly triggers the flow with appropriate perturbations. We have established that this instability is convective and started to quantify its spatial development (wavelength, phase velocity and spatial growth rate) in its linear regime. We have thus determined the stable and unstable modes and characterized the most amplified mode for a given set of parameters (J. Fluid Mech., 542 (2005), 175-194).

Objective: The aim of this study is to continue this investigation in order to improve our understanding of these convective instabilities for ultimately designing an open loop control. The method is to make use of the convective nature of the instabilities to carry out flow manipulations in order to master the growth of the oscillations, the transition location and the subsequent evolution of the instability waves. In particular this study will focus on the non-linear regime of large amplitude waves, possibly saturated, in order to reach the most efficient configuration from the standpoint of the homogeneity of the local mass transfer. This study will be carried out experimentally using PIV measurements as well as flow structure visualizations, in conjunction with efficient quantitative image analysis.

Collaborations: This work will benefit from collaborations with two others laboratories which carry out coordinated numerical simulations, addressing both the temporal as well as the spatial growth of this instability.

Profil: We are looking for an experimental scientist qualified in fluid dynamic area. Preference will be given to candidates with experience in thermo convective flows and their instabilities. An experience in MATLAB would be valuable

Applications will have to be addressed as soon as possible with a detailed CV to: Sophie Mergui:
Tél.: 01 69 15 80 47
@ mail: mergui@fast.u-psud.fr

Deadline for application: march 30th, 2008.
Web site : https://www2.cnrs.fr/DRH/post-docs08/?pid=8&lang=en